A requirement for antigen-specific helper T cells in the generation of cytotoxic T cells from thymocyte precursors

Thymocytes cultured with irradiated, allogeneic stimulator cells yield no cytotoxic effector cells after a period in culture. If, however, a population of irradiated spleen cells syngeneic to the responder cells are added to these cultures, cytotoxicity is generated. The helper activity present in the irradiated syngeneic spleen cells was found to be mediated by a cell bearing theta antigens. Furthermore, it was found to be antigen specific; helper cells which were tolerant of the stimulator cell antigens were unable to help the thymocyte responder cells, although these tolerant cells did contain helpers specific for a third party antigen. These experiments are consistent with a requirement for associative recognition of linked determinants in the induction of killer precursors which is thus strictly analogous to the induction of B-cell precursors via collaboration with helper T cells. In more extensive studies, it was found that histoincompatible helper cells (H-2b, H-2p, H-2q) were able to help a cytotoxic T cell (H-2k) response to a third party stimulator cell antigen (H-2d); that is, the helper T cells which interact with cytotoxic T-cell precursors are not strain specific. It seems likely that the histocompatible helper cells induce killer precursors in an antigen-specific cooperation event similar or identical to normal syngeneic cooperation.