Immunogenetic analysis ofH-2 mutations

A.BY, B10.LPa, and B10.129(5M) mice were presensitized in vivo against B10.A(5R) cells and then restimulated in vitro by the same cells in the standard CML assay. The effector cells thus generated lysed not only B10.A(5R), but also C57BL/6 targets, indicating that, in addition to anti-H-2D_d response [measured on the B10.A(5R) targets], response to minor histocompatibility (H) antigens (measured on the C57BL/6 targets) also occurred. The latter response was directed against multiple minor H antigens in the case of the A.BY effectors, and against H-1 and H-3 antigens in the case of B10.129(5M) and B10.LPa effectors, respectively. The sensitization against minor H antigens occurred in the context of H-2K^b H-2D^d antigens, but by testing the response on C57BL/6 targets, only cells reacting with minor H antigens in the context of H-2K^b were assayed. The same effector cells were then tested against H-2^b mutant strains, in which theH-2K ^b allele was replaced by a mutant one. All three effector types [A.BY, B10.LPa, and B10.129(5M)] behaved in a similar way: they all reacted with theH-2 ^bg1 mutant to the same degree as withH-2 ^b, they did not react at all or reacted only weakly with theH-2 ^bd andH-2 ^bh mutants, and they reacted moderately or strongly with theH-2 ^ba mutant. The degree of crossreactivity with the mutants reflects, with one exception, the degree of relatedness of these mutants toH-2 ^b, as established by other methods. The one exception is theH-2 ^ba mutant, which is the most unrelated toH-2 ^b, and yet it crossreacted strongly. Further testing, however, suggested that in this instance the crossreactivity was probably directed against H-2 antigens: the anti-H-2D^d effectors apparently crossreacted with the H-2K^ba antigens. This finding is an example of cell-mediated crossreactivity between the products of two differentH-2 genes (H-2K andH-2D). It is also an example of anH-2 mutation generating an antigenic determinant known to be present in another strain.     

CML typing of serologically identicalH-2 mutants

Cell-mediated lympholysis (CML) reactions were studied among four strains of C57BL/6 (B6) mice carrying mutant alleles (H-2 ^ba ,H-2 ^bd ,H-2 ^bg , andH-2 ^bh ) at thez1 locus in theK end ofH-2 ^b and the original B6 (H-2 ^b ) strain. Cross killing of target cells from lines that had not participated in the mixed lymphocyte reaction (MLR) was extensive, but usually less intense than that of target cells of stimulator cell genotype. The extent of CML crossreactivity could be limited by using cells from F₁ hybrid mice as responders in MLR. In a comprehensive analysis of the cytotoxicity exerted by 20 MLR combinations with homozygous, and 10 MLR combinations with F₁ hybrid responder cells, 19 different CML cytotoxicity patterns were identified, corresponding to at least 19 different CML target specificites. When the number of CML mismatches of each mutant with the originalH-2 ^b was calculated,H-2 ^ba was found to be most distinct fromH-2 ^b ,H-2 ^bs andH-2 ^bd were closest toH-2 ^b , andH-2 ^bh occupied an intermediate position. The validity of this sequence of relatedness is supported by published reports on skin graft survival times and on the interaction of T lymphocytes with virus-infected target cells using cells fromz1 locus mutants.     

H-2 effects on cell-cell interactions in the response to single non-H-2 alloantigens

Immune response (Ir) genes mapping in theI region of the mouseH-2 complex appear to regulate specifically the presentation of a number of antigens by macrophages to proliferating T cells. We have investigated the possibility that similarIr genes mapping in theH-2K andH-2D regions specifically regulate the presentation of target antigens to cytotoxic effector T cells. We report that the susceptibility of targets expressing specific non-H-2 H alloantigens to lysis by H-2-compatible, H-antigen-specific cytotoxic effector T cells is controlled by polymorphicH-2K/D genes. This control of susceptibility to lysis is accomplished through what we have defined operationally as antigen-specific regulation of non-H-2 H antigen immunogenicity. High immunogenicity of the H-4.2 alloantigen is determined by a gene mapping in theH-2K region ofH-2 ^b . However, high immunogenicity of H-7.1 is determined by a gene mapping in theH-2D region ofH-2 ^b . High immunogenicity of the H-3.1 alloantigen is determined by genes mapping in both theH-2K andH-2D regions ofH-2 ^b . Therefore, genes mapping in theH-2K andH-2D regions serve a function in presenting antigen to cytotoxic effector T cells. This function is analogous to that played byI-regionIr genes expressed in macrophages which present antigen to proliferating T cells. We present arguments for classification of theseH-2K/D genes as a second system ofIr genes and discuss the implications of twoH-2-linkedIr-gene systems, their possible functions, and their evolution.     

