Improving the dromedary antibody response: The hunt for the ideal camel adjuvant

Research has highlighted the benefits of camels as a model of immunological disease and as a production source of single domain antibodies. However, even after decades of research, little investigation has been conducted into the most suitable vaccine adjuvant for camel antibody production. Through different modes of action, adjuvants enhance the immune response to a co-injected antigen, although often at the price of increased local and systemic adverse reactions. In this study, we sought to find the best adjuvant for camel vaccines, capable of inducing high vaccine immunogenicity but without excess reactogenicity. We tested 7 different adjuvants, with a viral (African Horse Sickness Virus; AHSV) or a bacterial (Burkholderia mallei) antigen. Antigen-specific antibody responses and measures of reactogenicity (inflammation, skin thickness and pyrogenicity) were assessed. As previously reported with other species, oil-based emulsion adjuvants such as Gerbu Vet and Montanide ISA enhanced antigen-specific antibody production but suffered from high reactogenicity. By contrast, two newer particulate adjuvants, the polysaccharidebased adjuvant, Advax HCXL™, and the Poly-gamma-glutamic acid adjuvant, Montanide PGA were well tolerated and non-reactogenic. Of all the adjuvants, Advax HCXL™, showed the most favourable overall response, enhancing high levels of specific antibody to both AHSV and B. mallei whereas Montanide PGA only induced antibodies to AHSV but not against B. mallei. This study identified at least one promising non-reactogenic vaccine adjuvant with high potential for future use in camels and related species.