Functionally distinct ERAP1 and ERAP2 are a hallmark of HLA-A29-(Birdshot) Uveitis.

Birdshot Uveitis (Birdshot) is a rare eye condition that affects HLA-A29-positive individuals and could be considered a prototypic member of the recently proposed 'MHC-I (major histocompatibility complex class I)-opathy' family. Genetic studies have pinpointed the endoplasmic reticulum aminopeptidase (ERAP1) and (ERAP2) genes as shared associations across MHC-I-opathies, which suggests ERAP dysfunction may be a root cause for MHC-I-opathies. We mapped the ERAP1 and ERAP2 haplotypes in 84 Dutch cases and 890 controls. We identified association at variant rs10044354, which mediated a marked increase in ERAP2 expression. We also identified and cloned an independently associated ERAP1 haplotype (tagged by rs2287987) present in more than half of the cases; this ERAP1 haplotype is also the primary risk and protective haplotype for other MHC-I-opathies. We show that the risk ERAP1 haplotype conferred significantly altered expression of ERAP1 isoforms in transcriptomic data (n = 360), resulting in lowered protein expression and distinct enzymatic activity. Both the association for rs10044354 (meta-analysis: odds ratio (OR) [95% CI]=2.07[1.58-2.71], P = 1.24 × 10(-7)) and rs2287987 (OR[95% CI]: =2.01[1.51-2.67], P = 1.41 × 10(-6)) replicated and showed consistent direction of effect in an independent Spanish cohort of 46 cases and 2103 controls. In both cohorts, the combined rs2287987-rs10044354 haplotype associated with Birdshot more strongly than either variant alone [meta-analysis: P=3.9 × 10(-9)]. Finally, we observed that ERAP2 protein expression is dependent on the ERAP1 background across three European populations (n = 3353). In conclusion, a functionally distinct combination of ERAP1 and ERAP2 are a hallmark of Birdshot and provide rationale for strategies designed to correct ERAP function for treatment of Birdshot and MHC-I-opathies more broadly.

Properties of STAT1 and IRF1 enhancers and the influence of SNPs.

BACKGROUND: STAT1 and IRF1 collaborate to induce interferon-γ (IFNγ) stimulated genes (ISGs), but the extent to which they act alone or together is unclear. The effect of single nucleotide polymorphisms (SNPs) on in vivo binding is also largely unknown. RESULTS: We show that IRF1 binds at proximal or distant ISG sites twice as often as STAT1, increasing to sixfold at the MHC class I locus. STAT1 almost always bound with IRF1, while most IRF1 binding events were isolated. Dual binding sites at remote or proximal enhancers distinguished ISGs that were responsive to IFNγ versus cell-specific resistant ISGs, which showed fewer and mainly single binding events. Surprisingly, inducibility in one cell type predicted ISG-responsiveness in other cells. Several dbSNPs overlapped with STAT1 and IRF1 binding motifs, and we developed methodology to rapidly assess their effects. We show that in silico prediction of SNP effects accurately reflects altered binding both in vitro and in vivo. CONCLUSIONS: These data reveal broad cooperation between STAT1 and IRF1, explain cell type specific differences in ISG-responsiveness, and identify genetic variants that may participate in the pathogenesis of immune disorders.

Characterization and locus-specific typing of MHC class I genes in the red-billed gull (Larus scopulinus) provides evidence for major, minor, and nonclassical loci.

A major challenge facing studies of major histocompatibility complex (MHC) evolution in birds is the difficulty in genotyping alleles at individual loci, and the consequent inability to investigate sequence variation and selection pressures for each gene. In this study, four MHC class I loci were isolated from the red-billed gull (Larus scopulinus), representing both the first characterized MHCI genes within Charadriiformes (shorebirds, gulls, and allies) and the first full-length MHCI sequences described outside Galloanserae (gamebirds + waterfowl). Complete multilocus genotypes were obtained for 470 individuals using a combination of reference-strand conformation analysis and direct sequencing of gene-specific amplification products, and variation of peptide-binding region (PBR) exons was surveyed for all loci. Each gene is transcribed and has conserved sequence features characteristic of antigen-presenting MHCI molecules. However, higher allelic variation, a more even allele frequency distribution, and evidence of positive selection acting on a larger number of PBR residues suggest that only one locus (Lasc-UAA) functions as a major classical MHCI gene. Lasc-UBA, with more limited variation and PBR motifs that encompass a subset of Lasc-UAA diversity, was assigned a putative minor classical function, whereas the divergent and largely invariant binding-groove motifs of Lasc-UCA and -UDA are suggestive of nonclassical loci with specialized ligand-binding roles.