Analysis ofH-2 mutants: Evidence for multiple CML target specificities controlled by theH-2K b gene

C57BL/6 (H-2 ^b ) mice and two mutants derived from this strain, B6.C-H-2 ^ba (Hz1) andE6-H-2 ^bd (M505), were studied in a number of functional tests, in vitro and in vivo, that assay for differences at theH-2 complex. All three strains give rise to reciprocal mixed lymphocyte reactivity (MLR) and cell-mediated lympholysis (CML) in vitro as well as graft-host reactivity (GVHR) and skin graft rejection in vivo. Analysis for cross-reactivity between these strains in CML revealed that the gained antigens in each mutant do not cross-react, and that Hz1 has lost an antigen shared by C57BL/6 and M505 strains. In addition, spleen cells from B10.A(4R) mice, which differ from theH-2 ^b haplotype only at theK end of theH-2 complex, recognize a common antigen shared by all three strains tested. Provided that the mutations occurred in theH-2K ^b gene, these data indicate that a) there are at least three antigenic specificities coded for by theH-2K ^b gene(s) that serve as targets for receptors on thymus-derived (T) cells in CML; b) since C57BL/6 strain mice and the mutants are serologically indistinguishable on a qualitative basis, the antigens recognized by the receptors on T cells and by humoral H-2 antibody are nonidentical; and c) mutation in theH-2K ^b locus itself can give rise to allogeneic recognition phenomena such as MLR and GVHR.     

Evidence for a third,Ir-associated histocompatibility region in theH-2 complex of the mouse

Skin grafts transplanted from B10.HTT donors onto (A.TL × B10)F₁ recipients are rapidly rejected despite the fact that the B10.HTT and A.TL strains should be carrying the sameH-2 chromosomes and that both the donor and the recipient contain the B10 genome. The rejection is accompanied by a production of cytotoxic antibodies against antigens controlled by theIr region of theH-2 complex. These unexpected findings are interpreted as evidence for a third histocompatibility locus in theH-2 complex,H-2I, located in theIr region close toH-2K. The B10.HTT and A.TL strains are postulated to differ at this hypothetical locus, and the difference between the two strains is explained as resulting from a crossing over between theH-2 ^t1 andH-2 ^s chromosomes in the early history of the B10.HTT strain. TheH-2 genotypes of the B10.HTT and A.TL strains are assumed to beH-2K ^s Ir ^s / ^k Ss ^k H-2D ^d andH-2K ^s Ir ^k Ss ^k H-2D ^d , respectively. Thus, theH-2 chromosomes of the two strains differ only in a portion of theIr region, including theH-2I locus. The B10.HTT(H-2 ^tt) and B10.S(7R)(H-2 ^th) strains differ in a relatively minor histocompatibility locus, possibly residing in theTla region outside of theH-2 complex.     

Further evidence for two separate loci (H-2D andH-2L) in theD region of theH-2 complex

The differential redistribution method was used to analyze the relationships between the antigens of the H-2.1 and H-2.28 families and the K- and D-region H-2 specificities on the lymphocyte surface. The experiments were performed on T peripheral lymphocytes of B10. AKM mice (H- 2^m), where the H-2.28 specificity is controlled by theD region; C3H.OL mice (H- 2^0l), where the H-2.28 specificity is controlled by theK region and the H-2.1 specificity by theD region; and B10.A mice (H- 2^a) where the H-2.1 specificity is controlled by theK region. The results show the following:

1. In the D-region products, the redistribution of the private specificities fails to induce the redistribution of the H-2.1 or H-2.28 specificity. Antibodies against the H-2.1 or H-2.28 specificity provoke the redistribution of the D-region private specificities.
2. In the K-region products, the H-2.1 or H-2.28 specificity cocaps with the private specificities.
3. In both K- and D-region products, the public specificity H-2.5 always cocapped by antibodies against the private specificity.

