Factors Inducing Cytolytic Activity

There is good evidence that cytolytic T lymphocytes (CTL) 1 are derived from cytotytically inactive precursors (CTL-P). In most experimental systems for the activation of the precursors to CTL, extensive proliferation of the CTL-P is induced. Tbis proliferation depends on T cell growth factor (TCGF or interleukin 2), which acts only on T cells that have become TCGF responsive as a result of their interaction with antigenic stimulator cells. Whether such an interaction is sufficient to induce TCGF responsiveness or whether other signals or factors are required is still controversial (for discussion see 1). In any case, it is not known whether TCGF is required only for growth promotion or whether it has other effects on the maturation of CTL-P. CTL activity can be induced in the presence of a drug that blocks DNA synthesis (2) but it has not been tested whether TCGF is required for CTL activation under these conditions. Recently, several groups have confirmed the evidence obtained by Raulet and Bevan (3) that the induction of a proliferative response in cell populations containing CTL-P, by exposure to mitogenic lectins and a source of TCGF is not sufficient for the appearance of CTL activity in such populations. The experiments suggest that the maturation of CTL-P requires one or several factors found in the supernatant of spleen cells stimulated with concanavalin A (CS) or of cloned alloreactive T cells stimulated with appropriate stimulators. To determine the precise requirements for the activation of CTL-P and to elucidate the mode of action of the required signals, it would be useful to establish cell lines with the characteristics of CTL-P. The cell lines derived from CTL described in the literature so far, with the exception of some recently