Innate immunity is critical in the early containment of influenza A virus (IAV) infection， and surfactant protein D (SP-D) plays a crucial role in the pulmonary defense against IAV. In pigs， which are important intermediate hosts during the generation of pandemic IAVs， SP-D uses its unique carbohydrate recognition domain (CRD) to interact with IAV. An -linked CRD glycosylation provides interactions with the sialic acid-binding site of IAV， and a tripeptide loop at the lectin-binding site facilitates enhanced interactions with IAV glycans. Here， to investigate both mechanisms of IAV neutralization in greater detail， we produced an -glycosylated neck-CRD fragment of porcine SP-D (RpNCRD) in HEK293 cel... More
Innate immunity is critical in the early containment of influenza A virus (IAV) infection， and surfactant protein D (SP-D) plays a crucial role in the pulmonary defense against IAV. In pigs， which are important intermediate hosts during the generation of pandemic IAVs， SP-D uses its unique carbohydrate recognition domain (CRD) to interact with IAV. An -linked CRD glycosylation provides interactions with the sialic acid-binding site of IAV， and a tripeptide loop at the lectin-binding site facilitates enhanced interactions with IAV glycans. Here， to investigate both mechanisms of IAV neutralization in greater detail， we produced an -glycosylated neck-CRD fragment of porcine SP-D (RpNCRD) in HEK293 cells. X-ray crystallography disclosed that the -glycan did not alter the CRD backbone structure， including the lectin site conformation， but revealed a potential second nonlectin-binding site for glycans. IAV hemagglutination inhibition， IAV aggregation， and neutralization of IAV infection studies showed that RpNCRD， unlike the human analogue RhNCRD， exhibits potent neutralizing activity against pandemic A/Aichi/68 (H3N2)， enabled by both porcine-specific structural features of its CRD. MS analysis revealed an -glycan site-occupancy of >98% at Asn-303 of RpNCRD with complex-type， heterogeneously branched and predominantly α(2，3)-sialylated oligosaccharides. Glycan-binding array data characterized both RpNCRD and RhNCRD as mannose-type lectins. RpNCRD also bound Lewis structures， whereas RhNCRD bound polylactosamine-containing glycans. The presence of the -glycan in the CRD increases the glycan-binding specificity of RpNCRD. These insights increase our understanding of porcine-specific innate defense against pandemic IAV and may inform the design of recombinant SP-D-based antiviral drugs.