CTL clones isolated from an HLA-Cw-mismatched bone marrow transplant recipient with acute graft-versus-host disease.

HLA-Cw disparity in a donor increases the risk of acute graft-vs-host disease (GVHD) after bone marrow transplantation. Acute GVHD is mediated by donor CTLs. However, mismatched HLA-Cw-specific CTLs generated in posttransplant recipients who developed acute GVHD have not been characterized in detail. In this study, CTL clones isolated from a recipient at the onset of acute GVHD who was transplanted from an HLA-A, -B, and -DRB1-matched, HLA-Cw-mismatched (recipient, Cw*0303/Cw*0702; donor, Cw*0801/Cw*0702), unrelated donor were characterized. The seven isolated CTLs, including CD4(+), CD8(+), and CD4(+)CD8(+) T lymphocytes, lysed recipient cells, HLA-Cw*0303-transfected 721.211 cells, and HLA-Cw*0303-transfected donor cells, but not untransfected 721.211 cells or donor cells. Thus, all CTLs recognized the mismatched Cw*0303 molecule as an alloantigen. The sequences of Cw*0303 and Cw*0801 differ by 16 aas. Stimulation of CTLs by COS cells transfected with Cw*0303 cDNA constructs demonstrated that Cw*0303 mutants in which individual amino acids constituting peptide-binding pockets were substituted with the corresponding Cw*0801 amino acids significantly decreased IFN-gamma production by all CTLs, whereas Cw*0303 mutants bearing Cw*0801 amino acids outside the positions constituting peptide-binding pockets stimulated all CTLs to the same degree as the wild-type Cw*0303 construct. These data suggest that all CTLs recognized the Cw molecule in a peptide-dependent manner. ELISPOT revealed that Cw*0303-reactive T cells accounted for one-half of the total of alloreactive T cells in the blood during GVHD. Taken together, non-self Cw-specific CTL clones with a variety of phenotypes and peptide specificities can be generated in posttransplant recipients with acute GVHD.

Cellularly defined minor histocompatibility antigens are differentially expressed on human hematopoietic progenitor cells.

Previously, five CTL lines directed against minor histocompatibility (mH) antigens designated HA-1-5 have been established from peripheral blood of patients after allogeneic bone marrow transplantation (BMT), and have been characterized using population and family studies. All cell lines showed specific HLA class I-restricted lysis of PHA-stimulated peripheral blood target cells from donors positive for the particular mH antigens. After 4 h of incubation of the mH antigen HA-3-specific CTL line with bone marrow cells from HA-3+ donors, complete class I-restricted inhibition of colony growth of the hematopoietic progenitor cells was observed even at low E/T ratios, indicating that the HA-3 antigen is strongly expressed on hematopoietic stem cells. Therefore, this antigen may be a target structure in the immune-mediated rejection of the hematopoietic graft in case of incompatibility for this determinant between donor and recipient in allogeneic BMT. In contrast, incubation of bone marrow cells with the antigen-specific anti-HA-1, -2, -4, and -5 CTL lines did not result in growth inhibition of the hematopoietic progenitor cells tested. After a prolonged incubation time and using a very high E/T ratio, progenitor cells from HA-2+ or HA-5+ donors were killed to some extent by the anti-mH-specific CTL lines, although the growth inhibition observed was minor and variable. Our results show that mH antigens are differentially expressed on human hematopoietic progenitor cells. Therefore, only some of these antigens may be targets in immune-mediated rejection of the bone marrow graft.

Role of the H-2 complex in the induction of T cell tolerance to self minor histocompatibility antigens.

The present study has utilized cytotoxic T lymphocyte (CTL) responses specific for minor histocompatibility (minor H) antigens as an experimental approach to determining whether recognition of self MHC determinants is involved in the induction of T cell tolerance to self antigens. It was observed that C3H.SW splenic T cells from C3H.SW leads to B10 X B10.BR radiation bone marrow chimeras contained CTL precursors (pCTL) reactive against self C3H minor H antigens + H-2k but were tolerant to self C3H minor H antigens + H-2b. Precursor CTL with the reciprocal reactivity pattern were observed for C3H leads to B10 X B10.BR chimeras. In addition, it was observed that C3H.SW thymocytes from C3H.SW leads to B10 X B10.BR chimeras could generate minor H-specific CTL responses and were reactive against self C3H minor H antigens + H-2k, but were tolerant to self C3H minor H antigens + H-2b. Thus, the present study demonstrates that for peripheral and intrathymic T cell populations at least a component of T cell tolerance to self antigens is restricted by products of the MHC.

