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15. T-lymphocytes, ontogenesis, surface markers. Subpopulations of

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T-lymphocytes, ontogenesis,
surface markers. Subpopulations of
T-lymphocytes and their functions.
T lymphocytes- ontogenesis
• The undifferentiated stem cell in BM gives rise to
the lymphoid precursor cell which matures into 3
types of lymphocytes:
• T lymphocytes
• B lymphocytes
• Natural killer (NK) cell
• Pro-thymocytes come to the thymus where
•
continue the maturation into T lymphocytes
Maturation of B lymphocytes continue in BM
Surface markers of T cells
• CD (cluster of differentiation) proteins-
molecules on the cells membrane, allow
the identification of cells
• TCR- receptor for antigen
• MHC gp I or II class
CD proteins
• allow an identification of T-cell subsets
• CD 3 = important in intracellular signaling to
•
•
initiate an immune response; closely associated
with TCR
CD 4,8 = are expresed on subclasses of mature
T cells; CD4 reacts with MHC gp II.class),CD8
reacts with MHC gp I. class on macrophages
CD 28- receptor for costimulator molecules
CD80 and 86
Maturation of T lymphocytes
Consist of three types of processes:
• Proliferation of immature cells
• Expression of antigen receptors genes
• Selection of lymphocytes that express
useful antigen receptor (TCR)
TCR
• Antigen receptors are encoded by several gene
•
•
•
segments that recombine during lymphocyte
maturation
Heterodimer consisting of 2 nonidentical
polypeptide chains linked together by disulfide
bonds
TCR heterodimer is noncovalently associated
with the Оі,Оґ,Оµ chains of the CD3 molecule
COMPLEX TCR- CD3 makes contact with both
the Ag and MHC gp
Subpopulation of T cells
• Subpopulations of T cells have been defined
according to their particular function and their
CD membrane markers
• Cytotoxic T lymphocytes = Tc;CD8+
- recognize the foreign epitope in association
with class I MHC molecules
• Helper T-lymphocytes = Th; CD4+
- recognize the epitopes in association with
class II MHC molecules
Cytotoxic T lymphocytes
(Tc;CD8+)
• cause lysis of target cells; are active against
•
•
•
tumors, virus-infected cells, transplanted
allogenetic tissue
release TNF- depresses proteosynthesis
recognize the foreign epitope in association with
class I MHC molecules
destroy their target cells by releasing perforin
(create poresin the cell membrane and
cytoplasm escapes) and granzymes (degrading
essential macromolecules)
Helper T-lymphocytes
(Th; CD4+)
• recognize the epitopes in association with
class II MHC
• help B cells to produce antibodies and
help phagocytes to destroy ingested
microbes
• subsets of Th cells: Th1, Th2 cells
Th1 cells
secrete:
• INF-γ (gamma interferon) : activates macrophages to
become more effective at killing phagocytosed microbes,
supresses the development of Th2 cells
• IL- 2 : stimulates survival and proliferation of T cells,
called T-cell growth factor
• TNF (tumor necrosis factor)- stimulates the recruitment
of neutrophils and monocytes to sites of infection,
activates these cells to eradicate microbes
• IL-3 : promotes expansion of immature marrow
progenitors of all blood cells
• GM-CSF : acts on progenitors in the bone marrow to
increase production of neutrophils and monocytes
Th1 based immune reaction
• Th1 cells stimulate phagocytes to
eliminate ingested microbes.
• Interferon gamma is the main Th1
cytokine.
Th2 cells
secrete:
• IL-4 : induces differentiation of Th2 cells from
naive CD4+ precursors, stimulation of IgE
production by B cells
• IL-5 : activates mast cells
• IL-6 : stimulates the synthesis of acute phase
proteins by hepatocytes
• IL-10 : inhibits activated macrophages,
supresses Th1 production
• IL-3, GM-CSF
Th2 based immune reaction
• Th 2 response provide help for B cells
and are essential for antibodymediated immunity
• Antibodies are needed to control
extracellular pathogens
• The Th2 - type cytokines include IL 4, 5,
and 13.
Regulatory T cells
• Express CD4, CD25, FoxP3
• Regulate the activation or effector
function of other T cells
• Are necessary to maintain tolerance to self
antigens
The role of thymus.
Positive and negative selection
of T lymphocytes.
The role of thymus
• In the two thymic lobes, lymphocyte
precursors from the bone-marrow become
thymocytes, and subsequently mature into
T cells
• Once mature, T cells emigrate from the
thymus and constitute the peripheral T cell
repertoire responsible for directing many
facets of the specific immune system
Phases of thymocyte
maturation
• A rare population of hematopoietic progenitors
enters the thymus from the blood, and expands
by cell division to generate a large population of
immature thymocytes
• Immature thymocytes each make distinct T cell
receptors by a process of gene rearrangement.
• This process is error-prone, and some
thymocytes fail to make functional T cell
receptors, whereas other thymocytes make T
cell receptors that are autoreactive
Positive and negative
selection
• Immature thymocytes undergo a process
of selection, based on the specificity of
their T cell receptors.
