A NEW TYPE IN THE Gm SYSTEM

Identification of IgG1 and IgG3 Allotypes by PCR and Sanger Sequencing

The immunoglobulin heavy constant gamma (IGHG) gene cluster encoding immunoglobulin G (IgG) subclasses is highly polymorphic, resulting in amino acid variation along the antibody constant heavy chain referred to as allotypes. IGHG1 and IGHG3 are the two most polymorphic IgG subclasses in humans, with 4 classical IgG1 allotypes and 13 allotypes described for IgG3, though recent studies suggest greater allelic diversity, especially in underrepresented ethnic populations. Polymerase chain reaction (PCR) and Sanger sequencing of IGHG amplicons allow for the identification of the single nucleotide polymorphisms (SNPs) responsible for the observed amino acid substitutions. Here, we provide a detailed protocol for the amplification of IGHG1 and IGHG3 segments by PCR, sample preparation for Sanger sequencing, and analysis of sequencing data to identify SNPs associated with different IgG1 and IgG3 allotypes.

Human Gm, Km, and Am allotypes and their molecular characterization: a remarkable demonstration of polymorphism

Human immunoglobulin allotypes are antigenic determinants (or "markers") determined serologically, classically by hemagglutination inhibition, on the human immunoglobulin (IG) heavy and light chains. The allotypes have been identified on the gamma1, gamma2, gamma3, and alpha2 heavy chains (they are designated as G1m, G2m, G3m, and A2m allotypes, respectively), and on the kappa light chain (Km allotypes). Gm-Am allotypes are inherited in fixed combinations, or Gm-Am haplotypes, owing to the linkage of the human IGHC genes (IGHG3, IGHG1, IGHA1, IGHG2, IGHG4, IGHE, and IGHA2 from 5' to 3' in the IGH locus on chromosome 14). Gm and Am allotypes have been one of the most powerful tools in population genetics and very instrumental in molecular characterization of the human IGHC genes (gene conversion, copy number variation, gene order). They represent a major system for understanding immunogenicity of the polymorphic IG chains, in relation with amino acid and conformational changes. The correlation between G3m allotypes and amino acid changes has been possible with the sequencing of many alleles of the IGHG3 gene, from individuals from different populations and with known allotypes. In this chapter, we integrate genetics and sequence data and provide an updated overview of the Gm-Am haplotypes and Km allotypes. We propose, for the first time, a complete elucidation of the G3m allotypes, illustrated by the "IMGT G3m allele butterfly" concept that allows a graphical representation of the G3m alleles (variants of a gene expressing a given set of allotypes). Knowledge of allotypes is important in antibody engineering and humanization of monoclonal antibodies to improve immunotherapy.

Specificity of the reaction between rheumatoid factors and gamma globulin

Rheumatoid factors in the sera of patients with rheumatoid arthritis appear to be specifically directed against genetically determined "antigens" in human γ-globulin. At least eight rheumatoid factors of differing specificity exist; usually several are present in combination in the same serum. The different rheumatoid factors can be readily detected through their pattern of reactivity with anti-Rh antibodies from different individuals. Rheumatoid factors in diseases other than rheumatoid arthritis were found to have a more restricted specificity, contrasted to the broader reactivity of the factors in most rheumatoid arthritis sera. A specificity similar to that for incomplete antibodies was not demonstrated for the reaction of rheumatoid factors with aggregated γ-globulin or with γ-globulin to form the "22S complex." In certain instances, using the anti-Rh system, rheumatoid factors were found to react poorly with the patient's own γ-globulin, compared to that of other individuals of different genetic γ-globulin types. These results, as well as additional indirect evidence, indicate that the rheumatoid factors can possess isospecificity. However, a certain degree of autospecificity was also found which was most clearly evident through complex formation with the patients own γ-globulin and in the reaction with aggregates. The relevance of these findings to possible isoantibody as well as autoantibody concepts is discussed.

