Identical RT1 class II molecules are expressed by rat RT1^m and RT1(c) haplotypes

The RT1^m haplotype of MNR rats has been suggested to be a recombinant RT1 haplotype inheriting RT1.A (class I) alleles from RT1^a (DA) and RT1.B (class II) alleles from RT1(c) (AUG). Additional serologic and biochemical assays, however, have suggested that RT1^m and RT1(c) share a single identical RT1.B molecule, although differing in the expression of the second RT1.B molecule. To resolve this contradiction, RT1.B class II molecules, comparable to I-A and I-E molecules in mice, expressed by the RT1(c) and RT1^m haplotypes were immunoprecipitated by cross-reactive mouse anti-Ia antibodies and were compared by two-dimensional gel electrophoresis and by high pressure liquid chromatographic separation of tryptic peptides. Respective subunits expressed by the two haplotypes co-migrate on two-dimensional gels and have identical tryptic peptide maps. The results at the protein level were confirmed at the DNA level by Southern blot analysis of MNR and AUG genomic DNA. Identical restriction fragments associated with the RT1^m and RT1(c) haplotypes hybridized with each of the DC(1β), DR(α), and DRβ cDNA probes. The results at both the protein and DNA levels suggest that the RT1^m and RT1(c) haplotypes share identical expressed alleles at the RT1.Ba, RT1,Bb, RT1.Bc, and RT1.Bd loci.