Pathogenesis of a Chinese strain of bovine adenovirus type 3 infection in albino guinea pigs

Recombinant hexon protein as a new bovine adenovirus type 3 subunit vaccine candidate

Introduction

Bovine adenovirus (BAdV) type 3 causes respiratory and gastroenteric diseases of varying severity in cattle, particularly newborn calves. Trials have been conducted of a vaccination against the diseases caused by BAdV using both modified live-virus and inactivated-virus preparations in cattle, but no commercial BAdV-3 vaccine has yet reached the market. Therefore, there is an urgent need to develop new, safe, and effective vaccines against BAdV-3.

Material and Methods

Recombinant hexon protein (rhexon) of BAdV-3 was expressed in the E. coli system to evaluate immune response in mice and goats. Antibody responses and cytokine levels were analysed and the effects of administrations of different amounts of recombinant protein compared. Long-term antibody production was evaluated by indirect ELISA, and the total immunoglobulin G secreted by goats and mice immunised with the purified rhexon protein was determined.

Results

The immunised mice had a stronger antibody response than the control group at eight weeks post vaccination. The immunised groups also showed significantly higher (P ˂ 0.05) expression of interferon-γ, interleukin 2 (in mice), and interleukin 21 (in goats) at four weeks. Furthermore, vaccination with rhexon was able to induce long-term antibody production for at least 16 weeks in mice and goats.

Conclusion

The rhexon protein induced immune responses, especially long-term antibody production and T helper 1 cell cytokine production in mice and goats. The immunogenic properties of this protein make it a promising subunit vaccine antigen.

Pathogenicity of a novel bovine adenovirus type 3 with a natural deletion partial fiber gene in BALB/c mice

Objective

A novel Bovine adenovirus type 3 (BAdV-3) with a natural deletion partial fiber gene was isolated in 2020 and named BO/YB24/17/CH. The objective of this study was to understand the pathogenicity of this virus.

Methods

Thiry-two 3-week-old BALB/c mice were divided into three experimental groups and a control group. Mice in the experimental groups were intranasally inoculated with virus, and mice in the control group were inoculated with MDBK cell supernatant. Mice were weighed and clinically examined daily for appearance of any clinical signs. Three infected mice and one control mouse were euthanized at 1, 3, 5, 7, 9, 11, 13, and 15 days after inoculation. Tissue samples were collected for histopathological examination, immunohistochemical staining, and detection of the replication dynamics of virus.

Results

All infected mice developed mild clinical signs such as lethargy, weight loss, loss of appetite, and a rough hair coat, and gross lesions were observed as pulmonary punctate hemorrhage, lobular atrophy and splenomegaly. Histopathological examination revealed thickening of alveolar septa and mildly dilated splenic nodules and blurred red-white medullary demarcation in the spleen. Immunohistochemical results further confirmed that the production of the above lesions was due to viral infection. Importantly, unlike previously reported BAdV-3 detection only in the lungs and trachea, this isolate could be detected in multiple organs such as the heart, liver, spleen, kidney, and even blood by virus isolation and titration and real-time PCR methods.

Clinical significance

This study provides further insight into the pathogenicity of the fiber region deletion strain BO/YB24/17/CH in BALB/c mice, which provides a reference for the prevention and control of BAdV-3 as well as the development of vaccines.

Prevalence and characteristics of a novel bovine adenovirus type 3 with a natural deletion fiber gene

Bovine adenovirus type 3 (BAdV-3) is an important pathogen causing bovine respiratory disease complex (BRDC). From Jun 2016 to Jun 2018, 108 nose swab samples were collected from cattle with BRDC from 11 farms in five cities, and 78.7% (85/108) samples were detected as BAdV-3 positive by Real-time PCR. Interestingly, the sequences of 7/10 fiber (852 bp) and hexon (785 bp) fragments cloned from 10 positive samples from eight farms were clustered into a single branch of the evolutionary tree. A BAdV-3 strain (BO/YB24/17/CH) was successfully isolated. The isolate caused pathological changes of lung, trachea and spleen in BALB/c mice. Notably, 79 amino acid deletions in the shaft domain and 74 unique amino acid mutations were found in the fiber gene of the isolate compared with the available complete sequences for fiber genes in the GenBank database. These characteristics indicated that the isolate may represent a novel fiber genotype of BAdV-3. A pair of specific primers covering the deletion region in the fiber gene was designed to screen the prevalence of BAdV-3 encoding the novel fiber gene. The results showed that 7 of the 10 strains possessed the novel fiber gene, and these novel fiber strains were detected from six farms in which calves were just imported from five provinces, indicating that this BAdV-3 with the natural deletion fiber gene has a wide geographical distribution in China. In conclusion, our results reveal that BAdV-3 is widespread in China and a pathogenic BAdV-3 strain with a novel fiber gene has been detected at high frequency, which is beneficial to understand the prevalence and genetic evolution of BAdV-3.