Targeted Prevention of Human Ehrlichiosis

The incidence of tick-borne diseases has risen dramatically in the past two decades, and continues to rise. Human monocytic ehrlichiosis caused by Ehrlichia chaffeensis (Ech) is one of the most prevalent, life- threatening, emerging tick-borne zoonoses in the US. Ech is an obligatory intracellular bacterium of the order Rickettsiales. Therapy of choice is the broad-spectrum antibiotic doxycycline, which is effective only if initiated early. Currently there is no FDA-approved vaccine for Ech. Our long-term goal is to develop an evidence- based vaccine approach to effectively protect humans by targeting multiple critical steps of the rickettsial infection cycle. Toward this goal, we identified four Ech surface-exposed proteins that have known functions required for Ech survival, and that also lack homology to human proteins, OMP-1/P28, Entry triggering protein of Ehrlichia (EtpE), and VirB2. OMP-1/P28s are immunodominant surface-exposed outer membrane proteins that have porin activity essential for bacterial nutrient acquisition. P28 and OMP-1B are predominantly expressed in mammals and ticks, respectively. EtpE is an invasin that uses its C-terminus (EtpE-C) to bind the host cell receptor to trigger Ech entry. We have shown that the type IV secretion system (T4SS) is essential for Ech survival within the host cell. VirB2 is a T4SS pilus protein that is part of the T4SS machinery. Immunization of mice with recombinant P28, EtpE, or VirB2 proteins generated Ech-specific antibody responses that prevented Ech infection. These data support our premise that these proteins serve as rational vaccine candidates for targeting non-overlapping processes in Ech infection of mammalian host cells. DNA vaccines offer a number of potential advantages over traditional vaccines, including the stimulation of both humoral and T-cell-mediated responses, improved vaccine stability, the absence of any infectious agent, and the relative ease of packaging multi-components and large-scale manufacture. We showed the feasibility of an Ech DNA vaccine in dogs by safely immunizing dogs with the DNA vaccines by percutaneous needle-free jet injection and demonstrating humoral and cell-mediated immune responses to the DNA vaccines. Our hypothesis is immunization with plasmid DNA vaccine encoding P28, OMP-1B, EtpE and VirB2 singly or in combination prevents Ech transmission from ticks to mammals. To test this hypothesis, our Specific Aims are: 1. To construct DNA vaccines encoding P28, OMP-1B, EtpE-C, and VirB2, determine the development of humoral and cell-mediated immune responses in immunized mice, and evaluate protection of immunized mice from infection with Ech cultured in tick cells. 2. To test if immunization of dogs with P28, OMP-1B, EtpE-C and VirB2 can prevent Ech transmission from infected ticks. The immediate outcomes of the proposed studies will be to provide proof-of-principle for a DNA vaccine approach to the Ech vaccine candidates for blocking of Ech transmission from ticks to dogs. The long-term outcome will be development of an anti-infective vaccine against HME in humans that does not cause adverse effects.