Human astrovirus coat protein binds C1q and MBL and inhibits the classical and lectin pathways of complement activation

Human astroviruses (HAstVs) constitute a family of non-enveloped, RNA viruses which cause infantile gastroenteritis. We have previously demonstrated that purified HAstV coat protein (CP), multiple copies of which compose the viral capsid, bind C1q resulting in inhibition of classical complement pathway activity. The objective of this study was to further analyze the mechanism by which CP inhibits C1 activation. CP inhibited C1 activation, preventing cleavage of C1s to its active form in the presence of heat-aggregated IgG, a potent classical pathway activator. CP also inhibited generation of the potent anaphylatoxin C5a. CP dose-dependently bound to C1q, the isolated globular heads and the collagen-like regions of the C1q molecule. When CP was added to C1, C1s dissociated from C1q suggesting that CP functionally displaces the protease tetramer (C1s–C1r–C1r–C1s). Given the structural and functional relatedness of C1q and MBL, we subsequently investigated the interactions between CP and MBL. CP bound to purified MBL and was able to inhibit mannan-mediated activation of the lectin pathway. Interestingly, CP did not bind to a variant of MBL that replaces a lysine residue (Lys55) critical for binding to MASP-2, a functional homolog of C1s. Finally, CP was shown to cross the species barrier to inhibit C3 activation and MAC formation in rat serum. These findings suggest CP inhibits C1 and MBL activation via a novel mechanism of interference with the normal interaction of the recognition molecule with its cognate serine proteases.