Recombinant Ehrlichia TRP75 Rescues Molting Failure of Ticks Infected with TRP75 Mutant Ehrlichia

Ehrlichia species are obligatory intracellular bacteria that exist in nature through a cycle of infection and transmission between ticks and their mammalian hosts, potentially causing a severe febrile illness called ehrlichiosis in humans and animals. Roles of Ehrlichia molecules in tick-mammal Ehrlichia transmission are mostly unknown. Tandem repeat protein (TRP) 75 is a lipoprotein conserved among Ehrlichia species. We identified and cloned a Himar1 transposon insertional mutant of Ehrlichia japonica that lacks C-terminal intrinsically disordered region of TRP75 (TRP75{Delta}C). This mutant was competent in infecting and replicating in both mammalian and tick cells in culture, and causing fatal illness similar to wild-type (WT) E. japonica. Upon feeding on TRP75{Delta}C and WT Ehrlichia-infected mice, both larval- and nymphal-stage of Ixodes scapularis ticks ingested similar amounts of blood meal as those fed on uninfected mice, and acquired similar levels of infection. However, most of TRP75{Delta}C Ehrlichia-infected ticks failed to complete molting, in contrast to successful molting of WT Ehrlichia-infected and uninfected ticks by 10 weeks post-feeding. In both WT Ehrlichia-infected and uninfected ticks, ~1,000-fold exponential increase in ecdysone receptor (EcR) mRNA occurred at 15 days post-feeding, whereas EcR mRNA was not upregulated in TRP75{Delta}C Ehrlichia-infected ticks. Upon microinjection of recombinant full-length TRP75 protein into engorged ticks infected with TRP75{Delta}C Ehrlichia, molting efficiency was ~90% restored, and there was significantly increased EcR mRNA compared with solvent microinjection control. This suggests that the endoparasitic bacterial molecule TRP75 manipulates the molting success of infected ticks and EcR mRNA levels for the survival of bacteria-hosted ticks and transstadial transmission.