BOVINE AND MOUSE SERUM BETA-INHIBITORS OF INFLUENZA-A VIRUSES ARE MANNOSE-BINDING LECTINS

Normal bovine and mouse sera contain a component, termed β inhibitor, that inhibits the infectivity and hemagglutinating activity of influenza A viruses of the H1 and H3 subtypes. To investigate the nature of the interaction of β inhibitors with influenza A viruses we isolated a mutant of the virus Mem71(H)-Bel(N) (H3N1) that could grow in the presence of bovine serum. The mutant virus was resistant to hemagglutination inhibition by mouse serum as well as by bovine serum and had undergone changes in the receptor-binding and the antigenic properties of this hemagglutinin (HA) molecule. Sequence analysis of the HA genes of parent and mutant viruses revealed a single nucleotide change in the mutant, resulting in the substitution Thr → Asn at residue 167 of the HA1 chain of HA. This change leads to loss of the potential glycosylation site Asn-165 - Val-166 - Thr-167 at the tip of the HA spike, which in viruses of the H3 subtype is known to bear a high-mannose (type II) carbohydrate side chain N-linked to Asn-165. The association of β inhibitor resistance with loss of this carbohydrate side chain suggested that β inhibitors may be lectins. In support of this hypothesis, treatment of the β inhibitor-sensitive parent virus Mem71(H)-Bel(N) with periodate converted it to the resistant state. Furthermore, the inhibitory activity of both bovine and mouse sera for the parental virus was abrogated by D-mannose. We conclude that the β inhibitors in bovine and mouse sera are mannose-binding lectins that inhibit hemagglutination and neutralize virus infectivity by binding to carbohydrate at the tip of the HA spike, blocking access of cell-surface receptors to the receptor-binding site on HA.