Host-specific differences in the membrane fusion activity of influenza A viruses
The transmission of influenza A viruses from avian to other species involves numerous adaptive processes to overcome the species barrier. One major determinant of host-range restriction is the viral hemagglutinin (HA). HA plays a crucial role in virus entry into the host cell by mediating receptor-b...
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|The transmission of influenza A viruses from avian to other species involves numerous adaptive processes to overcome the species barrier. One major determinant of host-range restriction is the viral hemagglutinin (HA). HA plays a crucial role in virus entry into the host cell by mediating receptor-binding and membrane fusion. Virus adaptation to mammals results in alteration of receptor-binding specificity. There is growing evidence that the HA-mediated membrane fusion activity contributes to host range restriction as well. This study aimed to identify host specific differences in membrane fusion properties and to characterise potential alterations during interspecies transmission.
In the first part of the thesis Eurasian avian-like swine viruses that emerged by transmission of an avian H1N1 virus in pigs in the late 1970s in Europe were shown to have a higher pH optimum of HA-mediated fusion (pH 5.1-5.4) and a decreased HA stability when compared to avian precursors (pH 4.9-5.2). These results indicate that this avian-to-swine transmission was accompanied by changes in HA stability. Sequence comparison revealed eight amino acid substitutions that separate the HA of early avian-like swine viruses from their putative avian precursor. Furthermore, mutations in one of these positions contribute to the low stability phenotype. In agreement with natural avian-to-swine transmission, experimental adaptation of a potential avian precursor of the avian-like swine lineage to pigs resulted in a decreased HA stability. This states the first formal proof that viral membrane fusion and stability properties change during interspecies transmission.
The second part of the thesis investigated differences in membrane fusion activity among different avian virus species. Comparison of H7 viruses from wild birds and domestic poultry suggests that Eurasian H7 poultry viruses have a higher pH optimum of membrane fusion (pH 6.2) and thus possess a lower stability than H7 viruses from wild birds (pH 5.2). Moreover, all tested Eurasian H7 viruses express a lower HA stability than HAs from other subtypes (H2, H3, H4, H5, H13, H14 und H16). Previous studies indicate that H5 viruses with low HA stability replicate but do not transmit via respiratory droplets in the ferret model (Imai et al., 2012; Herfst et al., 2012). Thus, it is feasible, that H7 viruses originated from poultry are restricted in ferrets and humans to similar extend.
In the last part, fusion properties of human pandemic and zoonotic viruses were studied. HAs of pandemic viruses from the last century initiated fusion in a narrow pH range between pH 5.0 and 5.2. In contrast, the swine-origin 2009 pandemic virus HA starts to fuse at a pH 0.2 units higher, which might be due to the swine origin of this HA. This further suggests that fusion characteristics continue to adapt in the course of subsequent circulation. The pH optimum of fusion of a zoonotic human H7N9 (2013) virus represents an intermediate in that it is lower when compared to putative ancestors circulating in wild birds, but still higher than that of typical human-adapted viruses. This may account for limited human-to-human transmission observed for this virus.
In order to further investigate which changes in HA are needed for the emergence of avian viruses in humans, HA substitutions separating the 1968 Hong Kong pandemic virus HA from the putative avian precursor were examined. In addition to the well-known switch in receptor specificity, binding avidity changed prior to or during the emergence in humans. In this study, no difference in viral stability was observed between the pandemic virus and the putative avian precursor. This indicates that the avian ancestor was already sufficiently stable to facilitate replication and transmission in humans.
In sum, this study shows that membrane fusion properties vary between host species and alter during influenza A virus emergence in new hosts. This suggests HA-mediated fusion and HA stability to act as host range restriction factors. Alterations in membrane fusion activity and viral stability may not be essential for initial infection of new host individuals. However, an optimal stability seems to be necessary to facilitate transmission within populations of new host species.