Cell surface α₁-protease inhibitor on human peripheral mononuclear cells in culture

We have studied expression of α₁-protease inhibitor (α₁-PI) by human mononuclear cells. α₁-PI was detected on 50% of freshly isolated peripheral mononuclear cells. Unless a proliferative stimulus was provided, α₁-PI subsequently disappeared from the cell surfaces. Plant mitogens, periodate, neuraminidase-galactose oxidase, or allogeneic cells all were effective stimuli of α₁-PI expression. Concanavalin A stimulated de novo synthesis of α₁-PI in cell cultures containing both lymphocytes and mononuclear phagocytes, and α₁-PI simultaneously appeared on surfaces of activated lymphocytes. Inhibition of protein synthesis by cycloheximide or monocyte depletion abolished de novo α₁-PI synthesis, but only monocyte depletion inhibited α₁-PI expression. Lymphocytes, but not monocytes, displayed saturable binding of radioiodinated α₁-PI. The data are consistent with the interpretation that human mononuclear phagocytes synthesize and secrete α₁-PI. When protein synthesis is inhibited, mitogenic stimuli may provoke release of previously synthesized α₁-PI from mononuclear phagocytes. Secreted α₁-PI then may bind to specific lymphocyte cell surface receptors. This pattern of α₁-PI synthesis, secretion, binding, and expression on lymphoid cell surfaces appears to be a common characteristic of many immunologic reactions in vitro.