The traffic of the NKG2D/Dap10 receptor complex during natural killer (NK) cell activation

NKG2D is an important activating receptor for triggering the NK cell cytotoxic activity, although chronic engagement of specific ligands by NKG2D is also known to provoke decreased cell surface expression of the receptor and compromised NK cell function. We have studied the dynamics of surface NKG2D expression and how exposure to the specific ligand major histocompatibility complex class I chain-related molecule B (MICB) affects receptor traffic and fate. While in the NKL cell line and "resting" NK cells NKG2D was found principally at the cell surface, in activated primary NK cells an intracellular pool of receptor could also be found recycling to the plasma membrane. Exposure of NK cells to targets expressing MICB resulted in degradation of approximately 50% of total NKG2D protein and lysosomal degradation of the DAP10 adaptor molecule. Consistent with these observations, confocal microscopy experiments demonstrated that DAP10 trafficked to secretory lysosomes in both transfected NKL cells and in activated primary NK cells upon interaction with MICB-expressing target cells. Interestingly, polarization to the synapse of secretory lysosomes containing DAP10 was also observed. The implications of the intracellular traffic of the NKG2D/DAP10 receptor complex for NK cell activation are discussed. We propose that the rapid degradation of NKG2D/DAP10 observed coincident with recruitment of the receptor to the cytotoxic immune synapse may explain the loss of NKG2D receptor expression after chronic exposure to NKG2D ligands.