Identification of a polymorphic gene, BCL2A1, encoding two novel hematopoietic lineage-specific minor histocompatibility antigens.

We report the identification of two novel minor histocompatibility antigens (mHAgs), encoded by two separate single nucleotide polymorphisms on a single gene, BCL2A1, and restricted by human histocompatibility leukocyte antigen (HLA)-A*2402 (the most common HLA-A allele in Japanese) and B*4403, respectively. Two cytotoxic T lymphocyte (CTL) clones specific for these mHAgs were first isolated from two distinct recipients after hematopoietic cell transplantation. Both clones lyse only normal and malignant cells within the hematopoietic lineage. To localize the gene encoding the mHAgs, two-point linkage analysis was performed on the CTL lytic patterns of restricting HLA-transfected B lymphoblastoid cell lines obtained from Centre d'Etude du Polymorphisme Humain. Both CTL clones showed a completely identical lytic pattern for 4 pedigrees and the gene was localized within a 3.6-cM interval of 15q24.3-25.1 region that encodes at least 46 genes. Of those, only BCL2A1 has been reported to be expressed in hematopoietic cells and possess three nonsynonymous nucleotide changes. Minigene transfection and epitope reconstitution assays with synthetic peptides identified both HLA-A*2402- and B*4403-restricted mHAg epitopes to be encoded by distinct polymorphisms within BCL2A1.

Second class minors: molecular identification of the autosomal H46 histocompatibility locus as a peptide presented by major histocompatibility complex class II molecules.

CD4 T cells regulate immune responses that cause chronic graft rejection and graft versus host disease but their target antigens remain virtually unknown. We developed a new method to identify CD4 T cell-stimulating antigens. LacZ-inducible CD4 T cells were used as a probe to detect their cognate peptide/MHC II ligand generated in dendritic cells fed with Escherichia coli expressing a library of target cell genes. The murine H46 locus on chromosome 7 was thus found to encode the interleukin 4-induced IL4i1 gene. The IL4i1 precursor contains the HAFVEAIPELQGHV peptide which is presented by A(b) major histocompatibility complex class II molecule via an endogenous pathway in professional antigen presenting cells. Both allelic peptides bind A(b) and a single alanine to methionine substitution at p2 defines nonself. These results reveal novel features of H loci that regulate CD4 T cell responses as well as provide a general strategy for identifying elusive antigens that elicit CD4 T cell responses to tumors or self-tissues in autoimmunity.

T cell receptors for responses to Mls determinants and allo-H-2 determinants appear to be encoded on different chromosomes.

Previous studies have shown that T cell clones specific for strong Mlsa,d determinants concomitantly display apparently random reactivity to allo-H-2 determinants. One explanation for this finding is that T cell recognition of Mlsa,d and allo-H-2 determinants is controlled by separate sets of receptors. If these receptors were chromosomally unlinked, karyotypically unstable T cell hybrids with dual reactivity for Mlsa,d and particular allo-H-2 determinants would be expected, occasionally, to lose reactivity for one set of determinants, but not the other. The results presented here provide direct support for this prediction.

Selective decreases in T cell receptor V beta expression. Decreased expression of specific V beta families is associated with expression of multiple MHC and non-MHC gene products.

Previous reports of TCR V beta usage, studying either expression of a single V beta in a wide panel of strains (6, 7, 10, 12, 13), or expression of multiple V beta s in a very limited strain distribution (14, 15), have identified instances of clonal deletion of potentially autoreactive T cells specific for either self E alpha E beta or minor lymphocyte stimulatory (Mls) antigens. The present study has investigated the range of self antigens that can influence V beta usage by evaluating expression of 16 V beta families in 30 strains of mice. It was found that significant decreases in expression occur in at least 8 of the 16 V beta families and that dominant influences on the T cell V beta repertoire are exerted by expression of Mlsa, Mlsc, and MHC gene products. Decreased expressions of V beta 5, -11, -12, and -16 were influenced by MHC gene products. The patterns of decreased expression seen in intra-MHC recombinant strains and strains of different non-MHC background were distinct for V beta 11, -12, and -16, suggesting that different ligands are involved in the deletion of T cells expressing each of these V beta genes. Mice expressing Mlsa show decreased expression of V beta 9 as well as V beta 6. Mlsc mice lacked V beta 3 expression in those strains where the expressed MHC type was compatible with a strongly stimulatory Mlsc phenotype. V beta 7 was strongly influenced by both MHC and non-MHC products that are not yet identified. These results demonstrate that strain-specific decreases of mRNA expression occur in a major portion of the TCR repertoire. Self antigens including Mlsa, Mlsc, and E alpha E beta, as well as additional MHC and non-MHC products, appear to induce these decreases in expression in the process of eliminating self-reactive T cells from the mature T cell pool.

