Apparent interleukin 2 (IL-2) inhibitory activity of human serum is due to rapid killing of IL-2-dependent mouse cells.

A number of investigators have reported that human serum inhibits the proliferation of IL-2-dependent mouse cells in IL-2 bioassays, but the mechanism of inhibition has not been carefully examined. We noticed that IL-2-dependent mouse cells (CTLL-2) are killed within 30 min in the presence of a 1/10 dilution of human serum. However, CTLL-2 cells totally deprived of IL-2 did not begin to die until at least 6 h in culture. Thus, even complete inhibition of IL-2 by human serum could not account for the rapid cytotoxicity caused by human serum. Since humans have 'natural' antibodies which react with mouse cells, it seemed possible that the cytotoxicity was due to antibody/complement-mediated cell lysis. This was supported by the observation that EDTA (at a concentration sufficient to inhibit complement) protected CTLL-2 cells from the cytotoxic effects of human sera from four normal donors. In addition, preincubation of CTLL-2 cells with heat-inactivated human sera at 4 degrees C rendered them much more susceptible to lysis with rabbit complement than cells which were preincubated with complete culture medium. The cytotoxicity of human serum is not limited to IL-2-dependent mouse cells but was also observed with EL4 and Ag8.653 cells as well as normal splenocytes. The cytotoxic effect of human serum was lost upon removal of IgM, but not upon removal of IgG. These results strongly suggest that the inhibition of proliferation of IL-2-dependent mouse cells by human serum is due to antibody/complement-mediated lysis of those cells. In addition, non-heat-inactivated human serum did not inhibit the IL-2-mediated proliferation of human PHA blasts, indicating that there is no inherent inhibitory activity in human serum apart from the cytotoxic effect on xenogeneic cells. Thus the reported IL-2 inhibitory activity of whole human serum is probably not biologically relevant.