Pattern of the action of a suppressor factor produced by a human macrophage-like cell line, U937

A U937 suppressor factor (U937SF) was purified from crude supernatant by sequential chromatography using fast protein liquid chromatography. The molecular weight and isoelectric point of U937SF were 69 kDa and 4.5, respectively. The U937SF preparation inhibited the proliferative response in human PBMC stimulated with an antigen tuberculin purified protein derivative, tetanus toxoid) or a mitogen (phytohaemagglutinin concanavalin-A). U937SF depressed both interleukin-2 (IL-2) production and IL-2 receptor (CD25) expression in peripheral blood mononuclear cells (PBMC) stimulated with an antigen but not with a mitogen. Anti-CD3 monoclonal antibody-induced responses including a proliferative response, IL-2 production and CD25 expression were suppressed by U937SF. In contrast, U937SF did not affect monocyte functions such as antigen processing and IL-1 production. Neither did it modulate the expression of T cell receptor (TCR) or CD3 molecules on the surface of lymphocytes. Moreover it did not inhibit CD25 expression in PBMC stimulated with phorbol myristate acetate plus A23187. These results suggest that U937SF prevents both IL-2 production and CD25 expression in lymphocytes activated through the TCR/CD3, but not through the other receptors or molecules. In addition, U937SF does not block the early activation events following TCR-mediated stimulation, nor affect the pre-TCR activation steps.