Genetic (Gm) determinants of the gamma-2c (Vi) subclass of human IgG immunoglobulins. A study with special reference to Gm(c3) and Gm(c5), and their relationship with the Gm(b) determinants

FURTHER STUDIES ON THE gammaG-HEAVY CHAIN GENE COMPLEXES, WITH PARTICULAR REFERENCE TO THE GENETIC MARKERS Gm(g) AND Gm(n)

The recently described Gm (g) and Gm (n) genetic markers of the γG3- and γG2-subgroups of γ-globulin were characterized in detail primarily through studies of myeloma proteins, their polypeptide chains and fragments. Antisera derived from rabbits, non-human primates and rheumatoid arthritis patients gave identical results. This contrasted with the Gm (b) system where the rabbit antisera react with a different genetic determinant (b⁰) than the sera from rheumatoid arthritis patients (b). The Gm (g) and Gm (n) antigens were detected both by precipitin analysis and by hemagglutination inhibition. The Gm (g) antigen was not associated with any of the other genetic antigens of the γG3-proteins which all belonged in the Gm (b) class. The genes for the latter were always allelic to the gene coding for Gm (g), with that for Gm (b⁰) constantly present when that for Gm (g) was absent. The Gm (g) and Gm (n) markers were of particular value in tracing the various gene complexes made up of the closely linked subgroup genes. Further support was gained for the concept that the different gene complexes of various population groups arose primarily through crossing-over. The Gm^g and Gm^b genes for the γG3-subgroup were extremely closely linked to those for the γG1-subgroup. However the Gm (n) marker indicated that the γG2-subgroup genes were probably further separated on the chromosome. Additional evidence was obtained for the γG2-γG3-γG1-order of the subgroup cistrons. Among the wide range of gene complexes a new type (γG2,—,γ/G1) was described. This complex appeared to have a deletion of the γG3-cistron. Lower levels of γG3-globulin were found in the sera of the individuals with this gene in the heterozygous state. The possibility that this unusual complex arose through an unequal nonhomologous crossing-over is discussed.

Human IgG allotypes co-occurring in more than one IgG subclass

Inheritance of an excess of immunoglobulin allotypes in one haplotype was encountered which could not be explained by the assumption of a duplicated locus. The surplus of allotypes was related to markers on the CH3 domain of gamma 3 chains. Two such cases were investigated extensively. The IgG3 molecules were isolated by gel filtration and by absorption on protein A. Only the usual combination of allotypes appeared to be present on the IgG3 molecules. The supernumerary markers were found in one case on IgG2 molecules and in the other case on IgG1 molecules. This followed from investigations of eluates after separation of the subclasses by immune absorptions. A hypothesis was proposed to explain these events by mutation of a particular position of an otherwise homologous stretch of gamma-subclass DNA.

Human IgG3 allotypes, with special reference to a new allotype related to G3m(g) (G3m21)

The IgG3 allotype described as L1 (Blanc et al., 1976) occurs on the CH3 region of G3m(g) proteins, in contrast to G3m(g) that is known to be present on the CH2 region. G3m(g) and L1 are, as a rule, present on the same gamma3 heavy chain, just like the G3m(b) subspecificities of the CH2 region, (b1) and (b4), with those of the CH3 region (b0), (b3) and (b5). Several families were investigated that showed inheritance of rare combinations of IgG3 allotypes. The data obtained are suggestive for notation of L1 as (g5), since L1 probably occupies a position antithetical to (b5). The relation of amino acid substitutions to allotypes and isoallotypes is discussed.

Immunological studies of the human placenta. Characterization of immunoglobulins on trophoblastic basement membranes

Immunohistological and elution studies of the human placenta revealed the presence of IgG on the trophoblastic basement membrane (TBM) which demonstrated specificity for placental but not lung, thyroid, or kidney basement membranes, suggesting the presence of a placenta-specific antigen in TBM. IgG comprised the bulk of immunoglobulin in eluates, and small amounts of IgA, trace amounts of IgM, but no IgE or IgD were identified in eluates. The distribution of IgG subclasses in eluate was not unusual as compared to maternal and neonatal sera, and Gm and Inv typing of eluates indicated that it was of maternal origin. Small amounts of eluate-IgG effectively inhibited the blastogenic response of unrelated lymphocytes to old tuberculin, phytohemagglutinin, and in one- or two-way mixed lymphocyte culture reactions. The inhibition was distinct from nonspecific inhibitors, and dose-response analysis indicated that eluate was very much more potent as an inhibitor than were the nonspecific inhibitors. Inhibition was shown to not be due to anti-HL-A activity, and was probably not due to aggregated IgG or immune complexes. Binding of eluate to lymphocytes was very loose as shown by washing experiments, and no binding could be shown by immunofluorescence. The capacity of eluate IgG to inhibit MLC was retained after pepsin digestion to F(ab')(2), suggesting that the inhibition reactions were immunological. It is suggested that eluate-IgG is maternal blocking antibody to a hitherto uncharacterized trophoblast antigen, and it is speculated that either abnormal antigen or aberrant responses to antigen could result in fetal wastage.