Inhibition of MHC class II-restricted T cell response by Lyt-2 alloantigen.

T cell hybridomas were established by fusing a CD8+ V beta 8.1+ CTL clone and a CD4+ V beta 8.1+ helper T lymphocyte (HTL) clone to the thymoma cell line BW5147. In contrast to the HTL x BW hybridomas, which retain the same antigen specificity as the original T cell clone, the CTL x BW hybridomas lost the class I MHC-restricted antigen response but acquired a new specificity to Mlsa antigen. Mlsa reactivity of CTL x BW hybridomas was shown to be mediated by the CTL TCR as assayed by inhibition using an anticlonotypic antibody to the CTL clone. Since hybridomas established with BW5147 lose CD8 expression, we have introduced the CD8 molecule into CTL x BW5147 hybridomas by gene transfection. The CD8+ V beta 8.1+ hybridoma was no longer capable of reacting to Mlsa antigen but exhibited the same antigen specificity as the parental CTL clone. Furthermore, the presence of the transfected CD8 molecule in the HTL x BW hybridomas was found to be inhibitory to class II MHC-restricted antigen reactivity. These results demonstrate that, besides its role in increasing the overall avidity of T cell-class I MHC/antigen interaction, the CD8 molecule inhibits T cell-class II MHC gene product/antigen interaction. This negative effect of the CD8 molecule on a class II MHC-restricted response may account for the failure of CD8+ T cells using either V beta 8.1 or V beta 6, which impart reactivity to the Mlsa antigen on CD4+ T cells, to respond to the Mlsa antigen.

Analysis of Mlsc genetics. A novel instance of genetic redundancy.

The identity of the self determinants involved in the selection of the T cell repertoire has been a matter of considerable interest. In addition to the apparent critical role of MHC gene products, accumulated experimental results indicate the importance of non-MHC gene products in T cell repertoire selection. In particular, murine Mlsa and Mlsc determinants have been shown to be highly stimulatory to allogeneic T cells and to be involved in the negative selection (elimination) of self-reactive T cells expressing selected TCR V beta segments. In this work, a unique phenomenon of genetic redundancy is described in the control of Mlsc expression: Mlsc appears to be controlled by at least two unlinked loci, and the product of either one of these loci is sufficient to evoke Mlsc-specific T cell response and to act as a ligand in the deletion of self Mlsc-reactive V beta 3+ T cells. Based on these findings, we propose a possible explanation for the fact that Mls-like genes or gene products have not been identified in other species such as man.

Negative selection in vivo reveals expression of strong Mls determinants in mice with X-linked immunodeficiency.

Evidence is presented that mice with X-linked immunodeficiency (xid) express strong Mlsa,d determinants, a putative marker of the mature subset of B cells. Although young (3-5 wk) (CBA/N X DBA/2)F1 male (xid+) mice stimulated only very weak mixed lymphocyte reactions (MLR) to Mlsa,d determinants, older mice (greater than 7 wk) regularly elicited conspicuous responses, despite being totally unresponsive to TNP-Ficoll. Expression of Mlsa,d determinants by xid+ mice was also detected by the procedure of negative selection in vivo. Thus BALB/c T cells were totally depleted of Mlsa,d reactivity after blood to lymph recirculation through 10-wk old irradiated xid+ (CBA/N X DBA/2)1 male mice. Significantly, a marked (90%) reduction in the anti-Mlsa,d response also occurred with T cell filtration through 3-wk xid+ mice, i.e., mice that elicit only minimal primary MLR; filtration through 3-wk xid- normal female mice led to near-complete (99%) negative selection. Collectively these data indicate either, (a) that xid+ mice contain appreciable numbers of cells with at least some of the properties of mature B cells, or (b) that the expression of Mlsa,d determinants is not restricted to mature B cells. In either case, B cells from xid mice cannot be viewed as a simple model for immature normal B cells.

