Activation of the classical complement pathway by homogeneous anti-SIII antibody bound to bivalent or trivalent oligosaccharide antigens

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The capacity of a homogeneous rabbit anti-SIII antibody complexed with oligosaccharide antigens of different size to activate the classical complement pathway was tested by a C1 binding and a C4 consumption test. Complexes formed with a bivalent (16-sugar unit) or with a bigger antigen led to a significant C1 binding and C4 consumption over a broad range of antigen-antibody ratios. Bivalent antigen induced a smaller effect in both tests than the trivalent antigen (26-sugar unit). Fractionation of the antigen-antibody complex mixtures by gel-chromatography led to C4 consumption on patterns differing according to the antigen size. In soluble complexes containing bivalent antigen the highest activity was recovered in fractions corresponding to IgG hexamers whereas no significant activity was found in fractions containing dimers and/or monomers. In contrast, when trivalent antigen was present, the decrease of C4 consuming capacity with the decreasing size of the complexes was much less steep since measurable activity was detected even in fractions containing IgG dimers and monomers. The experiments reported here are consistent with the contention that the size of soluble antigen-antibody complexes is the major factor determining its capacity to trigger the classical complement pathway activation and they also suggest that the geometrical arrangement of IgG molecules clustered in small aggregates is of paramount importance.