Tissue distribution and quantitation of la‐like antigens in the rat

The tissue distribution of a major histocompatibility complex (MHC)‐linked alloantigen in the rat was studied using antiserum raised by alloimmunization in a MHC‐congenic pair of rat strains. HO (Ag‐B5) rats were immunized with HO.B2 (Ag‐B2) skin grafts, and the tissue distribution of antigens recognized by the resultant antiserum was determined by cellular radioimmunoassay using different HO.B2 tissues as targets or immunoabsorbent. Linkage to the rat MHC was formally established by backcross analysis. The results obtained indicated that the tissue distribution of antigens recognized by this antiserum was closely similar to that reported for the distribution of Ia antigens in the mouse. Thus, T cells and thymocytes bound, respectively, 2% and 10% as much antibody as peripheral B cells while no binding to erythrocytes was detectable. Quantitative immunoabsorption studies revealed that thymocytes and kidney tissue had, respectively, 20% and 2% of the absorbing capacity of spleen cells. However, both tissues were capable of completely absorbing out the anti‐B cell activity showing that they both possessed all the Ia‐like specificities detectable by this antiserum on peripheral B cells. A variety of procedures aimed at reducing the passenger leukocyte content of kidneys had only a slight effect on the absorbing capacity. The cellular radioimmunoassay results indicated that on B cells, Ia‐like molecules are a major membrane component, present in amounts comparable with surface immunoglobulin molecules. Studies at the single cell level, using a fluorescence‐activated cell sorter, indicated that the expression of Ia‐like alloantigens by rat thymocytes was very heterogeneous and that large thymocytes were the most heavily labeled. No Ia‐like alloantigens were detected on alloantigen‐activated T cells. The findings are compared with published data on the mouse.