The proteolytic activation of A (H3N2) Influenza virus hemagglutinin is facilitated by different type II transmembrane serine proteases.

Cleavage of influenza virus hemagglutinin (HA) by host cell proteases is necessary for viral activation and infectivity. In humans and mice, members of the type II transmembrane protease family (TTSP), e.g. TMPRSS2, TMPRSS4 and TMPRSS11d (HAT), have been shown to cleave influenza virus HA for viral activation and infectivity in vitro. Recently, we reported that inactivation of a single HA-activating protease gene, Tmprss2, in knock-out mice inhibits spread of H1N1 influenza viruses. However, after infection of Tmprss2 knock-out mice with H3N2 only a slight increase was observed in survival and mice still lost body weight. In this study, we investigated an additional trypsin-like protease, TMPRSS4. Both TMPRSS2 and TMPRSS4 are expressed in the same cell types of the mouse lung. Deletion of Tmprss4 alone in knock-out mice does not protect them from body weight loss and death upon infection with H3N2 influenza virus. In contrast, Tmprss2(-/-)Tmprss4(-/-) double knock-out mice showed a remarkably reduced virus spread and lung pathology in addition to reduced body weight loss and mortality. Thus, our results identified TMPRSS4 as a second host cell protease that, in addition to TMPRSS2, is able to activate the HA of H3N2 influenza virus HA in vivo