A calcium-redox feedback loop controls human monocyte immune responses: The role of ORAI Ca2+ channels

Monocytes adjust deadly reactive oxygen species production by altering the subunit composition of the ORAI calcium channel. Keeping monocytes in business When monocytes of the immune system encounter a bacterial pathogen, they activate a calcium signaling pathway called SOCE, which is mediated by the ORAI/STIM complex. ORAI subunits form the Ca2+ channel; STIM is its activating partner. This calcium signal activates a plasma membrane enzyme that produces toxic reactive oxygen species (ROS) to kill the bacteria. Given that ROS also inactivate ORAI channels, how do the monocytes maintain signaling? Saul et al. found that human monocytes respond by altering the composition of the ORAI channel. ROS inhibit ORAI channels composed of ORAI1 subunits; however, if just one of the six channel subunits is ORAI3, then the channel resists this ROS-mediated inactivation. Human monocytes exposed to bacterial peptides in culture and phagocytes isolated from mice infected with bacteria increased the amount of ORAI3 and reduced the amount of ORAI1. This change in the ORAI3/ORAI1 ratio meant that more ROS-insensitive channels were present and the cells could continue to produce the appropriate calcium signal and mediate bacterial killing. In phagocytes, pathogen recognition is followed by Ca2+ mobilization and NADPH oxidase 2 (NOX2)–mediated “oxidative burst,” which involves the rapid production of large amounts of reactive oxygen species (ROS). We showed that ORAI Ca2+ channels control store-operated Ca2+ entry, ROS production, and bacterial killing in primary human monocytes. ROS inactivate ORAI channels that lack an ORAI3 subunit. Staphylococcal infection of mice reduced the expression of the gene encoding the redox-sensitive Orai1 and increased the expression of the gene encoding the redox-insensitive Orai3 in the lungs or in bronchoalveolar lavages. A similar switch from ORAI1 to ORAI3 occurred in primary human monocytes exposed to bacterial peptides in culture. These alterations in ORAI1 and ORAI3 abundance shifted the channel assembly toward a more redox-insensitive configuration. Accordingly, silencing ORAI3 increased the redox sensitivity of the channel and enhanced oxidation-induced inhibition of NOX2. We generated a mathematical model that predicted additional features of the Ca2+-redox interplay. Our results identified the ORAI-NOX2 feedback loop as a determinant of monocyte immune responses.