Inactivation of human gamma interferon by Pseudomonas aeruginosa proteases: Elastase augments the effects of alkaline protease despite the presence of α2-macroglobulin

Abstract

Pseudomonas aeruginosa alkaline protease (AP) has recently been shown to produce limited proteolysis of human gamma interferon (IFN-γ) and thereby destroy the antiviral and macrophage-activating activities of the lymphokine. In the present study we describe some of the charcteristics of pseudomonas elastase (E) with regard to inactivation of human IFN-γ. The inihibitory effect of E on IFN-γ bioactivity differed from that of AP in that the direct effects of E were reduced in the presence of human serum. That this property of human serum was in large part attributable to the protease inhibitor α2-macroglobulin (α2-M) was suggested by the following observations: (i) methylamine treatment of serum reduced its effect on E, (ii) E interacted directly with α2-M to induce a characteristic conformational change in the protease inhibitor, and (iii) preformed E-α2-M complexes lacked IFN-γ-degrading activity. Despite these findings, anti-E antiserum partially neutralized the effect that Pseudomonas filtrate showed on IFN-γ, suggesting that E contributes to the activity of bacterial filtrates. Treatment of IFN-γ with E in the presence of a suboptimal concentration of AP resulted in an E dose-dependent inactivation of the lymphokine. Preformed E-α2-M complexes, although ineffective by themselves at cleaving IFN-γ, degraded the lymphokine, providing AP was also present in the reaction mixture. These data demonstrate that the destruction of small, biologically significant peptides by Pseudomonas proteases can involve protease-protease synergy that acts even in the presence of the serum protease inhibitor α2-M.