SURFACTANT PROTEIN-A, BUT NOT SURFACTANT PROTEIN-D, IS AN OPSONIN FORINFLUENZA-A VIRUS PHAGOCYTOSIS BY RAT ALVEOLAR MACROPHAGES

Surfactant protein A (SP-A) and surfactant protein D (SP-D) are collec tins, and both proteins were shown to interact with influenza A virus and alveolar macrophages. However, it is not known whether SP-A and SP -D can serve as opsonins for the phagocytosis of influenza A virus by alveolar macrophages. In the present study, we investigated the opsoni c activities of SP-A and SP-D for phagocytosis of fluorescein isothioc yanate (FITC)-labeled influenza A (H3N2) virus by rat alveolar macroph ages using flow cytometry. SP-A enhanced the association of the virus with macrophages in a dose-dependent manner, reaching a maximum at an SP-A concentration of 60 mu g/ml. An approximate threefold increase in association of influenza A virus with alveolar macrophages in the pre sence of SP-A over control incubations which contained no SP-A was obs erved. Half of the total cell-associated fluorescence could be quenche d as demonstrated using the extracellular quenching dye trypan blue. T hese results indicate that SP-A mediates internalization of FITC-label ed influenza A (H3N2) virus by alveolar macrophages. Removal of the ca rbohydrate moiety of SP-A by N-glycosidase F treatment or cleavage of its sialic acid residues by neuraminidase abolished the enhancement of the phagocytosis of FITC-labeled influenza A virus by alveolar macrop hages. Mannan, a mannose homopolysaccharide known to bind to the carbo hydrate-binding domain of SP-A, did not affect the SP-A-mediated phago cytosis of FITC-labeled influenza by alveolar macrophages. In contrast , SP-D neither enhanced the association of FITC-labeled influenza A vi rus with alveolar macrophages nor affected the opsonic activity of SP- A for FITC-labeled influenza A (H3N2) virus at the SP-D concentrations tested. It is concluded that SP-A acts via its sialic acid residues a s an opsonin in the phagocytosis of influenza A virus by alveolar macr ophages.