Binding of human collectins (SP-A and MBP) to influenza virus

Collectins are a group of soluble proteins each of which has collagenous domains and non-collagenous globular domains, the latter containing the consensus residues found in C-type lectins. Members of the collectin family are the serum proteins mannan-binding protein (MBP), conglutinin, CL-43, and the lung-associated proteins surfactant protein A (SP-A) and surfactant protein D (SP-D). MBP and conglutinin have been shown previously to bind to influenza viruses and to inhibit the infectivity and haemagglutinating activity of influenza viruses. We report here that the lung protein SP-A, like MBP, can bind to influenza virus (strain A/X31) through its lectin domain and inhibit the virus-mediated agglutination of red cells. The binding of SP-A or MBP to influenza virus was saturable, concentration-dependent, and required the presence of Ca2+ ions. Ligand-blot analysis, using MBP as ligand, of the virus lysate indicated that MBP binds to a 68 kDa viral species. The 68 kDa species was isolated to homogeneity and was shown to be the viral neuraminidase. The purified 68 kDa species inhibited the binding of both MBP and SP-A to influenza virus.