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The Lymph Node RevisitedDevelopment Morphology Functioning and Role in Triggering Primary Immune Responses.

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THE ANATOMICAL RECORD 293:320–337 (2010)
The Lymph Node Revisited:
Development, Morphology, Functioning,
and Role in Triggering Primary Immune
Responses
GUY SAINTE-MARIE*
Département de pathologie et biologie cellulaire, Université
de Montréal, Montréal, Québec, Canada
ABSTRACT
Scenarios have been proposed to explain how lymphoid components
of a lymph node favor the encounter of a drained antigen with a circulating competent naı̈ve lymphocyte to trigger a primary immune response.
However, these scenarios rest on incorrect concepts about the organ. This
situation resulted from a loss of interest for studies on in vivo lymphoid
organs due to a widespread switch, decades ago, to work on suspended
lymphoid cells. However, an in vivo holistic study of the organ continued
in our laboratory. The present review synthesizes resulting knowledge on
lymph node morphology and global functioning. We show that the opening of an afferent lymphatic vessel into the subcapsular sinus is the focal
point from which the related portion of a lymph node—a node compartment—is developed. As to the formation of a compartment’s lymphoid
components, it is neonatally orchestrated by the dichotomic nature and
distribution of antigens in this subcapsular sinus, which determines a dichotomic recruitment of circulating cells and the compartment’s architectural complexity. The transport process of an antigen from a given tissue
territory into restricted sites of the draining compartment further defines
its local morphological features and activities, while providing the possibility to reduce the wandering of a short-lived naı̈ve cell through innumerable target-devoid sites. We also explain that the nodal lymphoid
components are not implicated in the triggering of primary responses, but
are rather products of such responses. Scenarios for the triggering of primary responses, consistent with real node morphology and functioning,
C 2010 Wiley-Liss, Inc.
are proposed. Anat Rec, 293:320–337, 2010. V
Key words: lymph node development; lymph node morphology;
lymphnode functioning; lymphocyte quest; triggering of primary immune responses; rat
In 1984, Nossal wrote ‘‘A readership consisting of primarily anatomists has every right to question the favorite sport of research workers in cell immunology. This is
to take a lymphoid tissue and totally destroy its beautiful and elaborately designed architecture to obtain simple cell suspension of lymphocytes, which are then asked
to do more or less all the jobs of the original anatomic
masterpiece.’’ Nonetheless, in vitro work prevailed
largely because of the great difficulty of in vivo experimentation on lymphoid tissues. More recently, Gretz
et al. (1996) recalled that while in vitro studies yielded
notions on the activation of T and B cells at the subcelluC 2010 WILEY-LISS, INC.
V
*Correspondence to: Guy Sainte-Marie, Faculté de médecine,
Département de pathologie et biologie cellulaire, Université de
Montréal, C.P. 6128 succursale Centre-ville, Montréal, QC, Canada H3C 3J7. Tel: 450-449-1319. Fax: 418-775-0740.
E-mail: bernard.sainte-marie@dfo-mpo.gc.ca
Received 23 December 2008; Accepted 18 September 2009
DOI 10.1002/ar.21051
Published online in Wiley InterScience (www.interscience.wiley.
com).
THE LYMPH NODE REVISITED
lar level, the efficiency of the immune system in vivo
depends on a higher level of orchestration that occurs in
lymph nodes, where most primary immune responses
are presumed to be triggered. They added that activation requiring the encounter of an antigen (or an antigen-presenting cell: APC) and a rare competent
circulating naı̈ve lymphocyte—and how that encounter
occurs—depend on morphological features of the lymph
node’s parenchymal components, so that knowledge of
lymph node morphology is essential for understanding
its immune function. This opinion was shared by Cyster
(1999), Crivaletto et al. (2004), and Sixt et al. (2005)
who along with Gretz et al. (1996) proposed scenarios for
the triggering of a primary response in the lymph node.
However, these scenarios may directly transpose in vitro
findings to the in vivo condition, while ignoring the
organ’s complexity. Moreover, as will be demonstrated
below, the lymph node’s assumed morphology rests on
some inadequate morphological concepts and it is inconsistent across the various scenarios, reflecting confusion
in the field.
This review describes the basic morphology of a normal lymph node. It explains how some of the more popular misconceptions about lymph node morphology and
functioning arose, and why they are inadequate as a basis for elaborating scenarios on the triggering of a primary response. Alternative views are presented that are
derived mainly from a synthesis of our long-term, mostly
in vivo work on the rat lymph node undergoing various
spontaneous immunological states that reveal natural
reactions. We explain how drained antigens and the
influenced subcapsular sinus play a primordial role in
the neonatal emergence of nodal lymphoid components
and in orchestrating their activity. We show how antigen
dichotomy—the partitioning of antigens into those causing either a cellular or humoral response—can account
for the organ’s complexity and dichotomic recruitment of
circulating lymphocytes. These features facilitate the encounter of lymph-carried and blood-borne related immunogenic elements, and therefore, the triggering of a
response. We also consider how the recruitment of circulating cells may have become more efficient through the
elimination of time-consuming aspects of a lymphocyte
quest for its target. Finally, we elaborate new and morphologically more realistic proposals on the triggering of
primary cellular and humoral responses in the lymph
node.
MORPHOLOGY OF THE LYMPH NODE1
Since the morphological features of the lymph node
are essential for understanding its functions, they are
summarized here. Though our work focused on the rat,
nodal morphology is basically similar among eight examined mammal species including the human and the
mouse (Bélisle and Sainte-Marie, 1981a). However,
interpretation of single lymph node sections can be complicated by: (1) irregular occurrence, but more often in
1
Note that this review uses the terminology we have adopted
over the years to designate lymph node components; other terms
may be used when necessary but are put into quotation marks.
Lymph node terminology has been quite labile over the years, and
changes in terminology often have not been well justified, contributing to confusion in the field.
321
some species and in large than in small lymph nodes, of
variably developed connective tissue trabeculae or septae
arising from the capsule, which may partition a lymph
node to various extents (Yoffey and Courtice, 1970); (2)
changes in the appearance of structural components
with cutting angle because lymph node and compartment architectures are non-uniform; (3) difficulty of
clearly outlining the diverse nodal lymphoid components
in standard tissue sections; and (4) intrinsic and extensive variability in the number, size, shape, and content
of these components within and among compartments,
lymph nodes, and individuals (see for example, the
extreme variability in lymphocyte content of the subcapsular sinus described in Sainte-Marie et al., 1982;
Sainte-Marie and Peng, 1990b; Sainte-Marie, 2001).
These complications, combined with often limited organ
samples, contribute to explain why the lymph node has
remained poorly understood and why contradictory
reports, conclusions, and functional hypotheses abound
in the literature.
Although random sections of lymph nodes may be
variable and puzzling in appearance, their basic architectural organization is simple and consists of a structure called a compartment. The compartment is the
portion of a lymph node associated with the opening of
each afferent lymphatic vessel, or each of its terminal
branches, into the subcapsular sinus (Bélisle and SainteMarie, 1981b; Aijima et al., 1986; Hoshi et al., 1997).
The smallest lymph nodes have a single afferent lymphatic vessel and a single compartment (Bélisle and
Sainte-Marie, 1981b). In larger organs, the number of
compartments equals the number of afferent lymphatics
in small mammals or the number of terminal branches
of afferent lymphatics in large mammals (Bélisle and
Sainte-Marie, 1981b; Aijima et al., 1986). Here and there
at the margin of each compartment, cortical gaps connect the subcapsular sinus to medullary sinuses (Fig.
1A, B). A compartment also has a subsinus layer, a peripheral and a deep cortex, as well as medullary cords
and sinuses (Fig. 1C, D).
Compartmentation was demonstrated by the subcutaneous injection of a small dose of tracer (Sainte-Marie
et al., 1982). The tracer entered the subcapsular sinus of
the single compartment associated with the lymphatic
vessel draining the injected tissue territory. The tracer
then flowed from the lymphatic opening towards and
into the closest cortical gaps (Fig. 1C). Thus, the drained
material from a given tissue territory usually influences
only those components of the connected compartment
(Sainte-Marie et al., 1982). This is contrary to the common view of the lymph node being one functional unit,
such that the content of any of its afferent lymphatics is
assumed to spread throughout and influence the whole
organ. This view arose from observations made after
locally injecting a large dose of tracer that filled all
sinuses of the draining lymph node (Drinker et al.,
1934). The dose was administered to demonstrate the
lymph node’s great filtration capacity, but it was physiologically unrealistic.
Recently, Gretz et al. (1997) recognized the existence
of the compartment, but renamed it a ‘‘lobule.’’ Their
description of a ‘‘lobule’’ in the rat lymph node is compatible with ours; however, their statement that the
mouse lymph node has a single ‘‘lobule’’ disagrees with
observations in Bélisle and Sainte-Marie (1981a). Gretz
322
SAINTE-MARIE
et al. (1997) reported that in larger mammals, the
‘‘lobules’’ are outlined by radial fibrous trabeculae (more
correctly, septae). Although septae may indeed be seen
in some species (Yoffey and Courtice, 1970), they are not
a basic feature of lymph nodes with multiple compartments (Sainte-Marie et al., 1982). For example, there
are few septae in the rat’s multi-compartmented lymph
nodes.
Peripheral Cortex and Subsinus Layer
Fig. 1. Lymph node compartment. A: Minute lymph node with a single compartment. (c) Indicates the capsule, (s) subcapsular sinus, (o)
opening of the afferent lymphatic into the sinus, (p) peripheral cortex,
(d) deep cortex, and (e) efferent lymphatic. The drained lymph, carried
into the subcapsular sinus via the opening, enters medullary sinuses at
the margin of the peripheral cortex (arrows). B: A lymph node with three
compartments. The drained lymph further reaches medullary sinuses
via cortical gaps (g) between individual compartments. An often wider
expansion of peripheral cortex beyond the deep cortex center and
details of the medulla are not shown for the sake of simplicity. C: Compartment details. The peripheral cortex has folliculo-nodules (fn) and an
extrafollicular zone (e). The deep cortex has a center (DCC) and a periphery (DCP). The arrowhead points to a deep cortex sinus arising as a culde-sac in the periphery. A compartment also has anastomosed medullary cords (mc) surrounded by medullary sinuses (ms) that fuse to form
the efferent lymphatic at the lymph node’s hilus. The horizontal arrows
above the figure delimit the two domains of the subcapsular sinus and
peripheral cortex: one above the deep cortex center (aDCC) and one
beyond it (bDCC). The thick arrows trace lymph flow whereas the
others show the migration pathways of recruited circulating lymphocytes. From the subcapsular sinus and HEVs (ovals) of the extrafollicular zone–aDCC, cells committed to cellular responses (thin, solid
arrows) migrate to the DCC. Cells committed to humoral responses
(thick, stippled arrows) migrate to medullary cords by way of the DCP
for those recruited aDCC. Unconcerned drained cells (broken arrow)
leave the subcapsular sinus via cortical gaps. D: Left: HEVs (dark) run
in the extrafollicular zone and deep cortex periphery. At the corticomedullary junction, they become regular medullary venules (shaded). Right:
Capsule-borne coarse reticular fibers extend in parallel across the subcapsular sinus and extrafollicular zone; those above the deep cortex
center bend and extend into the deep cortex periphery. Reproduced
with permission from Anat Rec;209:95–104 and Anat Rec;245:593–620.
