MECHANISM BY WHICH U937 PROMONOCYTIC CELLS INACTIVATE HUMAN INTERFERON-GAMMA

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