High and low density PHA- (but not ConA-) activated T cells stimulate the autologous mixed lymphocyte reaction.

In this study, PHA- and ConA-activated cells (PAC and CAC) were used as stimulators in mixed lymphocyte reactions (MLR) using autologous (auto) and allogeneic (allo) peripheral mononuclear cells as responders. PAC, but not CAC, were stimulatory in allo- and auto-MLR, and this stimulation was not due to residual PHA. In PAC which have been activated for 96 h, auto-MLR was due to determinants present on low density T-cell blasts, while with PAC which had been stimulated for more than 192 h, the determinants seemed to be associated with high density T cells. Anti-T3 monoclonal antibodies and certain anti-DR suppressed auto- and allo-MLR mediated by PAC when present throughout the entire MLR assays. CAC suppressed PAC-mediated auto-MLR in a dose-dependent fashion. This inhibition was not DR-restricted and was reversed by the addition of exogenous IL-2. Our results indicate that: depending upon the length of activation, both low density and high density PHA-activated T cells exhibited strong stimulatory capacity in auto-MLR; ConA-activated T cells failed to stimulate auto- or allo-MLR and suppressed MLR mediated by PAC; this suppression was due to suppressor cells, not to suppressor factors, and was readily reversed by exogenous IL-2; pretreatment of CAC with anti-TAC did not reverse the inhibition.