Rational design of vaccines for the control of Campylobacter in chickens
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Campylobacter is the leading cause of bacterial food-borne diarrhoeal disease in the developed world and a significant cause of infant morbidity and mortality in developing countries. Epidemiological studies implicate poultry as a key source of infection, with up to 80% of human cases being attributable to the avian reservoir. An effective vaccine for broilers is predicted to limit the incidence of human campylobacteriosis. Vaccination of chickens with CjaA, either in recombinant form or vectored in live-attenuated Salmonella, has been reported to significantly reduce caecal colonisation by C. jejuni, with more invasive carriers eliciting greater protection. However, protection remains modest and is slow to develop. I therefore sought to improve such vaccines, first by vectoring codon-optimised CjaA in a licensed avian pathogenic E. coli ΔaroA vaccine. In two independent trials, White Leghorn birds were vaccinated subcutaneously on the day of hatch and 14 days later then challenged with C. jejuni M1 at 28 days post-hatch. No protection was observed despite significant induction of CjaA-specific serum IgY, however, a previously described S. Typhimurium ΔaroA vaccine vectoring CjaA also failed to protect. Owing to the variability observed with live CjaA-based vaccines in these and previous studies, other candidate antigens were sought and evaluated as subunits. Twenty-one candidate C. jejuni antigens were cloned and expressed as glutathione-S-transferase (GST) fusions. Nine of these could be purified in adequate soluble quantities to be tested in vivo. The intervals of vaccination and challenge were as above, with GST alone or GSTCjaA acting as negative and positive controls, respectively. Each antigen was administered subcutaneously in TiterMax Gold® adjuvant at the molar equivalent of the doses of GSTCjaA. Repeated testing of initially promising candidates revealed that, when averaged across three independent trials, GST-SodB and GST-FliD induced statistically significant reductions in caecal colonisation of 1-2 log10 colony-forming units of C. jejuni at 48 and 56 days post-hatch compared to negative controls. Induction of antigen-specific serum IgY was measured by enzyme linked-immunosorbent assays using maltose-binding protein fusions to each antigen. This revealed significant induction of antigen specific serum IgY for the majority of the antigens tested, even when no protection was observed. In the SodB- and FliD-vaccinated groups, the peak of antigen-specific serum IgY was not coincident with the onset of protection and the fold-change in specific IgY levels in individual birds did not correlate with caecal Campylobacter numbers. Furthermore, sera from SodB-vaccinated birds failed to detect SodB in the outer membrane or surface of Campylobacter cells, indicating that SodB-specific antibodies are unlikely to be neutralising. Taken together, these studies identified two novel protective antigens that, with further optimisation, could form part of an anti-Campylobacter vaccine for broilers. However further studies are required to define the nature and consequences of immune responses required for protection.