A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity.

KEY POINTS: Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are required to eliminate cancer cells. We analysed the Ca2+ dependence of CTL and NK cell cytotoxicity and found that in particular CTLs have a very low optimum of [Ca2+ ]i (between 122 and 334 nm) and [Ca2+ ]o (between 23 and 625 μm) for efficient cancer cell elimination, well below blood plasma Ca2+ levels. As predicted from these results, partial down-regulation of the Ca2+ channel Orai1 in CTLs paradoxically increases perforin-dependent cancer cell killing. Lytic granule release at the immune synapse between CTLs and cancer cells has a Ca2+ optimum compatible with this low Ca2+ optimum for efficient cancer cell killing, whereas the Ca2+ optimum for CTL migration is slightly higher and proliferation increases monotonously with increasing [Ca2+ ]o . We propose that a partial inhibition of Ca2+ signals by specific Orai1 blockers at submaximal concentrations could contribute to tumour elimination. ABSTRACT: Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are required to protect the human body against cancer. Ca2+ is a key metabolic factor for lymphocyte function and cancer homeostasis. We analysed the Ca2+ dependence of CTL and NK cell cytotoxicity against cancer cells and found that CTLs have a bell-shaped Ca2+ dependence with an optimum for cancer cell elimination at rather low [Ca2+ ]o (23-625 μm) and [Ca2+ ]i (122-334 nm). This finding predicts that a partial inhibition of Orai1 should increase (rather than decrease) cytotoxicity of CTLs at [Ca2+ ]o higher than 625 μm. We tested this hypothesis in CTLs and indeed found that partial down-regulation of Orai1 by siRNA increases the efficiency of cancer cell killing. We found two mechanisms that may account for the Ca2+ optimum of cancer cell killing: (1) migration velocity and persistence have a moderate optimum between 500 and 1000 μm [Ca2+ ]o in CTLs, and (2) lytic granule release at the immune synapse between CTLs and cancer cells is increased at 146 μm compared to 3 or 800 μm, compatible with the Ca2+ optimum for cancer cell killing. It has been demonstrated in many cancer cell types that Orai1-dependent Ca2+ signals enhance proliferation. We propose that a decrease of [Ca2+ ]o or partial inhibition of Orai1 activity by selective blockers in the tumour microenvironment could efficiently reduce cancer growth by simultaneously increasing CTL and NK cell cytotoxicity and decreasing cancer cell proliferation.