Generation of alloreactive cytotoxic T lymphocytes: evidence for a differentiation factor distinct from IL 2

In this report we provide evidence that the generation of cytotoxic T lymphocytes (CTL) involves a differentiation signal in addition to a proliferative signal, and that the differentiation signal is mediated by a factor distinct from interleukin 2 (IL 2). Generation of CTL from nylon wool-nonadherent splenocytes stimulated by heat-treated thymocytes did not correlate with the addition of IL 2-containing supernatants to this factor-dependent bioassay. IL 2 from two different sources and prepared by different methods (purified IL 2 from the LBRM33 cell line and partially purified IL 2 from the EL4 cell line) was not sufficient to generate a cytotoxic response in the bioassay. The inability of IL 2 to generate a cytotoxic response was not due to the use of heat-treated stimulator cells because similar results were obtained with UV-treated cells. As we previously reported, supernatant fluids containing a differentiation/activation factor did generate CTL from nylon wool-nonadherent splenocytes. Supernatant fluid from co-cultures of activated T cells and M phi as well as from cultures of activated M phi contained this CTL helper factor; however, only the former supernatant contained IL 2 activity. These results suggest a factor distinct from IL 2 is providing a differentiation/activation signal that is necessary for CTL development. Although IL 2 was not effective at generating CTL from nylon wool-nonadherent splenocytes, it was capable in some, but not all, experiments of generating a cytotoxic response from unseparated splenocytes. The ability of IL 2 to induce a cytotoxic response from unseparated splenocytes appeared to correlate with the presence of a weak background response seen when unseparated cells were used as responder cells.