Protective immunity against Rickettsia heilongjiangensis in a C3H/HeN mouse model mediated by outer membrane protein B-pulsed dendritic cells

Rickettsia heilongjiangensis is an obligate intracellular bacterium that causes Far-Eastern tick-borne spotted fever. Outer membrane protein B（Omp B） is an important surface protein antigen of rickettsiae. In the present study, the omp B gene of R. heilongjiangensis was divided into four fragments, resulting in four recombinant proteins（OmpB-p1, Omp B-p2, Omp B-p3, and Omp B-p4）. Each Omp B was used in vitro to stimulate murine bone marrow-derived dendritic cells（BMDCs） of C3H/He N mice, and the Omp B-pulsed BMDCs were transferred to naive C3H/He N mice. On day 14 post-transfer of BMDCs, the mice were challenged with R. heilongjiangensis and the rickettsial loads in the mice were quantitatively determined on day 7 post-challenge. Mice receiving BMDCs pulsed with Omp B-p2, Omp B-p3, or Omp B-p4 exhibited significantly lower bacterial load compared with mice receiving Omp B-p1-pulsed BMDCs. CD4＋ and CD8＋ T cells isolated from the spleen of C3H/He N mice receiving BMDCs pulsed with each OmpB were co-cultured with BMDCs pulsed with the respective cognate protein. In flow cytometric analysis, the expression level of CD69 on CD4＋ or CD8＋ T cells from mice receiving BMDCs pulsed with Omp B-p2, OmpB-p3, or Omp B-p4 was higher than that on cells from mice receiving Omp B-p1-pulsed BMDCs, while the expression level of tumor necrosis factor（TNF）-α on CD8＋ T cells and interferon（IFN）-γ on the CD4＋ and CD8＋ T cells from mice receiving Omp B-p2,-p3, or-p4 was significantly higher than on cells from mice receiving Omp B-p1-pulsed BMDCs. Our results suggest that the protective Omp Bs could activate CD4＋ and CD8＋ T cells and drive their differentiation toward CD4＋ Th1 and CD8＋ Tcl cells, respectively, which produce greater amounts of TNF-α and, in particular, IFN-γ, to enhance rickettsicidal activity of host cells.