The effect of C3 levels on yeast opsonization by normal and pathological sera: identification of a complement independent opsonin.

The ability of 150 normal and pathological sera to opsonize yeast for uptake by isolated human polymorphonuclear leucocytes has been studied. Phagocytosis was measured by an assay depending on electronic particle counting to measure yeast uptake and by a new fluorometric assay which is able to distinguish particle adherence from true phagocytosis. The opsonic activity of normal human serum was heat labile and depended markedly on C3 levels. The sera of 6% of healthy young adults (three from 45) and 8% of healthy donors aged 65-88 (four from 57) showed defective opsonization in spite of normal C3 and CH50 levels. Thirty-eight pathological sera with low C3 levels showed defective opsonization. However, three sera with low C3 levels showed apparently normal opsonic activity. The opsonin from two of these sera was characterized. It was heat stable, had apparent mol.wt of 400,000 and was active in serum free media. The opsonin was absorbed by immobilized Staphylococcus aureus and by gelatin-Sepharose, suggesting that it is related to fibronectin. Fibronectin from normal human sera prepared by affinity chromatography on gelatin-Sepharose was not opsonic in this system.