• This involves selection of T cells that are
functional (positive selection), and
elimination of T cells that are autoreactive
(negative selection)
Thymus – positive selection of T - cells
1. precursor T cells enter thymus from the blood
2. they are presented with self-antigens
3.
4.
complexed with MHC molecules on the surface
of cortical epithelial cells
only those thymocytes which bind the
MHC/antigen complex with adequate affinity
will receive a vital "survival signal"
the other thymocytes die (>95%)
Thymus – negative selection of T - cells
1. thymocytes that survive positive selection
migrate towards the boundary of the thymic
cortex and thymic medulla
2. they are again presented with self-antigen in
complex with MHC molecules on antigenpresenting cells
3. thymocytes that interact too strongly with the
antigen receive an signal for apoptosis
Humoral immune response
• The binding of antigen cross-links Ig
receptors of specific B cells and then
activation signals are delivered inside the
B cell; the necessary second signal is
provided by a breakdown product of the
complement protein C3
• This process results in clonal expansion of
B cells and secretion of low levels of IgM
Affinity maturation
= affinity of antibodies for protein antigens
increases with prolonged or repeated
exposure to the antigens
• B cells migrate into follicles and form
germinal centers, proliferate rapidly
• B cells that recognize the antigen with
high affinity are selected to survive
Phases of humoral immune
responses
Primary immune response
• Occurs during the first antigen exposure
• The amounts of antibody produced is
smaller, class IgM
2 types of antigens:
• T- dependent – help from T helper cells
is required; protein antigens
• T- independent – antibody production
is induced directly, without the
involvement of T helper cells; typical are
polysaccharides, lipids
Secondary immune
response
• Occurs during subsequent antigen
exposure
• Higher amount of antibodies is produced
• secondary responses show increased
isotype switching (IgG, IgA, IgE) and
affinity maturation (= production of
antibodies with increased affinity to
antigen)
• Gives arise memory cells involvement
Physiological mechanisms of
regulation of the immune system.
Immune mechanisms of
regulation
Immune system is regulated by:
• Antigen
• Antagonistic peptid
• Antibodies
• Cytokines and intercellular contact
• T lymphocytes
• Neuroendocrine regulation
REGULATION BY ANTIGEN
• Antigen competition – peptides from different
antigens compete for binding-sites on the MHC
gp = antigen is able to suppress expression of
other antigen
IMPORTANT IS :
• the binding strenght of peptide to MHC gp II
• density of peptid-MHC gp II complex on the
surface of APC
Immune response finishes after extinction of
antigen – due to a short life-span of effector
lymphocytes
ANTAGONISTIC PEPTIDES
• Agonists - peptide fragments of antigen with
•
•
adequate binding to MHC gp, recognition by T
cells with sufficient affinity – induce full
response of T cells
Partial agonists – induce a qualitative
different response of T cells - peptides have
a similar structure, bind adequately to MHC gp,
but make too weak or too strong interaction
with T cells
Antagonists – induce anergy of T cells
REGULATION MEDIATED BY
ANTIBODIES
• Antibodies have an effector and regulatory
functions
• Secreted antibodies – compete with
BCR for antigen = negative regulators of
B cells stimulation
• Immune complexes of antibody and
antigen – bind to the surface of B cells –
inhibit B cells activation
REGULATION MEDIATED BY
CYTOKINES AND INTERCELLULAR
CONTACT
• Inhibition of cytokine effect by endocytosis of
•
•
their receptors or by binding of inhibitors to
their receptors
Inhibitory receptors – protection against too
easy T cells activation
Apoptotic receptor (Fas) mediates a negative
regulation after binding to ligand FasL on
activated T cells – causes – lysis of cell
NEGATIVE REGULATION
mediated by T lymphocytes
• Th2 cells produce IL-4, IL-10- suppress immune
•
•
response of Th1 cells
CD8+ T cells – secrete soluble forms of TCR compete with TCR on the surface of other cellsinhibition
Regulatory Tr1 cells (CD4+)- secrete IL-10-antiinflammatory effect, induce tolerance to
autoantigens
NEUROENDOCRINE REGULATION
• Neurotrasmiters influence leukocytes by
•
•
•
•
binding to the specific receptors
(noradrenalin)
CS, growth hormons, thyroxin, endorfins –
influence the leukocytes by binding to the
specific receptors
Leukocytes produce endorfins, TSH, growth
hormon, vitamin D3, ACTH
Cytokines influence nerve system (IL-1, IL-6)
Stress influences immune systemcortocosteroids-activity of phagocytes, NK
cells
OTHER FACTORS
• The same antigen can induce active immune
•
•
•
•
response or active tolerance – depends on:
Condition of immune system
Character of antigen (size, structure of
molecule)
Dose of antigen (too low or too high dosesinduce tolerance)
Route of antigen administration (s.c.-induce
immune response, p.o. or i.v.- induce tolerance)
Cytokines (overview, disposal
according their function)
Cytokines
• Humoral factors - provide intercellular
•
•
•
•
communication between immune cells +
communication between immune system and
other body systems
Proteins secreted by leukocytes (and by other
cells of immune system)
Influence different cells of immune system
through specific receptors
Forms- secreted or membrane
Effects - pleiotropic (1 cytokine has a
several fysiological effects)
Function of cytokines
• Activating signals for cells – activate, regulate
•
•
•
•
cell cycle, mitotic activity
Cause changes of the cell membranes – increase
of cytokine receptors expression
Participate in reparation of tissues in the
terminal phase of inflammation
Regulate immune cells proliferation and
differentiation in the immune organs
Influence migration of the immune cells
Classification of cytokines
• Pro-inflammatory
• Anti-inflammatory
• With growth activity factor
• Participating in humoral response IS
• Participating in cell response IS
• With antiviral effect
Proinflammatory cytokines
• IL-1 – produced by macrophages and T cells;
•
•
•
activates neutrophils, endothelial cells and T cells,
induces synthesis of acute phase proteins in liver,
causes fever
IL-6 – produced by T and B cells, monocytes;
regulates B cell differentiation and proliferation,
synthesis antibodies; stimulates hepatocytes to
produce acute phase proteins
IL-8 – produced by monocytes, macrophages,
endothelial cells; chemotactic factor for
neutrophils
TNF
TNF
• TNF α – produced by macrophages (+T a B
cells, NK cells, neutrophiles,..)
