Inhibition of cytotoxic T-cell clonal expansion by cyclophosphamide and the recovery of cytotoxic T-lymphocyte precursors by supernatants from mixed-lymphocyte cultures.

Spleen cells from mice treated with cyclophosphamide (150 mg/kg) and cultured at suboptimal concentrations do not generate a cytotoxic T-lymphocyte (CTL) response to allogeneic tumor cells. The reduced response of spleen cells from cyclophosphamide-treated mice is not due to the elimination of CTL precursors because normal responses are obtained by the addition of a helper factor(s) derived from mixed lymphocyte culture supernatants. The results indicate that helper cells, required for development of CTL responses to tumor alloantigens, are eliminated by cyclophosphamide in the absence of evident toxicity to CTL precursors.