Contribution of NK cells to the innate phase of host protection against an intracellular bacterium targeting systemic endothelium

We investigated the mechanisms by which natural killer (NK) cells mediate innate host defense against infection with an endothelium-targeting intracellular bacterium, Rickettsia. We found that a robust Rickettsia-induced innate response in resistant mice cleared the bacteria early in the infection and was associated with significantly higher frequencies of splenic interferon (IFN)-γ (+) CD8 ⁺ T cells and cytotoxic NK cells compared with susceptible mice. More importantly, NK cell-deficient Rag ^-/-γc ^-/- animals displayed significantly increased susceptibility to Rickettsia infection compared with NK cell-sufficient Rag ^-/- mice, as evidenced by impaired bacterial clearance, early development of severe thrombosis in the liver, and a decreased serum level of IFN-γ. Furthermore, the lack of NK cells also impaired host resistance of CB-17 scid mice to Rickettsia, similar to what was observed in Rag ^-/-γc ^-/- mice. Interestingly, perforin deficiency in Rag ^-/-Prf1 ^-/- mice resulted in greater thrombosis and insignificantly different systemic levels of IFN-γ compared with Rag ^-/- mice, suggesting that perforin, which is mainly produced by NK cells, is involved in the prevention of vascular damage. Together, these findings reveal that NK cells mediate the innate phase of host protection against infection with rickettsiae, most likely via IFN-γ production. Furthermore, NK cells are involved in preventing rickettsial infection-induced endothelial cell damage, possibly via perforin production.