These data suggest that the D-region H-2.1 specificity is, like the H-2.28 specificity, controlled by gene(s) different from theH- 2.D gene for the private, and most of the public, specificities. However, in the K-region products, the H-2.1 or H-2.28 specificity and the private specificities are either controlled by the same gene or expressed on two different molecules associated on the cell surface. These results provide evidence for the existence of two separate loci in theD region: the classicalH-2D locus, controlling the expression of the private specificity and most of the public specificity, and theH-2L locus, controlling the expression of the H-2.1 or H-2.28 specificity.     

The genetic control of the immune response to murine histocompatibility antigens

The genetic control of the immune response to H-4 histocompatibility alloantigens is described. The rejection of H-4.2-incompatible skin grafts is regulated by anH-2-linkedIr gene. Fast responsiveness is determined by a dominant allele at theIrH-4.2 locus. TheH-2 ^b ,H-2 ^d , andH-2 ^s haplotypes share the fast response allele;H-2 ^a has the slow response allele. Through the use of intra-H-2 recombinants, we have mapped theIrH-4.2 locus to theI-B subregion of theH-2 complex; theH-2 ^h4 ,H-2 ¹⁵, andH-2 ^t4 haplotypes are fast responder haplotypes. These observations suggest that the strength of non-H-2 histocompatibility antigens is ultimately determined by the antigen-specific recipient responsiveness.     

Generation of cytotoxic lymphocytes in mixed lymphocyte reactions II. Importance of private and publicH-2 alloantigens on the expression of cytotoxicity

MLC were established to test for the generation of specific cytotoxic effector cells in CML. The target cell used to assay for CML in the five combinations tested was of a differentH-2 haplotype from the stimulating cell population. Cytotoxicity was observed against this target only when it shared private alloantigens (antigens that are specific for theH-2D andH-2K region of differentH-2 haplotypes) with the stimulating cell population. Very weak or no Cytotoxicity was found when such alloantigens were not shared, although cross-reactive publicH-2 specificities were. These findings indicate that T cells display a cytotoxic potential against privateH-2 antigens in a primary response in vitro and are not capable of responding to publicH-2 specificities to the same level.BSS balanced salt solution - CML cell-mediated lympholysis - GPC guinea pig complement - ¹²⁵IUdR ¹²⁵I-iodo-deoxyuridine - MLC mixed lymphocyte culture - SE standard error     

BALB/c-H-2 db : A newH-2 mutant in BALB/cKh that identifies a locus associated with theD region

A newH-2 mutant, BALB/c-H-2 ^db , is described. This mutant originated in BALB/c, is inbred, and is coisogenic with the parental BALB/cKh strain. The mutation is of the loss type since BALB/c-H- ^db rejects BALB/c, but not vice versa. Complementation studies have localized the mutation to theD region of theH-2 complex. A cross between BALB/c-H-2 ^db and B10.D2-H-2 ^da failed to complement for either BALB/c or B10.D2 skin grafts, indicating that these are two separate mutations at the same locus (Z2). Direct serological analysis and absorption studies revealed that, with one exception, theH-2 andIa specificities of BALB/c and BALB/c-H-2 ^db are identical. In particular,H-2.4, the H-2D^d private specificity, is quantitatively and qualitatively identical in the two strains. The exception is that of the specificities detected by antiserum D28b: (k×r)F₁ anti-h, which contains anti-H-2.27, 28, and 29. These specificities appear to be absent from theH-2 ^db mutant since they are not detected directly or by absorption. Other public specificities are present in normal amounts,e.g., the reaction with antisera to H-2.3, 8, 13, 35, and 36. The reaction with antiserum D28 (f×k)F₁ anti-s, which contains antibodies to H-2.28, 36, and 42, is the same in both strains. Antiserum made between the two strains (H-2 ^db anti-H-2 ^d ) reacts like an anti-H-2 serum, in that it reacts with both T and B cells by cytotoxicity, but is not a hemagglutinating antibody. The serum reacts as does the D28b serum in both strain distribution and in cross-absorption studies. We conclude that theH-2 ^db mutation occurred at a locus in theD region, resulting in the loss of the H-2.28 public serological specificity and of a histocompatibility antigen. Whether these are one and the same antigen is not yet known. The data, in view of other evidence, imply that the public and private specificities are coded for by separate genes.Abbreviations used in this paper are as follows CML cell-mediated lysis - MLR mixed lymphocyte reaction - GVHR graft-versus-host reaction - RFC rosette-forming cells - RAM-Ig rabbit anti-mouse IgG     

