The specificity of secondary MLC assayed by titration of primed lymphocyte populations

The specificity of responses in secondary MLC was studied by titration (100 x 10(3) to 5 x 10(3)) of responder primed lymphocytes. In all instances, significant proliferative responses of responder primed lymphocytes to the specific stimulator occurred using lower responding cell numbers. When tested against allogeneic stimulating donors, three patterns of responses were observed: no significant responses, responses only at higher (100 x 10(3)) responding cell densities, and responses at lower responding cell densities, similar to those with the priming donor. In instances where primed lymphocytes responded significantly to allogeneic stimulating cells at lower cell densities, the responses were considered positive and the stimulating cells positive for the PL determinant. In several instances, primed lymphocytes responded significantly to allogeneic stimulators negative for the specific HLA-D and/or HLA-DR specificities. On the other hand, in experiments where allogeneic stimulating donors shared either HLA-D or DR specificity with the priming donor, a significant response was always observed. Thus, the PL determinant was present on stimulating allogeneic cells negative for specific HLA-D and DR specificities. The present data suggest that an analysis of the specificity of primed populations could be profoundly affected by the responder cell density at which the assay is performed. Also, the data suggest that other MHC determinants, including non-HLA loci may be important in secondary MLC.