Genetic characters of human gamma-globulins in myeloma proteins

The genetic factors Gm(a), Gm(b), Gm(x), and Inv(a), Inv(b) described for normal human γ-globulin were all found in different myeloma proteins. A single myeloma protein never contained more than one product of alternate alleles even in heterozygous individuals. However, factors determined by the two different loci were often found in the same myeloma protein. The Gm(a) character of the myeloma protein parallelled that of the normal γ-globulin of the same serum in most cases. In contrast, the Gm(b) character was usually absent in the myeloma protein when it was directly demonstrable in the normal γ-globulin. The myeloma proteins from six Negroes were Gm(a+b-), whereas the normal γ-globulin was Gm(a+b+). This indicates that the effect of gene Gm^b is similar in Negroes and whites, even though its relation to gene Gm^a is different in the two races. Gm factors were found only in the 7S γ-globulin type myelomas and not in other products of plasma cell tumors. Inv characters were, however, present in all four types of proteins studied, namely 7S and 19S γ-globulins, β_2A-globulins, and Bence Jones proteins. In two instances, genetic heterogeneity of the protein products was demonstrated suggesting the proliferation of more than one clone of plasma cells in some multiple myeloma patients. The accumulated evidence obtained in this study strongly suggested that the presence and absence of genetic characters was compatible with the concept that myeloma proteins were closely analogous to individual moieties in the spectrum of normal γ-globulins rather than truly abnormal proteins. Their study offered evidence of a heterogeneity of genetic characters among the normal γ-globulins in a given individual. It also appears probable that in normal individuals single plasma cells have a restricted capacity to express genetic information in their protein product.

PROPERTIES OF VARIOUS ANTI-GAMMA-GLOBULIN FACTORS IN HUMAN SERA

The serological and physicochemical properties of the following three forms of human anti-γ-globulin factors were compared: (a) rheumatoid factors; (b) Milgrom type anti-γ-globulin factors; and (c) factors directed against an antigen in human γG-globulin that is hidden in the intact molecule and revealed by enzymatic digestion at low pH. The property common to these factors is ability to interact with human γG-globulin; they are distinguishable because they react with different antigenic groups on this molecule. In all of five sera, the Milgrom type anti-γ-globulin factors were γM-globulins. They reacted with various human γG-globulin antibodies but failed to interact with γM-globulin type antibodies in agglutination and absorption experiments. When isolated from other anti-γ-globulin factors, they agglutinated red cells coated with intact anti-Rh antibodies, but failed to react with cells cells coated with pepsin-digested anti-Rh antibody. These observations indicate that the agglutinator reacts with the crystallizable, inert fragment of γG-globulin. Anti-γ-globulin activity directed against an antigen in human γG-globulin revealed by pepsin digestion was demonstrated in γG-, γA-, and γM-globulins. This anti-γ-globulin factor could be absorbed by antigen-antibody precipitates containing human antibody, which shows that the hidden antigen in human γG-globulin is revealed not only by enzymatic digestion at low pH, but also when γG-globulin is present as antibody in an antigen-antibody precipitate. Rheumatoid factors and Milgrom type anti-γ-globulin factors were also absorbed by antigen-antibody precipitates containing human antibody. The results indicate that the three distinct forms of antiγ-globulin factors may all be produced as a result of antigenic stimulation by autologous antigen-antibody complexes.

Immunogenetic markers as probes for polymorphism, gene regulation and gene transfer in man--the Gm system in perspective

The genetic markers of immunoglobulins (Ig) demonstrable by immunological methods have shown their usefulness as genetic probes. The study of these allotypes originally proved that Ig production is under conventional genetic control and also established that allelic exclusion is valid for the key molecules of the immune response. The codons responsible for the G1m(a) marker and their position in the human genome are precisely known. This knowledge implies that Gm typing may be used as a convenient and reliable means of following the fate of IgG constant gene segments. Anti-Gm's are common in rheumatoid arthritis. They appear early in the disease and may persist throughout life. The stimulus for their appearance has not yet been established. The anti-Gm's in the allegedly autoimmune disease rheumatoid arthritis are commonly and apparently paradoxically specific for Gm gene products of other persons. Another apparent paradox brought to light by Ig allotype research is the occasional appearance of non-nominal allotypes in contradiction to Mendelian laws. It is proposed that a plausible explanation for these two paradoxes is Ig gene transfer between individuals with viral vectors. Reasons for this proposal and some possible consequences of gene transfer in a polymorphic species are delineated. Immunogenetics and DNA technology in combination provide powerful tools to elucidate the fate of genes. A method allowing the assignment of G1m(a+) and G1m(a-) at the gene level by polymerase chain reaction analysis has recently been established and is briefly described.

