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 (Mφ‐SF). The Mφ‐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 Mφ‐SF, suggesting that de novo synthesis is not required. The factor is not preformed, for virgin maerophages do not contain Mφ‐SF. but it appears in macrophage cell lysates after exposure to T‐SF. The production of the Mφ‐SF is inhibited by the presence of 2‐mercaptoethanol. Both T‐SF and Mφ‐SF are l‐rhamnose inhibitable and the Mφ‐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. ∼20,000. The T‐SF induced Mφ‐SF has some similarities with soluble immune response suppressor (SIRS) but differs from this factor in (a) its lack of effect upon lymphocyie proliferation (b) failure to induce conversion of T‐SF to Mφ‐SF by treatment with H2O2.