Functional characterization of the killer-helper factor responsible for the induction of cytotoxic T lymphocytes from thymocytes, and evidence for the nature of this factor as distinct from T cell-replacing factor (TRF) in regard to B cell triggering.

The functional relationship between the T cell-derived helper factor responsible for the induction of cytotoxic T lymphocytes (CTL) and for the triggering of B cells to form antibody-producing cells was analyzed. The cell-free supernatant (CFS) from PPD-stimulated Tbc-primed T cell culture contained both a killer-helper factor acting on CTL precursors and a T cell-replacing factor (TRF) causing B cell triggering. In the presence of the CFS, thymocytes generated efficient CTL responses against TNP-modified syngeneic target cells. Although CFS acts on thymocytes across the H-2 barrier, killer T cells that are induced act on the target cells in an H-2 restricted manner. Among the thymocyte subsets, peanut agglutinin-binding (PNA+) thymocytes were capable of generating CTL responses, this being contingent on the presence of factor. The T cells producing the killer-helper factor are sensitive to treatment with both anti-Lyt 1 and anti-la antiserum plus complement. Functional analysis of the T cell-derived factors mediating such activities revealed that killer-helper factor and TRF act on mutually distinct acceptor sites on the CTL precursors and B cells, respectively. This was substantiated by the facts that 1) thymocytes from TRF-nonresponder DBA/2Ha mice, whose B cells are genetically defective in the TRF-acceptor site, were able to respond to killer-helper factor and generate killer T cells, and 2) (DBA/2Ha × BALB/c)F1 male anti-BALB/c B cell antiserum, which contains specific antibody for the TRF-acceptor site on responder B cells, blocked TRF action on B cells but did not inhibit the activity of killer-helper factor in relation to CTL-precursors. Moreover, that these activities are mediated by functionally distinct molecules was indicated by the facts that 1) the activity of the killer-helper factor was lost after absorption with Con A-blasts from splenic T cells but not after absorption with splenic antigen-primed B cells, whereas TRF activity was absorbed out when using the latter but not the former cell type, and, 2) the CFS from TRF-producing monoclonal hybridoma T cells did not exert killer-helper activity. These results indicate that the T cell-derived helper factor responsible for the CTL response is distinct from the TRF responsible for B cell triggering, and that the acceptor sites for the respective factors are under distinctly separate gene control.