H-2 hemizygous mutants from a heterozygous cell line: role of mitotic recombination

Variants that no longer express an entire H-2 haplotype were readily isolated, by immunoselection with antisera directed against the haplotype, from an H-2b/H-2d heterozygous Friend leukemia cell line carrying a Robertsonian translocation of the chromosomes bearing the H-2 genetic region. These variants can be denoted as being of the phenotype H-2b- H-2d+ or H-2b+ H-2d-. Some of the H-2b- H-2d+ variants: (1) lack the restriction enzyme fragments characteristic of the missing H-2b haplotype, as assessed by Southern blot analysis; (2) express more cell surface H-2d antigens than wild-type cells, as assessed by flow microfluorimetry; and (3) appear to have become homozygous for the more active H-2d-linked allele at the Glyoxalase I locus. These variants thus seem to have lost genetic material corresponding to the H-2b haplotype and may have gained genetic material corresponding to the H-2d haplotype. These results are consistent with the possibility that these variants were generated by mitotic recombination.     

CTL and serologically defined antigens of B2m,H-3 region

The antigens of the B2m,H-3 region of 13 chromosome 2 congenic strains and seven inbred strains have been studied by using CML and serologic techniques. Nine patterns of cross-reactivity have been defined by CML assays. These results are in agreement with an extend previously described cross-reactivity studies. The reactivities of three monoclonal antibodies previously thought to be reacting with B2M-B are shown to differ: Ly-m11 and J-5 react with cells of strain B10-pa,at and clone 23 does not. Two H-3 region loci are hypothesized on the basis of CML and serologic activity: B2m and H-3. The CTL responses to the B2M antigens are H-2K restricted; the CTL responses to H-3 antigens are H-2D restricted. The restriction of the response to the H-3 antigen requires effector-target identity of the H-2D molecule but not the B2M molecule of the class I antigen. These loci have been separated by recombination from H-42 in the production of the congenic strain B10.FS-a. A gene order of B2m, H-3, H-42 is suggested.     

Modulation of natural cytotoxicity by alloantibodies: III. Augmentation of natural killer activity by alloantisera directed against K,D, and I regions of the murine H-2 complex

Natural killer activity of mouse spleen cells toward a human myeloid leukemia cell line, K562, can be enhanced by alloantisera directed against individual antigens in the H-2 region. By using a panel of 13 antisera (8 directed against antigens in the K and D regions and 5 directed against antigens in the I region) and four strains of mice (C57BL/6J, CBA, DBA/2, and A/J) it was found that certain antisera would stimulate target cell lysis by spleen cells only if the antisera had specificity for antigens which were a part of the haplotype represented on the spleen NK effector cells. Anti Ia antisera could stimulate the anti K562 NK activity of nude mouse spleen cells which lack mature T cells. Depletion of B cells and macrophages from nude spleen cells, by passing through a nylon-wool column also did not abolish the effect of anti-Ia antiserum. It appears likely therefore that the anti-Ia antibodies exert this effect directly on NK cells and that Ia antigens may be expressed on NK cells. Since the antisera directed against different antigens in H-2 complex irrespective of subregion specificity (K, D, or I) stimulated the NK activity of mouse spleen cells, the phenomenon offered an interesting method for testing the presence of a given alloantigen on mouse spleen cells. Log-dose response curves for the augmentation of lysis induced by appropriate alloantisera were linear over a dilution range of 1:320 to 1:5120. By using the dose-response curves, potency ratios of two preparations of antisera (directed against antigen 33 of the K region) could be successfully determined. Besides the K562 cell line, many human lymphoblastoid cell lines could also be used as target cells in this assay system.     

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

An (H-2k/H-2d)F1 sarcoma cell line was subjected to immunoselection using ascites fluid from a mouse growing a hybridoma secreting an anti H-2Kk antibody.One hundred random clones were picked from the surviving population and screened by direct cytolysis using the hybridoma antibody or alloantisera against H-2Kk and H-2Dk. Fifty-nine clones were resistant to all three antisera, indicating that they no longer expressed the entire H-2k haplotype. Thirty-two were resistant to the ascites and to the anti H-2Kk alloantiserum, but sensitive to the anti H-2Dk serum, indicating that they had lost H-2Kk antigen, but retained H-2Dk. Nine clones were sensitive to the alloantisera, but resistant to the hybridoma, indicating that, though they retained the product(s) recognized by the alloantiserum against H-2Kk, they had lost the site(s) that bound the hybridoma antibody. Quantitative absorption assays using lymph-node cells from young BALB.K (H-2Kk) mice as targets show that one representative clone from the last group absorbs the anti H-2Kk activity in the alloantiserum. This implies that the sensitivity of the variant clone to the alloantiserum is not due to contaminating anti C-type virus antibodies in the serum. The possible implications of these data are discussed.     

