Phagocytes, initially neutrophils and later macro- phages, ingest and attempt to destroy these microbes, but pathogenic intracellular bacteria are resistant to degradation within phagocytes. Products of these bacteria are recognized by TLRs and cytoplasmic proteins of the NOD-like receptor (NLR) family, resulting in activation of the phagocytes (see Chapter 4). Bacterial DNA in the cytosol stimulates type I interferon responses through the STING pathway.
Intracellular bacteria activate NK cells by inducing expression of NK cell–activating ligands on infected cells and by stimulating dendritic cell and macrophage pro- duction of IL-12 and IL-15, both of which are NK cell– activating cytokines. The NK cells produce IFN-γ, which in turn activates macrophages and promotes killing of the phagocytosed bacteria. Thus, NK cells provide an early defense against these microbes, before the development of adaptive immunity. In fact, mice with severe com- bined immunodeficiency, which lack T and B cells, are able to transiently control infection with the intracellular