Mechanism by Which U937 Promonocytic Cells Inactivate Human Interferon-γ

Abstract

Mononuclear phagocytes produce proteinases that are thought to play a role in regulating the activity of cytokines. Activated macrophages secrete urokinase-type plasminogen activator (uPA), which mediates the formation of the serine proteinase plasmin from the ubiquitous zymogen plasminogen. We previously observed a correlation between in vitro plasminogen activation by the promonocytic cell line U937 and the apparent ability of these cells to inactivate recombinant interferon-γ(rIFN-γ) by proteolysis. The present study was designed to test the hypothesis that plasmin, generated in U937 cell cultures, is both necessary and sufficient to inactivate rIFN-γ by limited proteolysis. The following observations are consistent with this hypothesis: (1) inactivation of rIFN-γ was prevented by inhibitors of serine proteinases or an antibody that specifically immunodepleted plasmin activity; (2) purified plasmin inactivated rIFN-γ as efficiently as U937 culture supernatants and was similarly sensitive to serine proteinase inhibitors; and (3) plasmin removed an 11 amino acid carboxyl-terminal peptide from rIFN-γ, which included a region known to be required for bioactivity. Overall, these data indicate that plasminogen activation by U937 promonocytic cells leads to the proteolytic inactivation of IFN-γ by a process that only requires the production of plasmin.