T-T cell interactions during <i>in vitro</i> cytotoxic T lymphocyte (CTL) responses. II. Helper factor from activated Lyt 1+ T cells is rate limiting i) in T cell responses to nonimmunogenic alloantigen, ii) in thymocyte responses to allogeneic stimulator cells, and III) recruits allo- or H-2-restricted CTL precursors from the Lyt 123+ T subset.

Abstract

Abstract We have analyzed in two systems the role of alloanti-gen-primed Lyt 1+ T cells during the in vitro induction of primary T cell-mediated cytotoxic immune responses towards allogeneic, or syngeneic but TNP-conjugated or Sendai virus-infected lymphocytes. In the first system, splenic T cells were sensitized to nonimmunogenic alloantigen, such as represented by UV light-irradiated or glutaraldehyde-fixed or sonicated allogeneic lymphocytes. In the second system, thymocytes were cultured with allogeneic or TNP-conjugated or virus-infected syngeneic stimulator cells. Upon addition of graded numbers of irradiated alloantigen-primed T cells, an apparently antigen-specific helper effect on the CTL responses generated was observed. The use of double-chamber cultures, in which helper cells and prekiller cells were separated by a Nuclepore membrane, established that the helper effect per se is mediated by a nonspecific helper factor from Lyt 1+ T cells, and that the apparent antigen specificity observed in the cell-mixing experiments reflected the necessity of antigen recognition by helper T cells in order to release the nonspecific helper factor. We show that the low immunogenicity of UV light-irradiated, glutaraldehyde-fixed, or sonicated stimulator cells resides in their incapacity to trigger Lyt 1+ T cells, and that the low CTL responsiveness of thymocytes is due to a relative lack of Lyt 1+ T helper cells. In the presence of Lyt 1+ T cell-derived helper factor, thymocytes generated efficient CTL responses to either allogeneic or TNP-conjugated or virus-infected syngeneic cells. Within thymocytes, also the peanut lectin binding (PLN+) cortical thymocytes generated effectively allo- and H-2-restricted CTL responses. Since all PNL+ thymocytes expressed the Lyt 123+ phenotype, these results evidence that Lyt 1+ T cell-derived helper factor not only augments CTL responsiveness of Lyt 23+ T cells, but also is able to recruit immunocompetent CTL precursors from the Lyt 123+ T cell subset (positive feedback loop). Lyt 1+ T cell-derived helper factor was equally effective in triggering Lyt 123+ thymocytes to mount in vitro allo- or H-2-restricted CTL responses. In either case, its presence was critical 24 to 48 hr after onset of the culture. Moreover, alloreactive and H-2-restricted thymic CTL precursors exhibited an identical 1 × G sedimentation pattern. According to these criteria, no fundamental difference exists between allo- and H-2-restricted CTL precursors.