Researchers Reveal Mystery Behind Important Poultr Vaccine

 

Infectious bronchitis virus (IBV), which causes respiratory disease in chickens, has spikes on its surface which are used to attach to the respiratory cells of chickens. The spike is made of two proteins call S1 and S2. S1 is the primary protein that the chicken's immune system recognizes and to which it produces an immune response. Small changes in the amino acid sequence of the S1 protein can make major differences in how the spike binds to chicken cells and how the chicken's immune system recognizes the virus. The amino acid sequence of the S1 protein is notoriously prone to variation, making it very difficult to provide immunity to the virus by vaccination.

 

The Arkansas (Ark) live IBV vaccines contain a virus isolate called ArkDPI. It has been shown that this vaccine contains virus subpopulations that vary in the amino acid sequence of the S1 protein. Field and laboratory studies have shown that vaccination with ArkDPI IBV vaccine may provide poor protection against field Ark IBV challenge and result in long term infection of chickens by the vaccine virus. Also, it has been shown that the unusual behavior of this vaccine is likely linked to the existence of virus subpopulations in the vaccine.

 

The objective of this study was to determine the effect of the two most common changes in the amino acid sequence of the S1 protein found in the virus subpopulations in the ArkDPI vaccine, mutations at amino acid position 43 and 344. It was found that the change at amino acid position 43 enhanced the virus's ability to bind adult chicken respiratory cells while the change at amino acid position 344 decreased the ability of the virus to attach to adult chicken respiratory cells and enhanced binding to chicken embryo cells.

 

This means that during vaccine production, which is performed in chicken embryos, a virus with the mutation at position 344 will likely reproduce to a higher concentration because of its enhanced affinity for embryo cells. Importantly, it was also found that viruses which carry the mutation at amino acid position 344 do not effectively induce antibodies which protect against field Ark IBV challenge. Therefore, the virus subpopulation with the ability to grow to a high titer in chicken embryos, the virus with the mutation at position 344, is not the most effective subpopulation for providing protection as a vaccine.