Activation co–activation and co–stimulation of resting human NK cells

摘要

NK cells possess potent perforin– and IFN-γ–dependent effector functions that are tightly regulated. Inhibitory receptors for MHC class I display variegated expression among NK cells, which confers specificity to individual NK cells. Specificity is also provided by engagement of an array of NK cell activation receptors. Target cells may express ligands for a multitude of activation receptors, many of which signal through different pathways. How inhibitory receptors intersect different signaling cascades is not fully understood. This review focuses on advances in understanding how activation receptors cooperate to induce cytotoxicity in resting NK cells. The role of activating receptors in determining specificity and providing redundancy of target cell recognition is discussed. Using Drosophila insect cells as targets, we have recently examined the contribution of individual receptors. Interestingly, the strength of activation is not determined simply by additive effects of parallel activation pathways. Combinations of signals from different receptors can have different outcomes: synergy, no enhancement over individual signals, or additive effects. Cytotoxicity requires combined signals for granule polarization and degranulation. The integrin LFA-1 contributes a signal for polarization, but not degranulation. Conversely, CD16 alone or synergistic combinations, such as NKG2D and 2B4, signal for PLC-γ and PI3K–dependent degranulation.