Direct and indirect NK cell responses during bacterial invasion. Direct NK cell activation by bacterial PAMPs was mediated through TLR and led to the secretion of cytokines (IFN-γ) and α-defensins that contribute, respectively, to activate surrounding cells (eg, macrophages, T lymphocytes) and to destroy (†) pathogens. In turn, surrounding cells that could be directly activated by PAMPs or environmental cytokines produced cytokines that synergized (IL-1β, IL-2, IL-12, IL-15; blue line) or inhibited (IL-10; red line) PAMP-mediated activation of NK cell functions.