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Humoral immune responses in koalas (Phascolarctos cinereus) either naturally infected with Chlamydia pecorum or following administration of a recombinant chlamydial major outer membrane protein vaccine

Khan, SA, Polkinghorne, A, Waugh, C, Hanger, J, Loader, J, Beagley, K & Timms, P 2016, Vaccine, vol. 34, no. 1, pp. 775-782.

Koalas that become infected with Chlamydia pecorum can produce a natural antibody response, but this response is not sufficient to adequately protect against the progression of the infection. Following immunisation with a recombinant chlamydial major outer membrane protein (rMOMP) vaccine, koalas exhibited a stronger humoral immune response to the infection than they were able to without the vaccine. Vaccinated koalas were also able to produce unique antibodies to epitopes, the parts of antigens that are recognised by the immune system, from non-vaccinated koalas. These unique antibodies effectively neutralised the C. pecorum bacterium in an artificial environment.

  Selected koalas from a free-ranging population in south-east Queensland were divided into categories for combinations of naturally infected/not infected with C. pecorum, and vaccinated/not vaccinated with the rMOMP vaccine, to examine the antibody-mediated immune response to C. pecorum in each group. Koalas that were infected with C. pecorum but not vaccinated produced very low levels of antibodies specific to the antigen; in fact, the greater the quantity of C. pecorum bacteria, the lower the neutralising antibody levels. After the rMOMP vaccination was administered, immunised koalas demonstrated an enhanced humoral immune response as the vaccine induced high levels of antigen-specific neutralising antibodies. Neutralising antibody levels for koalas that had previously been infected with C. pecorum and then received the rMOMP vaccination were the highest of all groups. In vitro, the antibodies found to be responsible for neutralising the C. pecorum infection were those that were specific to MOMP epitopes.

  Chlamydial infection is one of the greatest threats to free-ranging koala populations, and therefore the development of a vaccine that can effectively neutralise the pathogen is a priority for the protection of the threatened species. Although we are yet to develop a comprehensive understanding of how a vaccine should interact with the koala’s immune system to effectively combat the effects of chlamydial infection, the findings of this study demonstrate the significance of antibodies in immunity to C. pecorum and will inform future developments in this field.

 

 Summarised by Joanna Horsfall

 

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