CD56 regulates human NK cell cytotoxicity through Pyk2
Natural killer (NK) cells are innate immune cells that control viral infection and tumorigenic cell growth through targeted cell lysis and cytokine secretion. Human NK cells are classically defined as CD56+CD3- in peripheral blood. CD56 is neural cell adhesion molecule (NCAM1), and despite its ubiquitous expression on human NK cells, the role of CD56 in human NK cell cytotoxic function has not been fully explored. In non-immune cells, NCAM can induce signaling, mediate adhesion, and promote exocytosis, in part through interactions with focal adhesion kinase (FAK). Here we describe the generation and use of CD56-deficient human NK cell lines to define a novel requirement for CD56 in target cell lysis. Namely, we demonstrate that deletion of CD56 on the NK92 cell line led to impaired cytotoxic function against multiple susceptible target cell lines. Deletion of CD56 in a second NK cell line, YTS cells, led to a less severe cytotoxicity defect but impairment in cytokine secretion. Confocal microscopy of wild-type and CD56-KO NK92 cells conjugated to susceptible targets revealed that CD56-KO cells failed to polarize during immunological synapse (IS) formation and had severely impaired exocytosis of lytic granules at the IS. Phosphorylation of the FAK family member Pyk2 at tyrosine 402 was decreased in NK92 CD56-KO cells, demonstrating a functional link between CD56 and IS formation and signaling in human NK cells. Cytotoxicity, lytic granule exocytosis, and the phosphorylation of Pyk2 were rescued by the reintroduction of NCAM140 (CD56), into NK92 CD56-KO cells. These data highlight a novel functional role for CD56 in stimulating exocytosis and promoting cytotoxicity in human NK cells.