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Lack of antiviral antibody response in koalas infected with koala retroviruses (KoRV)

Fiebig, U, Keller, M, Moller, A, Timms, P & Denner, J 2015, Virus Research, vol. 198, pp. 30-34.

The purpose of this study was to screen koala sera for antibodies against endogenous koala retrovirus, KoRV-A. Polymerase chain reaction (PCR) identified endogenous KoRV-A from the sera of all 16 koalas tested, with two koalas positive for exogenous KoRV-B. Further protein analyses with Western blot failed to identify antibodies specific for KoRV-A, which could indicate a state of tolerance.

  Antibodies are important antiviral factors and are used as a diagnostic marker for viral infection in many animals, including koalas, pigs and humans. In this study, a PCR analysis using primers specific for KoRV-A and KoRV-B determined that all tested koalas were positive for KoRV-A, while two were positive for KoRV-B. Using a lysate of KoRV-A particles or KoRV-A-specific recombinant proteins, a Western blot analysis determined that all KoRV-A infected koalas had no antibody response to KoRV-A. Neutralising antibodies were also determined to be absent from the koala sera, evident from a neutralisation assay.

  The findings of this study conclude that koalas do not have an antibody response to KoRV-A, which suggests a tolerance to the viral infection. Considering that KoRV-A is an endogenous virus, meaning that it can be spread through the germline, it is not surprising that all koalas tested in the study were positive for KoRV-A infection. As the two koalas positive for KoRV-B were housed in European zoos but previously transported from an American zoo where no KoRV-B infections were reported, it is not known whether the presence of KoRV-B was a result of infection in Europe or recombination of KoRV-A and other endogenous sequences. The lack of antibody response to KoRV-A infection was also seen in previous studies involving other animals such as pigs; however, antibody production against endogenous viruses has been seen in humans, cats and mice. While the reason for this difference is still not entirely clear, it is hypothesised that KoRV-A might be expressed in the early development of the koala, leading the immune system to identify KoRV-A retroviral proteins as ’self’.

  Koalas’ tolerance to endogenous KoRV-A could hinder efforts to develop effective therapies and vaccinations against KoRV-A for koalas. It should be noted that this study did not examine the KoRV-B infected koalas for an antibody response against KoRV-B. Further research is required to determine the effectiveness of a recombinant KoRV vaccine in inducing the production of neutralising antibodies against KoRV-A and KoRV-B in infected and uninfected koalas.

 

Summarised by Daniel Chew

 

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