No abstract

SUMMARY Two antisera to Ia antigens, products of the H‐2 complex I‐Cd and I‐JkEk subregions, respectively, have been obtained by immunization of the F1 hybrids of recombinant strains of mice. These antisera are shown to display a 50% cytotoxic effect in vitro, in the presence of complement, upon lymphocyte populations immune to the H‐2‐complex antigens and enriched for specific suppressor T cells (SSTC) by fractionation on a monolayer of target cells. The specificity of anti‐Ia cytotoxins is shown by cross‐antibody absorption with T and B cells of mice originating from the recombinant H‐2 haplotypes and bearing either particular I‐Cd, I‐Jk and I‐Kk antigens, or their combinations. Anti‐I‐Cd cytotoxins were found to react with both B and T cells, but at a different rate, and the anti‐I‐JkEk serum contains two antibody types directed to I‐Ek and I‐Jk products, respectively, the latter being able to react preferently with T cells. Although both antisera do inactivate the in vitro SSTC function in the presence of complement to a similar degree, the inactivating action of the anti‐I‐Cd serum, but not that of the anti‐I‐JkEk serum, occurs without complement. SSTC are shown to bear both Ia‐antigens, I‐J and I‐C, as shown by both inactivation of the anti‐suppressor effect of the antisera absorbed with spleen cells of different H‐2 origin, and variation of the H‐2 origin of SSTC pretreated with the intact antisera. It is suggested that these two markers, located on the same SSTC, function differently in SSTC immune to the H‐2 antigens, and I‐C antigen expression on the SSTC surface is presumed to be required for their interaction with the inhibited responder T cells proliferating in MLC.