Until about 1962, the lymphoid population present
between the subcapsular sinus and the medulla of a
lymph node was termed cortex, which was further
resolved into a peripheral and a vaguely demarcated
deep cortex. It was then found that lymphocytes comprise B and T cells, and that T cells, responsible for cellular responses, are located in the deep cortex (Oort and
Turk, 1965). Subsequently, components that had been so
far termed peripheral and deep cortex became known as
the ‘‘cortex’’ and ‘‘paracortex,’’ respectively. For reasons
that will become apparent below, we continued to use
the original terms of cortex including a peripheral and a
deep cortex.
The peripheral cortex underlies the subcapsular sinus
entirely. It tends to stretch symmetrically around the
lymphatic opening and to thin with increasing distance
(Fig. 1A). It contains a superficial row of folliculo-nodules, each formed of a B-cell follicle covering to various
extents a nodule (or germinal center) (Sainte-Marie and
Sin, 1970). Folliculo-nodules are separated by an extrafollicular zone (Fig. 1C, D); they are not discussed further since they are not implicated in primary responses.
An elusive subsinus layer is sandwiched between the
subcapsular sinus and the peripheral cortex. Its fiber
network supports reticular-like cells interspaced by lymphocytes and APCs (Lind, 1968). The subsinus layer is
about 25 lm thick over the extrafollicular zone but thins
over outwardly bulging folliculo-nodules. This layer complements the screening activity of the sinus wall
(Sainte-Marie and Peng, 1985a).
Deep Cortex
The ‘‘paracortex’’ was initially defined as a uniform
layer of T cells underlying the whole peripheral cortex of
a lymph node (Gutman and Weisman, 1972). Actually,
the deep cortex of a compartment is hemispherical or
semi-ovoid, centered on the lymphatic opening (Fig. 1A),
and it usually does not extend horizontally as much as
the peripheral cortex above (Bélisle and Sainte-Marie,
1981c; Aijima et al., 1986; Okada et al., 2002; WillardMack, 2006). The deep cortex has a center, where T cells
of cellular responses are engendered, and a periphery
underlying the center (Fig. 1C) (Bélisle and SainteMarie, 1981d). The periphery is actually an extension of
the extrafollicular zone of the overlying peripheral cortex, with the additional feature of lymphatic culs-de-sac,
termed deep cortex sinuses, which are continued by
medullary sinuses. Hence, the deep cortex is not totally
unlike the peripheral cortex and it is, therefore, inappropriate to call it ‘‘paracortex,’’ which etymologically
implies a different component situated beside (para) the
cortex. Note that because the deep cortex does not form
a continuous layer in a lymph node, we previously
THE LYMPH NODE REVISITED
referred to its discontinuous portions as deep cortex
‘‘units’’ (Bélisle and Sainte-Marie, 1990). Considering
that each lymph node compartment has a single portion
of deep cortex, the word unit is superfluous and is now
discarded except when quoting.
Representation of the deep cortex and of its relationship to the peripheral cortex in single tissue sections
may be puzzling, due to variable development of individual compartments and the cutting plane (Fig. 2A–D). In
some lymph nodes, the peripheral cortex of neighboring
compartments does not extend beyond their respective
deep cortex, and therefore, these deep cortices may
appear to form a continuous layer (Bélisle and SainteMarie, 1981a). In some compartments, to the contrary,
the peripheral cortex extends far beyond the limit of the
deep cortex, and then, depending on the cutting plane,
tissue sections may exhibit only peripheral cortex so
that a compartment appears to be devoid of deep cortex
(Fig. 2C). Various other configurations may occur
(Sainte-Marie et al., 1990).
The fact that the deep cortex usually underlies only
part of the peripheral cortex of a compartment makes the
cortex as a whole non-uniform, contrary to the state of
uniformity implicit in the popular concept of ‘‘paracortex.’’
This difference is of paramount importance, for either
state implies different patterns for recruitment of circulating cells. A uniform cortex would result in uniform
recruitment. Instead, the peripheral cortex and subcapsular sinus are actually divided into domains, one above the
deep cortex center (aDCC) and one extending beyond the
deep cortex center (bDCC), that recruit differently (Fig.
1C). The aDCC recruits elements involved in both cellular
and humoral responses, whereas the bDCC recruits only
elements committed to humoral responses (see below). A
third domain located above the deep cortex periphery was
also distinguished (Sainte-Marie and Peng, 1996), but it
is not essential to the present discussion.
High Endothelial Venules
These peculiar vessels recruit blood cells and were
called ‘‘postcapillary venules’’ (Pirro, 1954; Burwell, 1962)
before the term HEV was largely adopted (Anderson and
Anderson, 1975). HEVs were thought to be restricted to,
and scattered throughout, the ‘‘paracortex’’ (Anderson
and Anderson, 1975; Ford and Smith, 1982). It was and
continues to be claimed that lymphocytes enter lymph
nodes via HEVs only and at random (Gowans and
Knight, 1964; Rouse et al., 1984). How these misconceptions arose will be examined in a later subsection.
Actually, HEVs occur in the extrafollicular zone of the
peripheral cortex and the continuing deep cortex periphery; they transform into regular venules upon entering
the medullary cords (Fig. 1C) (Sainte-Marie and Sin,
1970). Exceptionally, an HEV may cross the deep cortex
center and directly enter a cord; nonetheless, lymphocytes traveling in the HEV perivascular channels do not
mix with lymphocytes in the center as demonstrated in
athymic animals (Sainte-Marie et al., 1984). In addition,
HEVs are commonly described as being uniform, but
they have generally been studied under conditions that
do not reveal the great plasticity in the features of their
individual endothelial (or HEV) cells. Striking differences may be apparent even in adjacent cells (for a general
review on HEVs, see Sainte-Marie and Peng, 1996).
323
Fig. 2. Normal rat lymph nodes stained using the Dominici technique. A: Cervical lymph node cut in a plane showing a single compartment whose peripheral cortex extends greatly beyond the deep cortex
(d) (about 60). B: Parathymic lymph node. The cutting plane shows
the deep cortex of two compartments but no cortical gap in the upper
part of the organ; gaps are at other levels (about 20). C: Cervical
lymph node cut in a plane showing no deep cortex, the peripheral cortex appears underlaid by medulla only; a small deep cortical element is
at another level (about 15). D: Cervical lymph node whose cutting
plane shows the deep cortex (d) of three compartments overlaid by a
seemingly undisrupted peripheral cortex. The differences in the density
of their dark lymphocyte population betray variations in the intensity of
their individual activity (about 60). Reproduced with permission from
Am J Anat;164:275–309 and Anat Rec; 199:45–59.
Hence, HEV cells can vary markedly in shape and size:
the lowest are squamous, the highest are narrowed and
may exhibit a blastoid aspect (Dabelow, 1938; Burwell,
1962; Sainte-Marie, 1966). Lymphocytes are present in
variable numbers in individual interendothelial spaces
(Norberg and Rydgren, 1978) and in the expandable subendothelial space present between each HEV cell and
the basement membrane (Fig. 3A) (Sainte-Marie, 1966;
Mikata and Niki, 1971). Note that in comparison to the
rat, the architecture of the lymph node of the commonly
investigated mouse is more difficult to interpret because
the peripheral cortex and associated HEVs are less
developed, and this in turn may explain why the opinion
that HEVs are absent from the peripheral cortex is so
persistent.
Reticular Fiber Network and Pathways of
Transcortical Cell Migration
Coarse fibers 1–4 lm wide, lined by pleomorphic fibroblast- or reticular-like cells, arise perpendicularly from
324
SAINTE-MARIE
Fig. 3. A: HEV wall. Between each distorted HEV cell and its very
dark basement membrane, an expandable subendothelial space shelters subendothelial lymphocytes in varying number per space; one
space has an unusual abundance of them. Perivascular channels, outlined by reticular fiber material, cuff the vessel. The first channel, next
to the endothelium, is narrower and exhibits flattened lymphocytes. B:
Prevailing view of the process of random blood lymphocyte recruitment. A luminal lymphocyte (0) is attracted to an HEV cell and rolls on
it (1–2), crossing the basement membrane at the interendothelial junction (3). C: Author’s proposal: The lymphocyte (0) is already rolling on
the endothelium. At step 3, rather than crossing the basement membrane, it may enter a subendothelial space (4). After interaction there,
it crosses the basement membrane (5) and enters the first perivascular
channel (6) where also it might interact and/or enter the second channel (7). Reproduced with permission from Anat Rec; 245:593–620.
the capsule but almost only over the extrafollicular zone
(Fig. 1D). They run parallel through the subcapsular
sinus, subsinus layer, and extrafollicular zone. Between
the coarse fibers in the extrafollicular zone, fine fibers
knit small communicating alveoles, each hosting a few
lymphocytes (Sainte-Marie and Sin, 1970). Fiber material outlines one, but more often two or three, roughly
concentric perivascular channels that surround each
HEV (Sainte-Marie, 1966). The first channel, immediately cuffing the HEV endothelium, is narrow and hosts
distorted lymphocytes. The second and third channels
are dilated to varying degrees and contain more rounded
lymphocytes. At the flattened face of the deep cortex center, the parallel fibers and HEVs bend and course into
the deep cortex periphery to join medullary cords below.
