Carbohydrate-binding molecules inhibit viral fusion and entry by crosslinking membrane glycoproteins.

Defensins are peptides that protect the host against microorganisms. Here we show that the theta-defensin retrocyclin 2 (RC2) inhibited influenza virus infection by blocking membrane fusion mediated by the viral hemagglutinin. RC2 was effective even after hemagglutinin attained a fusogenic conformation or had induced membrane hemifusion. RC2, a multivalent lectin, prevented hemagglutinin-mediated fusion by erecting a network of crosslinked and immobilized surface glycoproteins. RC2 also inhibited fusion mediated by Sindbis virus and baculovirus. Human beta-defensin 3 and mannan-binding lectin also blocked viral fusion by creating a protective barricade of immobilized surface proteins. This general mechanism might explain the broad-spectrum antiviral activity of many multivalent lectins of the innate immune system.