T cell activation

First, a large immune cell, called a macrophage, engulfs a tumor cell or virus. The macrophage then displays a protein molecule or antigen of the engulfed material on its surface, becoming an antigen-presenting cell APC. This conglomerate then binds to a T cell receptor, which leads to T cell activation.

Unlike growth factor receptors whose intrinsic enzymatic activity is enhanced by tyrosine phosphorylation, the CD3 molecules have no such effector function on their own. It was speculated, therefore, that tyrosine phosphorylation of the ITAMs might serve as docking sites for interactions with other proteins.

This converted the TCR with no intrinsic enzymatic function to an active PTK able to phosphorylate a spectrum of substrates leading to a myriad of downstream signals that, when integrated appropriately along with signals from other co-receptors , leads to T cell activation The basic tenets of this model have stood the test of intensive investigation.

In the 15 years since ZAP was cloned, investigators have filled in many of the gaps between the TCR and initiation of effector functions. Much has been learned about substrates of the PTKs including src, syk, and more recently tec family members activated by the TCR and how these molecules participate in T cell activation, about how signaling complexes are organized by adapter proteins to bring effector proteins together, and about the unexpected intersection of particular signaling pathways.

With the accumulation of data, it has also become clear that signaling via the TCR complex is not a linear event starting at the receptor and ending in the nucleus. Instead, there appears to be complex feedback and feedforward regulation at each step. Ironically, one of the most central signaling questions that remains is how does receptor binding translate most proximally into an activating signal.

Many models have been proposed, but none has yet withstood the rigor of subsequent investigation. This review summarizes our current understanding of many of these issues and poses some of the intriguing questions that remain. Although it has been clear for more than 15 years that CD3 transduces signals from the engaged receptor via its ITAMs, exactly how ligation of the TCR is translated into the first signal remains controversial. Current models suggest that both TCR aggregation and conformational changes may play roles in signal initiation.

Two separate but not mutually exclusive conformational changes within the CD3 cytoplasmic tails have been proposed as mechanisms for TCR-inducible ITAM phosphorylation.

However, a rigorous test for this model in T cells has not yet been provided. It was speculated that Nck could then recruit and activate effector molecules required for subsequent ITAM phosphorylation. This observation must be considered in light of the T cell developmental abnormalities in the knock-in mice. Resolving the importance of the PRR for mature T cell function requires temporal control of expression of the mutant.

Ongoing work will provide insights into solving these controversies. Such a model would require external forces that could potentially be provided by cell-to-cell contact, but this possibility is difficult to reconcile with the ability of soluble ligands to activate the TCR.

This interaction results in a rotation of the TCR heterodimers around one another, displacing the extracellular domains of the associated CD3 molecules. Aggregation is supported strongly by observations that soluble multimeric but not monomeric pMHC can trigger TCR activation.

However, recent biochemical and microscopic studies suggest that preformed TCR aggregates are present on non-activated T cells reviewed in Inhibitory phosphatases with long extracellular domains such as CD45 are excluded due to their size. TCR complexes that engage pMHC on the APC surface remain in the close contact zone, where they are segregated from phosphatases and are able to initiate signal.

Shortening the extracellular domains of inhibitory molecules or lengthening the extracellular domains of adhesion or accessory molecules can abrogate TCR signaling, suggesting that kinetic segregation is an important aspect of TCR signal initiation reviewed in Another model has been proposed that suggests that endogenous pMHCs amplify signals produced by the rare agonist pMHCs by promoting TCR aggregation and a subsequent phosphorylation cascade Many of these models are not mutually exclusive, and it is likely that TCR aggregation, conformational changes within the TCR complex, and exclusion of inhibitory molecules are all required for TCR triggering, perhaps in a stepwise fashion.