Dermal histocompatibility and in vitro lymphocyte reactions of three newH-2 mutants

Histocompatibility of skin grafts and in vitro lymphocyte reactions of three new H-2 mutants (B6-H-2 ^bg1, B6-H-2 ^bg2, and B6-H-2 ^bh ) are described. Complementation tests indicated thatH-2 ^bg1,H-2 ^bg2, andH-2 ^bh were mutations at the same locus (z-1) and allelic withH-2 ^ba . Two of the lines, B6-H-2 ^bg1 and B6-H-2 ^bg2, appeared identical, although they were of spontaneous and independent origin. The third line, B6-H-2 ^bh , was unique. Such graft rejection was paralleled by reactivity in the MLR and by the generation of cytotoxic lymphocytes in vitro. The pattern of crossreactivity among antigens specified by these mutant genes supports the notion that thez-1 locus controls multiple specificities. The results indicate that a single point mutation can simultaneously affect histocompatibility, MLR, and CML reactions.The genetic nomenclature for theH-2 complex is that of Kleinet al. (1974a).     

Identification of specificityH-2.7 as an erythrocyte antigen: Control by an independent locus,H-2G,between theS andD regions

SpecificityH-2.7 is expressed predominantly on erythrocytes and controlled by a gene that maps within theH-2 gene complex at a locus, designated asH-2G, which apparently lies between regionsS andD. Three phenotypes have been observed with respect to this antigen: a) positive by direct test and absorption (haplotypesH-2 ^f ,H-2 ^j ,H-2 ^p ,H-2 ^s ); b) positive only by absorption (H-2 ^k ); and c) negative (H-2 ^b ,H-2 ^d ,H-2 ^q ). New crossover positions have been established for severalH-2 recombinants based on classifications for theH-2G locus.     

Identification of a new CML target antigen controlled by a gene associated with theQa-2 locus

BALB/cBy anti-BALB/cJ spleen cells were tested in a secondary cellmediated lympholysis assay. The effector cells generated displayed a positive cytotoxic effect against Con A lymphoblasts from only those strains that were typed serologically as having theQa-2 ^a allele. Confirmation that the target antigen is controlled by a locus closely associated with or identical toQa-2 was obtained by the findings that target cells from B6.K2 (Qa-2 ^a,Qa-3 ^a) mice were lysed by the effector cells, while those from theQa-2, 3 congenic strain B6.K1 (Qa-2 ^b,Qa-3 ^b) were not. The fact that target cells from aQa-2-positive/Qa-3-negative strain (DBA/1,Qa-2 ^ai,Qa-3 ^b) were killed indicates that the target antigen is controlled, at least in part, by theQa-2 locus, not the Qa-3.There is no observedH-2 genetic restriction for this cytotoxic effect, since target cells which have theQa-2 ^a allele but differ from the stimulator cells at theH-2K, D, andI regions were lysed efficiently.     

The role ofH-2 regions in overcoming tissue incompatibility

Neonatal transplantation tolerance to the products of theH-2 ^b complex was induced in B10.A (H-2 ^a ) mice. On the basis of the survival of skin allografts it was found that antigens determined by theD region of theH-2 ^b complex (of the B10.A(2R) strain) were most easily overcome and that tolerance to the products of theD end of theH-2 complex (of the B10.A(4R) strain) was also easy to induce. The antigens produced by theK end ofH-2 (of the B10.A(5R) and B10.A(3R) strains) represented a stronger incompatibility barrier and a difference in the entireH-2 ^b complex caused strongest resistance to tolerance induction. When tolerance to the products of the entireH-2 ^b complex was induced in newborn B10.A mice, and the neonatally treated animals were grafted simultaneously with five different grafts, those disparate at theK end ofH-2 and in the entireH-2 region were rejected in some animals, while the grafts disparate at theD end of H-2 remained intact in the same mice. No dependence on theI-J subregion was observed in this system. Furthermore, tolerance was more easily inducible in male than in female B10.A mice.     

