Calcium signals regulate the functional differentiation of thymic iNKT cells

Abstract

How natural or innate‐like lymphocytes generate the capacity to produce IL‐4 and other cytokines characteristic of type 2 immunity remains unknown. Invariant natural killer T (iNKT) cells differentiate in the thymus into NKT1, NKT2, and NKT17 subsets, similar to mature, peripheral CD4+ T helper cells. The mechanism for this differentiation was not fully understood. Here, we show that NKT2 cells required higher and prolonged calcium (Ca2+) signals and continuing activity of the calcium release‐activated calcium (CRAC) channel, than their NKT1 counterparts. The sustained Ca2+ entry via CRAC pathway in NKT2 cells was apparently mediated by ORAI and controlled in part by the large mitochondrial Ca2+ uptake. Unique properties of mitochondria in NKT2 cells, including high activity of oxidative phosphorylation, may regulate mitochondrial Ca2+ buffering in NKT2 cells. In addition, the low Ca2+ extrusion rate may also contribute to the higher Ca2+ level in NKT2 cells. Altogether, we identified ORAI‐dependent Ca2+ signaling connected with mitochondria and cellular metabolism, as a central regulatory pathway for the differentiation of NKT2 cells. Invariant Natural Killer T (iNKT) cells differentiate into mature functional subsets (NKT1, NKT2 and NKT17) that differ for cytokine production in the thymus. We reported here that calcium signaling connected with mitochondria and metabolism regulates this process. Thymic NKT1 and NKT2 cells have distinct Ca2+ signals upon stimulation. NKT2 cells require higher level of Ca2+ for proliferation and thymic differentiation. NKT2 cells require continuing activity of CRAC/ORAI channels for thymic differentiation. Mitochondrial content, respiration and Ca2+ uptake in thymic NKT2 cells are uniquely high. Mitochondria in thymic NKT2 cells regulate CRAC/ORAI dependent Ca2+ homeostasis. The differentiation of invariant natural killer T cell subsets depends on Ca2+ signaling in a manner dependent on ORAI activity and Ca2+ sequestration at the mitochondria.