Cell biological steps and checkpoints in accessing NK cell cytotoxicity.

Natural killer (NK) cell-mediated cytotoxicity is governed by the formation of a lytic immune synapse in discrete regulated steps, which give rise to an extensive array of cellular checkpoints in accessing NK cell-mediated cytolytic defense. Appropriate progression through these cell biological steps is critical for the directed secretion of specialized secretory lysosomes and subsequent target cell death. Here we highlight recent discoveries in the formation of the NK cell cytolytic synapse as well as the molecular steps and cell biological checkpoints required for this essential host defense process.

NK cell lytic granules are highly motile at the immunological synapse and require F-actin for post-degranulation persistence.

The formation of a dynamic, actin-rich immunological synapse (IS) and the polarization of cytolytic granules toward target cells are essential to the cytotoxic function of NK cells. Following polarization, lytic granules navigate through the pervasive actin network at the IS to degranulate and secrete their toxic contents onto target cells. We examined lytic granule motility and persistence at the cell cortex of activated human NK cells, using high-resolution total internal reflection microscopy and highly quantitative analysis techniques. We illustrate that lytic granules are dynamic and observe substantial motility at the plane of the cell cortex prior to, but not after, degranulation. We also show that there is no significant change in granule motility in the presence of Latrunculin A (which induces actin depolymerization), when added after granule polarization, but that there is a significant decrease in lytic granule persistence subsequent to degranulation. Thus, we show that lytic granules are highly dynamic at the cytolytic human NK cell IS prior to degranulation and that the persistence of granules at the cortex following exocytosis requires the integrity of the synaptic actin network.