Study of H-2 mutations in mice. IV. A comparison of the mutants M505 and Hz1 by skin grafting and serological techniques

The line B6.M505 is congenic with C57BL/6JY and carries a mutant form of theH-2 ^b haplotype designatedH-2 ^bd . The mutant site 505 was located by the F₁ tests in theK end of theH-2 gene complex. The M505 mice are histoincompatible with the B6.C(Hz1) line (haplotypeH-2 ^ba ) carrying another mutation in theK end ofH-2 ^b . Inability of M505 to complement Hz1 in tests with B6 skin grafting is considered as an evidence that the same gene was altered by both mutations. The gained H antigens of two mutants can cross-react in vivo as revealed by accelerated rejection of Hz1 skin grafts by B6 recipients presensitized with M505 spleen cells. The lost antigenic determinants are not identical as shown by accelerated rejection of B6 skin grafts by Hz1 hosts preimmunized with M505 spleen cells. Absorptions of the antiserum ASY-015, (d×a) anti-i, anti-H-2.33 with M505 spleen cells did not clear forH-2 ⁱ ,H-2 ^b andH-2 ^ba , and absorptions with Hz1 did not clear forH-2 ⁱ ,H-2 ^b , andH-2 ^bd . These results show that changes of histocompatibility determinants may be accompanied by loss of some haptenic determinants in the Hz1 and M505 mutations.     

Responses against antigens encoded by theH-3 histocompatibility locus: Antigens stimulating class I MHC- and class II MHC-restricted T cells are encoded by separate genes

The purpose of this work was to study the genetic basis of histocompatibility antigens encoded by the mouse minor histocompatibility (H) locusH-3. Both class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL) and class II MHC-restricted helper T cells (TH) specific for antigens encoded by genes within theH-3 locus were isolated and analyzed. Typing a number of mouse strains for expression of antigens recognized by these TH and CTL suggested that there was a different strain distribution pattern of expression of the antigens recognized by TH compared with those recognized by CTL. Separation of the genes whose products stimulate TH from those whose products stimulate CTL was suggested by: (1) analysis of the strain B10.FS(92NX)/Grf that has undergone recombination within theH-3 region; (2) genetic segregation studies of (B10.UW-H-3 ^b/Sn×C57BL/10Sn)F₂ mice; and (3) F₁ complementation studies in which CTL specific for products of the TH-defined gene(s) could not be detected, even in the absence of immune responses to products of the CTL-defined genes. Taken together, these data suggest that in addition to two genes (B2m andCd-1) within theH-3 region whose products typically stimulate class I MHC-restricted CTL, there is at least one additional gene whose product selectively stimulates class II MHC-restricted TH. This new gene is located telomeric from the CTL-defined genes and between the lociwe andun on chromosome 2. These data demonstrate a novel degree of complexity of theH-3 “locus” and suggest selective presentation of minor H gene products in the context of class I or class II MHC proteins.     

The histocompatibility-2 system in wild mice

The I-region gene products of 29 wild-derivedH-2 haplotypes on a B10 background (B10.W congenic lines) were typed with alloantisera which detect 17 inbred I-region antigens. Five new I-region antigens were defined by expanding the inbred line panel ofH-2 haplotypes to includeH-2 ^u , H-2^v, andH-2 ^j . Based on serological analyses of the inbred and B10.W lines, the polymorphism of theIA gene (or genes) is estimated to be at a minimum of 15 alleles and theIE gene (or genes) at a minimum of 4 alleles. These results indicate that theIA subregion is more polymorphic than theIE subregion. By combining the I-region typing data with theH-2K andH-2D region typing data reported previously, a total of 11 new natural recombinants of inbredH-2 alleles were detected among the B10.W lines.     

Genetic control of acquired resistance to visceral Leishmaniasis in mice

A series ofH-2 and non-H-2 congenic resistant (CR) strains on a C57BL/10Sn background were infected with 10⁷ amastigotes ofLeishmania donovani. Non-H-2 congenic strains B10.LP-H-3 ^b and B10.CE(30NX) and (B10.LP-H-3 ^b × B10)F₁ hybrids showed a very rapid decrease in liver-parasite burdens beyond day 21. Parasite counts for these strains at day 35 were significantly lower than for all other strains tested. The rapid decrease in parasite numbers, massive lymphocellular infiltration into the liver and strong delayed hypersensitivity reactions to parasite antigens in strains congenic for a portion of chromosome 2 indicated that acquired immunity toL. donovani was controlled by a dominant gene at or near theIr-2 locus. In addition, B10.129(10M) mice, which differ from C57BL/10Sn at theH-11 locus, showed highly significant increases in parasite numbers at day 35. Other observations supporting the absence of acquired immunity in B10.129(10M) included negative delayed hypersensitivity tests to parasite antigens and the absence of lymphocellular infiltrate into the liver. Although the differences were not as pronounced,H-2 CR strains withH-2 ^b ,H-2 ^a , andH-2 ^k haplotypes also showed significantly greater decreases in parasite numbers by day 35 as compared to otherH-2 CR strains.     

