Mechanism of binding of surfactant protein D to influenza A viruses: importance of binding to haemagglutinin to antiviral activity

Collectins are important in the initial containment of a variety of pathogens, including influenza A virus (IAV). We provide the first systematic evaluation of the oligosaccharide-binding sites for pulmonary surfactant protein D (SP-D) on specific IAV coat glycoproteins and define the relationship between this binding and antiviral activity. With the use of several techniques, SP-D was found to bind via its carbohydrate-recognition domain (CRD) to mannosylated, N-linked carbohydrates on the HA, domain of the haemagglutinin (HA) and on the neuraminidase of IAV. Using a set of IAV strains that differed in the level and site of glycosylation, and a panel of recombinant collectins, we found that binding of SP-D to the globular domain of the HA was critical in mediating the inhibition of viral haemagglutination activity and infectivity. We also demonstrated that the pattern of binding of a collectin to IAV glycoproteins can be modified by altering the monosaccharide-binding affinity of its CRD or by linking the CRD to a different N-terminal/collagen domain. These studies clarify the mechanisms of viral neutralization by collectins and might be useful in engineering collectins for enhanced antiviral activity.