Divalent metal requirement for soluble immune response suppressor (SIRS) activity

Abstract

Soluble immune response suppressor (SIRS) is an immunosuppressive protein which requires activation to SIRS ox by peroxide. This observation suggested that SIRS could be a metalloprotein. To investigate this possibility, purified hybridoma-derived SIRS was treated with chelating agents and dialyzed prior to activation with H 2O 2. EDTA, EGTA, or desferrioxamine prevented suppression of murine plaque-forming cell responses by SIRS ox, whereas sodium citrate and penicillamine did not inhibit suppression. Suppressive activity was reconstituted by subsequent treatment of SIRS with FeSO 4 (0.5 μ M), NiSO 4 (500 μ M), or MgSO 4 (500 μ M), but not by FeCl 3, MnSO 4, CuSO 4, ZnSO 4, CaCl 2, or CrCl 3. Reconstitution of activity occurred only if FeSO 4 was added at least 3 hr prior to treatment with H 2O 2. These data indicate that SIRS requires a divalent metal ion, probably ferrous iron, for activity, and suggest that SIRS is a metalloprotein.