Activation, binding, and processing of complement component 3 (C3) by Blastomyces dermatitidis

Abstract

Complement plays a key role in phagocyte recognition and killing of Blastomyces dermatitidis, but little is known about how complement components interact with the yeast. We report the characteristics of activation, binding, and processing of C3 by B. dermatitidis. In pooled normal human serum (NHS), deposition of C3 on the yeast was detectable within 2 min, whereas in NHS containing MgEGTA, deposition was delayed by 6 min, indicating that the yeast activates C3 by both classical and alternative pathways. When both pathways were operative, maximal binding of 4.5 x 106 C3 molecules/cell was achieved in less than 30 min. In the absence of the classical pathway, yeast cells bound 80% of the maximum C3, indicating that the yeast intrinsically activates the alternative pathway. Delayed deposition of C3 in NHS-MgEGTA was similar to that in NHS preabsorbed by the yeast or by immobilized protein A/G to remove serum immunoglobulin. Purified immunoglobulin restored C3 binding to NItS preabsorbed by the yeast, suggesting that antibody in nonimmune serum initiates the classical pathway. β-Glucan absorption of NHS abolished the classical pathway, suggesting that cell wall β-glucan is a target of initiating antibodies. Hydroxylamine treatment of NHS-opsonized yeast cells showed that 76% of C3 was bound to yeast cells by ester linkage, supporting a role for hydroxyl groups on cell wall polysaccharides. Hydroxylamine- cleaved fragments were chiefly C3b and iC3b; 70% of hydroxylamine-sensitive C3b was converted to iC3b within 1 min of opsonization, and the ratio was stable over 1 h. Our data predict that C3b and iC3b on opsonized yeast cells direct binding to CR1 and CR3 receptors on human phagocytes, which, in turn, may influence the fate of this host-pathogen interaction.