Cell surface alpha 1-protease inhibitor on human peripheral mononuclear cells in culture

We have studied expression of alpha 1-protease inhibitor (alpha 1-PI) by human mononuclear cells. alpha 1-PI was detected on 50% of freshly isolated peripheral mononuclear cells. Unless a proliferative stimulus was provided, alpha 1-PI subsequently disappeared from the cell surfaces. Plant mitogens, periodate, neuraminidase-galactose oxidase, or allogeneic cells all were effective stimuli of alpha 1-PI expression. Concanavalin A stimulated de novo synthesis of alpha 1-PI in cell cultures containing both lymphocytes and mononuclear phagocytes, and alpha 1-PI simultaneously appeared on surfaces of activated lymphocytes. Inhibition of protein synthesis by cycloheximide or monocyte depletion abolished de novo alpha 1-PI synthesis, but only monocyte depletion inhibited alpha 1-PI expression. Lymphocytes, but not monocytes, displayed saturable binding of radioiodinated alpha 1-PI. The data are consistent with the interpretation that human mononuclear phagocytes synthesize and secrete alpha 1-PI. When protein synthesis is inhibited, mitogenic stimuli may provoke release of previously synthesized alpha 1-PI from mononuclear phagocytes. Secreted alpha 1-PI then may bind to specific lymphocyte cell surface receptors. This pattern of alpha 1-PI synthesis, secretion, binding, and expression on lymphoid cell surfaces appears to be a common characteristic of many immunologic reactions in vitro.