Complement binding to Aspergillus conidia correlates with pathogenicity.

Complement has significant effects on the phagocytosis of Aspergillus organisms. We examined the amount and type of complement component C3 bound to the resting conidia of 29 isolates from nine Aspergillus species. The highly pathogenic species A. fumigatus and A. flavus bound fewer C3 molecules per unit of conidial surface area than did the less pathogenic species A. glaucus, A. nidulans, A. niger, A. ochraceus, A. terreus, A. versicolor, and A. wentii, as determined by quantitative flow cytometry. Immunoblot analysis of C3 fragments bound to conidia demonstrated that for all species most C3b was apparently converted to iC3b. For seven species, iC3b was clearly the major C3 product recognized by immunoblotting. However, A. niger and A. nidulans appear to promote further breakdown of opsonic C3 fragments to C3dg. We found significant variations in size and C3 binding among isolates within the same species. Intraspecies variation may contribute to seemingly discrepant results obtained in studies of Aspergillus phagocytosis.