Antigen-dependent memory NK cells: (Far left panel) Viral-induced NK cell memory is generated during MCMV infection after the cognate recognition of the m157 MCMV protein on infected cells by the activating Ly49H receptor. Memory generation is promoted by IL-12, IL-18, and IL-33 receptor signaling and co-stimulatory signals by DNAM-1, while memory NK cell survival is controlled by BNIP3 and BNIP3L-mediated mitophagy (shown in green). Memory NK cell survival is negatively regulated by BIM, Noxa, and SOCS1 (shown in red). (Middle left panel) Sensitization with haptens or specific antigens, in conjunction with CXCL16, IFNs, and IL-12 is required to generate CXCR6+ memory NK cells. These phenotypically distinct cells may be selected by the cognate recognition of the hapten, a hapten-modified self-protein, or virus-like particle, and develop into memory NK cells, although the receptors responsible for the antigen specificity have not been identified. Antigen-independent memory NK cells: (Middle right panel) Cytokine-induced memory NK cells are generated after exposure to IL-12, IL-15, and IL-18. These cells apparently do not require activating NK receptor triggering for their longevity or enhanced IFN-γ production upon re-stimulation; however, the mechanisms governing their longevity are unknown. (Far right panel) Mature NK cells undergo homeostatic proliferation when transferred into recipient Rag2 × Il2rγ-deficient mice. These NK cells persist and are long-lived, similar to MCMV-induced memory NK cells. The mechanisms driving NK cell homeostatic proliferation and survival in this model remain unknown.