The extrafollicular zone and continuing deep cortex
periphery connect both gateways—the subcapsular sinus
and HEV network—for entry of lymphocytes into the
compartment. Together, they form a continuing pathway
of parallel fibers and perivascular channels that orient
the migration of incoming lymph-carried or blood-borne
cells. This pathway guides a competent cell committed to
either a cellular or a humoral response towards the deep
cortex center or a medullary cord, respectively, where it
yields effector-cells of a response (Fig. 1C). In other
respects, it is noteworthy that the pattern of the fiber
network differs in the different components of the compartment (Fig. 1D), allowing them to be better delimited
with silver impregnation (Sin, 1967; Sainte-Marie and
Sin, 1970).
The Concept of ‘‘Paracortical Cords’’
Kelly (1975) suggested that ‘‘paracortical cords,’’ favoring T cell activation, are the basic elements of a ‘‘paracortical area.’’ A ‘‘cord’’ was said to comprise a central
HEV with a perivascular cuff of lymphocytes enclosed in
a reticulin framework surrounded by an alleged ‘‘paracortical sinus.’’ The area shown in Kelly’s supporting
Fig. 2 is in fact the deep cortex of a compartment, whose
center is the site of formation of effector-cells of cellular
responses. However, no ‘‘cord’’ is apparent in the illustrated deep cortex center and this is consistent with the
demonstrated absence of HEVs in the center (refer previous section).
More recently, Gretz et al. (1997) reviewed lymph
node organization and contemplated how a primary cellular response might be triggered in the ‘‘paracortex.’’
THE LYMPH NODE REVISITED
Their review was offered as a functionally-oriented summary of their understanding of ‘‘paracortex’’ architecture
and presented a hypothesis regarding its microanatomy
and the rules governing an orchestrated movement of
cells and soluble factors within it. Actually, they adopted
Kelly’s general concept of ‘‘paracortical cords,’’ while
stating as a caveat that they had not been able to identify deep cortex ‘‘units,’’ and therefore, could not integrate them in their hypothesis. However, Fig. 7B in
Gretz et al. (1997) shows an accumulation of lymphocytes under peripheral cortex, which is probably a deep
cortex ‘‘unit’’ that appears round due to the cutting
plane. Intriguingly, in an article published only a year
earlier, Gretz et al. (1996) had acknowledged the existence of deep cortex ‘‘units.’’ Moreover, as pointed out
above, the existence of deep cortex ‘‘units’’ was confirmed
by others through histology (Ajima et al., 1986) and
more recently by plastic casting of lymph nodes (Okada
et al., 2002).
Gretz et al. (1997) modified Kelly’s (1975) concept of
‘‘paracortical cords.’’ They correctly recognized the narrow first perivascular channel cuffing an HEV endothelium as described earlier (Sainte-Marie, 1966). However,
they renamed it ‘‘perivenular’’ channel, renamed the second and third channels ‘‘corridors,’’ and renamed Kelly’s
‘‘paracortical’’ sinus a ‘‘cortical’’ sinus and added that
such sinuses fill the extrafollicular zone. Figure 4 of
Gretz et al. (1997) was presented to support the existence of ‘‘cortical’’ sinuses, but none can be seen.
Recently, Sixt et al. (2005) reported that drained dextrans accumulate in ‘‘paracortical’’ sinuses, but none
were illustrated and to our knowledge they do not exist.
Gretz et al. (1997) said ‘‘that the elegant form’’ of the
paracortical cords ‘‘dictates, in large part, the marvelous
efficiency of secondary lymphoid tissues in facilitating
and regulating immune responses.’’ Ultimately, they
asked ‘‘If the foregoing description of the paracortical
cord is substantially correct, why has its existence been
such a well kept secret?’’ The answer was that its complex geometry probably ‘‘precludes simple assessment of
precisely how much of the paracortex consists of paracortical cords.’’ They stated that Kelly’s (1975) serendipitous study had revealed that cords are the essential
constituent of ‘‘paracortex.’’ It is not clear how this could
be concluded since Kelly had not carried out a tridimensional analysis of the presumed cords, a necessary task
given their reportedly complex geometry and the
acknowledged difficulty of distinguishing them in normal
lymph nodes (Kelly, 1975; Gretz et al., 1997). Moreover,
it is disconcerting that the ‘‘paracortical cords’’ were not
mentioned 1 year earlier in a proposal on how ‘‘paracortex’’ morphology facilitates the triggering of a cellular
response (Gretz et al., 1996) or a few years later in an
article examining APC-lymphocyte interaction (Gretz
et al., 2000). In our opinion, the existence of these cords
is unsubstantiated.
FUNCTIONAL ASPECTS OF THE LYMPH
NODE COMPARTMENT
Transport of Soluble Immunogenic Factors by
the Reticular Fiber Network
How do antigens penetrate the compartment’s parenchyma from the subcapsular sinus? An antigen causing
325
Fig. 4. Development of a compartment’s cortex; drawings based
on silver-impregnated sections, which blackens reticular fibers and
lymphocytic nuclei present in the fiber-poor deep cortex center and
folliculo-nodules. A: Rat, about 1 min old. The cortex to be has simply
a framework of fine fibers. B: Rat, about 1 day old. A tiny deep cortex
center has emerged, aligned with the overlying lymphatic opening. It
has a decreased density of fibers and a high level of large blastrelated lymphocytic cells. C: Rat, about 1 week old. The deep cortex
exhibits some mature features. Compared to day one, the density of
fibers and of blast-related cells have decreased. HEV-precursors
(oval-like) appear in the nascent peripheral cortex and deep cortex periphery, both of which have a high density of fibers. D: Rat, about 2
week old. The now typical deep cortex has a further decreased density of fibers and blast-related cells in its center but a greater level of
lymphocytes. Lymphatic sinuses, loaded with emigrating lymphocytes,
form as culs-de-sac in the deep cortex periphery. Follicles, still without
a nodule (or germinal center), have emerged in the peripheral cortex.
Modified with permission from Am J Anat;164:275–309 and Histol Histopathol;16:771–783.
a cellular response is carried by an APC, one causing a
humoral response is transported by fibers. In 1963, Moe
suggested that the lymph node’s reticular fibers transport various soluble substances. Anderson and Anderson
(1975) later reported that drained horseradish peroxidase is dispersed in the reticulum and sheaths around
HEVs before permeating their interendothelial spaces
and entering their lumen. However, we could not detect
the tracer in these spaces and lumen in their supporting
Fig. 19. Besides, permeation obtained with horseradish
326
SAINTE-MARIE
peroxidase was suspicious because this tracer alters vascular permeability (Contran and Karnovsky, 1967).
Injecting instead a foreign albumin, Sainte-Marie and
Peng (1986) observed its transport by fibers running
through the thickness of the subcapsular sinus, subsinus
layer, extrafollicular zone, deep cortex periphery, and
adjoining HEV basement membranes; no endothelial
permeation was detected. Recently, Gretz et al. (2000)
stated that drained fluorophore-labeled low MW dextrans and proteins diffused from peri-HEV fibers in
between HEV cells and into an HEV’s lumen. However,
in the absence of counterstaining with a standard technique, it is questionable whether the small pale structures in their supporting Fig. 3A, B are HEVs. This
uncertainty also applies to the only magnified structure
presented in their Fig. 3D, which shows no HEV cells—
an essential feature for identifying an HEV. In addition,
no interendothelial fluorescence is detectable to support
permeation as the delivery mode for the astonishingly
abundant fluorescent material seen in the presumed
HEV lumen. Hence, in our opinion, HEV permeation has
not been demonstrated. Still, a fiber-carried soluble factor can reach HEV cells, as confirmed by the presence in
fibers and HEV basement membranes of typical metachromatic products spontaneously released by mast cells
in the extrafollicular zone (Sainte-Marie and Peng,
1990a). Incidentally, on the basis of a conductive activity,
the reticular fiber network was renamed ‘‘reticular conduit’’ by Gretz et al. (1996), which is a reductionist term
unlike the accepted one.
Transport of soluble immunogenic factors is unidirectional. A barrier, present in each capsule-borne parallel
fiber at the outer wall of the subcapsular sinus, prevents
transported antigens from disseminating in the capsule
and perinodal tissue (Sainte-Marie and Peng, 1986).
Conversely, antigens deposited on the capsule spread in
its fibers but not in their sinusal extensions. By contrast,
antigens deposited on the thymic capsule are transported by capsule-borne fibers into the thymic parenchyma (Sainte-Marie et al., 1986a).
The quest is further facilitated by restricting the delivery of those antigens causing cellular responses only to
the domain of the subcapsular sinus and peripheral cortex above the deep cortex center (see below). This prevents the wasteful wandering of concerned T cells into
the peripheral cortex beyond the center, where only humoral responses are dealt with. Moreover, if T cells committed to cellular responses entered the peripheral
cortex extending beyond the deep cortex center, obstacles
would prevent them from reaching the center. First, T
cells would have to migrate around the large folliculonodules and HEVs in this domain of peripheral cortex
and cross its numerous reticulin walls set perpendicularly to the path of their migration towards the center.
Second, such a migration of T cells would be against the
flow of numerous cells committed to humoral responses
moving towards the medulla, thus hindering both migrations. Third, cell congestion would increase with proximity to the deep cortex center as all incoming T cells
moved towards an increasingly smaller volume of peripheral cortex, rendering opposing migrations even
more difficult. Clearly, the entry of concerned T cells in
the peripheral cortex above the deep cortex center, demonstrated below, provides the most expedient and effective pathway for these cells to reach the deep cortex
center.
The above constraints on T cell quest/migration have
not been perceived so far because of the widely held concepts that cells enter a lymph node only at HEVs and
that HEVs occur only in the deep cortex. Consequently,
a randomly incoming T cell would directly enter the
deep cortex. These concepts have endured, even though
a few studies have demonstrated them to be incorrect,
because it is difficult to perceive how unrealistic they
are, especially when one is unaware of their genesis as
is common nowadays. Therefore, it is appropriate to
review the origin of these concepts and examine alternative views.
Restricted Lymphocyte Wandering for a
Successful Quest
Around 1960, the role of thymic lymphocytes was
revealed by neonatal thymectomy (Miller, 1961); this
breakthrough eventually allowed great advances in
knowledge of immune reactions at the cellular and subcellular levels. But it also initially generated a rush of in
vivo experiments, implicating lymph nodes whose morphology and functioning were still poorly understood.
