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

Abstract

The obligately intracellular bacterium Ehrlichia 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 against Ehrlichia infection involve generation of Ehrlichia-specific gamma interferon (IFN-␥)-producing CD4 ؉ T cells and cytotoxic CD8؉ T cells, activation of macrophages by IFN-␥, and production of Ehrlichia-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 related Ehrlichia muris to stimulate long-term protective memory T and B cell responses and confer protection in mice. The spleens of mice vaccinated with E. 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 with E. muris had significantly lower bacterial loads than the spleens of mockvaccinated mice. Mice immunized with P28-9, P28-12, P28-19, or the mixture induced Ehrlichia-specific CD4 ؉ Th1 cells. Interestingly, mice immunized with P28-14, orthologs of which in E. chaffeensis and E. 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 the Ehrlichia 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 against Ehrlichia was associated with the generation of Ehrlichia-specific cell-mediated and humoral immune responses.

Ehrlichia chaffeensis Uses Its Surface Protein EtpE to Bind GPI-Anchored Protein DNase X and Trigger Entry into Mammalian Cells

“…Immunization of mice with recombinant P28, which functions as a porin [22], protects mice from E. chaffeensis challenge [21]. Additionally, in a mouse model of HME, immunization of mice with Ehrlichia muris P28 conferred protection from E. muris challenge [25]. As another control, to rule out the possibility that inhibition of binding is a general property of antibody neutralization of any E. chaffeensis cell surface proteins, we examined whether antibody against P28 blocks E. chaffeensis binding and entry.…”

Attenuated Mutants of Ehrlichia chaffeensis Induce Protection against Wild-Type Infection Challenge in the Reservoir Host and in an Incidental Host

“…A DNA vaccine targeting a p28 outer membrane protein homolog (MAP1) in another Ehrlichia species, E. ruminantium, also confers partial protection against infection challenges (16). Similarly, recent studies described that p28 outer membrane proteins of E. muris stimulate mouse memory B and T cell responses, reducing the bacterial burden after 10 to 14 days postchallenge with wild-type infection (17,18). Several studies of Ehrlichia species also demonstrated that attenuated strains offer a high level of protection.…”

Rickettsia rickettsiiouter membrane protein YbgF induces protective immunity in C3H/HeN mice

“…Antigen-specific CD4 C Th1 cells and CD8 C Tcl cells are well known to play key effector functions in the control of intracellular bacterial infection. 25 Antigen-specific CD8 C Tcl cells are critical for the clearance of intracellular bacteria. 26 During Toxoplasma gondii infections, IFN-g-producing CD8 C T cells are the major effectors, and IFN-g-producing CD4 C T cells play a synergistic role in protection against this pathogen.…”