CURRENT COMMENT The Gm Groups and Their Relation to Rheumatoid Arthritis Serology By R. GRUBB A NUMBER OF examples of genetic variants in human plasma proteins have been brought to light in recent years.1 The existence of such variation in gamma globulins was first realized in 1956.213 The gamma globulin groups thereby distinguished are designated the Gm groups. Owing to certain difficulties in the selection of proper reagents confirmation of the original findings appeared relatively slowly. The early findings however, have since been amply confirmed and e~tended.~-e It would appear that knowledge of the,Gm groups is of some interest to workers in the field of the serology of rheumatoid arthritis because human gamma globulin is, of course, a key substance in a number of serologic tests used in rheumatoid arthritis, and, secondly, because sera from patients with rheumatoid arthritis are generally used for demonstrating the Gm group differences. In most of the test procedures applied in the study of rheumatoid arthritis using human gamma globulin in the indicator system, the gamma globulins are prepared from pooled human plasma.. This also applies to the indicator in the euglobulin inhibition test.7 Gamma globulins from individual human samples have hitherto been little used except in the indicator consisting of human red cells coated with incomplete anti-% antibodies.YThis antigen-antibody complex, containing antibodies from a single donor, is used in Gm grouping. This is an important difference, and the utilization in the indicator of gamma slobulins from single donors was indeed a prerequisite for the detection of the Gm groups. In the opinion of the reviewer it would be of interest to pursue this individualistic line in some of the other tests-not for the purpose of elaborating a test giving a positive result in a high percentage of cases of rheumatoid arthritis, but to enable the study of the body constituents with which the socalled R.A. factor is able to interact in inhibition experiments. The Gm groups are determined by agglutination-inhibition tests. The test system consists of (1) an antigen-antibody complex in the form of human red cells coated with selected incomplete anti-Rh from a single individual; (2) a selected serum capable of agglutinating these coated cells, usually derived from a patient with rheumatoid arthritis; (3) the (normal) serum or preparation to be tested for Gm activity. The interaction between items (1) and ( 2 ) results in agglutination, which is inhibited if item (3) possesses Gm-activity. Readers interested in the technic are referred to references.80aJ'-11 The sera of Gm ( a + ) persons contain an inhibitor for the expected hemagglutination, whereas those of group Gm (a- ) do not or, if so, in much smaller amounts. The evidence that this inhibitor is a character of gamma globulin proper appears convincing (cf.ref.4). These group differences show up in normals, they are constant in normal adults, and they are hereditary. They are not developed 195 196 R. GRUBB at birth or during the first monthb of extrauterine life, the infant then having the Gm group of the mother.12-14The frequency distribution varies considerably between different populations. Frequency figures for Gm ( a+ ) are around 60 per cent in Western Europe, about 40 per cent in southern Italy, Poland and Greece.l6Js In the Negro populations studied, as well as in several other non-European stocks, the frequency is not far from 100 per cent. It has been shown that the Gm system is considerably more complex than a system made up of groups Gm ( a + ) and Gm ( a- ) . Two additional characters clearly belonging to the Gm system and called Gm ( b ) and Gm ( x ) are now ~ ~ o w I INearly . ~ ~ all - ~Gm ~ ( a - ) persons are Gm (b-t). The phenotype Gm (a+b+ ) is also common. The frequency and family data concerning Gm ( a ) and Gm ( b ) in whites do not differ significantly from calculations based on Gm(x) is related the assumption that genes Gma and Gmb are alleles.fR-zO to Gm(a) in the sense that nearly all Gm(x+) persons are also G m ( a + ) . Two additional factors probably related to the Gm system have recently been discovered. They have provisionally been called Gni-like21 and In( v ) ,22 It has also been shown that the characters Gm( b ) , Gm( x), Gm-like and In( v ) are inheritable. The approximate percentages of these characters in Western Europeans and in Western Africans (or their descendants) are given in the following compilation which is based on data given in reference^.