The Specificity of Secondary MLC Assayed by Titration of Primed Lymphocyte Populations

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Abstract
The specificity of responses in secondary MLC mixed lymphocyte culture was studied by titration (100 .times. 103 to 5.times. 103) of responder primed lymphocytes (PL). Significant proliferative responses of responder PL to the specific stimulator occurred using lower responding cell numbers. When tested against allogeneic stimulating donors, 3 patterns of responses were observed: no significant responses, responses only at higher (100 .times. 103) responding cell densities and responses at lower responding cell densities similar to those with the priming donor. In instances where PL responded significantly to allogeneic stimulating cells at lower cell densities, the responses were considered positive and the stimulating cells positive for the PL determinant. PL often responded significantly to allogeneic stimulators negative for the specific HLA-D and/or HLA-DR specificities. In experiments where allogeneic stimulating donors shared HLA-D or DR specificity with priming donor, a significant response was always observed. The PL determinant was present on stimulating allogeneic cells negative for specific HLA-D and DR specifities. An analysis of the specificity of primed populations could be profoundly affected by the responder cell density at which the assay is performed. Other MHC [major histocompatibility complex] determinants, including non-HLA loci, may be important in secondary MLC.