An Entry-Triggering Protein of Ehrlichia Is a New Vaccine Candidate against Tick-Borne Human Monocytic Ehrlichiosis

Ehrlichia chaffeensis is an obligatory intracellular bacterium that causes human monocytic ehrlichiosis, an emerging disease transmitted by the Lone Star tick, Amblyomma americanum. E. chaffeensis outer membrane protein entry triggering protein of Ehrlichia (EtpE) is necessary for bacterial entry into human cells. We investigated the role of EtpE in transmission of the bacteria from tick to human cells and whether or not vaccination with EtpE can prevent transmission of ehrlichiae from ticks to mammals. An antiserum against the recombinant C terminus of EtpE (rEtpE-C), which binds a mammalian cell-surface receptor and triggers bacterial entry, significantly inhibited E. chaffeensis transmission from infected tick cells to human monocytes in culture. Each of five specific-pathogen-free dogs were vaccinated with rEtpE-C along with an immunostimulating complex or were sham vaccinated with the complex alone. Dogs vaccinated with rEtpE-C developed high antibody titers against rEtpE-C and produced interferon-y-secreting cells, as assessed with the ELISpot assay. All 10 dogs were challenged with A. americanum adult ticks infected as nymphs by syringe inoculation with E. chaffeensis. Upon challenge, both the vaccinated and control dogs became infected by day 1 post-tick attachment, but the majority of rEtpE-C-vaccinated dogs rapidly cleared the infection from the bloodstream as soon as day 7, whereas most of sham-vaccinated dogs remained infected at day 35. Peripheral blood leukocytes from vaccinated dogs had significantly elevated interferon-gamma mRNA levels and secreted significantly elevated interferon-y soon after tick attachment. Thus, the EtpE-C vaccine represents the first ehrlichial protein vaccine demonstrated to reduce bacterial infection in mammals upon challenge with infected ticks. IMPORTANCE The incidence of tick-borne diseases has risen dramatically in the pasttwo decades and continues to rise. Discovered in 1986 and designated a nationallynotifiable disease in 1998 by the Centers for Disease Control and Prevention, humanmonocytic ehrlichiosis, which is caused by the bacterium Ehrlichia chaffeensis, is oneof the most prevalent, life-threatening, emerging tick-borne zoonoses in the UnitedStates. We investigated the role of the E. chaffeensis protein EtpE in transmission ofthe bacterium from tick to human cells and in vaccinated dogs with EtpE to assessthe efficacy of vaccination against E. chaffeensis-infected tick challenge. Our resultshelp fill gaps in our understanding of E. chaffeensis-derived protective antigens thatcould be used in a candidate vaccine for immunization of humans to counter ticktransmittedehrlichiosis.