Studies of a polymorphic Ehrlichia ruminantium gene for use as a component of a recombinant vaccine against heartwater

A previously identified polymorphic Ehrlichia ruminantium gene, Erum2510, was investigated to determine its ability to induce protective immunity in ruminants following two different DNA immunisation strategies; DNA-only and a DNA prime/recombinant protein (rprotein) boost immunisation. The DNA-only vaccine was also compared to a cocktail of three polymorphic E. ruminantium (Welgevonden) open reading frames (ORFs) adjacent to Erum2510 in the genome. Weak protection was observed in animals immunised with the pCMViUBs_Erum2510 construct alone, while none of the animals immunised with the DNA cocktail were protected. In contrast, all five animals immunised using a DNA prime/rprotein boost strategy survived challenge, thereby indicating that Erum2510 is a good candidate for inclusion in a recombinant vaccine against heartwater. One drawback of using polymorphic genes is a possible lack of cross-protection between genotypes, therefore the genetic diversity of Erum2510 was investigated to establish the degree of polymorphism among different E. ruminantium stocks. Three distinct genotypes were identified indicating that if this gene is used as a vaccine (prime/boost strategy) the vaccine should include a representative Erum2510 gene from each genotype.