Murine T lymphocytes sensitized in vitro against either allogeneic lymphocytes or syngeneic hapten-conjugated lymphocytes do differentiate into highly effective cytotoxic T lymphocytes (CTL) (1-3). In vivo immunization of T lymphocytes to the same antigens, however, results in the generation of only marginal cytotoxic activity (1,4,5). Recently we found that the weakness of in vivo generated cytotoxicity is not due to a failure of antigen-induced T-cell sensitization but rather due to suppression of the in vivo differentiation of sensitized CTL precursors into effective CTL(6). In keeping with this finding it was postulated that suppressor cells may regulate the in vivo differentiation of CTL.

We now report, that cyclophosphamide-sensitive T cells suppress the in vivo differentiation of antigen-specific CTL. Thus, pretreatment of mice with a single dose of cyclophosphamide (100 mg/kg) converts their state of low responsiveness to a state of high responsiveness.

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