Activation of abundant precursors associated with the response of Mlsa H-2k spleens to Mlsc antigen.

We have determined the frequency of splenic interleukin 2-producing T-cell precursors that respond to H-2-identical, Mls-disparate stimulating cells. Gene products of the Mlsa and Mlsd alleles are highly stimulatory in primary mixed lymphocyte reactions and activate substantial numbers of T-cell precursors in normal spleens. Although the product of the Mlsc allele has been described as weakly stimulatory in primary mixed lymphocyte reactions, we detected abundant T-cell precursors responsive to Mlsc under our limiting dilution conditions; H-2k Mlsa T-cell precursors responding to Mlsc were as frequent (2.605 per 1000) as H-2k Mlsb or Mlsc T-cells responsive to Mlsd (1.975 per 1000). In contrast, using the frequency of self-responding cells as a threshold, we were unable to detect precursors for Mlsa, Mlsb, Mlsc, or Mlsd in H-2b mice, for Mlsb in H-2k mice, or for Mlsc in H-2k Mlsb or Mlsd mice. Thus, we conclude that in select strain combinations and under our experimental conditions Mlsc stimulators can activate T-cell precursors that are as abundant as precursors reactive to strong stimulators Mlsa and Mlsd.