The Gm system. Anti-Gm's: characteristics in rheumatoid arthritis; experimental induction without resort to allotype; frequent occurrence in mononucleosis

Gm system: Some RFs specifically detect Mendelian markers of human Ig, originally proving that Ig production is gene controlled. The genetic marker systems of human Ig with 17 Gm, 2 Am, 3 Km and 1 Em markers are briefly described. Examples of the usefulness of the Gm system in medicine, immunology and molecular biology are mentioned.

CONCLUSIONS

1. Some RFs have been essential tools in elucidating genetic control mechanisms in Ig production. 2. Knowledge of the Gm system is extensive. Anti-Gm's: High-titered anti-Gm's are common in R.A. The anti-allotypes in R.A. are markedly restricted as to their specificity. They are usually not directed against the individuals own Gm markers. Anti-human-immunoglobulins were observed in 12 of 18 sera from patients with mononucleosis. The majority of these anti-immunoglobulins were inhibitable by native human Ig and showed restricted specificity.

CONCLUSIONS

1. There is a strong stimulus for production of particular anti-Gm's in a majority of R.A. cases. 2. Notions of an autoimmune origin for many anti-Gm's in R.A. are not in obvious agreement with experimental observations. 3. Anti-human-Ig's with restricted specificity are commonly induced in mononucleosis.

Rheumatoid factors in 129XB recombinant inbred strains. Igh-1-linked control of allotypic and isotypic specificities

To examine the role of autologous IgG in the induction of murine rheumatoid factors (RF) we have analyzed the allotypic specificity of anti-IgG2a RF in recombinant inbred strains derived from 129/Sv (Igh-1a) and C57BL/6 (Igh-1b) mice. In five of six Igh-1a strains, anti-IgG2a RF reacted with IgG2aa but failed to react with IgG2ab. In contrast, isotype-specific RF, which reacted equally well with a and b allotypes of IgG2a, represented the major RF species in one Igh-1a and all five Igh-1b strains tested. An additional form of RF specific for IgG2ab and not reactive with IgG2aa was detected in one Igh-1b strain. RF specific for a give allotype was thus only found in the presence of that allotype, which strongly suggests the involvement of autologous IgG in the induction of mouse RF synthesis. The specificity of RF was apparently further controlled by genes linked to but different from the Igh-C locus, as indicated by the absence of IgG2aa-specific RF in one of the 6 Igh-1a strains tested. Because this strain, 129XBG, has been shown to express idiotypic markers characteristic of Igh-1b mice, it is likely that the genes, which in the presence of a given allotype induce the production of isotype rather than allotype-specific RF, are identical to those that control the expression of idiotypes. Evidence was also obtained to indicate that Igh-1-linked genes influence the isotypic specificity and the isotype of RF itself: IgA anti-IgG2a predominated in Igh-1a strains and IgM anti-IgG1 in Igh-1b strains. Interestingly enough, total IgA and IgG2a levels also were higher in Igh-1a than in Igh-1b strains.