Immunogeneic analysis of H-2 mutations. II. Cellular immunity to the H-2DA mutation.

The H-2da haplotype was derived from the H-2d haplotype by a mutation localized to the D end of the H-2 complex. Coculture of H-2d and H-2da spleen cells gives rise to bidirectional MLR. However, the H-2d anti-H-2da response is much stronger than that of H-2da anti-H-2d. Both haplotypes give rise to reciprocal CML. B10.D2(R103) strain spleen cells, which differ only at the D end of the H-2 complex from the H-2d haplotype, kill H-2da target cells in CML when sensitized to H-2d stimulators and vice versa. Therefore, both the mutant and strain of origin share a D end CML specificity. H-2d and H-2da reject skin grafts in both directions, although some H-2d grafts show prolonged acceptance on H-2da recipients. These data are consistent with a mutation in the D end of the H-2d haplotype resulting in gain-loss of an antigen(s) that gives rise to reciprocal MLR, CML, and skin graft rejection. Further, the mutant can be distinguished from the strain of origin on the basis of the strength of immune response in MLR.     

Lymphokine-activated killer cell susceptibility and adhesion molecule expression of multidrug resistant breast carcinoma

Reports showing susceptibility of multidrug resistant (MDR) cancer cells to immune effectors, together with P-glycoprotein (P-gp) expression in immune effector subsets, including immature natural killer (NK) cells, and some activated T cells, suggest P-gp or some changes associated with it, have implications in immune-mediated mechanisms. A series of experiments were done to determine the nature of alterations associated with susceptibility to immune effector cells of MDR tumor cells. A cell line isolated from the malignant pleural effusion of a breast cancer patient was transfected with human and murine MDR1 genes, and four variants with different levels of MDR were obtained. Lymphokine-activated killer (LAK) activity was measured by a ⁵¹Chromium release, and conjugate formation assays. MDR1 transfectant P-gp⁺ breast carcinoma lines had increased LAK susceptibility compared to their parent line. Some part of the increased LAK susceptibility of drug-resistant cell lines was at the binding/recognition level as shown by conjugate formation assays. This suggests that differences may exist between paired cell lines with respect to the expression of cell adhesion molecules (CAMs). Monoclonal antibodies (mAbs) to CAMs and flow cytometry were used to quantitate these antigens. The CAMs studied were those previously found to be upregulated by stimulating NK cells with (interleukin-2) IL-2; ICAM-1 (CD54), LFA-3 (CD58), N-CAM (CD56), and the β chain of LFA-1 (CD18). Although no differences in these CAMs were found between the breast carcinoma line and its MDR1-transfected variants, the target susceptibility results given above suggest that IL-2 treatment could be effective in combination with current protocols using chemotherapeutics, monoclonal antibodies (mAbs) and stem cell transplantation.     

Isolation and characterization of an isoantigenic variant from a heterozygous mouse lymphoma in culture

A cell line from a mouse lymphoma heterozygous at the chromosome region for the H-2^d and H-2^k alleles was originally obtained from a transplantable lymphoma in the (C3H × DBA/2)F₁ hybrid (H-2^d/H-2^k) and cultured in vitro. The original cultured line, termed parent line, was susceptible to the cytotoxic action of antibodies directed against antigenic components of both the d and k alleles. The parent line also absorbed hemagglutinins from both anti-d anti-k antisera. A resistant, variant subline was selected from the original population by immunoselection in vitro with anti-H-2^d antibody and complement in a cytotoxic system. After one year in continuous culture in the absence of selecting antisera, the variant subline was still resistant to the cytotoxic action of anti-H-2^d antibody. Serologic analysis of the variant indicated that it had lost the D antigenic component of the d allele, had a reduced amount of the H component, controlled by both the d and k alleles, and had retained the K component of the k allele. Possible genetic mechanisms that might account for the emergence of the variant line are discussed. While the results do not necessarily support an analysis based on mitotic recombination, ascribing other mechanisms is also difficult because of aneuploidy in the cell line. Finally, the experiments point out the advantages of using in vitro immunoselective methods in the genetics of mammalian somatic cells.     

