Genomics and pathogenesis of the avian coronavirus infectious bronchitis virus

Abstract

Infectious bronchitis virus (IBV) is a member of the family Coronaviridae, together with viruses such as SARS-CoV, MERS-CoV and SARS-CoV-2 (the causative agent of the COVID-19 global pandemic). In this family of viruses, interspecies transmission has been reported, so understanding their pathobiology could lead to a better understanding of the emergence of new serotypes. IBV possesses a single-stranded, non-segmented RNA genome about 27.6 kb in length that encodes several non-structural and structural proteins. Most functions of these proteins have been confirmed in IBV, but some other proposed functions have been based on research conducted on other members of the family Coronaviridae. IBV has variable tissue tropism depending on the strain, and can affect the respiratory, reproductive, or urinary tracts; however, IBV can also replicate in other organs. Additionally, the pathogenicity of IBV is also variable, with some strains causing only mild clinical signs, while infection with others results in high mortality rates in chickens. This paper extensively and comprehensibly reviews general aspects of coronaviruses and, more specifically, IBV, with emphasis on protein functions and pathogenesis. The pathogenicity of the Australian strains of IBV is also reviewed, describing the variability between the different groups of strains, from the classical to the novel and recombinant strains. Reverse genetic systems, cloning and cell culture growth techniques applicable to IBV are also reviewed.

Keywords: IBV; avian coronavirus; cloning; genetics; infectious bronchitis; pathogenesis; protein functions.

Figure 1

Taxonomy of the members of the order Nidovirales, as recently approved by the international Committee on the Taxonomy of Viruses. On the left is the new classification of the complete order, while on the right the classification of the suborder Cornidovirineae. The blue arrow depicts the position of IBV. Adapted from Siddell et al.

Figure 2

Phylogenetic tree showing the old (current) and new (proposed) classification of families (left) and genera (right) in the order Nidovirales. The tree was inferred by maximum likelihood using concatenated multiple sequence alignments of 3CLpro, NiRAN, RdRp, ZBD and HEL1 domains. The blue arrow depicts the position of IBV. Adapted from Siddell et al.

Figure 3

Diagram showing the position of the coding sequences (CDS, grey arrows) and the mature peptides (black arrows) in the IBV genome. The numbers indicate the nucleotide position. The black dotted lines are used to magnify the annotations of the smaller peptides to increase clarity. Nsp, non‐structural protein. *, nsp 11. This genome arrangement is based on the nucleotide sequence of the Australian IBV vaccine strain VicS.

Figure 4

Electron micrographs of chicken embryo kidney (CEK) cells infected with the IBV strain VicS‐v for 48 h. The white arrow heads indicate the location of viral particles in some of the vesicular packets. The grey arrow head indicates the location of the junction of two adjacent cells, which are forming a syncytium. N, nuclei. Scale bars represent 500 nm (white) and 1 μm (black). Photograph, courtesy of Ms Liliana Tatarczuch, Melbourne Veterinary School, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne.

Figure 5

Details of the electron micrographs of chicken embryo kidney (CEK) cells infected with the IBV strain VicS‐v for 48 h. VP, vesicular packets; CM, cellular membrane; DMV, double‐membrane vesicles; M, mitochondria. Examples of DMVs are indicated by the black and white arrow heads. The reticular membranes associated with the vesicles are marked with an asterisk. Scale bars represent 100 nm (A), (B) and (D) and 200 nm (C). Photograph, courtesy of Ms Liliana Tatarczuch, Melbourne Veterinary School, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne.