Inhibition of Complement‐Mediated Red Cell Lysis by Immunoglobulins is Dependent on the IG Isotype and its Cl Binding Properties

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Abstract
We have investigated the effect on complement activation of human irnmunoglohulins (Ig) using several therapeutic Ig preparations including two 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. Purified IgG3 myeloma proteins were potent inhibitors whereas IgGl inhibited to a lesser extent and lgG2 and IgG4 did not inhibit at all. Inhibition 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 phase Ig after incubation revealed binding of Clq and activated C4 and C3 to the isotypes which inhibited lysis. Using selectively depleted sera and reconstitution with their respective purified components, efficient 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 complementmodulatory effect of Fg in vitro is isotype specific and dependent mainly on competitive C1 binding by the Ig molecule in the absence of antigen.

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