Analysis of somatic cell H-2 variants to define the structural requirements for class I antigen expression

We have taken the approach of producing somatic cell variants with altered H-2 products to study the structural requirements for cell surface expression of class I histocompatibility molecules. H-2 antigen variants generated by chemical mutagenesis of a cell line expressing the H-2b haplotype were first selected with alloantisera for their loss of H-2Kb expression, and then were analyzed by radioimmunoassay for the appearance of intracellular Kb antigen. For one such variant (69.9.15), whereas the H-2Kb antigen was absent from the cell surface as assayed by antibody-mediated complement-dependent cytotoxicity, an H-2Kb molecule was detected within the cell lysate as confirmed by direct immune precipitation with Kb-specific monoclonal antibodies. The product had an altered antigenic phenotype, since it reacted with only two anti-Kb monoclonal antibodies (Y-3 and EH-144) and not with a third (5F1.2). Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis identified the beta 2 microglobulin-associated, intracellular H-2Kb heavy chain to be slightly smaller in Mr than the H-2Kb of the parental cell line. Hybridization analysis revealed the Kb gene from the variant to be without gross alterations, and furthermore, identified a Kb mRNA species that was identical in size to wild-type Kb mRNA. Because complementation was not observed after somatic cell fusion of variant cells with BALB/c splenocytes, it appeared that the alteration in Kb expression was due to a cis-acting defect. In addition, DNA-mediated gene transfer of the wild-type Kb gene into the variant cell line resulted in expression of the Kb antigen on the cell surface, thus confirming that the defect in expression of the mutant Kb product was not due to other factors in the 69.9.15 cell line. Such findings are consistent with the conclusion that stable H-2Kb surface-negative somatic variants can arise due to limited alterations in the Kb gene, resulting in the synthesis of a class I molecule that is expressed only as an intracellular product.     

Mouse Hepa-1 tumor is rejected by H-2Db-restricted CTL despite decreased MHC class I antigen expression

It has recently been hypothesized that tumor cells with reduced levels of MHC class I antigens are more susceptible to NK-mediated lysis and are rejected by NK cells, whereas tumor cells with normal levels of class I are rejected by tumor-specific CTL. We have tested this hypothesis using a mouse hepatoma system. The Hepa-1 tumor is a spontaneous H-2Kb loss variant that arose from the BW7756 tumor, when BW7756 was adapted to growth in culture. Our studies have shown that despite the loss of H-2Kb antigen, Hepa-1 is not more susceptible to NK lysis than its H-2Kb-transfected variants. These studies also suggested that NK cells were not responsible for rejection of the Hepa-1 tumor. The Hepa-1 tumor, therefore, appears to contradict the hypothesized linkage of MHC levels and NK susceptibility. Because NK cells are not involved in immunity to this tumor, we have sought to identify the effector cell responsible for Hepa-1 rejection. Cytotoxic T lymphocyte assays demonstrate that in vitro, Hepa-1 cells are lysed by Hepa-1-specific H-2Db-restricted CD4-CD8+ T lymphocytes. Footpad assays demonstrate that in vivo, Hepa-1 rejection requires CD4+CD8- and CD4-CD8+ Hepa-1-primed splenocytes. These results indicate that immunity to Hepa-1 is T cell mediated. Hepa-1 is therefore an example of an unusual tumor in that down-regulation of MHC class I antigen expression is associated with increased CTL susceptibility.     

