On the heterologous interaction between beta 2-microglobulin and the heavy chain of rat major histocompatibility complex class 1 antigens.

The heterologous interaction between beta 2-microglobulin (beta 2m) and rat major histocompatibility complex (MHC) (RT1) antigens was measured in a two-step binding assay consisting of binding of radiolabelled beta 2m to RT1 antigens and immunoprecipitation of beta 2m-RT1 antigen complexes with RT1 antisera. The effects of varying the concentrations of the three reactants involved were studied. The molecular events taking place in the two steps were analysed by gel chromatography. The beta 2m-RT1 antigen complex had the apparent size of albumin and reacted completely with specific alloantisera. RT1 antigens prepared from Wistar/Furth (RT1u) and Brown Norway (RT1n), respectively, both effectively bound heterologous beta 2m. The times for association and dissociation, respectively, at 37 degrees C, were of the same order, but dissociation was slightly slower. Association was markedly temperature-dependent and was considerably slower at low temperatures. All these processes were slower for RT1n than for RT1u antigens. The association constant for the interaction between RT1u antigens and 125I-human beta 2m was estimated by Scatchard analysis to be about 10(9) M-1. Contribution to the heterologous interaction by products from various rat MHC subloci (A, B, and C) was investigated by the introduction of sublocus-specific antisera in step 2. The reaction apparently involved neither class 2 antigens (sublocus B) nor the presumed rat Qa homologue (sublocus C). Classical class 1 antigens (sublocus A) clearly contributed to the binding. However, a monoclonal antibody against products from rat MHC class 1 genes only precipitated less than half of the RT1 antigen-complexed beta 2m. Thus, at least two RT1u class 1 alloantigen molecules seem to participate in the reaction. This, in turn, indicates that the rat genome may contain multiple class 1 genes, as is the case for most other mammals investigated.