Characterization of inner capsid σA protein as a virulence factor of the pteropine orthoreovirus

Pteropine orthoreovirus (PRV) is an emerging zoonotic virus that causes pneumonia in humans. We previously isolated the PRV strain Nachunsulwe-57 (N57) from a Zambian fruit bat and demonstrated its low virulence in laboratory mice. Here, we have attempted to identify factors responsible for differences in the virulence between strain N57 and the human-derived clinical strain Miyazaki-Bali/2007 (MB). Characterization of the virulence of recombinant monoreassortant PRVs derived from highly virulent MB and low virulent N57 strains in mice revealed that compared with wild-type (WT) MB, MB-based monoreassortants carrying the L1, S1, or S2 segment from N57 exhibited attenuated virulence. Among these, the monoreassortants carrying the S1 or S2 segment exhibited reduced viral loads and reduced cytokine gene expression levels in the lungs. Genetic mapping of virulence determinants using the reciprocal monoreassortant viruses with increased virulence demonstrated that N57-based monoreassortants carrying the S1 or S2 segment of MB exhibited enhanced virulence, resulting in lower survival rate compared with WT N57. Unlike the S1 segment, the functions of the S2 segment in pathogenesis are unclear. Thus, we further investigated the functional region of the inner-capsid σA protein encoded by the S2 segment. Notably, Ser-46 of σA was identified as a key amino acid determinant of PRV virulence and is present in strains derived from humans, monkeys, and bat flies, but not those identified from bats (their natural host). Collectively, these findings demonstrate that PRV σA is one factor that regulates virulence, and that σA Ser-46 may be related to potential interspecies transmission events from bats.