As targets for the immunosuppressive drugs cyclosporin A and FK506, transcription factors of the NFAT (nuclear factor of activated T cells) family have been the focus of much attention. NFAT proteins, which are expressed in most immune-system cells, play a pivotal role in the transcription of cytokine genes and other genes critical for the immune response. The activity of NFAT proteins is tightly regulated by the calcium/calmodulin-dependent phosphatase calcineurin, a primary target for inhibition by cyclosporin A and FK506. Calcineurin controls the translocation of NFAT proteins from the cytoplasm to the nucleus of activated cells by interacting with an N-terminal regulatory domain conserved in the NFAT family. The DNA-binding domains of NFAT proteins resemble those of Rel-family proteins, and Rel and NFAT proteins show some overlap in their ability to bind to certain regulatory elements in cytokine genes. NFAT is also notable for its ability to bind cooperatively with transcription factors of the AP-1 (Fos/Jun) family to composite NFAT:AP-1 sites, found in the regulatory regions of many genes that are inducibly transcribed by immune-system cells. This review discusses recent data on the diversity of the NFAT family of transcription factors, the regulation of NFAT proteins within cells, and the cooperation of NFAT proteins with other transcription factors to regulate the expression of inducible genes.

Antigen stimulation of immune cells triggers Ca2+ entry through Ca2+ release-activated Ca2+ (CRAC) channels, promoting the immune response to pathogens by activating the transcription factor NFAT. We have previously shown that cells from patients with one form of hereditary severe combined immune deficiency (SCID) syndrome are defective in store-operated Ca2+ entry and CRAC channel function. Here we identify the genetic defect in these patients, using a combination of two unbiased genome-wide approaches: a modified linkage analysis with single-nucleotide polymorphism arrays, and a Drosophila RNA interference screen designed to identify regulators of store-operated Ca2+ entry and NFAT nuclear import. Both approaches converged on a novel protein that we call Orai1, which contains four putative transmembrane segments. The SCID patients are homozygous for a single missense mutation in ORAI1, and expression of wild-type Orai1 in SCID T cells restores store-operated Ca2+ influx and the CRAC current (I(CRAC)). We propose that Orai1 is an essential component or regulator of the CRAC channel complex.

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Stimulation of immune cells causes depletion of Ca2+ from endoplasmic reticulum (ER) stores, thereby triggering sustained Ca2+ entry through store-operated Ca2+ release-activated Ca2+ (CRAC) channels, an essential signal for lymphocyte activation and proliferation. Recent evidence indicates that activation of CRAC current is initiated by STIM proteins, which sense ER Ca2+ levels through an EF-hand located in the ER lumen and relocalize upon store depletion into puncta closely associated with the plasma membrane. We and others recently identified Drosophila Orai and human Orai1 (also called TMEM142A) as critical components of store-operated Ca2+ entry downstream of STIM. Combined overexpression of Orai and Stim in Drosophila cells, or Orai1 and STIM1 in mammalian cells, leads to a marked increase in CRAC current. However, these experiments did not establish whether Orai is an essential intracellular link between STIM and the CRAC channel, an accessory protein in the plasma membrane, or an actual pore subunit. Here we show that Orai1 is a plasma membrane protein, and that CRAC channel function is sensitive to mutation of two conserved acidic residues in the transmembrane segments. E106D and E190Q substitutions in transmembrane helices 1 and 3, respectively, diminish Ca2+ influx, increase current carried by monovalent cations, and render the channel permeable to Cs+. These changes in ion selectivity provide strong evidence that Orai1 is a pore subunit of the CRAC channel.

Store-operated Ca2+ entry through calcium release-activated calcium channels is the chief mechanism for increasing intracellular Ca2+ in immune cells. Here we show that mouse T cells and fibroblasts lacking the calcium sensor STIM1 had severely impaired store-operated Ca2+ influx, whereas deficiency in the calcium sensor STIM2 had a smaller effect. However, T cells lacking either STIM1 or STIM2 had much less cytokine production and nuclear translocation of the transcription factor NFAT. T cell-specific ablation of both STIM1 and STIM2 resulted in a notable lymphoproliferative phenotype and a selective decrease in regulatory T cell numbers. We conclude that both STIM1 and STIM2 promote store-operated Ca2+ entry into T cells and fibroblasts and that STIM proteins are required for the development and function of regulatory T cells.

