The importance of the ratio between effector and target cells for detection of serum blocking of tumor lymphocytolysis.

Sera from tumor-bearong guinea pigs were tested for their capacity to influence the magnitude of lymphocytolysis of tumor cells in vitro as measured by 51Cr release. Thirty-three sera were tested in 124 experiments in which the effector to target cell ratio (E:T) was 200:1. Significant blocking of cell-mediated lysis occurred in 62% of the experiments, whereas significant potentiation occurred in 7%. Six sera were found which showed significant blocking in some experiments and significant potentiation in other experiments. One of these sera was further evaluated by varying E:T, using lymphocytes from three immune donors. For each of the three populations of immune lymphocytes, significant blocking was demostrable only at one of three E:T tested. Blocking was observed only with an E:T of 200:1 in two cases. In a third case, blocking was observed at an E:T of 50:1, whereas an E:T of 200:1 gave rise to significant potentiation. The effect of these sera on lymphocytolysis in vitro is clearly dependent upon the E:T ratio.