Tropism and pathogenesis of influenza B virus in human and swine respiratory tracts

Abstract

Influenza B virus (IBV) infection causes regular seasonal epidemics resulting in substantial morbidity and mortality, but research to alleviate the disease burden is still lacking, and little is known if there are other IBV animal reservoirs/hosts apart from human and seals. Swine with the presence of both α2,6- and α2,3-linked sialic acid (SA) receptors in the respiratory tract and proposed role as a ‘mixing vessel’ for influenza A virus (IAV) reassortment prompted a possibility for it to be a ‘neglected’ IBV reservoir. In this project, tissue tropism, infectivity, and replication of IBVs in swine and human ex-vivo respiratory tract explants and in-vitro primary human respiratory epithelial cells were examined by immunohistochemistry (IHC)/immunofluorescence staining and infectious virus titration. Host innate immune response upon IBV infection in respiratory epithelial cells was assessed using RT-qPCR and multiplexed bead-based immunoassay. Besides, RT-qPCR screening for IBV non-structural (IBV-NS) gene in swine nasal and tracheal swabs and inhibitory effects of human and swine airway mucus on IBV infection were investigated. The study was carried out using a panel of 22 IBV strains from year 1940 to 2012, belonging to the early undiverged and more-recent Yamagata and Victoria lineages, in parallel with human seasonal and swine IAVs (H1N1 and H3N2). SA receptor binding specificity of IBVs was studied using haemagglutination assays. Whereas swine tracheal and bronchial explants were susceptible and supported most IBV replication, swine lung explants mainly did not. The better replication in the swine conducting airway could be linked to higher abundance of α2,6-linked SA receptors which matched with IBV binding preference than in the lung. 1 swine nasal swab (0.152%; 1/658) was found positive for IBV-NS gene. Human nasopharyngeal, bronchial, and lung explants, differentiated bronchial epithelial cells (dHBECs), and type I-like pneumocytes, all supported replication of most IBV strains, with stronger tropism for the conducting airway than lung. Earlier induction of pro-inflammatory cytokine and chemokine mRNA expression was observed in IBV-infected dHBECs and type I-like pneumocytes compared to human seasonal IAVs. In addition, IBVs induced significantly more IFNβ, IL29, CCL5, CXCL10, TNFα, MX1, and ISG15 mRNA than human seasonal IAVs in dHBECs at 24 hours post infection. Furthermore, both swine and human airway mucus inhibited IBVs and human seasonal IAVs in a concentration dependent manner. Overall, tropism and pathogenesis of IBVs in swine and human respiratory tracts were strain-specific rather than lineage-specific. Susceptibility of swine to IBV infection prompted necessity for more comprehensive surveillance with regards to its potential as an IBV host. Comparable replication and pathogenesis of IBVs to human seasonal IAVs in human respiratory tract ex-vivo and in-vitro highlighted their non-negligible virulence which needs further investigation and better intervention. Earlier induction of pro-inflammatory cytokine and chemokine by IBVs than human seasonal IAVs hinted differential underlying mechanisms, unravelling which may help better restrict viral replication and disease progression.