Natural killer (NK) cells are cytotoxic lymphocytes that detect and kill virally infected cells and malignant cells through the release of lytic granules. The NK receptor NKG2D binds to specific markers on the surface of the target cell, such as MICA on tumor cells, which then triggers adhesion of the NK cell to the target, polarization of the NK cell, and release of its granules. Although activation of NK cells by NKG2D is well characterized, how NKG2D signals to initiate the cytotoxic response is unclear. Segovis et al. showed that engagement of NKG2D on human NK cells triggered the activation of integrins containing β1 or β2 subunits and that integrins were recruited to the contact points (the cytotoxic synapses) between NK cells and target cells transfected to express MICA. Inhibitors of phosphoinositide 3-kinase (PI3K) blocked formation of conjugates between NK cells and target cells, as did short-interfering RNA directed against the adaptor protein CrkL, which plays a role in integrin activation in T cells. CrkL-deficient NK cells also showed decreased NKG2D-dependent release of cytotoxic granules compared with that of control cells. CrkL was required for the polarization of the microtubule-organizing center, a process that targets lytic granules toward the cytotoxic synapse before their release. Stimulation of NKG2D triggered an association between the p85 subunit of PI3K and the N-terminal Src homology 3 (SH3) domain of CrkL. An SH3 mutant form of CrkL could not rescue the cytotoxic defect of CrkL-deficient NK cells. Together, these data uncover a PI3K- and CrkL-dependent mechanism for NKG2D-dependent formation of NK cell–target cell conjugates.
C. M. Segovis, R. A. Schoon, C. J. Dick, L. P. Nacusi, P. J. Leibson, D. D. Billadeau, PI3K links NKG2D signaling to a CrkL pathway involved in natural killer cell adhesion, polarity, and granule secretion. J. Immunol. 182, 6933–6942 (2009). [PubMed]