Activation of the alternative pathway of complement by monosodium urate crystals.

Monosodium urate crystals (MSU) have been shown to activate the alternative pathway of complement in a dose- and time-dependent fashion at 37 degrees C. Activation was maximal upon addition of 10-20 mg/ml monosodium urate crystals to C2-deficient human serum (C2D) or normal human serum containing 5 mM MgEGTA. Immunoelectrophoretic analysis of such treated sera demonstrated cleavage of C3 and factor B. Incubation of highly purified C3 and factor B with 10 mg/ml MSU did not, however, affect their immunoelectrophoretic pattern, suggesting that cleavage of either factor B or C3 in serum requires an intact alternative complement pathway. The fluid-phase control proteins, Factor H and Factor I, were not found to be diminished upon incubation of C2D serum or NHS containing MgEGTA with MSU. Thus activation appeared to be surface dependent and not a consequence of control protein depletion. It was also found, in agreement with earlier observations, that the classical complement pathway is activated, with concomitant depletion of C1 and C4. We conclude that MSU crystals activate both the classical and alternative pathways, and that such activation may participate in the pathogenesis of gouty arthritis.