Immuno-pathogenesis study of local infectious bronchitis virus G1-1 lineage variant showed altered tissue tropism in experimental broiler chickens.

Infectious bronchitis (IB) is an acute contagious disease of poultry caused by infectious bronchitis virus (IBV). This study investigated the immunopathogenesis and tissue tropism of an Indian IBV field isolate (IBV/Chicken/India/IVRI/Rajasthan/01/2023) in experimental broiler chickens. This isolate belongs to the G1-1 lineage and is closely associated with the Mass genotype. 106.23 EID50/0.2 mL of the virus was administered intranasally and intraocularly to the IBV-challenge group, whereas uninoculated allantoic fluid was administered to the control group. Clinical signs, gross and histopathological lesions, immunohistochemistry (IHC), viral load, humoral responses, and the relative expression of immune response genes were evaluated at seven observation points. The infected group showed a significant reduction in weight gain from 3 dpi onwards, with clinical signs of varying severity from 3 to - 11 dpi. Gross lesions and microscopic changes were observed in the nasal turbinates, trachea, lungs, and kidneys, mainly representing epithelial degeneration and necrosis with mononuclear infiltrates. The caecal tonsils also showed microscopic lesions at 7-9 dpi. Absolute viral load estimation in the organs corroborated the lesion severity scores and IHC results. The expression of innate immune responses broadly demonstrated higher expression in the trachea and lungs of the IBV-infected group during the early phase of infection, whereas similar responses were observed in the kidneys and caecal tonsils during the later phases of infection. This study suggests that the given IBV isolate may cause significant production losses in broilers and exhibit tissue tropism for both respiratory and non-respiratory tissues, triggering varying innate and adaptive immune responses.