– participates on early phase of inflammation,
induces expression of adhesive molecules on
endothelial cells and leukocytes; stimulates
proinflammatory protein production
• TNF β- produced by T and B cells; similar
effects
Anti-inflammatory cytokines
• IL-10 – produced by Th2 cells, monocytes,
•
•
•
macrophages, activated B cells; inhibits
cytokine Th1, Tc and NK cells production;
inhibits synthesis of pro-inflammatory
cytokines by macrophages
TGF-β – growth activator but also inhibitor of
different cell types
Regulates damaged tissues reparation by
stimulation of intercellular substances
synthesis; can inhibite T and B cell
proliferation
IL-4
CYTOKINES with activity of growth
factors
• SCF- stimulates stem cell proliferation, their
•
•
•
•
release to the peripheral blood
IL-3 – influences maturation of all cell lines
IL-7 – growth factor of T cells
IL-11 – growth factor of megakaryocytes
IL-15 – induces proliferation of mast cells,
Th and Tc cells
CYTOKINES with activity of growth
factors
• G-CSF, M-CSF, GM-CSF – stimulate
granulocytes and/or monocytes/macrophages
proliferation and differentiation, prolongate their
survival and increase their functional capacity
during inflammation
• EPO = erythropoetin- stimulates red cells
differentiation
CYTOKINE participating in humoral
response of IS
• IL-4 – produced by Th2 cells, mast cells and
•
•
basophils; stimulates B and T cells proliferation,
activates macrophages; growth factor of mast
cells (anti-inflammatory effect)
IL-13 – see IL-4 + chemotactic factor for
monocytes and macrophages
IL-5 – produced by Th2 cells, lymphocytes and
mast cells; activates and stimulates B cells and
eosinophils proliferation, stimulates Tc cells
CYTOKINES participating in cell
mediated response
• IL-2 – produced by Th cells after stimulation by
•
•
antigen, autocrine effect to Th cells; activating
signal for Tc cells and NK cells; 2. signal for
activation and differentiation of B cells
IL-12 – produced by monocytes, macrophages;
stimulates maturation Th0 into Th1 cells;
activates NK cells; stimulates INF- Оі production
by Th1 cells; inhibites IgE secretion
IFN-Оі, GM-CSF
CYTOKINE with antiviral effect
• Interferons- anti-inflammatory, anti•
•
•
prolifferative
IFN-О±, IFN-ОІ - produced by cells with nucleus
after stimulation by viruses
Induce decomposition of viral nucleus acids and
influence into translation of viral proteins
IFN- α - in recombinant form – therapy of
chronic active hepatitis B, hepatitis C
Regulation of local
concentration of cytokines
• Biological effects are neutralized by
binding to nature inhibitors and soluble
forms of receptors for cytokines
• MMP (matrix metaloproteases) release
cytokines from molecules of intercellular
substance
• Elastases release binded cytokines
External regulation of immune
response
• Substitutive therapy –
immunoglobulins, C1-inhibitor
• Non-specific immunostimulation :
Isoprinosine, bacterial lysates, cytokines
• Ag- specific immunotherapy:
active immunisation – prophylaxis
pasive immunisation – prophylaxis,
therapy
Therapy- Non-specific
immunosupresion
• corticosteroids
• anti- inflamatory drugs (antihistaminics, inhibitors of pro-
•
•
•
inflamatory cytokines-monoclonal Ab agains TNF,
nonsteroid anti-inflamatory drugs- metabolites of
arachid acid)
drugs affecting the proliferation of both T cells and B
cells (Cyclophosphamide, Methotrexate, Azathioprine,
Mycophenolate mofetil)
drugs blocking the activation of lymphocytes
(Tacrolimus, Sirolimus, Cyclosporin A)
monoclonal antibodies – against IL-2 receptor
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