Interaction of a lymphokine with normal human macrophages results in release of a suppressor factor for mitogen-induced immunoglobulin synthesis.

Normal human macrophage/monocyte cultures exposed to a suppressor factor produced by concanavalin A-activated T cells (T-SF), respond by releasing after 72 h a macrophage-derived suppressor factor (M phi-SF). The M phi-SF inhibits pokeweed mitogen-induced Ig synthesis but not T- or B-cell proliferation. Cycloheximide treatment of the macrophages does not interfere with generation of the M phi-SF, suggesting that de novo synthesis is not required. The factor is not preformed, for virgin macrophages do not contain M phi-SF, but it appears in macrophage cell lysates after exposure to T-SF. The production of the M phi-SF is inhibited by the presence of 2-mercaptoethanol. Both T-SF and M phi-SF are L-rhamnose inhibitable, and the M phi-SF appears to be released as a high molecular weight complex which is dissociable into a low molecular weight form of a size similar to the T-SF, i.e. approximately 20,000. The T-SF induced M phi-SF has some similarities with soluble immune response suppressor (SIRS) but differs from this factor in its lack of effect upon lymphocyte proliferation failure to induce conversion of T-SF to M phi-SF by treatment with H2O2.