^^-^^ Western Europeans Western Africans Gm(n+) Gm(b+) Grn(x+) Grn-like(+) In(v+) 35-60 400 85 25 95-100 rare not observed 27-65 19 53 Owing to the complexity of the system, it is still too early to give a complete genetic schema. Even the nomenclature involves difficulties. Attempts have been made to overcome them by agreement between several workers in this field. The recommendations made, and observed in the present article, are to he found in reference 30. A further character of human gamma globulins, which varies with age and which is probably inheritable and possibly related to the Gm system, has been described by G0odman.3~.~* Interesting similarities exist between the thoroughly studied, complex "allotypes" of rabbit gamma globulins and the Gm groups (cf.ref.33-36). The heterogeneity within the gamma globulin family of molecules is indeed striking (cf.also ref.37-39). The study of the physicochemical basis of the Gm specifities is in its infancy and only a few data have been gathered.4s40Gammaz globulins (7s) contain the Gm( a ) specificity, which is not dependent on the classic combining site of antibody molecules. Evidence for the heterogeneity of the so-called R.A. factor The factors which cause agglutination of the coated red cells used in Gm grouping are called reagins anti-Gm or simply anti-Gm, the specificity being indicated by the addition of ( a ) , ( b ) or ( x ) as the case might be.30 Reagins anti-Gm may rarely be found in and sometimes in diseases other however, that reagins anti-Gm than rheumatoid a r t h r i t i ~ . ~It. is ~ ~impressive, -~~ 197 GM GROUPS are much more common in rheumatoid arthritis than in other states, as are also high titers. It has been shown that anti-Gm( a ) is made up of macroglobulin40.41and that its solubility characteristics in the cases studied are similar to, if not identical with, those of the factor(s) causing a positive Waaler-Rose tesP6,4’ and that the activity is destroyed by treatment with sulph-hydryl comp o ~ n d sOn . ~ ~the basis of these data it may be justified to class the anti-Gm’s in the R.A. factor family. Much evidence obtained in different ways suggests that what is somewhat ~ - ~ conclusion ~ is loosely referred to as the R.A. factor is not a single f a ~ t o r . 4This amply borne out by results obtained in studies using red cells coated with incomplete anti-Rh. It has been shown that the specificity may be anti-Cm( a ) , anti-Gm(b ) , anti-Gm( x) or anti-Gm-like occurring either singly or probably more frequently in varying combinations in one and the same serum.B4-Gu The factor(s) operative in the Waaler-Rose test is not identical with anti-Gm(a) (and hardly with anti-Gm( b ) or anti-Gm( x ) either), independence having been observed in direct agglutination reaction^^^*^^ as well as in some absorption testsG8and as regards thermo~tabi1ity.j~ To complicate matters still more, anti-Gm( a ) , for example, may occur by itself or in association with its specific gamma globulin inhibitor. The formula of the anti-Gm’s in a single individual may thus run: anti-Gm( a ) 4- anti-Gm( b) complexed with Gm( b ) anti-Gmlike. It is needless to enumerate all combinations, the possibility for wide variegation being obvious. Incidentally, this complexity explains some of the early (and still existent) difficulties in finding proper reagents for Gm grouping. It is clear that the patterns of agglutination and of agglutination inhibition obtained with some sets of anti-Rh and rheumatoid arthritis sera might not lend themselves well to interpretation in terms of one single Gm factor. The ideal combination for Gm( a ) grouping would be an anti-Rh emanating from a Gm( a+b-x-) individual and an anti-Gm from a Gm( a-) individual containing solely anti-Gm( a ) . Bewildering as this complexity may seem at present, I think it is highly desirably to find terms which are more precise than “the R.A. factor” if confusion is to be avoided. The terms used in Gm serology may be regarded as a beginning of such a precision of terms. + Gm-types and Anti-Gm Specificities in Patients with Rheumtoid Arthritis The frequency of group Gm( a+ ) in patients with rheumatoid arthritis has been studied by Grubb,G9 Podliachouk et a1.60 and by Tiilikainen.61 In two of the series the frequency distribution did not appear to differ from that found in normals, Tiilikainens series showing some excess of Gm( a+ ) in the rheumatoid arthritis patients. In the series studied by Podliachouk et al. there was some excess of Gm(a+) among those severely affected.62Further studies including collection of family data and with due attention to the “new” Gm factors and to the presence or absence of Gm-anti-Gm complexes in the patients are clearly desirable. An unusual feature of the Gm groups is the phenotypical delay in their expression. This delay in individual specificity plus the exposure of the fetusinfant to the mother’s gamma globulins may have implications as regards 198 R. GRUBB the status of immune tolerance for gamma globulins (cf.ref.32). It is felt that interesting possibilities amenable to study are concealed here. Extended prozones are frequently observed on titration of rheumatoid arthritis sera against red cells coated with anti-Rh. Evidence has been presented that these prozones may be ascribable to the simultaneous occurrence in the same individual of, e.g., anti-Gm( a ) and Gm( a + ) gamma glob~lin.'