A large part of complement components are synthetized by cells of the macrophage-monocyte type which are under narrow control by lymphokines. Preliminary results obtained in our laboratory have shown that addition of phytohemagglutinin M to a culture of whole normal guinea pig spleen cells decreased the amount of C2 detectable in the culture supernatant at the same time where it strongly stimulated lymphocyte proliferation, as shown by a high rate of cellular 3H-thymidine incorporation. One possible explanation for this phenomenon would be the inhibition by some lymphokine of C2 synthesis by macrophages.
In order to substantiate this hypothesis, a two step experiment was designed. Whole normal guinea pig spleen cells (0.5 ml of 20 × 106/ml suspension) were first cultivated for 24 hours in the presence of an optimal 50 µg dose of insoluble concanavalin A. The supernatant obtained was assayed for its lymphokine activity by a classical Macrophage Inhibition Test.