Cytotoxic T cells (CTLs) and natural killer cells (NKs) both kill virus-infected cells and tumor cells by releasing the cytoxic contents of their lytic granules. We recently demonstrated a role for calcineurin in lytic granule exocytosis in TALL-104 human leukemic CTLs[1]. However, whether calcineurin plays a similar role in NK lytic granule release is not known. We tested whether calcineurin is involved in lytic granule exocytosis in human leukemic NK-92 cells using immunosuppressive drugs that block calcineurin function and by overexpressing a constitutively active calcineurin fusion protein. Our results indicate that calcineurin does play a role in lytic granule exocytosis in NK-92 cells, and suggest that, as was the case in TALL-104 cells, there are likely to be multiple calcium-dependent steps.

Cytotoxic T cells (CTLs) kill target cells by releasing lytic agents via regulated exocytosis. Three signals are known to be required for exocytosis: an increase in intracellular Ca 2+ , activation of protein kinase C (PKC) and activation of extracellular signal regulated signal kinase (ERK). ERK activation required for exocytosis depends on activity of PKC. The simplest possibility is that the sole effect of PKC required for exocytosis is ERK activation. Testing this requires dissociating ERK and PKC activation. We did this using TCR-independent stimulation of TALL-104 human leukemic CTLs. When cells are stimulated with thapsigargin and PMA, agents that increase intracellular Ca 2+ and activate PKC, respectively, PKC-dependent ERK activation is required for lytic granule exocytosis. Expressing a constitutively-active mutant MAP kinase kinase activates ERK independent of PKC. However, activating ERK without PKC does not support lytic granule exocytosis, indicating that there are multiple effects of PKC required for granule exocytosis.