Recruitment of ZAP follows, leading to a cascade of phosphorylation events. The past decade has seen the description of a subcellular assembly and activation of an adapter protein nucleated multi-molecular signaling complex Figure 1b. Among the most important of the ZAP targets are the transmembrane adapter protein linker for the activation of T cells LAT and the cytosolic adapter protein src homology 2 SH2 domain-containing leukocyte phosphoprotein of 76 kDa SLP 42 , These two adapters form the backbone of the complex that organizes effector molecules in the correct spatio-temporal manner to allow for the activation of multiple signaling pathways.

While LAT and SLP serve to nucleate this large signaling complex, the effector molecules themselves also are important for stabilizing the complex.

These and other data suggest that the formation of the complex is more complicated than the linear model most often invoked for simplicity. Advancements in biochemical and structural techniques are needed to elucidate the precise allosteric and perhaps stochiometric changes within the multimolecular complex that allow for signal transduction.

To investigate more precisely the importance of these complex interactions in primary T cells, several laboratories have generated mice expressing transgenic or knock-in mutations in specific binding regions in various molecules involved in proximal signaling 53 — However, these tyrosine mutations still result in severe defects in downstream signaling pathways consistent with defective Vav1 or Itk activity.

Similarly, mutation of tyrosines on Vav1 does not result in a loss of interaction with their proposed binding partners, although they do result in abrogation of Vav1-dependent signaling Indeed, structural studies have suggested that the interaction between the SH2 domain of Itk and a phosphotyrosine results in a conformational switch allowing kinase activity Further studies are required to determine how the activities of molecules beyond Itk are affected by specific domain-domain interactions within the complex.

Ligation of costimulatory receptors such as CD28 augments these pathways. Below we discuss the mechanisms by which these pathways are activated and regulated. Ras is a guanine nucleotide-binding protein and is required for the activation of the serine-threonine kinase Raf-1 that leads to the activation of the mitogen-associated protein kinases MAPKs extracellular signal-regulated kinase 1 Erk1 and Erk2.

Erk activation results in transcriptional activation of Elk1 and signal transducer and activator of transcription 3 STAT3 and lck serine phosphorylation reviewed in Over the past several years, the identification and characterization of a lymphocyte-specific activation complex has provided some insight into this question. For decades the CRAC channels had only been identified by their biophysical properties, and it has just been in the past few years that the pore-forming subunit of the channels was identified as the four transmembrane domain-containing molecule ORAI 83 — Activated calcineurin dephosphorylates members of the nuclear factor of activated T cells NFAT family, leading to their translocation to the nucleus.

In the nucleus, NFATs can form cooperative transcriptional complexes with a variety of transcription factors, thereby integrating signaling pathways resulting in differential transcriptional patterns. When a T cell is presented with cognate antigen by an APC, signals from the TCR initiate a program of actin cytoskeletal rearrangements that results in polarization and activation of the T cell reviewed in These changes are thought to be dependent on a TCR-induced increase in plasma membrane fluidity and a decrease in cellular motility.

Plasma membrane fluidity is increased, in part, by the TCR- and Vav1-dependent transient dephosphorylation of ERM ezrin, radixin, and moesin proteins, resulting in the loss of their ability to link the plasma membrane to the actin cytoskeleton This result also suggests that other Vav-1 functions are important for TCR-induced actin changes.

Consistent with this is the observation that through its protein interaction domains, Vav1 may contribute to Wave2 and WASp activation through the recruitment of Dynamin2, a GTPase known to be important for TCR-induced actin dynamics Recent data suggest, however, that the adapter protein ADAP a component of the SLPnucleated complex may play a role through its interaction with the microtubule motor protein dynein Although the IS was described approximately 10 years ago, its precise role in T cell activation remains unclear.

Models have been proposed suggesting it is essential for both signal initiation and signal termination, while others suggest that the IS is irrelevant for signaling but essential instead for directed secretion of cytokines reviewed in It is likely that the IS is important for different aspects of each of these processes, likely depending on the precise conditions of cellular activation.

