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Vaccination of healthy and diseased koalas (Phascolarctos cinereus) with a Chlamydia pecorum multi-subunit vaccine: Evaluation of immunity and pathology

Kollipara, A, George, C, Hanger, J, Loader, J, Polkinghorne, A, Beagley, K & Timms, P 2012, Vaccine, vol. 30, no. 1, pp. 1875-1885.

A multi-subunit chlamydial vaccine has been developed that was safe for administering to both healthy and diseased koalas, and that produced strong antibody and lymphocyte proliferation responses.

  Most wild koala populations are affected by chlamydial infection or disease, and the primary pathogen is Chlamydia pecorum. In koalas, infection with this bacterium can lead to blindness, rhinitis, pneumonia, and urinary and genital tract disease. In vaccine-development studies, the leading vaccine antigen candidate is major outer membrane protein (MOMP) encoded by the ompA gene because it is conserved across multiple Chlamydia species and is highly immunogenic. Use of this antigen in a vaccine is challenged, however, by the genetic diversity of the surface-exposed variable domains of C. pecorum MOMP; thus, a vaccine based on only one C. pecorum MOMP would be unlikely to provide sufficient protection across the multiple strains that infect koalas. It may, therefore, be beneficial to include an additional antigen that is conserved across multiple koala isolates. Such a vaccine was trialled in this study, consisting of the antigens C. pecorum MOMP G and ribonucleotide reductase small chain protein (NrdB) and adjuvanted with immune stimulating complex. A three-dose regimen of the vaccine was administered subcutaneously to groups of healthy and diseased koalas. Trialling the vaccine in diseased koalas is important as vaccination can enhance inflammatory disease if the animal has been previously infected with the target pathogen. In this regard, the majority of previously infected animals did not experience any worsening of their symptoms during the 140-day observation period. Additionally, no koalas had adverse reactions to the vaccine. Both healthy and diseased koalas exhibited strong levels of antibody and lymphocyte proliferation responses following the vaccination regimen, although the neutralising antibody response in plasma was significantly higher in healthy animals than in diseased animals. Although the vaccine was based on one MOMP genotype, vaccinated koalas developed antibodies that recognised all of the four different MOMP types tested against, suggesting that the vaccine may be cross-reactive to genetically diverse antigens. Despite the initial effectiveness of this new vaccine, a three-dose regimen is not practicable for administration to free-ranging koalas; therefore, the efficacy of the vaccine was also examined after only two doses. After two doses, a similar response against both MOMP and NrdB was observed to that demonstrated after three doses, with equivalent levels of plasma neutralising antibody and lymphocyte proliferation. This two-dose regimen may, therefore, be suitable for eliciting the desired immune response in wild koalas.

  The creation of a safe and effective anti-chlamydial vaccine that can feasibly be administered to free-ranging koalas would be a significant development in the conservation of this threatened species. This study is the first to present a multi-subunit vaccine capable of eliciting an antibody response that can neutralise koala C. pecorum infection in vitro. The next steps in developing a vaccination protocol for managing disease in wild koalas are to commence a trial programme, attach radio collars to vaccinated koalas, and then re-capture animals after release to monitor the vaccine’s effectiveness.

 

Summarised by Joanna Horsfall

 

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