Natural Killer (NK) cells are cytotoxic lymphocytes that can recognize and kill virally infected or tumour transformed cells by the secretion of cytolytic granules containing perforin. An individual NK cell can kill several target cells sequentially. Each target cell can trigger NK cell activation via different activating ligands and here we report that the order in which ligands are encountered affects the NK cell response. When NK cells are repeatedly activated via their Fc receptor CD16, with the therapeutic antibody rituximab, perforin secretion decreases with each stimulation. However, perforin secretion is restored to its initial level upon subsequent activation by MICA, which ligates NKG2D. Repeated stimulation of NK cells via MICA also decreases the degranulation capacity of NK cells but, strikingly, this effect cannot be rescued by a subsequent stimulation with rituximab. The strength of perforin secretion is also translated to killing of Daudi target cells, expressing different ligands. When Daudi, opsonised with rituximab is the first target NK cell encounters, the sequential killing of another opsonised rituximab or Daudi, expressing MICA will not be affected. But, when Daudi-MICA is met first, the consecutive killing of Daudi-MICA as well as Daudi-rituximab will be impaired. We found that the mechanism underlying these differential outcomes involves shedding of CD16, which occurs upon NK cell activation through both, CD16 and NKG2D. Shedding of CD16 renders the cells insensitive to further activation via that receptor but they remain competent for further activation through NKG2D. Interestingly, however, we also identified the beneficial role of CD16 shedding for NK cell serial killing. NK cells are more motile on rituximab-coated surfaces than on MICA-coated surfaces and their migration speed decreases upon inhibition of CD16 shedding. Moreover, the inhibition of CD16 shedding also prevents the NK cell detachment from rituximab opsonised Daudi cells. Thus, the shedding of the receptor can serve to augment NK cell motility to move between target cells. Efficient NK cell detachment also correlated with their increased survival. Finally, we report that CD16 is constitutively organised in small, dense nanoclusters and that the ligation with rituximab does not affect their spatial distribution. Despite the shedding of the receptor, leading to less protein molecules at the surface, the area of these clusters remains the same. Together these data suggest that CD16 shedding hinders NK cell cytotoxicity against opsonised targets, but promotes their movements between different targets. Thus, receptor shedding is important for efficient NK cell serial killing. Manipulation of CD16 shedding, perhaps by boosting its recovery, might therefore represent an important target for NK cell-based therapies including treatments with therapeutic antibodies.

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