Evidence for influenza B virus hemagglutinin adaptation to the human host: high cleavability, acid-stability and preference for cool temperature

Influenza A virus (IAV) and influenza B virus (IBV) cause yearly epidemics associated with significant morbidity and mortality. Whereas IBV is confined to humans, IAV circulates in animal reservoirs from where it can enter the human population. When this occurs, adaptation of the viral hemagglutinin (HA) is required to achieve sustained human-to-human transmission. IBV has been circulating in humans for a long period of time. Whether this entailed adaptation of IBV HA to the human host is unknown. To address this question, we compared the two seasonal IAV (A/H1N1 and A/H3N2) and two IBV viruses (B/Victoria and B/Yamagata lineage) with regard to host-dependent activity of HA as the mediator of membrane fusion during viral entry. First, we investigated host cell protease-dependent activation of HA, required to render HA membrane fusion-competent. Our analysis covered all members of the type II transmembrane serine protease (TTSP) and kallikrein families, many of which proved to be expressed in human respiratory epithelium. We found that, compared to IAV, the IBV HA0 precursor is cleaved by a broader panel of TTSPs and is activated with much higher efficiency. In agreement with this finding, knockdown of a single HA-processing enzyme, TMPRSS2, was sufficient to abrogate spread of IAV but not IBV in human airway epithelial cells. Second, we showed that the acidic pH required to trigger membrane fusion by HA is similar for IBV and human-adapted IAVs (one noticeable exception being the HA of 1918 IAV), indicating that IBV HA is well-adapted to resist the mildly acidic pH in the upper part of the human respiratory tract. Finally, IBV HA exhibited higher protein expression at 33°C than at 37°C, and 33°C was strictly required to achieve membrane fusion driven by HA of the B/Victoria lineage. Preference for a cooler temperature fits with adaptation of IBV to the proximal airways. In sum, we revealed some distinct properties of the IBV HA protein, which are compatible with extensive adaptation to the human airways during prolonged circulation of IBV in the human population.