The S2 Glycoprotein Subunit Determines Intestinal Tropism in Infectious Bronchitis Virus

The molecular basis for the distinct intestinal tropism of infectious bronchitis virus (IBV) strains remains poorly understood. This study identifies the S2 subunit of the spike glycoprotein as the critical determinant conferring duodenal tropism to the IBV CSL strain. Comparative pathogenesis in specific-pathogen-free (SPF) chicks revealed that the CSL strain achieved significantly higher viral titers in the duodenum compared to strains D90, PYG QX1, and XXX QX5. This duodenal replication was associated with severe epithelial inflammation, characterized by upregulation of pro-inflammatory cytokines (IL-6, IL-17A, IL-22, TNF-α, IFN-β, IFN-γ) and disruption of barrier integrity via downregulation of tight junction proteins (Occludin, Claudin-1, ZO-1). Crucially, reverse genetics using the non-enterotropic D90 backbone demonstrated that recombinant viruses carrying the CSL-S2 gene (rD90-ΔS/CSL and rD90-ΔS2/CSL), but not those carrying CSL-S1 (rD90-ΔS1/CSL), replicated efficiently and induced inflammation in the duodenum, phenocopying wild-type CSL. In contrast, renal tropism was independent of the S2 subunit. These findings establish the S2 subunit as both necessary and sufficient for IBV duodenal tropism, uncoupling it from renal pathogenicity. This identifies S2 as a prime molecular target for developing next-generation vaccines against intestinal IBV pathotypes.