Studies on a serum factor which blocks the effect of human natural thymocytotoxic antibodies

Normal human serum contains an IgM associated cytotoxic activity acting on guinea pig thymocytes. The inhibition of this cytotoxic effect by normal guinea pig serum was investigated. The blocking effect could be detected within 1 min and seemed to act by interfering with the cytotoxic factor in solution, since preincubation of human serum with autologous guinea pig serum potentiated the blocking effect, whereas no irreversible blocking effect was obtained by pre-incubation of target cells with the blocking factor. The inhibitory factor was not absorbed by incubation with thymocytes. The blocking effect was heat stabile (56 degrees C, 30 min) and detectable at both 4 degrees C and 37 degrees C. The clear effect on the cytotoxic effect of heterologous serum at 4 degrees C was in contrast to the much lower effect on the cytotoxicity of autologous serum at this temperature.