Coxiella burnetii-infected NK cells release infectious bacteria by degranulation

Natural killer (NK) cells are critically involved in the early immune response against various intracellular pathogens, including Coxiella (C.) burnetii and Chlamydia (C.) psittaci. Chlamydia-infected NK cells functionally mature, induce cellular immunity, and protect themselves by killing the bacteria in secreted granules. Here, we report that infected NK cells do not allow intracellular multi-day growth of Coxiella, as is usually observed in other host cell types. C. burnetii-infected NK cells display maturation and IFN-γ secretion, as well as the release of Coxiella-containing lytic granules. Thus, NK cells possess a potent program to restrain and expel different types of invading bacteria via degranulation. Strikingly though, in contrast to Chlamydia, expulsed Coxiella largely retain their infectivity and, hence, escape the cell-autonomous self-defense mechanism in NK cells.