Pulmonary surfactant protein A binds to Cryptococcus neoformans without promoting phagocytosis

Background: Evidence is accumulating that the alveolar collectin surfactant protein A (SPA) plays an important role in the first line of defence against infiltrating pathogenic microorganisms and viruses. The ability of SP-A to facilitate the binding and uptake of acapsular Cryptococcus neoformans by monocyte-derived macrophages, human alveolar macrophages, monocytes and polymorphonuclear leucocytes was investigated.

Materials and methods: Binding, competition and phagocytosis experiments were performed using a flow cytometry technique.

Results: SP-A bound to both the acapsular and the encapsulated form of C. neoformans in a concentration-dependent manner. SP-A showed a threefold better binding to the acapsular yeast: this binding was partly calcium dependent and could be inhibited by mannose (ID50=3 mmol L-1) and glucose (ID50=2.1 mmol L-1) but not by galactose (ID50 = 391 mmol L-1). SP-A did not function as an opsonin in phagocytosis of acapsular C. neoformans for any of the phagocytes studied.

Conclusion: Our results indicate that SP-A binds in a concentration-dependent manner to both encapsulated and acapsular C. neoformans. Despite SP-A binding to the acapsular C. neoformans, phagocytosis by various phagocytes was not enhanced.