This resulted in many premature observations that
could not be put into proper morphological context, and
therefore, the interpretations or functional concepts that
followed were questionable. A striking example is an initial report that lymph nodes of thymectomized mice
‘‘showed no proper structures and no plasma cells, often
being reduced to a small piece of adipose tissue’’ (Miller,
1962). Reports followed stating instead that the deep
cortex of the lymph nodes of thymectomized mice is
devoid of lymphocytes, hence, it was called a ‘‘thymusdependent zone’’ (Parrott et al., 1966) but also a ‘‘T cell
area’’ or a ‘‘paracortex’’ (Goldschneider and McGregor,
1973), however, these terms still conveyed partly unrealistic concepts. The term ‘‘thymus-dependent zone’’ is a
misnomer (Fossum, 1990) because the deep cortex does
form in athymic animals, except that its center is devoid
of T cells while its periphery is still populated by B cells
To trigger a response, a naı̈ve lymphocyte must contact its target, which may or may not be present in a
body, and when present usually occurs in only a small
territory. In addition, at best one out of 100,000 circulating lymphocytes may be competent to react with any one
of a large mosaic of antigens (Ager, 1994). This context,
at face value, makes it highly unlikely that a naı̈ve cell
will encounter its target. Gretz et al. (1997) realized that
this apparent hurdle had been overcome by an evolved
process of antigen transport from tissues into secondary
lymphoid organs. Instead of a lymphocyte circulating
throughout the entire body, they argued, it needs only to
circulate through these organs and mainly through
lymph nodes, where virtually all primary responses
would be triggered. This is especially important for rare
circulating naı̈ve cells that survive only a few days if
inactivated (Stites et al., 1994). Sainte-Marie and Peng
(1996) further concluded that a circulating cell enters
only a compartment hosting its target, not excluding the
possibility that a cell might enter for an activity other
than triggering a response.
Origin of Popular Concepts
THE LYMPH NODE REVISITED
(Sainte-Marie et al., 1984). As for the term ‘‘T cell area,’’
it implies that the deep cortex contains only T cells,
which is true only of its center. The inadequacy of the
term ‘‘paracortex’’ was explained above. Incidentally, in
two of our earlier works (Sainte-Marie et al., 1981,
1984), we mistakenly labeled the area of peripheral cortex over the deep cortex center (aDCC) as being thymusdependent, because of the occurrence of numerous T
cells there. We should have stated simply that the extrafollicular zone aDCC is the site of entrance and migration of T cells towards the deep cortex center. In the
absence of a thymus, the extrafollicular zone aDCC is
still entered by B cells as is the continuing deep cortex
periphery.
Some of the most enduring misconceptions arose from
the results of an intravenous transfer of labeled thoracic
duct cells to determine whether lymphocytes re-circulate
from blood to lymph (Gowans and Knight, 1964). At the
organismal level, it was reported that labeled cells
appeared to occur in equal concentrations in the various
lymph nodes of the host, which was taken to indicate
random entry. This interpretation was not supported by
quantitative data. Regardless, even if the mix of T and B
cells of various antigen-specificities had been more or
less equally distributed among the various lymph nodes,
this would not prove or disprove either a random or an
antigen-specific entry. At the level of individual lymph
nodes, the earliest observation at 24 hr after transfer,
revealed that labeled cells present in a host’s lymph
node were mostly located in the deep cortex, where some
were associated with HEVs, and that only ‘‘trivial’’ numbers occurred in the subcapsular sinus and peripheral
cortex. These observations yielded the concepts that circulating lymphocytes enter lymph nodes only at HEVs
and that these venules are restricted to the deep cortex.
The concept that cells do not enter a lymph node at its
subcapsular sinus was immediately questionable because
in the course of a study of the thymus (Sainte-Marie and
Leblond, 1958) lymphocytes were observed in the subcapsular sinus of parathymic lymph nodes. This discrepancy prompted an analysis of the experimental basis for
this and other concepts summarized above, which
showed them to be unfounded, largely due to a lack of
observations made at short time-intervals within the
first 24 hr after the transfer (Sainte-Marie, 1975).
To resolve the mode of lymphocyte entry into lymph
nodes, we performed an intravenous transfer of labeled
thoracic duct cells and followed them through high-frequency observations starting soon after injection
(Sainte-Marie et al., 1975). Labeled cells were first seen
in the subcapsular sinus and peripheral cortex, and later
they accumulated in the deep cortex. However, concerns
about the mode of lymphocyte entry were cast into
shadow by a blinding interest for the process of lymphocyte recirculation. The early questionable concepts continued to be uncritically accepted and became
entrenched in the literature despite later divergent findings (Sainte-Marie et al., 1975; Elves, 1977), which were
overlooked but never rebutted. Moreover, the great difficulty of in vivo work on lymph nodes soon fostered a
switch of research efforts to in vitro work using suspended lymphoid cells. As a result, understanding of the
lymph node stagnated. Hence, Gowans wrote in 1996,
that it is still not possible to explain the evolution of any
immune response in terms of the highly dynamic struc-
327
ture of lymphoid tissue in vivo and of the migratory
pathways of co-operating lymphocytes within it, qualifying this situation a ‘‘disgraceful gap in medical knowledge.’’ His review unfortunately did not mention some
studies discussing the route of lymphocyte entry into
lymph nodes and intranodal migration pathways
(Sainte-Marie and Peng, 1980, 1983, 1990), which did
contribute to partly fill this gap (Fig. 2).
The entrenchment of the concept that cells enter the
lymph node only via HEVs diverted interest away from
the subcapsular sinus and subsinus layer. Similarly,
reported variability of individual HEV cells and the existence of subendothelial spaces and lymphocytes were
overlooked because the concept of random cell entry was
accepted. These oversights formed a serious obstacle to
understanding lymph node functioning since these elements and features are of primordial importance, as will
become apparent in the next three subsections.
Entry of Circulating Lymphocytes at the
Subcapsular Sinus
Recent findings revealed that for several days after
birth, lymphocytic cells colonizing the lymphocyte-free
lymph node compartment enter it virtually only at the
subcapsular sinus (Sainte-Marie, 2001). Thereafter, to a
variable extent, lymphocytes continue to enter the compartment via this route as evidenced by many facts
(Sainte-Marie and Peng, 1996), some of which are
recalled here. First, is the frequent occurrence of lymphocytes in the afferent lymphatic vessels (as seen with
a favorable cutting plane of tissue in Fig. 19 in SainteMarie and Sin, 1970) and the subcapsular sinus of older
animals. Entry by this route is also supported by the
presence in this sinus of newly formed marrow B cells
(Brahim and Osmond, 1973) and of lymphocytes soon after transfusion (Sainte-Marie et al., 1975). At times,
moreover, lymphocytes are retained on the wall of the
subcapsular sinus while their number in the corresponding area of subsinus layer and outer stratum of the
extrafollicular zone is reduced (Sainte-Marie and Peng,
1990b). Additionally, in some lymph node compartments
of Xid mice almost devoid of B and T cells, occasional B
cells occur in areas of the subcapsular sinus and accumulate in corresponding areas of the subsinus layer
while the almost HEV-devoid underlying peripheral cortex remains quite lymphocyte-free (Sainte-Marie and
Peng, 1985b).
A contribution of drained cells to the lymphocytic population of the adult lymph node is supported as well by
unusual events in some compartments under an athymic
state. One is the replacement of normal cells of this population by drained altered lymphocytes (Sainte-Marie
and Peng, 1990c). A second event is the atrophy of the
extrafollicular zone in a compartment whose subcapsular
sinus is deprived of drained lymphocytes (Sainte-Marie
and Peng, 1990d). A third event is the formation of a
compartment replica on the outer wall of the subcapsular sinus, which represents a compensatory reaction to
the degradation of the original compartment (SainteMarie and Peng, 1987). Briefly, a replica can arise next
to the lymphatic opening as an intracapsular lymphocytic islet with a high level of blast-related cells, which
recalls the neonatal situation. In the absence of HEVs in
328
SAINTE-MARIE
the capsule, the colonizing cells of the islet should come
from the sinus, as they do in the neonatal state.
Dichotomic Recruitment of Circulating
Lymphocytes due to Antigen Dichotomy
One factor that facilitates the encounter of related elements for triggering an immune response is the topographically dichotomic recruitment of circulating cells
arising from antigen dichotomy, that is, the partitioning
of antigens into groups causing either a cellular or a humoral response. After birth, the earliest drained antigens induce the lymphocytic colonization and
development of a compartment. As the compartment
develops, antigen dichotomy causes the functional division of both the subcapsular sinus and nascent peripheral cortex into two domains, with each domain having a
different recruitment pattern (Fig. 1C). The result is a
non-uniform cortex.
The formation of the two cortex domains is induced by
sequential delivery and differential spreading in the subcapsular sinus of antigens provoking cellular and humoral responses, as follows. Although popular concepts
imply that colonization happens by way of blood lymphocytes entering randomly at HEVs (Harris and Ford,
1963), in fact only T cells are initially implicated (Eikelenboom et al., 1979). Moreover, they enter at the subcapsular sinus, mostly as blast-related elements, settling
on a small surface of the sinus wall opposite the lymphatic opening (Sainte-Marie, 2001). From there, they
gather at a short distance beneath the wall and form a
tiny spheric deep cortex center within a day (Fig. 4A–D)
(Bélisle and Sainte-Marie, 1981e). Days after, the sinus
domain above the center begins to also handle late-arriving drained antigens causing humoral responses that
induce the emergence of peripheral cortex above the center, which is initially constituted of extrafollicular zone
alone. As the latter thickens, it flattens the outer half of
the center and extends under it, prefiguring the deep
cortex periphery. Hence, the tardy arrival of B cells prevents mixing of T and B cells in the emerging center,
whereas the subsequent formation of the deep cortex periphery continues to preserve the integrity of the center’s milieu: B cells do not cross the center to reach the
underlying medullary cords, traveling instead via the periphery as confirmed by immunofluorescence (van Roojen, 1987). Meanwhile, HEV-precursors of the nascent
peripheral cortex and deep cortex periphery develop
HEV traits and a cell-recruiting activity, thus enhancing
colonization. Progressively, the sinus domain above the
center becomes somehow saturated with antigens causing humoral responses. As such antigens continue to
enter the sinus, they spread beyond the center into a
nascent second sinus domain, which will thus deals only
with antigens causing humoral responses. Therefore,
beneath this second domain one finds only peripheral
cortex underlaid by plasmocytic medullary cords. Indeed,
antigens retained by a sinus site enter the portion of peripheral cortex directly beneath, stimulating the development of HEV-precursors and inducing them to recruit
relevant circulating cells. Thus, antigen dichotomy
establishes a topographically dichotomic pattern of antigen presence and influence in the sinus as well as in the
nascent peripheral cortex, which results in a correspond-
ing dichotomic pattern of recruitment of circulating
cells.
