Characterization of the Protective Efficacy Against QX Strain of a Recombinant Infectious Bronchitis Virus With H120 Backbone and QX Spike Gene

Abstract

Infectious bronchitis virus (IBV) has been prevalent in chicken farms for many years, and its control relies on extensive vaccine administration. The continuous emergence of new variants and the low cross-protection efficiency prompt the development of new vaccines. In this study, we develop a reverse genetics technique based on the classical vaccine strain H120 genome, via in vitro ligation method. Using the H120 genome as the backbone, we constructed the recombinant virus rH120-QX(S) by replacing the H120 S gene with the QX S gene, a prevalent strain in China. Biological characteristics of the rH120-QX(S) virus, such as 50% egg lethal dose (ELD50), 50% egg infectious dose (EID50), dwarf embryo, growth curve, and genetic stability, are measured, which are comparable to the parental virus H120. There are no clinical symptoms and tissue lesions in the trachea and kidney in the rH120-QX(S)-infected specific-pathogen-free (SPF) chickens, demonstrating that this recombinant virus does not confer pathogenicity. Furthermore, protection studies show that there is 100% homologous protection of rH120-QX(S) to the virulent QX strain, as shown by the absence of clinical signs and no lethality. Taken together, our results demonstrate that swapping the S gene onto the H120 genetic backbone is a precise and effective way to produce genetically defined IBV vaccine candidates.

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“…For IBV, SPF chicken embryos were inoculated, and numbers of embryos which were dwarf at 5 d. p. i. were calculated as 50% embryo infectious doses (EID 50 ). [ 50 ] HA assay was used for IAV and NDV. [ 51 ] Log 10 TCID 50 and Log 10 EID 50 per 0.1 mL were calculated using the Reed‐Muench method.…”