Studies on variant infectious bronchitis viruses: In vitro and in vivo comparison of virulence, immunopathogenesis and protection

Infectious bronchitis virus (IBV) variants have wide tissue tropism and high pathogenicity which provides a unique opportunity to dissect the molecular mechanisms contributing these differences. After infection of chicken embryo kidney (CEK) cells infected with IBV strains 885, QX and M41, to establish the role of host response signature to envisage the tissue tropism and pathogenicity, the apoptosis and innate immune responses were investigated. The CEK cells infected with 885 and QX shown higher cell death rate than M41. These differences in cell death rate were due to higher levels of apoptosis induction by 885 and QX as compare to M41. We found that up-regulation of toll like receptor 3 (TLR3), melanoma differentiation associated protein 5 (MDA5) and interferon beta (IFN-β) 9 hours after infection corresponded to IBV pathogenicity. In summary, higher apoptosis and elevated levels of TLR3, MDA5 and IFN-β expression correlates to pathogenicity of IBVs in kidney (CEK cells) tissues. To further analyse the differences in early host innate immune responses and apoptosis after IBV infection, we infected chicken tracheal organ cultures (TOCs) with IBV strains 885, QX and M41. We demonstrated that IBV strain M41 induced stronger innate immune response as indicated by up-regulations of TLR3, MDA5 and IFN-β and apoptosis than 885 or QX in TOCs. These observed effects suggest that higher apoptosis together with elevated levels of TLR3, MDA5 and IFN-β expression appears to be correlates to pathogenicity of IBVs in TOCs (respiratory tissue). For corroborating theses in vitro results further work involved the differential immunopathogenesis in chickens infected with IBV strains 885, QX or M41. We confirmed that the histopathological changes, proinflammatory and innate immune gene response could be induced to varied degrees, depending on the IBV strain. Essentially, our results indicates that higher upregulation of expression of proinflammatory cytokines (such as IL-6 and IL-1β) and lipopolysaccharide-induced tumor necrosis factor-alpha factor (LITAF) is induced by M41 in trachea and 885 and QX in the kidney, which seem to mainly coincide with tracheal and renal histopathological lesions caused by these strains, respectively. In addition, elevated levels of TLR3, MDA5 and IFN-β expression correlated with lesion severity in IBV infected trachea and kidney tissues. Overall, this study reports striking differences in the activation of host responses by diffrerent pathogenic IBV strains. Despite vaccination IBV 793B infections continue to be a major threat to the health of all types of chicken in many parts of the world. In this study, the immunopathogenesis of 793B serotype was comprehensively analysed after IBV infection either in vitro (CEK cells and TOCs) or in vivo (SPF chicks). In conclusion this study reports higher levels of apoptosis together with elevated TLR3, MDA5 and IFN-β expression in vitro which correlates to the pathogenicity of 793B in trachea and kidney in vivo. Furthermore, we observed that greater upregulation of proinflammatory cytokines (such as IL-6 and IL-1β) and LITAF is induced by 793B infection in trachea and kidney which appears to be partially implicated lesions caused by IBV. Our findings contribute to the underlying mechanisms of induction of host immune responses to IBV in SPF chicks in vivo as well as in vitro. The project aimed to assess the mucosal, cellular and humoral immune responses induced by two different infectious bronchitis virus (IBV) vaccination regimes and their efficacy against challenge by a variant IBV Q1. Day-old broiler chicks were vaccinated with live H120 alone (Group I) or in combination with CR88 (Group II). Both groups were again vaccinated with CR88 at 14 days of age (doa). One group was kept as a control (Group III). All groups were challenged oculo-nasally with a virulent Q1 strain at 28 doa, and their protection was assessed. Both vaccinated groups gave excellent ciliary protection against the Q1, though group II’s histopathology lesion scores and viral RNA loads in the trachea and kidney showed greater levels of protection compared to group I