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Antigenic specificity of a monovalent versus polyvalent MOMP based Chlamydia pecorum vaccine in koalas (Phascolarctos cinereus)

Kollipara, A, Wan, C, Rawlinson, G, Brumm, J, Nilsson, K, Polkinghorne, A, Beagley, K & Timms, P 2013, Vaccine, vol. 31, no. 1, pp. 1217-1223.

A comparison of two koala anti-chlamydial vaccines containing either one or multiple target antigens demonstrated that both were equally effective at eliciting a strong cell-mediated and humoral immune response. Both vaccines elicited similar results in terms of developing plasma antibodies, neutralising different strains of C. pecorum infections, and proliferating lymphocytes against different C. pecorum recombinant major outer membrane proteins (MOMPs) and whole chlamydial elementary bodies (EBs). These findings imply that it is possible to develop a vaccine that can offer suitable protection against a range of chlamydial strains.

  A vaccine can either be monovalent or polyvalent. Monovalent vaccines contain only a single strain of the target antigen whereas polyvalent vaccines contain two or more strains of the target antigen. For this study, captive female koalas were divided into four groups. Three groups received monovalent vaccines that were each based on a unique strain of koala C. pecorum, while the fourth group received a polyvalent vaccine that combined two strains. After immunisation with a monovalent vaccine, koalas developed antibodies that recognised a variety of MOMP types in addition to the single antigen used as the base of the vaccine. In an artificial environment, these antibodies proved to be functional as plasma samples of all koalas post-immunisation were equally effective in neutralising infections across different C. pecorum genotypes. All groups demonstrated a similar cell-mediated immune response following immunisation, measured by peripheral blood mononuclear cell (PBMC) concentrations. In all immunisation groups, 2.5 – 8% of these lymphocytes proliferated against intact chlamydial elementary bodies (EBs) of different strains.

  The majority of recent attempts to develop effective anti-chlamydial vaccines have concentrated on MOMP as the target antigen, and a multi-subunit vaccine based in this antigen has been shown to be both safe and effective for use in koalas against C. pecorum infection. This approach is challenged, however, by the genetic diversity of MOMP antigens within C. pecorum. Trials in humans have shown that vaccinations for genital C. trachomatis only offer protection against the strain upon which the vaccine is based, or very closely related strains. Given the variety of strains of C. pecorum that can infect koalas, it is therefore important that a vaccine developed to combat this threat is effective against multiple strains of the infection. This study is the first to report on the effects of a polyvalent anti-chlamydial vaccine in koalas, but also suggests that a monovalent vaccine is capable of producing a similar immune response.

  The findings of this study are significant for the development of an effective vaccine against chlamydial infections which pose one of the greatest threats to free-ranging koala populations. Further research is recommended to determine whether immune responses elicited would be similar in male koalas, whether the vaccination can reduce the infectious load of a naturally infected koala, and the efficacy of a single-dose vaccine.

 

Summarised by Joanna Horsfall

 

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