In the generation of allogeneic, hapten-modified and virus-specific cytotoxic T cell (CTL) responses there is usually a requirement for T-T interaction between the T helper cell (TH) and the precursor CTL (CTL). We have investigated the role of a TH signal in the induction of a xenogeneic mouse antihuman CTL response by using membranes and liposomes bearing the xenogeneic antigen to stimulate primed responders. The TH signal can be achieved by either an Ia-restricted, L3T4+ DR-specific T cell or by the addition of nonspecific T helper factors(s). This signal is delivered to an Lyt-2+, L3T4-DR-specific CTL to generate active xenogeneic (xeno-) CTL. The roles of the T cell accessory molecules L3T4, Lyt-2, and LFA-1 in the generation of and target cell lysis by xeno-CTL are investigated.

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