No abstract

The stages of human natural killer (NK) cell differentiation are not well established. Culturing CD34 ؉ progenitors with interleukin 7 (IL-7), IL-15, stem cell factor (SCF), FLT-3L, and murine fetal liver cell line (EL08.1D2), we identified 2 nonoverlapping subsets of differentiating CD56 ؉ cells based on CD117 and CD94 (CD117 high CD94 ؊ and CD117 low/؊ CD94 ؉ cells). Both populations expressed CD161 and NKp44, but differed with respect to NKp30, NKp46, NKG2A, NKG2C, NKG2D, CD8, CD16, and KIR. Only the CD117 low/؊ CD94 ؉ population displayed cytotoxicity and interferon-␥ production. Both populations arose from a single CD34 ؉ CD38 ؊ Lin ؊ cell and their percentages changed over time in a reciprocal fashion, with CD117 high CD94 ؊ cells predominating early and decreasing due to an increase of the CD117 low/؊ CD94 ؉ population. These 2 subsets represent distinct stages of NKcell differentiation, since purified CD117 high IntroductionNatural killer (NK) cells are CD56 ϩ CD3 Ϫ innate immune effector cells that recognize target cells that have undergone cellular stress, such as malignant transformation or viral infection. 1 Individual NK cells display a diverse repertoire of activating and inhibitory receptors, including the killer immunoglobulin-like receptors (KIRs), natural cytotoxicity receptors (NKp30, NKp44, and NKp46), and c-lectin receptors (NKG2A/ CD94, NKG2C/CD94, NKG2D, and CD161). The summation of activating signals, when not opposed by inhibitory signaling, leads to the release of granzymes and perforin and target cytolysis. 2 NK-cell activation also results in the elaboration of proinflammatory cytokines, including interferon ␥ (IFN-␥), 3,4 stimulating other compartments of the immune system.The stages of human NK-cell development are not well characterized. Since human NK cells can be differentiated from CD34 ϩ hematopoietic progenitor cells (HPCs) in vitro, such stages of differentiation may be elucidated. Early studies showed that interleukin 2 (IL-2) could induce the differentiation of NK cells from CD34 ϩ HPCs. 5,6 More recently, IL-15 has been found to be critical for NK-cell development since IL-15 Ϫ/Ϫ and IL-15R␣ Ϫ/Ϫ mice show a near-complete absence of NK cells. 7,8 Accordingly, culture of human HPCs with IL-15 gives rise to NK cells, 9 but other cytokines, such as IL-3, IL-7, stem cell factor (SCF), and FLT-3L increase the efficiency of in vitro NK-cell differentiation. [10][11][12] Both SCF and FLT-3L induce the expression of IL-15R␣ mRNA in developing progenitors, inducing IL-15 responsiveness. 11 CD34 ϩ cells cultured with cytokines and stromal cells give higher yields of NK cells that more accurately reflect peripheral blood NK cells with respect to KIR expression. [12][13][14][15] To prevent auto-aggression, NK cells express inhibitory receptors that recognize self major histocompatibility complex (MHC) class I. The current paradigm of NK-cell tolerance suggests that every NK cell must express at least one self MHC-specific inhibitory receptor, since this has been demonstrated at both t...