Immunization with Ehrlichia P28 Outer Membrane Proteins Confers Protection in a Mouse Model of Ehrlichiosis

The obligately intracellular bacteriumEhrlichia chaffeensis that resides in mononuclear phagocytes is the etiologic agent of human monocytotropic ehrlichiosis (HME). HME is an emerging and often life-threatening, tick-transmitted infectious disease in the United States. Effective primary immune responses againstEhrlichia infection involve generation ofEhrlichia -specific gamma interferon (IFN-γ)-producing CD4+ T cells and cytotoxic CD8+ T cells, activation of macrophages by IFN-γ, and production ofEhrlichia -specific antibodies of the Th1 isotype. Currently, there are no vaccines available against HME. We evaluated the ability of 28-kDa outer membrane proteins (P28-OMP-1) of the closely relatedEhrlichia muris to stimulate long-term protective memory T and B cell responses and confer protection in mice. The spleens of mice vaccinated withE. muris P28-9, P28-12, P28-19, or a mixture of these three P28 proteins (P28s) using a DNA prime-protein boost regimen and challenged withE. muris had significantly lower bacterial loads than the spleens of mock-vaccinated mice. Mice immunized with P28-9, P28-12, P28-19, or the mixture inducedEhrlichia- specific CD4+ Th1 cells. Interestingly, mice immunized with P28-14, orthologs of which inE. chaffeensis andE. canis are primarily expressed in tick cells, failed to lower the ehrlichial burden in the spleen. Immunization with the recombinant P28-19 protein alone also significantly decreased the bacterial load in the spleen and liver compared to those of the controls. Our study reports, for the first time, the protective roles of theEhrlichia P28-9 and P28-12 proteins in addition to confirming previous reports of the protective ability of P28-19. Partial protection induced by immunization with P28-9, P28-12, and P28-19 againstEhrlichia was associated with the generation ofEhrlichia- specific cell-mediated and humoral immune responses.