Myosin IIA is required for cytolytic granule exocytosis in human NK cells

Formation and function of the lytic NK-cell immunological synapse

The natural killer (NK)-cell immunological synapse is the dynamic interface formed between an NK cell and its target cell. Formation of the NK-cell immunological synapse involves several distinct stages, from the initiation of contact with a target cell to the directed delivery of lytic-granule contents for target-cell lysis. Progression through the individual stages is regulated, and this tight regulation underlies the precision with which NK cells select and kill susceptible target cells (including virally infected cells and cancerous cells) that they encounter during their routine surveillance of the body.

The killer's kiss: the many functions of NK cell immunological synapses

Natural killer (NK) cells comprise a subset of lymphocytes involved in protection against microbial pathogens and tumors. NK cells recognize host cells that are missing MHC class I molecules and eliminate them through localized delivery of lytic granules. The majority of NK cell effector functions require direct cell-to-cell contact. Binding to a target cell is accompanied by creation of complex structures at the cell-cell interface known as immunological synapses. Recent studies have contributed immensely to the characterization of several types of NK cell immunological synapses and understanding of the variety of processes originating at this intriguing place. The emerging picture illustrates NK cell immune synapses as the sites of highly complex regulation of NK cell activity.

Line of attack: NK cell specificity and integration of signals

Natural killer (NK) cells possess potent cytolytic activity and secrete immune modulating cytokines. The large repertoire of NK cell receptors provides versatility for the identification of infected and transformed cells and for their elimination by NK cells. NK cell responses also stimulate and regulate the adaptive arm of the immune system. We review current knowledge about the molecular specificity of NK cell receptors and about the regulation of NK cell effector functions upon encounter with target cells. Mechanisms of recognition, interplay among receptors, signal integration, and the dynamic fine-tuning of NK cell responses are discussed. New insights into the molecular checkpoints for NK cell effector function are highlighted, and underlying reasons for the complexity in NK cell recognition and signaling are proposed.

WIP is essential for lytic granule polarization and NK cell cytotoxicity

Natural killer (NK) cells play important roles in host immunity by killing virus-infected and tumor cells. Killing of the target cell is achieved by formation of an immune synapse and localized secretion of lytic granules containing perforin and granzymes. Here, we demonstrate that Wiskott-Aldrich syndrome protein (WASp)-interacting protein (WIP), important in generation of a large complex of proteins involved in actin cytoskeleton rearrangements, is indispensable for NK cell cytotoxicity. WIP knockdown completely inhibited cytotoxicity, whereas overexpression of WIP enhanced NK cell cytolytic ability. WIP was found to colocalize with lytic granules. WIP segregated to the lysosomal fraction, where granzyme B activity was also found, and the interaction between WIP and granules was independent of WASp. Importantly, WIP knockdown inhibited polarization of lytic granules to the immune synapse, but not conjugate formation. These results indicate that WIP is involved in lytic granule transport and is essential for regulation of NK cell cytotoxic function.