Isotypic and allotypic specificity of mouse rheumatoid factors

The specificity of polyclonal mouse rheumatoid factors (RF) was analyzed by competition experiments with heat-aggregated mouse IgG subclasses. The RF spontaneously produced by three normal mouse strains (129/Sv, CBA/Ht, and C57Bl/6) and by two strains with autoimmune diseases (MRL/l and NZB) were found to consist of distinct non-cross-reactive antibody subpopulations each specific for one IgG subclass. The sera of the normal strains contained IgG1- and IgG2a-specific RF. The autoimmune strains produced an additional variety of RF that was specific for The autoimmune strains produced an additional variety of RF that was specific for IgG2b. Also, the RF secreted by spleen cells of various normal strains after in vitro polyclonal activation with lipopolysaccharide could be resolved into distinct subpopulations specific for IgG1 or IgG2a. These results were confirmed by the analysis of monoclonal RF derived from BALB/c, C57Bl/6, CBA/Ht, and 129/Sv mice: of 73 hybridomas with RF activity, 71 displayed a strict subclass specificity. The subclass predominantly recognized depended on the origin of the spleen cells used to generate the hybridomas. After polyclonal activation in vitro, a broad spectrum of different specificities was obtained with 16 RF specific for IgG1, 13 for IgG2a, and 4 for IgG2b. In contrast, 27 of 28 monoclonal RF derived from 129/Sv and BALB/c mice without prior polyclonal activation were specific for IgG2a, and of these 75% were allotype specific since they failed to react with IgG2a of the b allotype. These results demonstrate the importance of subclass specificity in the production of RF in vivo. With the exception of the IgG2b-specific clones, all these monoclonal RF reacted preferentially with heat-aggregated or antigen-bound IgG. Among the hybridomas generated by the fusion of in vitro polyclonally activated spleen cells of 4-wk-old mice, the frequency of clones with RF activity was at least 40 times higher than that of clones specific for mouse IgM, human IgG, ovalbumin, and hen lysozyme.

Pi-Gm linkage: evidence for linkage in males but not in females and for an effect of the S allele of the Pi system

Linkage between the Pi (alpha 1-antitrypsin) and Gm (immunoglobulin heavy chain) loci was studied in thirty-four families including forty-one informative parents and 142 children. In females, the results did not provide evidence for linkage (posterior probability of non-linkage 0.98). In contrast, in males, there was strong evidence for linkage (peak lod 3.9 at theta = 0.18, posterior probability of linkage 0.98). The two populations appeared to be significantly different (0.001 less than P less than 0.01) with respect to the heterogeneity criterion of Morton. In addition, the effect of the possession of the S allele (associated with significantly decreased serum alpha 1-antitrypsin levels) was studied in fifteen informative parents and fifty-three children of the same group. No evidence for or against linkage was found in females, but in males close linkage between Pi S and Gm was demonstrated (peak lod 7.7 at theta = 0.05, posterior probability of linkage 0.9999). These data indicate significant linkage between Pi and Gm in males but not females and close linkage between the Pi S and Gm markers in males.

The genetic control of gamma-globulin heavy chains. Studies of the major heavy chain subgroup utilizing multiple genetic markers

The genetic control of gammaG1-heavy chains was investigated by taking advantage of two recently described genetic antigens, Gm(z) and Gm(y), both produced by heteroimmunization of rabbits with myeloma proteins. These were studied in conjunction with known genetic markers, Gm(a) and Gm(f). The results indicated that among Caucasians there are two major allelic genes, Gm(za) and Gm(fy), coding for distinct varieties of gammaG1-heavy chains. Each of these contains a pair of genetic antigens which are located on different fragments of the chain and can be separated by enzymatic splitting with papain. The different areas of the heavy chains appear to be under the control of the same gene. In Mongoloid populations a grouping of three genetic antigens, Gm(f), (y), and (a), was found on isolated myeloma proteins and normal gamma-globulins indicating the presence of a Gm(fya) gene. The possible genetic events leading to the contrasting Caucasian and Mongoloid genes are discussed. In the gamma-globulin system the occurrence of multiple genetic antigens in different positions of the same heavy chains is the general rule. A better understanding of the relationships between the genes for the gammaG1-subgroup to those for the gammaG2- and gammaG3-subgroup has been obtained through the use of the multiple genetic markers. Strong evidence was obtained for intergenic crossover mechanisms to explain racial differences in the relationships of these genes as well as certain unusual gene complexes found through family studies. Further evidence was obtained for mapping the closely linked genes for the three subgroups in a specific order.

[The Gm system in the population of Latium]

In order to contribute to the studies on the group substance properties of the Gm system, the Authors have carried out a research on 180 subjects living in Latium. The results [Gm(a) 48.8%; Gm(b) 95%; Gm(x) 9.4%], are very similar to those obtained for white subjects in other countries of Europe and North America.

The Authors underline the importance of this new system in the investigations for ascertaining the parental relationship.