Insulin-Dependent Diabetes Mellitus Is Associated With Genes That Map to the Right of the Class I RT1.A Locus of the Major Histocompatibility Complex of the Rat
Previous studies have demonstrated that the presence of at least one u-haplotype of the rat major histocompatibility complex (MHC), RT1, is a necessary but not sufficient condition for the development of overt diabetes mellitus. The present studies were undertaken to determine which portion of the RT1 gene complex is necessary for the occurrence of diabetes. We crossed hooded diabetic rats (RT1 .AaBuDu) with PVGr8 rats (RT1 .AaBuDu). F1 animals were mated to give 82 F2 animals and backcrossed with the hooded diabetics to produce 41 backcross animals. Diabetes occurred in animals with all three possible RT1.A genotypes. The diabetes was similar to that seen in BB rats and in hybrid strains developed from them. An immunoregulatory defect was marked by decreased percentage of peripheral blood lymphocytes staining with w3/25 monoclonal antibody, by an increased percentage of peripheral blood lymphocytes binding a mouse ascites control protein, and by decreased responsiveness of peripheral blood lymphocytes to stimulation by conconavalin A. We conclude that the u-allele of the class I A-locus gene product is not necessary for susceptibility to the development of diabetes in the rat. Therefore, either genes coding for the class II products of the uhaplotype or genes in linkage disequilibrium with these genes and mapping to the right of the A locus provide the permissive condition. Furthermore, the data suggest, but do not prove, that the u-haplotype derived from a strain remote from the BB rat can confer this susceptibility to the development of diabetes. We can also demonstrate that neither the susceptibility to diabetes nor the presence of peripheral blood lymphocyte characteristics, which mark the immunoregulatory defect, segregates with albinism.