Key Points• DOCK2-deficienct NK cells fail to effectively kill leukemia cells in vitro and major histocompatibility complex class I-deficient bone marrow cells in vivo.• Activating NK receptor-mediated Rac activation and the lytic synapse formation are severely impaired in DOCK2-deficient NK cells.Natural killer (NK) cells play an important role in protective immunity against viral infection and tumor progression, but they also contribute to rejection of bone marrow grafts via contact-dependent cytotoxicity. Ligation of activating NK receptors with their ligands expressed on target cells induces receptor clustering and actin reorganization at the interface and triggers polarized movement of lytic granules to the contact site. Although activation of the small GTPase Rac has been implicated in NK cell-mediated cytotoxicity, its precise role and the upstream regulator remain elusive. Here, we show that DOCK2, an atypical guanine nucleotide exchange factor for Rac, plays a key role in NK cell-mediated cytotoxicity. We found that although DOCK2 deficiency in NK cells did not affect conjugate formation with target cells, DOCK2-deficienct NK cells failed to effectively kill leukemia cells in vitro and major histocompatibility complex class I-deficient bone marrow cells in vivo, regardless of the sorts of activating receptors. In DOCK2-deficient NK cells, NKG2D-mediated Rac activation was almost completely lost, resulting in a severe defect in the lytic synapse formation. Similar results were obtained when the Rac guanine nucleotide exchange factor activity of DOCK2 was selectively abrogated. These results indicate that DOCK2-Rac axis controls NK cell-mediated cytotoxicity through the lytic synapse formation. (Blood. 2013;122(3):386-393) Introduction Natural killer (NK) cells are lymphocytes of the innate immune system that play an important role in protective immunity against viral infection and tumor progression but also contribute to rejection of bone marrow (BM) grafts via contact-dependent cytotoxicity.1,2 Activation of NK cells is regulated by a balance of activating and inhibitory signals delivered through the germ-line-encoded receptors that recognize ligands expressed on the surface of target cells.1,2 NK cells express multiple activating receptors including NKG2D, Ly49D, and the low-affinity receptor for immunoglobulin G (IgG), FcgRIIIa. 3 NKG2D recognizes endogenous major histocompatibility complex (MHC) class I-related molecules such as Rae1a-Rae1e, which were originally identified as retinoic acid-inducible proteins but were expressed at high levels primarily in virally infected cells and tumor cells. 4 On the other hand, Ly49D triggers the killing of Chinese hamster ovary (CHO) cells by recognizing hamster MHC class I molecules, 5 and FcgRIIIa is involved in antibody-dependent cell-mediated cytotoxicity (ADCC).1,2 Because these receptors lack cytoplasmic signaling elements, the delivery of activating signal is mediated through the associating transmembrane protein, such as DAP12, FcRg, CD3z, and DAP10, ...