Critical roles of Rickettsia parkeri outer membrane protein B (OmpB) in the tick host.

Rickettsia parkeri is a pathogen of public health concern and transmitted by the Gulf Coast tick, Amblyomma maculatum . Rickettsiae are obligate intracellular bacteria that enter and replicate in diverse host cells. Rickettsial outer membrane protein B (OmpB) functions in bacterial adhesion, invasion, and avoidance of cell-autonomous immunity in mammalian cell infection, but the function of OmpB in arthropod infection is unknown. In this study, the function of R. parkeri OmpB was evaluated in the tick host. R. parkeri wild-type and R. parkeri ompB STOP ::tn (non-functional OmpB) were capillary fed to naïve A. maculatum ticks to investigate dissemination in the tick and transmission to vertebrates. Ticks exposed to R. parkeri wild-type had greater rickettsial loads in all organs than ticks exposed to R. parkeri ompB STOP ::tn at 12 h post-capillary feeding and after 1 day of feeding on host. In rats that were exposed to R. parkeri ompB STOP ::tn -infected ticks, dermal inflammation at the bite site was less compared to R. parkeri wild-type-infected ticks. In vitro , R. parkeri ompB STOP ::tn cell attachment to tick cells was reduced, and host cell invasion of the mutant was initially reduced but eventually returned to the level of R. parkeri wild-type by 90 min post-infection. R. parkeri ompB STOP ::tn and R. parkeri wild-type had similar growth kinetics in the tick cells, suggesting that OmpB is not essential for R. parkeri replication in tick cells. These results indicate that R. parkeri OmpB functions in rickettsial attachment and internalization to tick cells and pathogenicity during tick infection.