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 (OmpB) is an important surface protein antigen of rickettsiae. In the present study, the ompB gene of R. heilongjiangensis was divided into four fragments, resulting in four recombinant proteins (OmpB-p1, OmpB-p2, OmpB-p3, and OmpB-p4). Each OmpB was used in vitro to stimulate murine bone marrow-derived dendritic cells (BMDCs) of C3H/HeN mice, and the OmpB-pulsed BMDCs were transferred to naïve C3H/HeN 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 OmpB-p2, OmpB-p3, or OmpB-p4 exhibited significantly lower bacterial load compared with mice receiving OmpB-p1-pulsed BMDCs. CD4(+) and CD8(+) T cells isolated from the spleen of C3H/HeN 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 OmpB-p2, OmpB-p3, or OmpB-p4 was higher than that on cells from mice receiving OmpB-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 OmpB-p2, -p3, or -p4 was significantly higher than on cells from mice receiving OmpB-p1-pulsed BMDCs. Our results suggest that the protective OmpBs 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.