Replication of influenza B virus in chicken embryos is suppressed by exogenous aprotinin.

Chicken embryo proteinases, one of which is a blood clotting factor Xa-like proteinase, are known to effectively cleave the haemagglutinin (HA) of Influenza B viruses to permit their replication in chicken embryonated eggs. Here we show that injection of the serine proteinase inhibitor, aprotinin, into the allantoic cavity of eggs infected with Influenza B/Hong Kong/73 and B/Lee/40 viruses suppresses the viral HA cleavage and reduces the virus proteolytic activation and replication. Effective inhibition dose was determined as approximately 10.0 micrograms of aprotinin per embryo that corresponds to 0.1 microM concentration. However, heparin, which is known to be a direct inhibitor of the Factor Xa, was not able to suppress Influenza B virus hemagglutinin cleavage and replication in chicken embryo system. These data shed light on the pattern of proteinases involved in the Influenza B virus proteolytic activation and indicate that aprotinin possesses antiviral potential against Influenza B viruses.