T cell tolerance to non-H-2-encoded stimulatory alloantigens is induced intrathymically but not prethymically.

The present report has evaluated the differentiation compartment in which T cells are tolerized to non-major histocompatibility complex (MHC)-encoded minor lymphocyte-stimulating locus (MLS) alloantigens. It was observed that T cell precursors are not tolerized prethymically to MLS alloantigens but are tolerized intrathymically and postthymically to MLS alloantigens. The failure of prethymic T cells to be tolerized indicates either that T cell precursors are unable to be tolerized to MLS alloantigens or that cells in the prethymic compartment are unable to induce MLS-specific tolerance. In either case, these results demonstrate that the thymus is the initial site in which T cell tolerance to MLS alloantigen is induced. The present results also demonstrate a striking disparity in the reactivity of thymocytes to MHC and MLS alloantigens expressed in the extrathymic host through which their precursors had migrated. In the experimental mice constructed for these studies, intrathymic T cells were tolerant to the MHC alloantigens but were reactive to the MLS alloantigens expressed by the extrathymic host. This observation is consistent with the concept that T cell precursors may be tolerized to MHC alloantigens at an earlier point in their differentiation than they are tolerized to non-MHC-encoded MLS alloantigens.

Influences of transporter associated with antigen processing (TAP) on the repertoire of peptides associated with the endoplasmic reticulum-resident stress protein gp96.

The endoplasmic reticulum (ER)-resident stress protein gp96 induces protective immunity and specific cytotoxic T lymphocyte (CTL) responses against antigens expressed in those cells it has been isolated from. This ability is based on peptides associated with gp96. Because gp96 is located inside the ER, our experiments address the question whether or not the repertoire of peptides associated with gp96 is influenced by the transporter associated with antigen processing (TAP). For this purpose, gp96 was isolated from cells with and without a TAP defect and used for immunization of mice. We found that for some antigens the association of peptides with gp96 required functional TAP molecules, whereas the association of peptides from other antigens was TAP independent. In the case of a TAP-dependent association of peptides with gp96, our results prove that peptide binding by gp96 in vivo occurs inside the ER and is not an artifact induced by cell lysis during the gp96 purification. The finding that some antigens can also associate with gp96 in the absence of functional TAP molecules indicates that the repertoire of peptides bound by gp96 truly reflects the entire repertoire of peptides present inside the ER and not only those peptides transported by TAP. These results, together with the earlier finding that the gp96 peptide repertoire is independent of the major histocompatibility complex molecules expressed by the cell gp96 is isolated from, give the theoretical foundation for the ability of gp96 to induce CTL responses against all kinds of intracellular antigens.

Thymic cytotoxic T lymphocytes are primed in vivo to minor histocompatibility antigens.

Potent cytotoxic T lymphocyte (CTL) activity can be derived from cultures of thymocyte responders and minor H different spleen cell stimulators. As is the case of the spleen cell response previously reported, this cytotoxic activity requires in vivo priming. We performed several experiments designed to determine whether the in vivo priming effect is due to the in situ priming of the thymocyte CTL precursors, to contamination of thymus cell preparations with cells of neighboring lymph nodes, or to the appearance in the thymus of antigen-reactive peripheral T cells. We show by depletion of peripheral cells with antilymphocyte serum and part body irradiation that recent thymic immigrants derived from the bone marrow contribute to the primed thymic response. Thymic CTL were primed in animals in which peripheral T cell responses were completely eliminated by repeated treatment in vivo with monoclonal anti-Thy-1 reagents. Primed, antigen-activated lymph node cells were also demonstrated to contribute to the thymus-derived CTL response. Thus, the minor H-specific thymic CTL response is due both to in situ priming and the immigration of activated peripheral T cells. We discuss the possible significance for models of T cell differentiation of the presence within the thymus of antigen and antigen-reactive mature T cells.

The minor histocompatibility antigen HA-1: a diallelic gene with a single amino acid polymorphism.

