Isolation of a more aggressive GVI-1 genotype strain HX of the avian infectious bronchitis virus

Abstract

The avian infectious bronchitis virus (IBV) poses a significant economic threat to the global poultry industry. Although in recent years, the GVI-1 lineage of IBV has proliferated throughout China, there is still a lack of comprehensive studies regarding the pathogenicity of this lineage, particularly with respect to infections of the digestive tract and the antigenic characteristics of the S1 gene. In this study, we investigated the effects of infecting 14-day-old chicks with the HX strain of the GVI-1 lineage over a 14-d period postinfection. Assessment of the pathogenicity of the HX strain included clinical observations; monitoring of body weight, organ viral load, viral shedding, and gross anatomy; histopathological analysis, and bioinformatics-based antigenic characterization of the S1 protein. The findings revealed that compared with previously reported GVI-1 lineage strains, the HX strain is characterized by greater virulence, with infection leading to approximately 26% mortality and extensive severe organ damage, including that of the proventriculus and kidneys. Moreover, at 14 d postinfection, 80% of oral swabs and 100% of cloacal swabs from chickens infected with the HX strain tested positive, indicating a prolonged period of viral shedding relative to that previously reported for GVI-1 lineage strains. Bioinformatic analysis of B-cell epitopes on the S1 protein revealed 7 potential antigenic epitopes. Collectively, our findings in this study provide clear evidence to indicate that compared previously reported GVI-1 lineage strains, chicks infected with the IBV GVI-1 lineage strain HX are characterized by heightened rates of mortality, more pronounced organ damage, and an extended period of viral shedding. This comprehensive characterization highlights the pathogenic potential of the GVI-1 lineage and its capacity to induce severe kidney and proventriculus damage, thereby emphasizing the imperative of early initiated preventive measures. Furthermore, on the basis of our analysis of the antigenic properties of the S1 protein, we have identified 7 potential linear B-cell epitopes, which will provide valuable insights for the development of epitope-based vaccines.

Keywords: GVI-1; HX; S1; antigenicity; infectious bronchitis virus.

Figure 1

Chicken embryo infected with IBV HX strain and PCR identification. (A) Inoculation of chicken embryos with the HX strain resulting in typical IBV infection-associated changes in the embryos; (B) PCR analysis for determining the presence of IBV and potential contaminant viruses including Newcastle disease virus (NDV), avian influenza virus (AIV), and infectious laryngotracheitis virus (ILTV).

Figure 2

A phylogenetic tree based on the S1 gene of the HX strain (red triangle) and 109 reference strains representing genotypes GI to GIX, among which 7 representative strains (black triangles) were used for whole-genome sequence comparisons.

Figure 3

Prediction of epitopes in the S1 protein of the HX strain. (A) Antigenicity prediction using BepiPred (threshold: 1.000). (B) Flexibility prediction using the Karplus and Schulz algorithm (threshold: 1.000). (C) Surface accessibility prediction using the Emini algorithm (threshold: 1.000). (D) Hydrophilicity prediction using the Parker hydrophilicity scale (threshold: 1.678). (E) Protein secondary structure prediction using the Chou–Fasman algorithm (CFSSP). (F) Homology modeling of the S1 protein using SWISS-MODEL, with visualization of the S1 protein and predicted antigenic epitope regions (colored purple) obtained using PyMOL.

Figure 4

Monitoring of clinical symptoms of chickens infected with IBV HX strain. Clinical scores (A), survival rate (B), tracheal ciliary stasis scores (C), and weight gain rate (D) of chickens challenged with the IBV-HX strain.

Figure 5

Gross pathological changes in the glandular stomach, trachea, kidneys, thymus, spleen, and liver of chicks infected with IBV. Lesions are indicated by black arrows.

Figure 6

Histopathological changes in the trachea, kidneys, thymus, spleen, bursa of Fabricius, liver, glandular stomach, and ovary of chicks infected with IBV. Hematoxylin and eosin staining at × 200 magnification. Lesions are indicated by black arrows.

Figure 7

Changes in viral loads in different tissues during the observation period. (A) trachea, (B) kidneys, (C) thymus, (D) spleen, (E) bursa of Fabricius, (F) liver, (G) glandular stomach, and (H) ovary.

Figure 8

Relative expression of immune-related genes in chickens. (A) TLR7, (B) MAVS, (C) MyD88, (D) IFN-α, (E) IL-1β, and (F) IL-6.