1,2 This specific protein–protein interaction needs at least 10 seconds to trigger TCR-dependent intracellular signalling pathways.3 To produce an effective TCR response, an additional interaction of the CD4 or CD8 co-receptors with invariant parts of the MHC–peptide complex is required to stabilize the TCR-agonist peptide–MHC complex. Upon TCR activation, the Src kinases Fyn and Lck phosphorylate the tyrosine residues in their immune-receptor tyrosine-based activation motifs (ITAMs), which allow activation

of the ζ-chain-associated protein of molecular weight 70 000 (ZAP-70).4,5 ZAP-70 phosphorylates the adaptor proteins LAT and SLP76, which activate phospholipase Cγ (PLCγ) through the Src-like tyrosine kinase Tec.3 The PLCγ cleaves phosphatidylinositol 4,5 bisphosphate and generates the second messengers inositol 1,4,5,-trisphosphate (InsP3) and diacylglycerol.5–8

selleckchem The InsP3 binds to the InsP3 receptor in the membrane of the endoplasmic reticulum, which is the main Ca2+ store, and initiates the release of its stored Ca2+.6–9 Depletion of Ca2+ from the endoplasmic reticulum induces stromal interaction molecule (STIM1)-dependent activation of store-operated calcium release-activated Ca2+ (CRAC) channels in the plasma membrane.6–11 ORAI (also called mTOR inhibitor CRACM) proteins have been shown to form the pore of the CRAC channel complex.12–15 STIM1 has been shown to activate CRAC/ORAI channels.16–18 The function of its close relative STIM2 is not as well understood.19–21 Analysis of STIM1- and STIM2-deficient mouse T cells revealed that they are

both important for Ca2+ influx, T-cell activation and the development and function of regulatory T cells, with STIM2 being less important than STIM1.22 Parvez et al.21 demonstrated that STIM2 activates CRAC channels but that DOK2 this activation is much more complicated because it involves store-dependent and store-independent processes. Influx of Ca2+ through STIM-activated CRAC/ORAI channels elevates the intracellular calcium concentration [Ca2+]i in T cells for times lasting from minutes up to hours.23 A rise of [Ca2+]i as the result of Ca2+ release and Ca2+ influx through store-operated CRAC channels is critically involved in the regulation of the three most important transcription factor families controlling transcriptional activity and T-cell proliferation.5,9,24,25 It is remarkable that 75% of all activation-regulated genes are dependent on Ca2+ influx through the plasma membrane via CRAC channels.26 Decreasing [Ca2+]i leads to inhibition or reduction of T-cell activation and proliferation,23,27–29 highlighting the great influence of [Ca2+]i on T-cell-based immune responses. While TCR stimulation alone activates many signalling cascades, including Ca2+ signalling, it is not sufficient for optimal T-cell activation in most circumstances and a costimulatory signal is required for adequate activation.