A few factors account for this scenario. At birth, antigens causing humoral responses are controlled for some
time by maternal antibodies, probably then halting their
drainage towards lymph nodes. This explains why APCcarried antigens causing cellular responses and relevant
T cells are the first to be drained from tissues and to
enter a lymph node compartment. There, APCs contact
fibers crossing the subcapsular sinus and settle, with
attracted T cells, on the sinus wall opposite to the lymphatic opening. This leads to the emergence beneath of
the deep cortex center with a peculiar milieu propitious
to the development of effector-cells of cellular responses.
By contrast, antigens causing humoral responses are minute, soluble, and possibly more abundant, so they eventually flow farther than APCs, that is, beyond the
center, where they promote the expansion of only peripheral cortex beneath the sinus. In axenic animals there is
very little such expansion (Bélisle et al., 1982) because
the sinus above the deep cortex can cope with an unusually small amount of antigens of both categories. Paradoxically at first glance, such expansion is also very
limited in some neighboring compartments of normal
animals, and particularly, in their mesenteric lymph
nodes. The reason for this peculiarity is a marked development of humoral responses directly in the gut, as
revealed by abundant plasmocytes there, which reduces
the arrival of antigens causing humoral responses to
draining mesenteric compartments. Consequently, the
peripheral cortex does not expand much beyond the deep
cortices, which are therefore close to one another
(Sainte-Marie et al., 1982). This situation is a vestige of
the phylogenesis of the immune system: the formation of
gut plasmocytes precedes the development of lymph
nodes (Good and Finstad, 1967). By contrast, the peripheral cortex expands considerably in some compartments
at other sites and this is matched underneath by formation of plasmocytic medullary cords (Fig. 2A, C). It thus
appears that the extent to which peripheral cortex and
plasmocytic cords are developed beyond the deep cortex
is proportional to the supply of antigens causing humoral responses to individual compartments.
The lymph node is to be viewed as a complement, not
a substitute, to ancestral immune tools. A multitude of
potential encounter sites of related elements improves
the chance of triggering a response. For greatest efficiency of the system, a naı̈ve cell should be activated
wherever it encounters its target and essential mediators within a basic territory of the immune system. Such
a territory comprises: (1) a portion of body tissue that is
drained by (2) a collecting lymphatic vessel, termed an
afferent lymphatic at its junction with (3) a compartment of a local lymph node, over which it spreads its
content. Within a territory, a continuum of events
unfolds that is initiated by the intrusion of an antigen in
the drained tissue, leading to the triggering of a
response in the tissue itself and/or in the compartment
where the lymphatic carries drained immunogenic elements. That activation can also occur in the drained tissue is evidenced postnatally by a late entry of blastrelated cells in the subcapsular sinus and their presence
in the sinus beyond the deep cortex, with these cells
yielding plasmocytes in underlying medullary cords
(Sainte-Marie, 2001). In the adult, moreover, humoral
THE LYMPH NODE REVISITED
responses appear to unfold to a greater degree in some
tissues rather than in draining compartments, as is the
case of the gut where plasmocytes are known to be
abundant.
Antigen-Specific Recruitment of Circulating
Lymphocytes
A process that would further favor the encounter of a
naı̈ve cell and its target is the antigen-specific recruitment of circulating cells, which provides a rational explanation for a suite of intriguing observations on HEVs
in particular (reviewed in Sainte-Marie and Peng, 1996).
As explained above, antigen-specific recruitment is a corollary of the beneficial directed transport of antigens
from a tissue territory into a single lymph node compartment or even limited sites of that compartment. Moreover, specificity is strongly supported, for instance, by
the lymphocytic colonization of neonatal compartments
(Sainte-Marie, 2001). In a random process, colonization
by B and T cells would be expected to occur concomitantly, as both cells would occur irrespective of the presence or absence of proper targets in a compartment.
Instead, T cells account for nearly all colonization activity during the first postnatal week (Eikelenboom et al.,
1979). Moreover, blast-related forms represent 84% of
colonizing cells at 16-20 hr after birth, declining to 21%
at day 21, mostly due to an accumulation of their maturing progeny-lymphocytes prior to emigration from the
lymph node. Such percentages are not compatible with a
random entry of circulating cells of which 0.001% at
most are competent to respond to any given antigen
(Goodman, 1994).
In other respects, a specific recruitment of lymph-carried cells is consistent with the observation that a
drained soluble antigen adheres to the wall of the subcapsular sinus (Sainte-Marie and Peng, 1985c). An APC
does so as well, and may also become associated with
the subsinus layer (Hendricks et al., 1980). There, an
APC could release exosomes by long cytoplasmic processes (Murk et al., 2002; Théry et al., 2002) to expose its
antigens at the surface of the sinus wall. A relevant lymphocyte, probing this wall, may thus contact its target
and be selectively recruited there. As to the specific
recruitment of blood-carried cells, it is supported by the
non-uniform (i.e., focalized and selective) adherence pattern of suspended live nodal lymphocytes to HEVs in
sections of frozen lymph nodes (Sainte-Marie and Peng,
1995) as well as by other observations on HEVs and
lymph nodes under various states, which can be
explained only by specific recruitment (Sainte-Marie and
Peng, 1996). In addition, the marked variations in the
features of individual and even adjacent HEV cells (Fig.
3A) as well as in the numbers of lymphocytes hosted in
their subendothelial spaces reflect the individualism of
their activity (Fig. 3C). By itself, the existence of subendothelial lymphocytes betrays a specific recruitment: the
presence of lymphocytes in a subendothelial space makes
little sense unless they are related cells, recruited by a
concerned HEV cell to participate in a specific interaction. Actually, these lymphocytes are in contact with
abluminal endothelial processes and, moreover, they
interact in groups (Soderström, 1967; Bailey and Weiss,
1975), which implies a common competence. In turn, the
interaction of related cells in a given space requires that
329
they group there, that is, that they be recruited specifically. Indeed, the number of circulating cells that can at
a given time enter one of a multitude of exiguous subendothelial spaces is quite limited, so that selection is the
only process by which rare cells competent to carry out a
particular interaction could become associated in a same
space. That HEV cells are stimulated individually and
react specifically is also indicated by an unusual excess
of lymphocytes in the subendothelial space of an isolated
HEV cell(s) of a restricted HEV area(s). Such excessive
loading betrays recruitment of related lymphocytes by
HEV cells influenced by the same material, the recruited
cells then unusually being retained in their subendothelial spaces due to an anomaly that prevents them from
entering the compartment’s parenchyma. This explanation is consistent with the fact that lymphocyte retention
is by far most often observed under the state of immunodeficiency (Sainte-Marie et al., 1986b); moreover, the
phenomenon mimics excessive lymphocyte retention in
the subsinus layer of immunodeficient Xid mice (SainteMarie and Peng, 1985b). Finally, it is noteworthy that
subendothelial lymphocytes have never been included in
models of random cell recruitment (see Ulrich and
Thorsten, 2003; and Fig. 3B), with which they are
incompatible.
It was suggested that drained cytokines permeating
into an HEV lumen can ‘‘either in solution or immobilized on the luminal endothelial surface act directly on
lymphocytes’’ of the blood (Gretz et al., 1996), but the
authors omitted to consider a comparable role for
drained soluble antigenic materials despite having
reported that they too permeate the HEV endothelium.
Although one may doubt that antigens usually do so,
they nonetheless can reach an HEV cell via its basement
membrane as seen above. There, the antigen could be
immobilized on the luminal face of the membrane and/or
the abluminal face of the HEV cell, becoming accessible
to blood cells probing its subendothelial space (Fig. 3C).
An antigenic material causing a cellular response could
similarly infiltrate a subendothelial space through the
release of exosomes by cytoplasmic processes of an APC
migrating in a perivascular channel. A contact between
a probing blood cell and its target would be the decisive
signal in the cascade of steps leading to cell recruitment
(Sainte-Marie and Peng, 1996).
DEVELOPMENT OF ENTRANCES FOR
CIRCULATING LYMPHOCYTES
Additional aspects of the subcapsular sinus and HEVs
will be examined, especially their interrelated and comparable mode of development or expansion. The almost
concomitant formation of a new sinus area and of fiberlinked HEV areas beneath, both influenced by the same
antigenic material, enhances the recruitment of competent cells and the chances of triggering of responses.
Moreover, consideration of the mode of expansion of the
subcapsular sinus and HEVs can provide clues on their
functioning.
Subcapsular Sinus
Despite rather uniform appearance, the subcapsular
sinus is functionally complex. Because it is shallower
above the bulging folliculo-nodules, lymph flows more
330
SAINTE-MARIE
freely over the extrafollicular zone where the cell-lined
fibers crossing the sinus are concentrated. This favors
the screening of the lymph content there, so that
selected drained elements by far mostly enter the extrafollicular zone (Sainte-Marie and Peng, 1985c, 1986a).
Besides, antigen distribution and recruitment of drained
cells differ in the two sinus domains, as discussed earlier. Furthermore, recruitment of particular cells can
involve only part of a domain: in some compartments,
for instance, the sinus contains numerous mast cells
entering the extrafollicular zone selectively next to cortical gaps (Sainte-Marie and Peng, 1990a).
How does the sinus develop or expand after antigen
dichotomy has neonatally delimited it into two functional domains? After the existing areas of the sinus domain above the deep cortex become almost or completely
saturated with antigens, the entry in the sinus of additional antigenic material causing humoral responses promotes its expansion. Sinus expansion results from the
addition of new sinus area at the margin of a compartment to handle2 excesses of such material and is accompanied by the emergence of peripheral cortex and
plasmocytic medullary cords beneath. This mode of
expansion strongly suggests that the added area selectively recruits drained cells related to the antigens that
initiated its emergence, an interpretation supported by a
neonatal event. Days after birth, there is a high level of
blast-related cells in the subcapsular sinus and the nascent peripheral cortex beneath. Thereafter, the lymphocyte level increases, greatly predominating at about day
13. Nonetheless, blastic forms in the sinus of some compartments are still abundant beyond the deep cortex,
where they can yield a trail stretching from a limited
sinus area to the plasmocytic medullary cords directly
beneath (Sainte-Marie, 2001). This betrays a focalized
and specific recruitment of these cells, which are probably activated in the drained tissue by the same antigens that contributed to induce the formation of the
sinus area where they are selectively recruited.
Expansion of the subcapsular sinus further indicates
that the capacity of a sinus area to handle lymph-carried
elements is limited. This capacity is at least partly constrained by the correlated diameter and length of local
fibers crossing the sinus. In the neonate, the foremost
antigens causing humoral responses are transported
beneath the subcapsular sinus by short extensions of the
fine sinus fibers. These extensions are connected to the
basement membranes of underlying HEV-precursors.
The antigens carried by these extensions stimulate the
development of HEV features in the linked areas of the
HEV-precursors and the recruitment of relevant blood
cells which, in conjunction with drained cells recruited
from the sinus, contribute to the emergence of a thin
nascent peripheral cortex. With further antigenic stimuli, the peripheral cortex thickens while fibers and
HEVs elongate, matching the growing thickness of the
peripheral cortex, as these elements evolve in unison
(Bélisle and Sainte-Marie, 1981e). Fiber diameter simultaneously increases, probably due to the increasing volume of transported drained factors. Fiber growth and
2
Handling refers to all the events implicated in the screening of
lymph-carried immunogenic elements for entry into the
parenchyma.
peripheral cortex thickening continue until antigenic
stimuli peak or some maximal handling capacity of a
sinus area is reached. These events apply as well to the
sinus fibers of the peripheral cortex above the deep cortex center. There, moreover, expansion of the sinus wall
favors more crossings of APCs at spaces between the
parallel fibers, thus allowing a matching expansion of
the deep cortex center.
Clearly, the neglected subcapsular sinus plays a primordial role in the organization and functioning of the
compartment, by screening the content of its afferent
lymph and offering passage to focally selected elements
into the compartment’s parenchyma. Moreover, the
rarely observed and almost ignored lymphatic opening is
the central point from which this architecture is spatially established; this knowledge is essential for understanding the lymph node compartment.
High Endothelial Venules
An HEV arises from the transformation of squamous
endothelial cells of a regular venule under the influence
of an antigenic stimulation. Observations made under
various conditions indicate that development of HEV
cells results from focal and individual cell stimulation,
which is manifest both neonatally and after wherever a
new HEV area emerges (Sainte-Marie and Peng, 1996).
Macrophages were proposed to be the main stimulators
of HEV formation (Hendricks et al., 1980). However,
HEV formation is hindered in Xid animals with abundant macrophages but few lymphocytes (Sainte-Marie
and Peng, 1985b) and more recent observations point to
an essential stimulatory role for drained lymphocytes
and, in particular, antigen-presenting T cells (Pichler
and Wisscoray, 1994). Hence, it appears that development of HEV cells necessitates stimulation involving antigenic material, drained lymphocytes, and mediators:
where they meet in a vessel, a squamous cell becomes
an HEV cell (Sainte-Marie and Peng, 1996).
Neonatally, the foremost antigens causing humoral
responses induce initial HEV cell development in a small
area of regular venules slightly below the subcapsular
sinus (Sainte-Marie and Sin, 1970), and possibly on one
side only of a vessel (Bailey and Weiss, 1975). Moreover,
the extent of individual cell transformation differs,
reflecting uneven individual stimulation. Subsequent
antigens can further transform the earliest HEV cells to
the extent that they can still be influenced; they also
transform squamous cells in a contiguous area of the
same vessels. Thus, HEV elongation mimics subcapsular
sinus expansion. Similarly, the HEV scenario also
implies limits to the number of HEV cells that can be
stimulated at a given time by a given material and to
the extent to which a given cell can be stimulated, with
these limits partly depending on the amount of material
handled by the sinus area above. The scenario of HEV
elongation is repeated until the associated sinus area
has attained a maximal handling capacity, thereby ending elongation of underlying HEVs. However, the HEV
does not remain in a steady state after. Since the influence of a stimulation is transient (Hendricks et al.,
1980), an HEV cell regresses when stimulation fades
and redevelops if re-stimulated. In fact, such dynamics
concern all cell populations of a compartment that are
involved in ongoing responses. Because these
THE LYMPH NODE REVISITED
populations have a defensive role, they would atrophy if
stimuli waned and were not followed by new ones.
Later in life, new antigenic stimuli accentuate the
transformation of lesser developed HEV cells (Henry
and Beverly, 1976), which are either variably understimulated or regressed cells. The stimulus can also form
new HEV cells in areas of transition from an HEV to a
regular vessel, mainly at the corticomedullary junction
(Burwell, 1962), where once again the neonatal features
betraying individual stimulation of squamous cells can
be observed. The fact that a new stimulation adds areas
of HEV cells at this junction should increase the recruitment of competent blood cells. This would likely be the
case if a new HEV area selectively recruits cells related
to the antigens having caused its emergence, as proposed for a sinus area added by the same stimulus.
Indeed, the HEV network starts as fine branches fusing
into progressively larger ones that form an HEV trunk
upon nearing the junction (Sainte-Marie and Sin, 1970).
Thus, if a competent blood cell has not been recruited
prior to reaching the trunk, it may be recruited there in
an area of new HEV cells; otherwise it would exit the
compartment through the continuing medullary venule
in an inactivated state—a waste of time and energy.
Hence, the HEV layout matches that of a new sinus
area under the influence of the same stimulation. Their
combined features and advantages should ensure the
triggering of a response.
Functional Link Between a Subcapsular Sinus
Area and Associate HEV Areas
As we have seen above, the handling of antigens causing cellular responses by the subcapsular sinus domain
around the lymphatic opening determines the emergence
of the underlying rounded deep cortex center where cells
of these responses form. Hence, the nature of the antigens handled in a sinus area determines the nature of
the responding effector cells formed beneath it. By analogy, since a newly formed sinus area and underlying
HEV area(s) are stimulated by the same antigens, it is
reasonable to propose that they recruit the same competent cells to yield the same responses. This would represent a further step in the optimization of the immune
process, consisting in the transport of a given tissue
antigen into a restricted site(s) of the same sinus and peripheral cortex of the draining compartment. Such a layout would increase the likelihood of the encounter of
related elements by eliminating their wandering in target-devoid sites of the peripheral cortex. A common stimulation of a sinus and HEV area(s) occurs because of
topographical and functional links between them, whose
modality will now be examined.
When a humoral response is in cause, fibers transport
the antigen and ensure a functional link between a new
sinus area and the new underlying area(s) of HEV(s).
When a cellular response involving an APC-carried antigen is in cause, two functional links are plausible. One
is the release of an antigen from exosomes extruded by
an APC associated with an area of sinus wall or subsinus layer. The antigen could then be transported by
local fibers as above. A second possibility is that the
APC migrates in the perivascular channel of an underlying HEV or HEV-precursor. Upon reaching one or more
endothelial cells open to stimulation, the APC influences
331
it or them by way of antigenic material released from
exosomes excreted by cytoplasmic processes extending
through the HEV’s basement membrane.
Since the influence of an antigen is transient, a functional link produced by a given antigen evolves following
scenarios orchestrated by the importance and duration
of its presence in the afferent lymph. For instance, when
an antigen handled by a given sinus area fades, that
area eventually can handle another material proportionally to its regained capability, with the cells of functionally linked HEV areas behaving likewise (Sainte-Marie
and Peng, 1996). Such a dynamic state causes variability
in the antigen-specificity of recruitment at a given sinus
and HEV sites, entailing a variably important distribution of sites recruiting the same cells. This makes it difficult to experimentally prove cell recruitment specificity,
as does the varying degree of stimulation of a given
HEV cell by diverse antigenic materials. Although not
strictly comparable, the transient neonatal recruitment
of neutrophils by HEVs illustrates a similar variability
in recruitment (Sainte-Marie and Guay, 1995). After
birth, neutrophils are recruited in the initial HEV
area(s) of a regular venule. Recruitment gradually
declines there and is shifted towards the venule’s next
developing HEV area(s); this is repeated for about a
month until neutrophil recruitment ceases altogether.
This betrays a temporal change in the nature of cells
recruited at a given HEV site.
To summarize, the proposed scenario represents a
more efficient mode for lymphocyte searching through
the transport of antigens from a tissue territory into
only one compartment of the draining lymph node and,
in the case of antigens causing cellular responses, into
only one domain of its subcapsular sinus and peripheral
cortex. One can reasonably infer that evolution also led
to a further improvement consisting in the handling of a
given antigenic material by a restricted site(s) of this
sinus and peripheral cortex, thus limiting the recruitment of relevant cells to these linked sites. Experimental
proof will be difficult. Meanwhile, considering that complex and efficient means to ensure the encounter of
related elements have evolved in the brain, one may
wonder why comparable refinements would not have
occurred in the lymph node compartment as well, especially considering that its reticular fiber network seems
well suited for that task and that prompt immune
responses can be critical to individual survival.
ANTECHAMBERS TO THE LYMPH NODE
PARENCHYMA
At both entrances by which circulating cells can access
the lymph node compartment, evolution may have further favored the formation of a confining place for the
rapid encounter of an antigen and recruited competent
cells, thus optimizing encounter. Such antechambers to
the parenchyma of a compartment exist: the subsinus
layer and subendothelial spaces.
Subsinus Layer
Because of the common belief that lymphocytes do not
enter the lymph node via the subcapsular sinus, this
elusive component receives very little attention. Overall,
observations indicate that it complements the screening
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SAINTE-MARIE
of drained cells by the sinus, which can explain why
some may be retained in this layer, delaying or preventing their passage into the extrafollicular zone. Thus, in
some lymph node compartments of Xid mice, B cells
accumulate in the subsinus layer while the cortex
remains lymphocyte-free (Sainte-Marie and Peng,
1985b). This means that an element that would normally
allow their entry into the cortex is lacking or deficient.
In fact, these B cells fail to undergo blastogenesis and do
not switch to IgG production (Mond et al., 1980). In
other respects, since drained immunogenic elements can
penetrate the subsinus layer, activation can occur there.
required for an essential interaction is lacking or inefficient, so that recruited cells are withheld. The B nature
of the retained cells is indicated by the predominance of
the phenomenon next to medullary cords, where plasmocytes form. The missing element is probably thymic
helper-cells and/or factors, especially since the phenomenon occurs mainly in athymic animals (Sainte-Marie
and Peng, 1996). In other respects, as factors essential
to activation can reach a concerned subendothelial space
and the narrow adjacent area of the first perivascular
channel where lymphocytes also interact (Schoefl, 1972),
activation can occur in both places.