An approach to the study of structure-function relationships of MHC class I molecules: isolation and serologic characterization of H-2Kb somatic cell variants

Somatic cell variants expressing an altered antigenic form of the H-2Kb molecule were isolated for the purpose of performing structure-function analysis of a class I MHC molecule. Over 25 independently isolated variants were derived from an Abelson virus transformed pre- B cell line (R8) by mutagenesis with ethyl methane sulfonate or ethyl nitrosourea. Negative selection was performed by complement-dependent cytotoxicity with anti-H-2Kb monoclonal antibodies subsequently followed by positive selection to separate the H-2Kb surface negative variants from structural variants. Biochemical characterization of a random selection of three independent variants indicated that the variant H-2Kb molecule was present in normal amounts in lysates, and was unchanged in size. Cytofluorometric analysis with the use of a panel of seven monoclonal antibodies against H-2Kb indicated that all of the variants had lost one or more alloantigenic determinants (monoclonal antibody binding sites). For these variants, the pattern of monoclonal antibody loss of recognition suggested that antibody defined alloantigenic determinants appear to be discretely localized to a single domain, either the alpha 1 or the alpha 2 domain, of the H-2Kb molecule. In contrast, CTL recognition of the Kb molecule of these variants depends on involvement of both alpha 1 and alpha 2 domains as shown in the companion paper.     

Dissociation of H-2 recognition by antibody and cytotoxic T cells of a cloned murine leukemia cell line

The differential expression of H-2 specificities recognized by antibody and by cytotoxic T lymphocytes (CTL) has been studied using a clone (FY7) of the C57BL/6 leukemia cell line FBL-3 (H-2b/H-2b). Unlike C57BL/10 spleen cells, EL-4 lymphoma cells and Y57-2C leukemia cells (all H-2b/H-2b), FY7 failed to induce the primary in vitro generation of anti-H-2b CTL by (B10.A x A)F1 (H-2a/H-2a) or B10.D2 x BALB/c)F1 (H-2d/H-2d) responder spleen cells. In addition, FY7 was not lysed by, and did not competitively inhibit anti-H-2b CTL. Quantitative absorption tests with H-2Kb and H-2Db antisera revealed that FY7 expressed these antigens in quantitatively similar amounts to EL-4. The H-2Kb product of FY7 appeared to be identical with that of C57BL/10 spleen cells both in apparent molecular weight and isoelectric point. Yet FY7 failed to inhibit anti-H-2Kb CTL competitively in a cold target inhibition assay. Possible mechanisms are discussed for the lack of T-lymphocyte recognition of the H-2Kb-gene product expressed by FY7.     

Properties of H-2L locus products in allogeneic and H-2 restricted, trinitrophenyl-specific cytotoxic responses.

The role of the recently defined L antigen (a second D region product) in allogeneic and TNP-specific syngeneic primary CML responses has been investigated. The lysis by anti-L specific cytotoxic effector cells was not inhibited when the target cells were pretreated with an antiserum directed against K and D, whereas an antiserum against L completely abrogated this response. Therefore, H-2L products are recognized on the target cell independently of H-2K and H-2D locus products. Both A.SW cells as well as B10 cells were found to respond to Ld alloantigens, in addition to Dd alloantigens when stimulated by cells differing only in the D region. The results of cold target blocking and antiserum inhibition experiments failed to detect cytotoxic cells with specificity of L antigens in association with TNP, under conditions in which TNP-specific effectors to K and D antigens were demonstrable. These findings suggest that there is a more limited involvement of H-2L locus products than the H-2K or H-2D locus products in the induction and specificity of these responses.     

Recombinant haplotype H-2 in mice of the congenic resistant strain B10.D1(R108)/Y

Recombinant H-2 haplotype of mouse strain B10.D1(R108)/Y (symbol R108) obtained in experiments with skin grafting in the course of developing the CR B10.D1/Y strain (strain DBA/LacY--the donor of H-2q) was studied. Strains with recombinant H-2 haplotypes a, h2, g1, i3, i5, i7, m, y1 were used. Alleles of different H-2 (K, I, D) regions were determined according to the presence or absence of genetic complementation in the F1 test with skin grafts. R108 recombinant was studied by serological methods with panel of anti-H-2 sera. Anti-H-2Kb (H-2.33) and anti-H-2Dq (H-2.30) monospecific antisera were used in microcytotoxicity test and in absorption experiments in vitro. It was concluded that crossing over between H-2b and H-2q chromosomes, which led to formation of recombinant H-2 haplotype of R108 mice, occurred at I region, between IA and IC subregions. The H-2 complex of R108 line has KbIAbIJ?IE?ICqSqDq alleles. bq1 symbol was proposed for the H-2 haplotype of B10.D1(R108)/Y strain.     

