A molecular hybrid of the H-2Dd and H-2Ld genes expressed in the dm1 mutant.

Sequential immunoprecipitates show that H-2dm1 mutant cells express a hybrid "H-2D/L" antigen exhibiting determinants normally associated with two different gene products of the parental d haplotype-i.e., the H-2Dd and H-2Ld antigens. The hybrid H-2D/Ldm1 antigen appears to consist of a portion of the NH2-terminal extracellular half of the H-2Dd antigen "fused" to a portion of the COOH-terminal extracellular half of the H-2Ld antigen. This structure is inferred from the reactivity of dm1 antigens with cytotoxic T lymphocytes specific for H-2Ld determinants and with monoclonal antibodies specific for determinants in the structural domains of H-2Ld or H-2Dd. The H-2D/Ldm1 molecule apparently retains all of the third external domain (C2 or alpha 3) and part of the second external domain (C1 or alpha 2) of H-2Ld, but its first external domain (N or alpha 1) derives from H-2Dd. From these findings and from previous peptide mapping studies, we propose that the H-2D/Ldm1 antigen is the product of a hybrid gene that has resulted from an unequal crossover between the parental H-2Dd and H-2Ld genes, leaving the N exon and part of the C1 exon of the H-2Dd gene joined to the H-2Ld gene beginning somewhere within its C1 exon.