Demonstration of fungal proteinase during phagocytosis of Candida albicans and Candida tropicalis

Abstract

The extracellular acid proteinase of Candida albicans and Candida tropicalis was monitored in vitro during phagocytosis by murine peritoneal macrophages. Fungal blastospores were quickly ingested by the thioglycolate-elicited macrophages and the intracellular blastospores partly resisted killing and started to grow out after 6 h incubation, causing destruction of the macrophage. Proteinase antigen appeared on fungal cells after 30 min in culture medium containing 10% fetal calf serum. The antigen was detected on ingested blastoconidia and filamentous cells of C. albicans serotype A. The proteinase antigen was also expressed by blastoconidia of C. albicans serotype B but was missing on the filamentous cells of this serotype. Isolates of C. tropicalis behaved similarly to C. albicans serotype A. The acid proteolytic activity of Candida cells was confirmed by the haemoglobin test on culture supernatants. Lysates of infected and noninfected phagocytes showed a differential acid proteolytic activity; noninfected macrophages revealed rising activity, while infected macrophages showed a distinct reduction of activity. The proteolytic activity of lysates of noninfected cells is due to lysosomal cathepsin-D. Cathepsin-D was also most likely to be responsible for the declining proteolytic activity in lysates from infected phagocytes; such lysates contained increasing amounts of fungal proteinase antigen. The differential kinetics of this antigen and the total acid proteolytic activity in the lysates suggest a conflict between microbial and lysosomal hydrolases in infected phagocytes. The outcome of this conflict depends on the number and hydrolytic activity of the ingested yeasts and may be decisive in the progress of infection.