Coxiella burnetii-Infected NK Cells Release Infectious Bacteria by Degranulation

Abstract

Natural killer (NK) cells are critically involved in the early immune response against various intracellular pathogens, including <i>Coxiella burnetii</i> and <i>Chlamydia psittaci</i><i>Chlamydia</i>-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 multiday growth of <i>Coxiella</i>, as is usually observed in other host cell types. <i>C. burnetii</i>-infected NK cells display maturation and gamma interferon (IFN-γ) secretion, as well as the release of <i>Coxiella</i>-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 <i>Chlamydia</i>, expulsed <i>Coxiella</i> organisms largely retain their infectivity and, hence, escape the cell-autonomous self-defense mechanism in NK cells.