Ca 2+ entry into cells of the peripheral immune system occurs through highly Ca 2+ -selective channels known as CRAC (calcium release-activated calcium) channels. CRAC channels are a very wellcharacterized example of store-operated Ca 2+ channels, so designated because they open when the endoplasmic reticulum (ER) Ca 2+ store becomes depleted. Physiologically, Ca 2+ is released from the ER lumen into the cytoplasm when activated receptors couple to phospholipase C and trigger production of the second messenger inositol 1,4,5-trisphosphate (IP 3 ). IP 3 binds to IP 3 receptors in the ER membrane and activates Ca 2+ release. The proteins STIM and ORAI were discovered through limited and genome-wide RNAi screens, respectively, performed in Drosophila cells and focused on identifying modulators of store-operated Ca 2+ entry. STIM1 and STIM2 sense the depletion of ER Ca 2+ stores, whereas ORAI1 is a pore subunit of the CRAC channel. In this review, we discuss selected aspects of Ca 2+ signaling in cells of the immune system, focusing on the roles of STIM and ORAI proteins in store-operated Ca 2+ entry. KeywordsCRAC channels; store-operated calcium entry; T cell activation; primary immunodeficiencies OVERVIEW Ca 2+ signaling is essential for diverse biological processes (reviewed in 1 -4). Ca 2+ ions are especially suited as intracellular second messengers because of the strong homeostatic mechanisms that maintain intracellular free Ca 2+ concentrations ([Ca 2+ ] i ) in resting cells at 100 nM or less, in the face of extracellular Ca 2+ concentrations ([Ca 2+ ] o ) that are four orders of magnitude higher (1-2 mM). Cytoplasmic Ca 2+ concentrations are maintained at low levels primarily through the action of plasma membrane Ca 2+ -ATPases (PMCAs) that pump Ca 2+ out of the cell across the plasma membrane, and the sarco-endoplasmic reticulum Ca 2+ -ATPases (SERCAs) that pump Ca 2+ into the lumen of the endoplasmic reticulum (ER) ( Figure 1). Secondary regulators of [Ca 2+ ] i include the mitochondrial Ca 2+ uniporter (MCU) that transports Ca 2+ across the inner mitochondrial membrane and the electrogenic Na + -Ca 2+

The contributors Christian U. Blank is a medical oncologist and principal investigator at the Netherlands Cancer Institute. He is Professor of Haematology/oncology at the university of Regensburg, Germany, and received an MBA degree from the university of Warwick, UK. His research interests include neoadjuvant immunotherapies, targeted and biological response modifiers, and prognostic markers for cancer immunotherapies. W. Nicholas Haining is a physician-scientist and vice-President for Discovery oncology and Immunology at Merck Research Laboratories. His former academic laboratory at the Dana-Farber Cancer Institute and the Broad Institute focused on understanding the transcriptional control of T cell exhaustion and on identifying regulators of the immune response to cancer in tumour and immune cells. Werner Held's laboratory has a long-standing interest in understanding the development, differentiation and function of natural killer cells and CD8 + T cells. Current work focuses on CD8 + T cell differentiation in response to acute and chronic infections as well as cancer. Patrick G. Hogan's research centres on mechanisms and regulation of cellular calcium signalling, the biology of the nuclear factor of activated T cells (NFAT) family of transcription factors and the transcriptional control of immune cell development and function. Axel Kallies is a professor at the University of Melbourne, Australia. His laboratory studies the molecular control of CD8 + cytotoxic T cell and regulatory T cell differentiation with a focus on populations residing in non-lymphoid tissue, including healthy tissues and tumours. The Kallies laboratory has developed and applied genetic and molecular approaches to this field, including novel gene reporters, metabolic techniques, transcriptional profiling, chromatin immunoprecipitation and accessible chromatin sequencing. Enrico Lugli's laboratory is focused on understanding the biological mechanisms at the basis of memory T cell responses and homeostasis in humans and how this information can be exploited to favour antitumour immune responses in patients with cancer. The group is specialized in single-cell technologies, in particular high-dimensional flow cytometry. Rachel C. Lynn is an associate director of research at Lyell Immunopharma. She received her PhD degree from the the university of Pennsylvania, where she developed multiple preclinical chimeric antigen receptor (CAR) T cell therapy platforms. During her postdoctoral work with Crystal mackall at Stanford university, she developed models to interrogate and strategies to mitigate CAR T cell exhaustion. At Lyell Immunopharma, her research group will continue to investigate optimal strategies for adoptive T cell therapy in cancer.

SUMMARY During persistent antigen stimulation, CD8+ T cells show a gradual decrease in effector function, referred to as exhaustion, which impairs responses in the setting of tumors and infections. Here we demonstrate that the transcription factor NFAT controls the program of T cell exhaustion. When expressed in cells, an engineered form of NFAT1 unable to interact with AP-1 transcription factors diminished T cell receptor (TCR) signaling, increased the expression of inhibitory cell surface receptors, and interfered with the ability of CD8+ T cells to protect against Listeria infection and attenuate tumor growth in vivo. We defined the genomic regions occupied by endogenous and engineered NFAT1 in primary CD8+ T cells, and showed that genes directly induced by the engineered NFAT1 overlapped with genes expressed in exhausted CD8+ T cells in vivo. Our data show that NFAT promotes T cell anergy and exhaustion by binding at sites that do not require cooperation with AP-1.