Serological and complementation studies in four C57BL/6H-2 mutants

C57BL/6 (H-2 ^b ) mice, and four mutants (B6.C-H-2 ^ba , B6-H-2 ^bg1 , B6-H-2 ^bg2 , B6-H-2 ^bh ) derived from this strain after separate mutations had occurred at the same locus within theH-2 complex, were analyzed to determine whether the mutations had led to anyH-2 (or Ia) difference which could be detected serologically. The strains were typed directly with antisera specific for H-2K and H-2D public and private specificities and for the Ia specificities; quantitative absorption studies were also performed for the relevant H-2K^b, H-2D^d and Ia^b specificities. In no case was any quantitative or qualitative difference detected serologically between any of the strains. In addition, by using a variety of techniques to produce and assay for antibody, we failed to produce any antisera between the parental strains and the four mutants. TheH-2 mutations therefore appear to give rise to a type of antigenic specificity which is recognized byT cells and which generateT, but notB cell responses; nor are they recognized by H-2 or Ia alloantisera. The location of the mutating locus within theH-2 complex was shown by the complementation method to be within theK orIA region and not in theIB region, since crosses of the mutant strains with B10.A(4R) or D2.GD failed to complement for a subsequent C57BL/6 skin graft.     

H-2 antigen variants in a cultured mouse leukemia cell line

We have investigated the effect of immune selection against a single gene product on a cultured mouse Friend leukemia cell line. The clonal cell line used is heterozygous at theH-2 complex and expresses theH-2 ^d andH-2 ^b haplotypes. The genes selected against were theH-2K locus alleles. Variants were obtained after a single-step selection using either antiH-2K^b or anti-H-2K^d serum. The phenotypes of the variants obtained showed an interesting asymmetry between the two haplotypes. Selection against theH 2K ^b allele resulted in the isolation of the two expected types of variant-those that had lost only H-2K^b and those that had lost both H-2K^b and the linked H-2D^b. Selection against H-2K^d yielded, exclusively, variants that had lost both the selected antigen and the linked H-2D^d. None of the variants showed an alteration in expression of antigens intrans configuration. Karyotypic analyses of the variants revealed that all the cells had retained both copies of chromosome 17 present in the wild-type cells. The results suggest that the variants did not emerge through chromosome loss.     

Histocompatibility-2 system in wild mice

Ninety-six wild mice trapped at 13 localities in the state of Texas were tested in the dye-exclusion cytotoxic test with a battery of 49 oligospecific H-2 antisera. The antisera detected 36 class I (K and D) and 10 class II (Ia) antigens. The phenotypic frequencies of private class I antigens ranged from 1 to 20%, the majority of them being in the range between 1 and 5%. At least some of the higher frequencies resulted from the presence of more than one antibody in the typing reagents, and from other factors complicating the typing. We estimate that the frequencies of most of the class I alleles among Texas wild mice are 1% or less. This estimate leads to the prediction that at least 200 alleles exist in Texas mice at theH-2K locus, and another 200 alleles exist at theH-2D locus. Frequencies of most of the class I public antigens were in excess of 20%. In the sample of 96 mice, 46 different phenotypic combinations of private class I antigens were found, and the frequency of blanks (mice unreactive with any of the antibodies to private class I antigens) was 27%. The frequencies of private class II antigens ranged from 5 to 15%. Some of the public class II antigens, in particular those controlled by theE region, occurred with frequencies of 80% or higher. The class II antigens were found in 26 phenotypic combinations. No striking linkage disequilibrium was found either between K and D antigens, or between class I and class II antigens. The polymorphism of theK, A, andD region appears to be higher than that of the corresponding regions of the human or rat major histocompatibility complex. The polymorphism of theE region is significantly lower than that of theA, K, andD regions. The polymorphism of theA region is extensive.