Subendothelial Spaces
ROLE OF THE LYMPH NODE IN TRIGGERING
PRIMARY IMMUNE RESPONSES
A ‘‘Paracortical’’ Hypothesis
The subendothelial spaces also are largely ignored, as
it is assumed that the interaction of circulating lymphocytes with an HEV cell happens exclusively at its luminal face. However, this interaction also proceeds at its
abluminal face that is exposed to subendothelial lymphocytes, to factors transported by fibers and conceivably to
antigens presented by APCs. In a subendothelial space,
endothelial processes contact probing blood lymphocytes.
There, these cells form groups that may include drained
lymphocytes having migrated into a first perivascular
channel (Sainte-Marie and Peng, 1996). The groups
reflect cell interaction (Soderström, 1967; Bailey and
Weiss, 1975), which is likely a prerequisite, allowing
passage into the parenchyma of blood cells entering an
endothelial space rather than directly entering the first
perivascular channel. Consistent with these observations, suspended live nodal lymphocytes bind selectively
to individual HEV cells or restricted HEV sites in sections of frozen lymph nodes (Sainte-Marie and Peng,
1995). They bind to the plasma membrane at both faces
of HEV cells as well as to subendothelial spaces, basement membranes and first perivascular channels, where
they likely become attached to lymphocytes or antigenic
materials. Moreover, lymphocytes that bind to a same
site are interconnected by cytoplasmic bridges. Note that
an endothelial space can also serve as a waiting room
for asynchronously recruited partner-cells (see below).
Subendothelial lymphocytes predominate in HEV
trunks, supporting the idea that a subendothelial space
is a site of essential cell interaction. HEV cells in trunk
areas have often been stimulated by new antigenic material, which suggests that blood cells entering their subendothelial spaces are often naı̈ve and in need of an
essential interaction. This idea is also supported by the
fact that unusually large accumulations of subendothelial lymphocytes occur primarily in HEV trunks where
they exhibit a peculiar distribution (Sainte-Marie and
Peng, 1996). Indeed, such accumulations can occur
under an isolated HEV cell, a few to several cells in a
small HEV area, and/or many cells in a larger area and
possibly on one side only of an HEV. This distribution
pattern is similar to that of the foremost HEV cells arising in a neonatal regular vessel under the influence of
the earliest antigenic stimuli (see above). By analogy,
therefore, the similar distribution pattern of accumulated subendothelial cells betrays a stimulation of the
implicated HEV cells by the same material and their
recruitment of the same lymphocytes. Like lymphocyte
retention in the subsinus layer of Xid mice, a subendothelial accumulation must imply that an element
A proposal by Gretz et al. (1996) dealt with features of
the ‘‘paracortex’’ that could facilitate the triggering of a
primary cellular response within it. The proposal is summarized using their terminology and conception of
lymph node organization as follows:
APCs, which have processed antigens in a tissue and
are lymph-carried into the subcapsular sinus of a local
lymph node, migrate from the sinus through the ‘‘intrafollicular’’ zone to the ‘‘paracortex’’ where they cling to
the fiber network. Circulating lymphocytes come from
the direction opposite to arriving APCs, by transmigrating into the ‘‘paracortex’’ from HEVs; then T cells committed to cellular responses progress along the fiber
network studded with APCs. Activation would occur in
this meshwork of intertwining fibers of the ‘‘paracortex’’
when a proper APC and naı̈ve T cell meet.
However, the fiber network spreading between the
subcapsular sinus and the HEVs in Gretz et al.’s supporting Fig. 1 lies next to a folliculo-nodule (‘‘follicle’’ in
their terminology), and therefore, represents a portion of
the extrafollicular zone of a compartment’s peripheral
cortex, not its deep cortex (‘‘paracortex’’ in their terminology). Hence, if both APCs and transmigrating naı̈ve T
cells did indeed move into the deep cortex, which in fact
lies below the extrafollicular zone, then both would
move in the same direction, not in opposite directions. In
a later article, Gretz et al. (1997) repositioned HEVs
inside presumptive ‘‘paracortical cords’’ forming the
‘‘paracortex’’ (see above). In this case, a cell leaving an
HEV would settle within the ‘‘paracortex,’’ thus not
needing to move towards it from any particular
direction.
Gretz et al. (1996) remarked that the dense fiber network of the structure presumed to be ‘‘paracortex’’ in
their Fig. 1 could facilitate contact between a clinging
APC and a relevant wandering naı̈ve T cell. We argued
earlier that this structure is in fact a portion of the
fiber-rich extrafollicular zone. The deep cortex center
itself is actually poor in fibers (Bélisle and Sainte-Marie,
1981d; Okada et al., 2002). It is therefore unlikely that
these scarce fibers play an important role in mediating
contact between rare wandering competent T cells and
numerous APCs. Moreover, a quest in the deep cortex
center would be hindered by congestion arising from the
many proliferating or maturing lymphocytes and by
abundant randomly-incoming circulating cells. This is
particularly true if one considers the minuteness of a
naı̈ve cell with respect to the large volume of a deep
THE LYMPH NODE REVISITED
cortex center to be probed. Moreover, one wonders what
mechanism could guide a minute rare cell through this
large volume and permit it to encounter a relevant APC
that may be present, but most likely is not. Almost as a
closing remark, Gretz et al. (1996) recognized that the
fiber network is looser in areas of cell proliferation such
as the deep cortex ‘‘units,’’ adding that activation in
these areas has already progressed enough that rapid
access to information transported by fibers is no longer
required. Since the fiber-poor center represents a substantial portion of the deep cortex, the question of where
the site of activation is localized in Gretz et al.’s (1996)
scheme remains wide open.
In another respect, Gretz et al. (1996) initially considered that T cells enter a lymph node only at HEVs, in
agreement with a popular view, but soon after Gretz
et al. (1997) stated that the afferent lymph carries lymphocytes from tissues and upstream lymph nodes. Why
then did they not examine the possibility of a triggering
of responses by lymph-carried T cells? Probably because
one of the authors had postulated that virtually only
memory cells exercise tissue surveillance (Adams and
Shaw, 1994). If so, naı̈ve cells would be absent from the
afferent lymph so that drained cells need not be considered in a proposal dealing with primary responses. However, the concept of tissue surveillance based on memory
cells alone is questionable, especially since it is assumed
to ensure an optimal immune function. Quite to the contrary, such a surveillance mode would compromise the
survival of neonates in particular, for memory cells do
not exist when an antigen intrudes for the first time.
Besides, lymphocytes occur in the gut lamina propria
and in the subcapsular sinus of draining lymph nodes
soon after birth, indicating that naı̈ve cells assume surveillance during this crucial period (Sainte-Marie, 2001).
In addition, Gretz et al. (1996) recognized that the afferent lymph of central lymph nodes can contain some
efferent lymph from peripheral lymph nodes. On the basis of random entry of cells into lymph nodes, a concept
accepted by Gretz et al. (1996), this afferent lymph
would contain naı̈ve T cells having already entered and
left a peripheral lymph node because it was devoid of
their target. Hence, the participation of lymph-carried T
cells should have been considered in the elaboration of a
proposal on T cell activation. However, such participation implies lymphocyte intranodal migration in the
same direction as lymph-carried APCs, not the opposite
direction as proposed by Gretz et al. (1996).
A Chemokine Hypothesis
Cyster (1999) ascribed a central role to chemokines in
the intranodal encounter of circulating cells with their
target. It was proposed that T and B cells leaving HEVs
at random would be oriented towards a T- or a B-area,
respectively, by corresponding attracting chemokines
produced there. It is hardly possible to conceive how
such an orientation process could work in vivo. For
instance, to attract into the deep cortex center T cells
committed to cellular responses—which are hypothetically recruited by HEVs situated anywhere in the peripheral cortex—chemokines produced in the center
would need to diffuse throughout the peripheral cortex.
Since the center underlies only a portion of the peripheral cortex, chemokines would have to first diffuse out-
333
wardly into this portion and then laterally while
forming a gradient potent enough to attract all incoming
T cells from as far as a possibly distant compartment
margin. These numerous cells would then all have to
migrate towards the center, something also inconceivable as explained above. Thereafter, each cell would
have to wander throughout the center in search of a relevant APC, a quest hindered by cellular congestion. In
addition, an encounter could happen only if a naı̈ve cell
had randomly entered not only a rare relevant lymph
node hosting its target, but also the node’s relevant compartment. This seems unlikely, for the blood vascular
network of a compartment represents a small fraction of
the network of a whole lymph node which is usually
multi-compartmented, and an even quite smaller fraction of the body’s whole vascular network. Hence, such a
scenario would be grossly inefficient for rare competent
short-lived naı̈ve T cells, which with increasing age
become even rarer due to thymic involution. In other
respects, it must be considered how a naı̈ve T cell, having randomly entered a center devoid of its target, could
succeed in exiting it rather than endlessly and vainly
wandering in it due to the local elevated concentration
and steep gradient of attracting chemokines. If somehow
the cell succeeded in exiting, it would have to repeat,
over and over again, the same hazardous scenario in
any of a great number of compartments, few (if any) of
which contain its target. The cell would likely die before
such an encounter.
Cyster (1999) stressed that a lymph node contains an
immensely complex chemokine landscape, its multiple
gradients attracting a cell in opposite directions. If so, a
resulting erratic migration would jeopardize the benefit
provided by a compartment architecture well organized
for an efficient directed migration. He added that it will
be exciting to see how many ‘‘turns’’ a cell might make
as it travels through such multiple gradients. He did not
indicate where a cell would turn. In the case of a T cell
committed to a cellular response, turns would not be
useful outside of a deep cortex center where there is at
least a minute probability that its target might be present. In addition, the randomness of such turns would
decrease a cell’s chance of encountering its target. Turns
would furthermore represent an additional waste of
time, which would be a detrimental behavior, especially
for short-lived cells. All of this does not fit with a condition identified by Cyster (1999), that, for a mature dendritic cell to function in an immunogenic manner, it
must interact rapidly with competent T cells. In fact,
Cyster’s proposal could work only if the compartment
morphology was as simple as presented in his Fig. 3,
where a ‘‘T area’’ is reduced to a cell islet close to the
subcapsular sinus with HEVs conveniently situated next
to it. The reality, as we have seen, is far different, yet
this does not preclude a role for chemokines. However,
this role must be consistent with the orderly design of
the compartment, which imposes definite patterns for
factor transport and cell migration. This context suggests that a chemokine has a focal or local influence, as
illustrated by the recruitment of T cells committed to cellular responses occurring only above the deep cortex
center.
