Safety and efficacy of an attenuated Chinese QX-like infectious bronchitis virus strain as a candidate vaccine

Abstract

Infectious bronchitis (IB) is a highly contagious respiratory and urogenital disease of chickens caused by infectious bronchitis virus (IBV). This disease is of considerable economic importance and is primarily controlled through biosecurity and immunization with live attenuated and inactivated IB vaccines of various serotypes. In the present study, we tested the safety and efficacy of an attenuated predominant Chinese QX-like IBV strain. The results revealed that the attenuated strain has a clear decrease in pathogenicity for specific-pathogen-free (SPF) chickens compared with the parent strain. Strain YN-inoculated birds had clinical signs of varying severity with 30% mortality, while the attenuated group appeared healthy, with less tissue damage. The attenuated strain also had relatively low tissue replication rates and higher antibody levels. The superior protective efficacy of the attenuated strain was observed when vaccinated birds were challenged with a homologous or heterologous field IBV strain, indicating the potential of the attenuated YN (aYN) as a vaccine. Producing a vaccine targeting the abundant serotype in China is essential to reducing the economic impact of IB on the poultry industry.

Analysis of antigenicity and pathogenicity reveals major differences among QX-like infectious bronchitis viruses and other serotypes

“…The M41 and 4/91 strains belong to genotype Mass-type and 4/91-like, respectively. The YN strain (GenBank accession no: JF893452) is a YN-type originating from a H120 vaccinated broiler flock with a death rate of 30% in the Yunnan Province of China Zhao et al, 2015). The GD strain was isolated from IB-affected chickens in the Guangdong Province of China, and is closely related to the TW-type according to phylogenetic analysis .…”

“…The inaccuracy of the coronavirus RNA-dependent RNA polymerase and high frequency of genetic changes in IBV (e.g., gene insertion, mutation, deletion, and reconstruction), promote the emergence of new variant strains, genotypes and serotypes of IBV, and these are continuously reported (Cavanagh et al, 1986;Chen et al, 2017;Yan et al, 2016;Zhao et al, 2016). IBV immunity is serotype-specific, and Mass-type vaccines (H120) can be ineffective against infection with different IBV isolates in China (Chen et al, 2017;Sun et al, 2011;Zhao et al, 2015). Therefore, the continuous testing of pathogenicity in new isolates, epidemic serotype determination, and typing new IBV strains remains crucial for better epidemiological understanding and control of IB in regions and/or countries.…”

Successful establishment of a reverse genetic system for QX-type infectious bronchitis virus and technical improvement of the rescue procedure

“…Reverse transcription was conducted at 37 ℃ for 15 min, with 3 μg of total RNA using the PrimeScript™ RT reagent Kit (Takara Bio Inc, Beijing, China). The cDNA samples were submitted for SYBR Green I real-time RT-qPCR to detect the viral load for IBV N gene as described before (Zhao et al, 2015). All quantitative PCR reactions were carried out in triplicate and repeated at least twice, and the expression of IBV was calculated according to a standard curve.…”

“…Finally, we assessed growth kinetics of IBV-rYN and IBV-YN in ECEs and CEK cells. For the replication profile comparison in ECE, 200 μl PBS containing 10 2.0 EID 50 of IBV-rYN or IBV-YN were inoculated into the allantoic cavities of 10-day old ECEs, and the allantoic fluid of five eggs from each group were harvested at the time points of 12, 24, 36, 48, 60 and 72 hpi and pooled for the real-time PCR detection assay (Zhao et al, 2015). As shown in Fig.…”

“…Zhao, et al Virus Research 272 (2019) 197726 were harvested at the time points of 6, 12, 24, 36, 48, and 60 hpi. Viral load was determined by a real-time PCR detection assay quantifying IBV N gene as previously described (Zhao et al, 2015). As shown in Fig.…”