Evaluation of E. ruminantium Genes in DBA/2 Mice as Potential DNA Vaccine Candidates for Control of Heartwater

: Heartwater caused by the rickettsia Ehrlichia ruminantium (E. ruminantium) is an acute and fatal tick-borne disease of domestic and some wild ruminants. A user-friendly vaccine does not exist. We selected and tested nine genes of E. ruminantium for protection against challenge in a DBA/2 mouse model, in order to identify candidate genes for incorporation into a recombinant vaccine. Of the nine DNA vaccine constructs tested, four DNA constructs 14HWORF1/VR1012, 14HWORF2/VR1012, 27HWORF1/VR1012, and HSP58/VR1012 were not protective and were excluded from the study. The remaining five DNA constructs—MAP2/ VR1012, 1HWORF3/ VR1012, 4HWORF1/ VR1012, 18HWORF1/ VR1012, and 3GDORF3/ VR1012—offered partial protection against lethal challenge demonstrated by reduced mortalities compared to control groups. Protection was augmented when DNA primed mice were boosted with a respective homologous recombinant protein. Protection in these five groups was associated with the induction of cell-mediated or T helper 1 (Th1) type of immune responses characterized by the production of large amounts of interferon-gamma and interleukin-2 in in vitro proliferation assays using E. ruminantium antigens for stimulation. These responses were enhanced when the DNA-vaccinated DBA/2 mice were boosted with specific homologous recombinant protein vaccination. In a preliminary follow-up study, protection conferred by DNA vaccination with individual gene constructs was not enhanced when the protective constructs were administered in combination (including the map-1 gene of E. ruminantium). Further evaluation of these and other untested DNA constructs is necessary to optimize their expression in vivo in the presence of molecular adjuvants, such as the IFN-gamma gene, GM-CSF gene, IL-12 gene, and CpG motifs to fully evaluate their protective value.