Evidence for two suppressor factors secreted by a single cell suggests a solution to the J-locus paradox.

The hybridoma produced by the fusion of lactate dehydrogenase-B (LDH-B)-primed B10.A(2R) mouse suppressor T (Ts) cells with the BW5147 thymoma secretes two kinds of T suppressor factors (TsF), TsF-A and TsF-E. The TsF-A suppresses A beta-restricted and the TsF-E, E beta-restricted helper T (Th) cells. Each of the two factors consists of two polypeptide chains, an antigen-binding chain (ABC) and a major histocompatibility complex (MHC) chain. The ABC binds LDH-B, which is then recognized by one of the two receptors of the Th cell and an antigen bridge is formed between the factor and the Th cell. This chain is presumably identical in both factors. The MHC chain of the TsF-A carries antigenic determinants recognized by three sets of monoclonal antibodies: antibodies specific for the A beta chain, antibodies specific for class II determinants expressed in T cells and controlled by the A beta-E beta chromosomal segment, and antibodies crossreacting with J determinants. The MHC chain of the TsF-E carries determinants recognized by E beta-specific and by J-specific antibodies. Only some of these serologically detectable determinants reside in the region of the TsF molecule recognized by Th cells. These findings suggest that the J determinants are carried by the modified E beta and also by the modified A beta chains.