Genetic, pathogenic, and antigenic characterization of GX-1, a novel infectious bronchitis virus genotype identified in South Korea

Infectious bronchitis virus (IBV), a highly contagious avian coronavirus, remains a major threat to global poultry production due to its rapid genetic evolution and frequent antigenic shifts. During a nationwide surveillance program in South Korea (2024-2025), we identified a previously unrecognized IBV lineage within dead birds in commercial chicken flocks. The Spike 1 (S1) gene of the IBVs belonging to this lineage exhibited less than 75 % nucleotide identity with those of any previously reported IBV genotypes. Phylogenetic analysis demonstrated that this virus forms a distinct monophyletic clade, with no evidence of recombination detected within the S1 gene. On the basis of these genetic and phylogenetic characteristics, we propose that this virus represents the first lineage of a novel tenth IBV genotype, herein designated GX-1. Complete genome analysis further revealed a high degree of similarity to the recent East Asian GI-19 lineage, except within the Spike (S) gene, suggesting recombination on an East Asian GI-19 genomic backbone. Experimental infection of specific-pathogen-free (SPF) chickens with the proposed GX-1 IBV resulted in milder clinical manifestations, lower lesion severity, and lower mortality compared with infection with the GI-19 IBV isolate. Notably, GX-1 did not induce severe nephritis or renal lesions, indicating a loss of nephropathogenicity, a hallmark of GI-19 IBV infections. Although the virus was detected in respiratory, gastrointestinal, and internal organ tissues, its load and persistence were consistently lower than those of GI-19. Antigenic analysis further revealed limited cross-reactivity between GX-1 and GI-19 IBVs, suggesting distinct serotype characteristics. These results indicate that the proposed GX-1 IBV is a genetically distinct genotype with reduced pathogenicity, altered tissue tropism, and distinct antigenic properties. The emergence of this strain highlights the need for ongoing genomic surveillance and genotype-matched vaccines to manage the evolving IBV landscape.