Endothelial Mechanistic Target of Rapamycin Activation with Different Strains of R. rickettsii: Possible Role in Rickettsial Pathogenesis

Rickettsia rickettsii is an obligate intracellular pathogen that primarily targets endothelial cells (ECs), leading to vascular inflammation and dysfunction. Mechanistic target of rapamycin (mTOR) regulates several cellular processes that directly affect host immune responses to bacterial pathogens. Here, we infected ECs with two R. rickettsii strains, avirulent (Iowa) and highly virulent Sheila Smith (SS) to identify differences in the kinetics and/or intensity of mTOR activation to establish a correlation between mTOR response and bacterial virulence. Endothelial mTOR activation with the highly virulent SS strain was significantly higher than with the avirulent Iowa strain. Similarly, there was increased LC3-II lipidation with the virulent SS strain compared with the avirulent Iowa strain of R. rickettsii. mTOR inhibitors rapamycin and Torin2 significantly increased bacterial growth and replication in the ECs, as evidenced by a more than six-fold increase in rickettsia copy numbers at 48 h post-infection. Further, the knockdown of mTOR with Raptor and Rictor siRNA resulted in a higher rickettsial copy number and the altered expression of the pro-inflammatory cytokines interleukin (IL)-1α, IL-6, and IL-8. These results are the first to reveal that endothelial mTOR activation and the early induction of autophagy might be governed by bacterial virulence and have established the mTOR pathway as an important regulator of endothelial inflammation, host immunity, and microbial replication.