Tropism and pathogenesis of influenza B viruses in human respiratory tract, an in vitro and ex vivo study

Abstract

Background: Although influenza B virus (IBV) causes regular global seasonal epidemics in human leading to substantial hospitalization and death, research on IBV is still overshadowed by the prominence of influenza A (IAV). Yet, IBV and IAV are evolutionarily diverged and genetically incompatible. Research results of IAV are not completely applicable to IBV. As evidence on the prevalence and severity of IBV infections accumulates, more thorough studies of IBV need to be carried out. Our study uses human respiratory explant cultures and primary respiratory epithelial cells to examine tissue and cellular tropism as well as pathogenesis of IBVs from both Yamagata and Victoria lineages of different years. Method: Human bronchus and lung explants were prepared from non-tumour residual tissue of patients undergoing lung resection. Primary human welldifferentiated bronchial epithelial cells (dHBECs) and alveolar epithelial cells (pneumocytes) were isolated from corresponding tissue using well-established protocols as part of a study approved by the Institutional Review Board of the University of Hong Kong and Hospital Authority, Hong Kong West Cluster. Panels of Yamagata and Victoria lineage IBVs from different years were used to infect the explant tissues and cells. Seasonal IAVs were used as controls. Viral replication kinetics and tropism were evaluated through TCID50 assay and IHC. Host innate immune response in infected cells was investigated at mRNA and protein levels using RT-qPCR and BD Cytometric Bead Array. Conclusion: Most IBVs replicate in both human bronchus and lung, with a trend suggesting a higher preference in bronchus tropism. Consistently, IBVs showed better replication in dHBECs than pneumocytes. The differential replication of IBVs in human respiratory tract maybe explained by the binding preference of IBVs towards α2,6-linked sialic acids, which is predominantly found in bronchus but much less in alveoli. Furthermore, similar replication patterns and comparable viral titres were observed between most IBVs and IAVs in both bronchus and lung explants as well as pneumocytes. Interestingly, there was a trend suggesting earlier pro-inflammatory cytokine and antiviral gene induction at 6 hours post infection (hpi) in IBV-infected dHBECs and pneumocytes compared to IAV-infected cells. Besides, there was a significantly higher expression of IFNβ, TNFα, IL6, IL29, CXCL10, RANTES, Mx1 and ISG15 in IBV-infected dHBECs at 24hpi. These results suggested that IBVs differ in their pathogenesis from IAVs in human respiratory tract which may result in differential disease presentation and clinical outcomes. Therefore, more investigation into IBVs is required to acquire better prevention and treatment to alleviate the disease burden.