Cytokine-dependent thymocyte responses. II. Generation of cytotoxic T lymphocytes from immature thymocytes.

The ability of murine thymocyte subpopulations to give rise to alloreactive cytotoxic T lymphocytes (CTL) in vitro in the presence of purified preparations of IL 2 was examined. Fractionation of thymocytes by selective agglutination with peanut lectin (Arachius hypogeae) resulted in separation of two cell populations, each with distinctive cytotoxic responses to allogeneic cells. The nonagglutinated fraction (PNA− cells) was capable of generating cytolytic activity after stimulation with mitomycin C-inactivated allogeneic splenocytes in the absence of any exogenous factors. However, the majority of thymocytes agglutinated by the lectin (PNA+ cells) were unable to generate CTL activity to the same stimulatory antigen unless preparations containing IL 2 activity were additionally provided. The factor responsible for the generation of CTL by PNA+ responders was further characterized and was found to possess both a m.w. (approximately 30,000) and an isoelectric mobility (pl 4.5 and 5.0) similar to that of conventional IL 2. However, identical experiments aimed at generating CTL from PNA+ cells stimulated with heat-treated, allogeneic splenocytes revealed that highly purified preparations of IL 2 were unable to give rise to appreciable CTL activity. Furthermore, a separate molecule present in PHA-activated LBRM-33 5A4 cell line conditioned medium was necessary in addition to IL 2 for the generation of CTL from PNA+ thymocytes stimulated with heat-treated alloantigen. However, IL 2 by itself was capable of inducing generation of alloreactive CTL from both PNA− and splenic T cell responder populations in mixed lymphocyte cultures with heat-treated allogeneic spleen cells. These results suggest the inability of PNA+ thymocytes to generate CTL may not be due solely to their failure to produce IL 2, but that these cells may require additional soluble signals to promote the efficient differentiation of mature effector cell function.