Wheat germ agglutinin-resistant variant of EL4 containing altered oligosaccharides as a target cell for cytotoxic T cells

A lectin-resistant variant of the murine EL4 lymphocytic leukemia cell line was selected in the presence of wheat germ agglutinin for low levels of cell-surface sialic acid. H-2Kb was the major internally radiolabeled H-2b molecule on the cell-surface of WD1, and it was not sialylated, as determined by two-dimensional gel analysis. Endo-beta-N-acetylglucosaminidase H treatment of the WD1 membrane fractions suggested that the oligosaccharides on the cell-surface H-2Kb molecule were complex, but nonsialylated. Monoclonal antibody inhibition of the allogeneically primed cell-mediated cytotoxicity (CMC) reaction indicated that the T cells (BALB/c anti-EL4; H-2d anti-H-2b) were specific only for the H-2Kb target cell antigen. These WD1 variant cells were used as targets in the CMC assay using anti-H-2Kb T cells and compared with the parent EL4 in vitro line. The change in the cell-surface oligosaccharide did not affect the susceptibility to lysis by the cytotoxic T lymphocytes even though there were 2.5-fold more H-2Kb antigens on the WD1 variant cell (1.5 X 10(5) sites/cell) than on the parent EL4 in vitro cell (5.9 X 10(4) sites/cell). It was possible to isolate highly purified preparations of H-2Kb from either the EL4 or the WD1 line using a monoclonal antibody affinity column. Interestingly, the variant WD1 cell would no longer grow in the peritoneal cavity of the syngeneic C57BL/6 mouse.     

Synergistic interactions between lymph node and thymus cells in response to antigenic differences limited to selected regions of the H-2 complex.

Thymus and lymph nodes from the A.TL recombinant line were utilized as sources of responding cells in MLR (mixed lymphocyte response) assays to MHC-determined (major histocompatibility complex) antigenic differences. Cells from both sources were stimulated to proliferate by antigenic determinants controlled by the H-2K region alone, H-2D region and the H-2I-H-2S regions. Nylon-fiber-adherent splenic cells from each of the stimulating cell strains stimulated T-cell-dependent responses. Synergistic interactions between A.TL thymus and lymph node cells were initiated by antigenic products limited to single H-2 regions. Antigenic differences determined within the H-2I region were not required for synergistic responses to H-2K-controlled products or for the generation of cytotoxic killer cells to H-2D-associated antigens. The H-2I-region-associated products also were very effective in stimulating T-cell synergy. These data demonstrate that the two responsive T-cell subpopulations can both be stimulated by alloantigens coded within a single known H-2 region.     

YAC-1 MHC class I variants reveal an association between decreased NK sensitivity and increased H-2 expression after interferon treatment or in vivo passage

Two H-2 negative variants of the YAC-1 lymphoma were selected by mutagenization and sequential in vitro selections and compared with wild-type cells for changes in NK sensitivity and H-2 expression after interferon treatment or in vivo passage. The H-2 negative variants and the low H-2 expressor YAC-1 wild-type cells had similar NK sensitivity. However, IFN-beta or recombinant IFN-gamma pretreatments increased the H-2 expression of YAC-1 and protected them from NK lysis, whereas the H-2 variants, which remained H-2 negative, were not protected and often more sensitive to NK lysis. The H-2 variants were similarly susceptible as wild-type cells to three other cellular effects of interferon: protection from virus infection, modulation of Con A capping, and inhibition of cell proliferation. Thus, the only interferon-mediated effect that distinguished the H-2 negative variants from wild-type cells was the inability of the former to increase their H-2 expression and decrease their NK sensitivity. The wild-type YAC-1 line showed increased H-2 expression and decreased NK sensitivity after in vivo passage. In contrast, in vivo passaged H-2 variants showed no reexpression of H-2, and remained NK sensitive. The altered responses to interferon and in vivo passage were specific for loss or down-regulation of H-2, because Thy-1 loss (H-2 positive) YAC-1 variants behaved as the wild-type cells in all respects. This study supports the hypothesis that NK cells may function in vivo to eliminate host cells that fail to express H-2 after interferon stimulation during an immune response; such cells are a potential threat because they may escape recognition by T lymphocytes despite the expression of viral or tumor-associated antigens.     

