TRPML1-mediated Modulation of Dense-core Granules Tunes Functional Potential in NK Cells

Inhibitory signaling during natural killer (NK) cell education translates into increased responsiveness to activation; however the intracellular basis for functional tuning by inhibitory receptors remains unclear. We found that NK cells expressing self-MHC specific inhibitory killer cell immunoglobulin-like receptors (KIR) show a greater accumulation of dense-core secretory granules, converged closer to the centrosome in resting NK cells, which are released upon recognition of target cells. This discrete morphological phenotype persists in self-KIR+ NK cells independently of transcriptional programs that regulate metabolism and granule biogenesis and indicates an intrinsic role for lysosomal homeostasis in NK cell education. Upon activation, interference of signaling from acidic Ca2+ stores reduced both target-specific Ca2+-flux, degranulation and cytokine production. Furthermore, inhibition of PI(3,5)P2 synthesis or genetic silencing of the PI(3,5)P2-regulated lysosomal Ca2+-channel TRPML1 in primary NK cells led to an increase in granular load and enhanced functional potential. These results suggest a model where continuous unopposed signaling through activating receptors render NK cells hypofunctional through TRPML1-mediated modulation of acidic Ca2+ stores.