Secretion of a suppressor cell-inducing factor by an interleukin 3-dependent cell Line with natural cytotoxic activity. II. Range, potency, and kinetics of suppressive activity

Abstract

We have shown previously that the interleukin 3 (IL 3)-dependent, M1-A5 cell line hasnatural cytotoxic (NC) activity and secretes a small molecular weight factor which activates suppressor cells capable of inhibiting the in vitro generation of antibody forming cells (PFC). In this paper, we show that the M1-A5 cells are capable of suppressing the generation of both PFC and cytotoxic T cells (CTL) as well as the proliferative response in a mixed lymphocyte reaction (MLR). Very little suppression of Concanavalin A (Con A)-stimulated proliferation is seen. Suppression is seen with as few as 10 2 M1-A5 cells, a 10 −4 dilution of a cell-free supernatant from the M1-A5 cells, or as few as 104 suppressor cells activated by the suppressor cell inducing factor (SIF) secreted by the M1-A5 cell line. Kinetic studies show that, in all cases, the cells or suppressive factors must be present in the initial 24 to 48 h of the culture for significant suppression. In contrast, a second IL 3-dependent cell line FDC-P1 is directly suppressive, but this suppression is clearly due to a different mechanism, since cell-free supernatants are not inhibitory and do not activate suppressor cells.