Regulation of idiotope expression. III. H-2 influences the magnitude and the idiotypy of a T-independent antibody response in mice of certain genetic backgrounds

Antibody response to the phosphocholine (PC) epitope on Streptococcus pneumoniae R36a (Pn), a T-independent Ag type 2, was studied in H-2 congenic mouse strains. The PC-specific antibody plaque-forming cells (PFC) were enumerated in the spleen at various intervals after the primary Pn injection, and the proportion of PFC that produced antibody expressing the AB1-2 idiotope (Id) was determined by using the corresponding monoclonal anti-Id. AB1-2 is a cross-reactive Id, detectable on germline-encoded PC antibody of the T15 family, and on most, but not all, somatic variants of that antibody. The specific PFC responses in BALB/c (H-2d) and BALB.B (H-2b) strains were of comparable magnitude and most, if not all, PFC were ABl-1 Id-positive (AB1-2+). This was not the case in the responses of the B10D2 (H-2d) vs C57BL/10 (H-2b) strains and the D1.C (H-2d) vs D1.LP (H-2b) strains (on DBA/1 background). In each of these pairs, the H-2d mice were high responders, and the response was dominated by AB1-2 Id (greater than or equal to 80% AB1-2+ PFC at the peak, on day 5). The H-2b mice were low responders, and only a minor proportion of PFC (less than or equal to 30%) were AB1-2+; an increase of AB1-2+ was seen later in the response (d.10). The results of PFC assays were confirmed by measuring the PC-binding antibody and AB1-2 Id in the sera of D1.C and D1.LP mice immunized repeatedly with Pn. Moreover, D1.LP mice that had very low levels of AB1-2 Id had higher serum levels of antibody expressing two other T15 Id, B36-82, and B24-44. The B36-82 and B24-44 Id have been previously found on somatic variants of PC antibody expressed independently of the Ab1-2 Id. The concentrations of these two Id in D1.LP mice after repeated immunization approached those in D1.C. These results indicate that 1) the H-2 allelism may have a significant effect on TID antibody response in mice of a certain genetic background, but not in the BALB/c; and 2) the idiotypic repertoire of the response may be influenced by H-2 at the level of clonal variants of PC-reactive cells.     

T cell-mediated cytotoxicity against trinitrophenyl-modified cells: effect of glutaraldehyde treatment on the immunogenicity and antigenicity of trinitrophenyl-modified cells.

Cells treated with low concentrations of glutaraldehyde for a 10-sec interval were unable to incorporate 3H-leucine into TCA precipitable protein, respond to H-2 allogeneic cells in mixed lymphocyte reactions (MLR) and cell-mediated lympholysis (CML) assays, or display capping of cell surface immunoglobulin (Ig) with a fluoresceinated anti-Ig reagent. Such cells could stimulate and specifically block H-2 allogeneic CML activity but could not stimulate an H-2 allogeneic MLR response. Trinitrobenzenesulfonic acid (TNBS) treated spleen cells were used to sensitize syngeneic splenocytes into displaying a cytotoxic effect against trinitrophenyl (TNP)-modified target cells. Treatment of the stimulator cells with glutaraldehyde immediately after modification with TNBS did not impair their immunogenic activity. Similar treatment of TNP-modified concanavalin A-stimulated lymphoblasts that were used as inhibitors in a CML cold target competition assay allowed such cells to retain their antigenicity. Cells treated with glutaraldehyde before TNP-modification, however, were not antigenic in the cold target competition assay. These data are compatible with TNBS acting on plasma membrane molecules directly to cause cells to be antigenic and immunogenic in the CML assay rather than affecting internal cellular components.     

Intra-H-2 recombination in the mouse. III. I-Region characterization of haplotypes H-2at2, H-2at3 and H-2a2

Serological characterization of three K-S interval recombinant strains, TBR2 (H-2at2), TBR3 (H-2at3) and AIR 1 (H-2a2) was performed using anti-H-2, Ia, Ss and Slp antisera. The data presented here reveal that the crossover events in both TBR2 and TBR3 occurred between the I-A and I-E subregions. In both cases, the H-2K and I-A subregions were derived fron the H-2t1 of chromosome, while the I-E, S and H-2D regions were derived from the H-2b chromosome (KsAkEbSbDb). The H-2a2 chromosome resulted from a crossover event between the H-2a1 and H-2i9 chromosomes. Ia and Ss typing of AIR 1 suggested that the K to I-E regions originated from H-2a1 and the S and D regions originated from H-2i9 (KkAkEkSbDd).