INHIBITION OF COMPLEMENT-MEDIATED RED-CELL LYSIS BY IMMUNOGLOBULINS IS DEPENDENT ON THE IG-ISOTYPE AND ITS CL BINDING-PROPERTIES

We have investigated the effect on complement activation of human immu noglobulins (Ig) using several therapeutic Ig preparations including t wo for intravenous use (IVIG), and various purified myeloma proteins. Ig inhibited lysis in a dose-dependent manner in the classical pathway assay whereas no alternative pathway inhibition was observed. The Fc part of the molecule was responsible for all the inhibitory effect. Pu rified IgG3 myeloma proteins were potent inhibitors whereas IgG1 inhib ited to a lesser extent and IgG2 and IgG4 did not inhibit at all. Inhi bition was obtained both when Ig was added to the solution and when it was coated onto a solid matrix. Analysis of the soluble and solid pha se Ig after incubation revealed binding of C1q and activated C4 and C3 to the isotypes which inhibited lysis. Using selectively depleted ser a and reconstitution with their respective purified components, effici ent inhibition of lysis was seen when Ig was added prior to serum (C1) , some inhibition was seen at the C4 level, whereas no effect was seen when Ig was added at the C9 level. We conclude that the complement-mo dulatory effect of Ig in vitro is isotype specific and dependent mainl y on competitive C1 binding by the Ig molecule in the absence of antig en.