The proliferation of unrealistic proposals based on a
misunderstanding of lymph node morphology and oversight of relevant literature is self-fostering and will
334
SAINTE-MARIE
continue. This trend is illustrated by a recent proposal
by Crivaletto et al. (2004). These authors also emphasized that a proper understanding of lymph node morphology is paramount for proposing relevant hypotheses
and theoretical models for immune responses based on
in vitro experiments in cell and molecular immunology.
However, their conception of lymph node morphology is
inadequate in many ways. For instance, in their Fig. 1,
the afferent lymphatics are incorrectly shown to be as
numerous as folliculo-nodules whereas the extrafollicular zone between these structures is delimited by an ‘‘intermediate sinus’’—a component that simply does not
exist.
A Morphologically Realistic Proposal
Previous proposals on the triggering of primary
immune responses attempted to explain how morphological features of components of a developed lymph node,
mainly its HEVs and ‘‘paracortex,’’ allow triggering of a
primary response, concluding that ‘‘forms dictate function’’ (Gretz et al., 1997). In reality, antigens (i.e., function) dictate compartment morphology as we have seen
above. From the outset, previous proposals are fundamentally unrealistic because the components (i.e., forms)
invoked do not exist prior to the occurrence of the first
postnatal antigenic stimuli and immune responses: these
components are products of responses, not prerequisites.
Therefore, to be plausible, a proposal on the triggering
of primary responses must above all be consistent with
the level of compartment organization at birth, a
requirement not considered so far. At birth, only lymphatic sinuses exist (Fig. 4A) and among them the subcapsular sinus is the first to be exposed to the foremost
antigens of the afferent lymph. Sinusal cells handling
these incoming antigens contribute to the foremost
immunological reactions of the lymph node compartment. This neonatal potential for triggering primary
responses in the absence of developed nodal components
(i.e., independently of form) persists throughout life.
Since each primary response is specific, its triggering at
all ages requires particular lymphocytes to be recruited
by sinusal or HEV cells influenced by a new antigen.
These recruiting cells are newly formed and/or existing
cells having regained the ability to be influenced by new
antigenic material. Hence, existing sinusal and HEV
cells of a developed compartment can be implicated in
the triggering of a primary response because of a newly
acquired recruiting property that is elicited by a new
antigen.
To formulate a proposal on events associated with the
triggering of a response in a developed lymph node compartment, one must consider that the latter is basically
constituted of (1) a receptacle for drained immunogenic
elements—the subcapsular sinus, (2) distinct entrances
for lymph-borne and blood-carried lymphocytes—the
subcapsular sinus and HEVs, respectively, (3) an antechamber to the parenchyma for each type of entrance—
the subsinus layer and subendothelial spaces, (4) a
migration pathway—the extrafollicular zone and the
continuing deep cortex periphery—which is connected to
the antechambers and guides a recruited competent lymphocyte to the structure where it will yield its progeny,
and (5) distinct structures propitious to the production
of effector-cells of either cellular or humoral responses—
the deep cortex center and medullary cords, respectively.
Where does activation occur? Observations indicate
that a recruited T or B cell is activated before it enters
the structure where it yields its progeny. Neonatally, a
large fraction of the cells entering the nascent deep cortex center or medullary cords are blast-related, that is,
already activated cells. Thereafter, plasmablasts still
congregate in the initial portion of cords, followed deeper
by progressively less immature plasmocytes (SainteMarie, 1964). These facts point to an activation occurring somewhere upstream of the center and cords. The
immediate upstream component is the migration pathway. For sake of efficiency, this pathway for cell traffic
should not be a place where a rare short-lived naı̈ve cell
searches for a related, yet by far most often absent, element; it should instead serve only to guide an already
activated cell directly to either the center or a cord.
Therefore, activation should ideally happen even more
upstream, in components providing a confined milieu
favoring the rapid encounter of related elements and
where a naı̈ve cell can be exposed to all factors essential
to activation. Such components are the subsinus layer
and the subendothelial spaces of relevant HEV cells or
the adjacent first perivascular channel. This allows the
subsequent direct migration of an activated cell towards
either the center or a cord, the cell being oriented by
fibers and/or perivascular channels of the migration
pathway studded with guiding determinants. On its
way, the activated cell undergoes blastogenesis, thus
explaining the presence of possibly numerous blastrelated cells in the extrafollicular zone and deep cortex
periphery (Sainte-Marie, 1966). If unfinished, blastogenesis is completed in the center or a cord.
Cellular Response
After having processed an antigen in a tissue and having been carried by the lymph into the subcapsular sinus
of the draining compartment, an APC probes the sinus
over the extrafollicular zone present above the deep cortex center until it reaches a receptive site where it
attaches to or enters the subsinus layer. The APC’s
attracting chemokines induce drained T cells committed
to cellular responses to probe the sinus there. Chemokines are also transported by local fibers towards the
center. Somewhere along their course, these fibers join
the basement membrane of HEVs overlying the center,
so that the transported chemokines attract similar blood
T cells to probe the endothelium of these same HEVs, as
can do chemokines released by APCs migrating in their
perivascular channels. Thus, the recruitment of T cells
committed to cellular responses is restricted to the domain of subcapsular sinus and peripheral cortex present
above the center, so as to reach the center with minimal
delay (Fig. 1C).
A lymph-carried naı̈ve T cell encountering a relevant
APC associated with the sinus wall or the subsinus layer
can be activated in either place, where they are exposed
to all essential factors (Fig. 5A, left). This ensures that a
mature APC and a competent T cell will interact quickly.
The activated T cell then migrates to the center where it
engenders its progeny. The APC can likewise travel to
the center (Fig. 5A, left), so that, from a given site of the
subsinus layer, both cells are guided by the same fibers
335
THE LYMPH NODE REVISITED
Fig. 5. A–E: Schematics of proposed modes of intranodal encounter of cooperating elements and their migration pathways for the triggering of primary immune responses. Explanations are in the last two
subsections of this review.
to the same place in the center where their interaction
may continue. An observation suggests that an APC
may cling to an element of the loose fiber network of a
deep cortex center, from where it could influence related
T cells close by. Indeed, in a large center with a reduced
population of T cells, the latter concentrate into broadly
spaced ribbons (Bélisle and Sainte-Marie, 1981d). Conceivably, factors from APCs clinging to distant fibers
attract T cells in their proximity explaining the fewer T
cells between ribbons.
As for competent naı̈ve blood T lymphocytes, they are
recruited by relevant HEV cells, that is, cells that are
presenting the implicated antigenic material (Fig. 5A,
right). Activation can occur in the subendothelial spaces
or in the adjacent first perivascular channel since both
places may contain all essential factors. The activated T
cell (and perhaps a related APC) then migrates to the
center. This leads to their aggregation at the same place
of the center where related drained cells also gather, all
being guided by the same fibers or channels. Finally, it
is conceivable that in the center an APC might activate
newly formed memory cells, rapidly reinforcing a
response by sparing them the quest for their target.
Humoral Response
In the case of a thymo-independent response, a
drained naı̈ve B cell can be recruited at a site of the sub-
capsular sinus exposing its target. Activation may occur
there or next to it in the subsinus layer (Fig. 5B, left).
The activated cell is then guided by local fibers to an
underlying medullary cord, either indirectly via the deep
cortex periphery for a cell recruited above the deep cortex center or directly for a cell recruited beyond it. The
activation of a B cell recruited by a relevant HEV cell
can occur in the subendothelial space or in the adjacent
perivascular channel; the activated B cell then migrates
to the underlying medullary cord (Fig. 5B, right). Reaching a cord, the activated cell completes blastogenesis if it
is unfinished. Thus, resulting plasmoblasts congregate
at the entrances of cords.
In the case of a thymo-dependent response, triggering
implicates a T helper-cell and a naı̈ve B cell. In the subcapsular sinus, both cells can be recruited over the
extrafollicular zone at a site presenting their target and
there, they can enter the subsinus layer. When all essential factors are encountered in the sinus or the layer,
activation unfolds. The activated B cell then travels to
an underlying medullary cord, guided by local parallel
fibers (Fig. 5C). If instead either only the B or the T
helper-cell is recruited on the sinus wall where the target is encountered, the recruited cell may wait there or
in the subsinus layer for the arrival of a partner. Alternatively, it may leave this site in search of a partner,
which could have been recruited by an underlying relevant HEV cell (Fig. 5D, E). To do so, the recruited cell
moves along local fibers connecting to a perivascular
channel until it reaches a relevant HEV cell. If, instead,
a partner is present in the subendothelial space or the
first perivascular channel, activation can happen when
all essential factors are present. The activated B cell
then proceeds towards an underlying medullary cord. If
a partner is absent where a relevant HEV cell is first
encountered, the recruited cell may still adhere to its
target, bind to the abluminal face of the relevant HEV
cell or a facing site of its basement membrane, and wait
for a partner to arrive. Alternatively, in the absence of a
partner there, the recruited B cell may eventually move
to a relevant HEV cell located deeper, thus possibly
reaching the HEV trunk where recruitment of cells concerned in the triggering of a primary response is most
intense. In a simpler scenario, a naı̈ve blood B cell and T
helper-cell are both recruited by a same relevant HEV
cell (Fig. 5F).
To conclude, although knowledge of the lymph node
compartment is still incomplete, it is possible to elaborate proposals on its global functioning and on basic tissue and cellular events related to the triggering of
primary responses there. Nevertheless, these proposals
would benefit from improved knowledge, in particular,
on the neonatal emergence of the compartment parenchyma, which could provide further clues on the unfolding of primary responses. A full understanding of the
triggering of a response will depend on knowledge of the
cellular and subcellular events occurring in the subsinus
layer and subendothelial spaces, where triggering would
commonly take place. Such understanding is a remote
goal.
ACKNOWLEDGEMENTS
The author is grateful, in alphabetical order, to Dr. C.
Bélisle, Mrs. G. Guay, Mr. F.-S. Peng, and Dr. Y.M. Sin
336
SAINTE-MARIE
for their contributions to data and discussions. The
author thanks Dr. É. Gagnon for helpful information,
Dr. J. Ménézes for constructive comments, and Dr. L.
Weiss for his enthusiastic and long-term support of their
work. He is most indebted to his son Dr. B. Sainte-Marie
for reviewing the manuscript.
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