br Introduction Natural killer NK cells have an unique
Introduction Natural killer (NK) cells have an unique ability to directly lyse transformed, virus-infected or stressed cells without prior sensitization or major histocompatibility complex (MHC) class I restriction , . They were initially defined and shown to be important effectors of the innate immune system and are included in the recently redefined family of innate lymphoid cells (ILC) as type 1 ILC . Upon activation by target cells, NK cells may also produce large amounts of IFNγ. All NK cells express both activating and inhibitory receptors and the balance between the signals triggered by these receptors dictates whether NK cells will become activated and display effector functions . NK cells are characterized by a CD3–CD56+CD16+ and NKp46 natural cytotoxicity receptor (NCR)-positive phenotype and comprise 5–20% of peripheral blood lymphocytes in normal individuals. Morphologically, resting human NK cells have been identified as large granular lymphocytes (LGL) distinct from T and Bacitracin . According to the density of expression of the CD56 receptor, NK cells are divided into two subsets that differ in terms of phenotype, effector function and tissue localization. The CD56dim subset is cytotoxic and represents a majority of peripheral blood NK cells, expresses high levels of CD16, mediates antibody-dependent cell-mediated cytotoxicity (ADCC)  and is characterized by HLA class I specific inhibitory receptors, KIRs, that prevent killing of autologous normal cells. In contrast, a minor immunoregulatory CD56bright NK cell subset (CD16-/lowKIR-/low, perforin low) subset expresses distinct cytokine and chemokine receptors and is poorly cytotoxic. However, these cells are major producers of cytokines in response to IL-12, IL-18 or IL-15 and can therefore regulate the adaptive immune response . Until recently, NK cells have been considered as innate immune cells that lack immunogenic specificity in terms of clonal antigen receptors or memory of prior activation, which are considered as the main attributes of the adaptive immune system. However, increasing evidence indicates that NK cells do exhibit memory that results from prestimulation with different cytokines including IL-12, IL-18 and IL-15 which is manifested by enhanced functional capacities after restimulation . The differential expression of various cytokine receptors during NK cell development implies that different cytokines are relevant for transition from one development stage to the next. Human NK cells develop from hematological stem cells (HSCs) in the bone marrow and complete their differentiation and maturation in peripheral organs, especially lymphoid tissues . The proliferation and differentiation of HCSs requires fms-like tyrosine kinase 3 ligand (FL), kit ligand (KL), IL-3, and IL-7, which interact with their respective receptors. There are five grading stages in NK cell development . “Stage I” or pro-NK cells characterized as CD34+ and c-Kit cytokine receptor positive acquire IL-2/15Rβ cytokine receptor and are able to give rise to IL-15 responsive, “stage II” pre-NK cells. These pre-NK cells differentiate to “stage III” immature NK cells (iNK) that represent committed NK cell lineage. iNK cells subsequently differentiate to “stage IV” NK cells, also defined as CD56bright NK cells. Finally, CD56bright NK cells differentiate into “stage V” mature cytotoxic CD56dim NK cells , , . Complete NK cell development and maturation occurs in peripheral organs, especially lymphoid tissues. During this process, NK cells are rendered tolerant and licensed for functional activation via interaction of NK cell inhibitory receptors with self MHC class I molecules . However, for additional full activation, NK cells need to undergo “priming” by different cytokines, one of the most important being IL-15 that is trans-presented by dendritic cells (DC)s . NK cells are not only found in peripheral blood but also populate different organs as well as most peripheral tissues under steady-state conditions. NK cells enter the tumor site by extravasation through the tumor vasculature. The major chemokine receptor involved in NK cell migration toward the tumor is CXCR3 that binds to the tumor-derived chemokine (C-X-C motif) ligands CXCL9, 10, and 11 , . In melanoma, increased CXCL10 expression results in increased infiltration of adoptively transferred CXCR3-positive expanded NK cells, reflecting the role of CXCL10-induced chemoattraction . However, tumor infiltrating NK cells often display a suppressed function and phenotype. Accumulating evidence indicates that tumor cells, various immunosuppressive cells as well as residing cells produce microenvironmental factors such as cytokines and other immunosuppressive mediators that negatively affect NK cell function (Fig. 1). Immunosuppressive cytokines, transforming growth factor beta (TGFβ), interleukin (IL)-10 and IL-6 inhibit NK cells directly as well as indirectly by affecting antigen presenting cells (APC) or regulatory T cells (Treg)s and myeloid-derived suppressor cells (MDSC)s to produce additional immunosuppressive factors . Attenuated NK cell function in the tumor microenvironment may be restored by stimulatory cytokines such as IL-2, IL-15, IL-18, IL-21 and interferon (IFN)α. Although these cytokines can act independently, owing to some redundancy in their effects on NK cell activity, some of them, like IL-18 and IL-12, may also exert cooperative effects . Interestingly, other cytokines like IL-23 and IL-27 may enhance or suppress NK cell function depending on the context, in particular the presence of a protective acute or aberrant chronic inflammation in the tumor microenvironment (Table 1) .