The minor histocompatibility antigen (mHag) HA-1 is the only known mHag for which mismatching is correlated with the development of severe graft versus host disease (GvHD) after human leukocyte antigen-identical bone marrow transplantation. HA-1 was found to be a nonapeptide derived from an allele of the KIAA0223 gene. The HA-1-negative allelic counterpart encoded by KIAA0223 had one amino acid difference from HA-1. Family analysis with HA-1 allele-specific polymerase chain reaction showed an exact correlation between this allelic polymorphism and the HA-1 phenotype. HA-1 allele typing of donor and recipient should improve donor selection and allow the determination of bone marrow transplantation recipients with high risk for HA-1-induced GvHD development.

Minor histocompatibility antigen H-Y is expressed on human hematopoietic progenitor cells.

Polymorphic minor transplantation antigens probably play an important role in immune mediated graft rejections of bone marrow transplants. Mapping of these antigens on hematopoietic progenitor cells (HPC) is important since these antigenic determinants may serve as target structures in the rejection process, and it ultimately opens the possibility to match for these antigens. Using a cell-mediated cytotoxicity assay with H-Y-specific cytotoxic T lymphocytes as effector cells, a dose-dependent growth inhibition up to 100% of myeloid (CFU-GM), erythroid (BFU-E) and multipotential (CFU-GEMM) HPC of male donors was obtained, indicating expression of the H-Y antigen on these progenitor cells. In contrast, inhibition of relatively mature erythroid and myeloid progenitor cells was only 40-50%, indicating that the recognition of the H-Y antigen diminished during maturation of erythroid and myeloid HPC. Our results show that the H-Y antigen can be recognized on HPC as a target for cytotoxic T cell responses. This may be important in graft rejection of male donor bone marrow grafts by female recipients.

Identification of HLA class II-restricted H-Y-specific T-helper epitope evoking CD4+ T-helper cells in H-Y-mismatched transplantation.

BACKGROUND: Stem-cell grafts between HLA-identical siblings are less likely to succeed when there is a sex mismatch. This lack of success can be interpreted as enhanced activity directed against minor histocompatibility antigens encoded by the Y chromosome (H-Y). So far, in man, only cytotoxic T lymphocytes (CTLs) specific for several minor histocompatibility antigens have been reported. We aimed to identify and clarify the role of MHC class II-restricted H-Y-specific T-helper cells in these transplant settings. METHODS: H-Y-specific MHC class II-restricted CD4+ T cells were isolated from blood of a female patient who rejected an HLA-identical male stem-cell transplant. By molecular cloning of H-Y genes and functional T-helper experiments, we elucidated antigen specificity and the functional properties of these H-Y-specific T-helper cells. FINDINGS: CD4+ T-helper cells recognise the Y gene-encoded peptide VIKVNDTVQI presented by HLA-DRbeta3*0301. These T-helper cells mature dendritic cells and enhance expansion of minor histocompatibility antigen-specific MHC class I-restricted CD8+ CTLs. INTERPRETATION: Characterisation of an MHC class II-restricted H-Y epitope that evoked CD4+ T-helper responses adds a novel cellular component to the alloimmune response against Y chromosome-encoded minor histocompatibility antigens. This component completes the H-Y-directed alloimmune response and aids understanding of the poorer outcome of sex-mismatched transplants.

The original pink-eyed dilution mutation (p) arose in Asiatic mice: implications for the H4 minor histocompatibility antigen, Myod1 regulation and the origin of inbred strains.

Allelic variation of the mouse pink-eyed dilution (p) gene in common laboratory strains and wild mice was examined by Southern blot and by polymerase chain reaction. In these assays the original p mutation allele found in strains SJL/J, 129/J, B10.129(21m), P/J and FS/Ei most closely matches an Asian Mus musculus allele, confirming anecdotal accounts of the Asian origin of this mutation. In contrast, the wild-type allele found in other common laboratory strains was apparently derived from Mus domesticus. Analysis of chromosome 7 loci both proximal and distal to the p locus demonstrates that strains SJL/J, 129/J, B10.129(21M), P/J and FS/Ei contain DNA segments of varying length derived from M. musculus. Strains 129/J and B10.129(21M) contain the largest segment of M. musculus-derived DNA (about 5 cM), including the loci Myod1, p, three clustered GABAA receptor subunit loci (Gabrg3, Gabra5 and Gabrb3), and Snrpn. The difference in the species origin of genes from this region of chromosome 7 may underlie the basis of the antigenicity of the minor histocompatibility antigen H4, defined by the strain B10.129(21M), and may account for the enhanced Myod1 activity observed in SJL/J mice.