Initiation of the alternative pathway of murine complement by immune complexes is dependent on N ‐glycans in IgG antibodies

Objective Collagen antibody–induced arthritis in mice exhibits a requirement for amplification by the alternative pathway of complement. Although the alternative pathway is activated by spontaneous hydrolysis, it is not known whether this pathway can also be initiated directly by IgG antibodies in immune complexes (ICs). IgG lacking terminal sialic acid and galactose (G0 IgG) can activate the lectin pathway of complement, but it is not known if G0 IgG can also activate the classical or alternative pathway. The purpose of this study was to examine the mechanism of initiation of the alternative pathway of complement by ICs. Methods We used adherent ICs containing bovine type II collagen (CII) and 4 monoclonal antibodies (mAb) to CII (adCII‐IC). C3 activation was measured in the presence of sera from wild‐type C57BL/6 mice or from mice deficient in informative complement components. The mAb were used intact or after enzyme digestion to create G0 IgG or to completely remove the N ‐glycan. Results Both the classical and alternative pathways, but not the lectin pathway, mediated C3 activation induced by the adCII‐IC. Mannose inhibited the alternative pathway–mediated C3 activation but had no effect on the classical pathway, and N ‐glycans in IgG were required by the alternative pathway but not the classical pathway. Both the classical and alternative pathways mediated C3 activation induced by G0 IgG. Mannose‐binding lectin bound avidly to G0 IgG, but lectin pathway–mediated C3 activation was only slightly increased by G0 IgG. Conclusion The alternative pathway of complement is capable of initiating C3 activation induced by adCII‐IC and requires the presence of N ‐glycans on the IgG. G0 IgG activates both the classical and alternative pathways more strongly than the lectin pathway.