Serum factors affecting the incorporation of (3H)thymidine by lymphocytes stimulated by antigen. 3. Evidence for a role of complement from studies with specific complement inhibitors.

To obtain further evidence that complement is involved in both the early, non-specific and the late, specific, components of the heat effect (Forsdyke, 1973b), lymph node cells from preimmunized rabbits were cultured with varying concentrations of antigen in autologous preimmunization serum treated with one of three specific complement inhibitors, inulin, Zymosan or cobra venom factor. The Zymosan particles were removed before use of the serum in cultures, but the other inhibitors were not removed. Inulin only slowly removed haemolytic activity from serum and enhanced the late response to high concentrations of specific antigen. The inulin concentrations required to remove haemolytic activity were similar to those required to enhance the response to antigen. Zymosan-pretreated serum enhanced labelling with [3H]thymidine in both control and antigen-treated cultures to a proportionate extent. No late specific enhancement of labelling was detected, probably because of the secretion of complement by cultured cells. The interpretation of data obtained when Zymosan was left in cultures was complicated by Zymosan acting as an antigen and inducing a primary response to itself. Cobra venom factor reduced the labelling of both control and antigen-treated cultures so that the non-specific component of the heat effect was not detectable. However, the response to high concentrations of specific antigen was enhanced. It is concluded that although each complement inhibitor shows unique individual characteristics in the system, on balance the results support the view that the activity responsible for both components of the heat effect is complement.