The Hemagglutinin of Bat-Associated Influenza Viruses Is Activated by TMPRSS2 for pH-Dependent Entry into Bat but Not Human Cells

Hoffmann, Markus; Krüger, Nadine; Zmora, Pawel; Wrensch, Florian; Herrler, Georg; Pöhlmann, Stefan

The FLUAV-HA is sufficient to mediate viral binding and entry into target cells and our findings indicate that the same applies to batFLUAV-HAL. In contrast to FLUAV-HA, however, HAL does not depend on the presence of sialic acids for entry. Thus, treatment of EpoNi/22.1 cells with sialidase did not decrease HAL-mediated pseudotype entry. These findings are in keeping with the work by Maruyama et al. [64] and with structural data indicating that HAL does possess a distorted putative sialic acid binding site [17, 24] . Contrarily, high amounts of sialidase increased entry efficiency, potentially by increasing access to the elusive receptor. In addition, removal of sialic acids might increase electrostatic interactions of batFLUAV-HAL with cell surface factors, since sialic acids are negatively charged. Although HAL-driven entry was independent of sialic acids, it did require endosomal acidification (in accordance with Maruyama et al. [64] ), which is known to trigger the membrane fusion activity of HA. Most likely, protonation also triggers bat-FLUAV-HAL for membrane fusion. However, it cannot be disregarded that the inhibitory effect of ammonium chloride was due to blockade of pH-dependent endosomal cysteine proteases, which activate the surface proteins of several coronaviruses and ebolaviruses [65] [66] [67] [68] .