Surfactant protein A, but not surfactant protein D, is an opsonin for influenza A virus phagocytosis by rat alveolar macrophages

Abstract

Surfactant protein A (SP-A) and surfactant protein D (SP-D) are collectins, 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 opsonic activities of SP-A and SP-D for phagocytosis of fluorescein isothiocyanate (FITC)-labeled influenza A (H3N2) virus by rat alveolar macrophages 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 microg/ml. An approximate threefold increase in association of influenza A virus with alveolar macrophages in the presence of SP-A over control incubations which contained no SP-A was observed. Half of the total cell-associated fluorescence could be quenched as demonstrated using the extracellular quenching dye trypan blue. These results indicate that SP-A mediates internalization of FITC-labeled influenza A (H3N2) virus by alveolar macrophages. Removal of the carbohydrate 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 macrophages. Mannan, a mannose homopolysaccharide known to bind to the carbohydrate-binding domain of SP-A, did not affect the SP-A-mediated phagocytosis of FITC-labeled influenza by alveolar macrophages. In contrast, SP-D neither enhanced the association of FITC-labeled influenza A virus 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 as an opsonin in the phagocytosis of influenza A virus by alveolar macrophages.

Surfactant protein-A enhances respiratory syncytial virus clearance in vivo

“…When antibodies to SP-A were added to this in vitro system, the phagocytosis of herpes simplex virus by macrophages was abolished (16). The addition of SP-A to cell cultures infected with influenza A virus resulted in a reduction of viral infected cells (18), and SP-A enhanced the association of influenza A virus with alveolar macrophage in a dose-dependent manner (19). Deglycosylated SP-A did not bind viral infected cells, suggesting that the carbohydrate moiety of SP-A is involved in the recognition of viruses by SP-A (17,18).…”

“…In vitro, SP-A stimulates macrophage chemotaxis (11) and enhances the binding of serum-opsonized Staphylococcus aureus (12), non-serum-opsonized Escherichia coli, S. aureus, Hemophilus influenzae, and Mycobacterium tuberculosis to alveolar macrophages (13)(14)(15). SP-A also binds herpes simplex virus, herpes simplex virus-infected cells (16,17), and influenza A virus (18,19).…”

Expression of Surfactant Protein D in the Human Gastric Mucosa and duringHelicobacter pyloriInfection

“…Agglutination is thought to facilitate phagocytosis by polymor- (9). While some reports suggest that there may be a specific SP-D receptor on neutrophils and macrophages, this idea is controversial, and other studies suggest that SP-D does not act as an opsonin (1,12). Motility is thought to be an important virulence determinant in H. pylori, allowing the organism to migrate to suitable niches within the gastric mucosa; inhibition of motility prevents colonization (6,31).…”

Pulmonary Collectins Modulate Strain-Specific Influenza A Virus Infection and Host Responses

“…SP-A but not SP-D enhances rat alveolar macrophage uptake of IAV in vitro (3). The collectins may also influence the host immune response to IAV without attaching to IAV by interacting in nonopsonic fashion with lung macrophages, neutrophils, dendritic cells, and T cells.…”

“…In contrast, SP-A neutralizes IAV by directly occupying the HA cell attachment site with the terminal sialic acid on the oligosaccharide located on the CRD of SP-A, presumably blocking this site (3). Unlike the case with SP-D, the attachment of SP-A to influenza virus strains and SP-A-dependent neutralization in vitro are not influenced by the extent of HA glycosylation; in fact, SP-A is slightly more effective at neutralizing IAV strains that lack glycosylation completely (19).…”