T cell activation

Abstract

This year marks the 25th anniversary of the first Annual Review of Immunology article to describe features of the T cell antigen receptor (TCR). In celebration of this anniversary, we begin with a brief introduction outlining the chronology of the earliest studies that established the basic paradigm for how the engaged TCR transduces its signals. This review continues with a description of the current state of our understanding of TCR signaling, as well as a summary of recent findings examining other key aspects of T cell activation, including cross talk between the TCR and integrins, the role of costimulatory molecules, and how signals may negatively regulate T cell function.Acronyms and DefinitionsAdapter protein: cellular protein that functions to bridge molecular interactions via characteristic domains able to mediate protein/protein or protein/lipid interactions Costimulation: signals delivered to T cells by cell surface receptors other than the TCR itself that potentiate T cell activation cSMAC: central supramolecular activation cluster Immunoreceptor tyrosine-based activation motif (ITAM): a short peptide sequence in the cytoplasmic tails of key surface receptors on hematopoietic cells that is characterized by tyrosine residues that are phosphorylated by Src family PTKs, enabling the ITAM to recruit activated Syk family kinases Inside-out signaling: signals initiated by engagement of immunoreceptors that lead to conformational changes and clustering of integrins, thereby increasing the affinity and avidity of the integrins for their ligands NFAT: nuclear factor of activated T cells PI3K: phosphoinositide 3-kinase PKC: protein kinase C PLC: phospholipase C pMHC: peptide major histocompatibility complex (MHC) complex pSMAC: peripheral supramolecular activation cluster PTK: protein tyrosine kinase Signal transduction: biochemical events linking surface receptor engagement to cellular responses TCR: T cell antigen receptor

Figure 1. TCR proximal signaling then and now: filling in the gaps

Then: a. TCR signaling mid-1990s (adapted from 2). Ligation of the TCR/CD3 results in activation of src and syk family PTKs associated with the intracellular CD3 domains that then activate PLC and Ras-dependent pathways. Now: b–d. Current understanding of how the TCR couples to PLC activation (b), Ras (c), and the molecular basis for Ca²⁺ influx (d). b. The link between PTKs and downstream pathways (including those in c and d) is a multi-molecular signaling complex nucleated by the adapter proteins LAT, Gads, and SLP-76. Lck activates ZAP-70 to phosphorylate (p) tyrosine residues on LAT, which then recruits Gads and its constitutive binding partner SLP-76. ZAP-70-mediated phosphorylation of SLP-76 results in the recruitment of multiple SH2 domain-containing effector molecules (circles) and adapter proteins (octogons). SH3 domains (hatching) also link effectors to adapters and contribute to stabilization of the complex. c. Activation of Ras involves two Ras GEFs (Sos and RasGRP) in a positive feedback loop. TCR-induced production of DAG (see b) results in the membrane recruitment of RasGRP, where it is phosphorylated and activated by PKC. RasGRP then facilitates the removal of GDP from Ras, which can then bind GTP and become activated. GTP-bound Ras then binds Sos (bound constitutively to GRB2, which is inducibly recruited to LAT), increasing its GEF activity resulting in a positive feedback loop and robust Ras activity. d. The link between depletion of ER Ca²⁺stores and CRAC channel activation is Stim. TCR-induced IP₃ production (see b) results in the activation of IP₃ receptors (IP₃R), which release Ca2+ from the ER into the cytoplasm. Stim contains paired EF hands within the ER lumen, which bind Ca²⁺. Upon depletion of ER stores, Ca²⁺-free Stim molecules cluster and are thought to move to areas of ER/plasma membrane proximity where they co-localize with CRAC channels (the pore-forming subunit has been identified as Orai1) and contribute to their activation and the subsequent influx of Ca²⁺.

Figure 2. A model of integrin activation

Upon TCR ligation, the LAT/SLP-76-nucleated signalosome assembles. This complex allows for the activation of three pathways necessary for inside-out activation of integrins. Vav1 and Itk contribute to the actin reorganization required for integrin mobility (pathway A). Mobilization of the ADAP/SKAP-55/RIAM complex is necessary for activated Rap1 plasma membrane localization (pathway B). An active Rap1/RIAM complex induces the association of talin with integrin β tails, perhaps resulting in altered integrin affinity. Thirdly, PLCγ1-mediated activation of DAG leads to PKD1 activation and association with Rap1 (pathway C). This interaction is required for Rap1 activation and contributes to Rap1 recruitment. Rap1 in turn recruits RAPL, and subsequent RAPL binding to αL subunits results in integrin clustering and affinity changes.