~.~7 These findings could give some support to autoimmunization ideas. It should be mentioned, however, that strongly reacting anti-Gm( a ) without prozone is met with mainly in Gm( a- ) individuals/0*63In the reviewer's opinion the most straightforward, though not the only conceivable, interpretation is that the stimulus for anti-Gm(a) is hardly the individual's own gamma globulin. A possibly specially pertinent example of this independence is the rheumatoid arthritis serum "Bomb of Steinberg,21.55which contains anti-Gm-like although the donor is Gm-like( - ) and Caucasian, i.e., belongs to a population in which Gm-like( ) gamma globulin has not been encountered. The findings just cited might give an important hint, namely to look for the primary stimulus for the arisal of the anti Gm's outside the gamma globulins. + Capacity of Anti-Gm to Interact with Classical Antibodies and with Native Human Gamma Globulin There is evidence that some, perhaps most, R.A. factors interact with frankly denatured or aggregated human gamma g l o b ~ l i n . " ~Some ~ ~ alteration of gamma globulin appears essential for the manifestation in Epstein's precipitin reactions5 and probably also in the FII It is well known, particularly R.A. factors interact with some from the studies of V a ~ g h a n , ~that ~ , ~some * animal antibodies when complexed with their antigen. This applies also to some human antibodies in complex with their antigen, as is obvious from sevred eral agglutination reactions (Rh- anti-RhsO*70.2 Brucella-anti-Br~cella,~~ cells in hemoglytic anemia and possibly also some agglutination tests of cocci. ) 71-73 The possibility of antibodies being denatured in serological reaction has been speculated upon74,75and such denaturation might be conceived as a prerequisite for the reaction with a R.A. factor. Whether or not antibodies are denatured on reaction with their antigen, it appears that observations made in studies of the Gm system show that the anti-Gm's interact with nuiiue human gamma globulin of appropriate Gm-type. For inhibition is obtained by native serum in the system used in Gm grouping. The argument is valid to the extent that gamma globulins in a blood sample (also from the umbilical cord) are native and not existing as antigen-antibody-complexes. It would be of interest to know the relative affinity of various R.A. factors for native, slightly altered and frankly denatured gamma globulins with due allowance for the Gm's and anti-Gm. There is some evidence that the complex forming on interaction between anti-Gm and Gm is not firmly linked together since agglutinating capacity may reappear upon simple dilution of a non-agglutinating mixture of the t ~ ~ . ~ ~ q ~ g Findings made in studying the Gm system stress two points discussed below. 1. Some R.A. factors are able to interact with classical human antibodies of 199 GM GROUPS the 7s variety. This brings to mind the question whether the addition of these macromolecules (19s) to the 7s antibodies might alter some function or manifestation of the 7s antibody. We do indeed notice this when incomplete antiRh is, so to say, transformed into complete anti-Rh. Parenthetically it is well known that the pathophysiologic significance of complete anti-Rh, which is a macroglobulin, is nil, at least for a fetus. It would appear that here is a rather wide field for investigation. 2. Some R.A. factors are able to interact with native gamma globulin. This appears interesting because there is abundant experience on the effect of injections of native gamma globulin into rheumatoid arthritis patients, transfusions of whole blood being a fairly common therapeutic measure in rheumatoid arthritis. In other words, considerable amounts of Gm( a+ ) gamma globulin have often been injected into patients possessing reagin anti-Gm( a ) (or Gm( b+ ) gamma globulin into those possessing anti-Gm( b ) ). I have personally observed a number of cases in which recipients possessing anti-Gm( a ) active at 37” C. have received transfusions of whole blood from Gm(a+ ) donors. No manifest side effects have appeared. This is in contrast to what happens in serum sickness. 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