Formation and translocation of the clusters is dependent on F-actin dynamics, and new clusters continue to form even after the mature IS is established — Integrins play a key role in sustaining microclusters, emphasizing the importance of integrin activation for T cell function Although the role of the DPC is not known for certain, it is speculated that it is critical to sequester negative regulators away from the TCR activation complex Additionally, the DPC may contribute to the polarization of key signaling molecules that may be required for distinguishing memory versus effector fate decisions in recently divided cells Formation of this complex is dependent on F-actin and the rephosphorylation of ERM proteins that migrate to the distal pole linking signaling molecules to the cytoskeleton TCR signaling cascades and pathways downstream of actin reorganization are intertwined and difficult to tease apart, as many of the effector molecules involved have multiple enzymatic and adapter functions.

WASp, Wave2, and Vav1 signals play roles in TCR-induced signaling that appear to be independent of their roles in actin responses 50 , , Therefore, loss of different actin regulators may result in complex TCR signaling defects Future studies are required to fully understand the feedforward and feedback mechanisms that define the interdependence between cytoskeletal dynamics and T cell activation.

Key T cell integrins include leukocyte function-associated antigen-1 LFA-1 and very late antigen-4 VLA-4 , which bind their respective ligands intercellular adhesion molecule ICAM and vascular cell adhesion molecule VCAM and fibronectin on other immune cells, endothelial cells, fibroblasts, and extracellular matrix proteins. This conclusion is based on studies utilizing overexpression of dominant-negative and constitutively active forms of Rap1 , and more recently by analysis of mice deficient in Rap1A, in which TCR-induced adhesion to ICAM-1 is markedly reduced The importance of understanding the role of Rap1 is clear, as mutations in Rap1-mediated integrin activation have been linked to leukocyte adhesion-deficiency syndrome, a disease that can lead to severe bacterial infections reviewed in However, there are downstream effectors that have a more selective role in integrin activation, including the adapter protein ADAP.

Although mice expressing the reciprocal mutation in the SH2 domain of SLP have been generated and share several characteristics with ADAP-deficient mice, TCR-induced integrin activation in these mice has not yet been reported. 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.

This interaction is required for Rap1 activation and contributes to Rap1 recruitment. How RIAM itself relocalizes to the plasma membrane following T cell stimulation is currently unknown, although it is hypothesized that recruitment may be through the inducible interaction of phosphoinositide PI 3,4 P 2 a product of T cell activation and the pleckstrin homology PH domain of RIAM Interestingly, it is this interaction and not the kinase activity of PKD1 that is required for Rap1 membrane recruitment and activation.

Activated Rap1 can also associate with the effector regulator of cell adhesion and polarization enriched in lymphoid tissues RAPL. In cell line models, this association is important for LFA-1 clustering as well as affinity modulation In addition to regulating the activation of Rap1, signals from the TCR also regulate cytoskeletal attachments to integrins. One cytoskeletal binding protein important for integrin activation is talin.

Talin is not the only actin-binding protein implicated in TCR-induced integrin activation. One central tenet of T cell activation is that signaling solely through the TCR results in a non-responsive state anergy in which T cells fail to respond and are then refractory to restimulation. Co-ligation of other cell surface receptors provides additional signals required for anergy avoidance and productive T cell activation.

Although many cell surface receptors can enhance signaling through the TCR, CD28 does so more robustly than other costimulatory molecules. Numerous studies have shown that CD28 promotes T cell proliferation, cytokine production via gene transcription and mRNA stability , cell survival, and cellular metabolism reviewed in Akt phosphorylates multiple proteins enabling it to affect numerous cellular responses.

This function, coupled to the ability of Akt to inhibit transcription factors that promote cell cycle arrest, results in Akt-driven cell survival and proliferation Unbound pNFAT would then be susceptible to calcineurin phosphatase activity and nuclear entry.

Whether this pathway is Akt dependent remains to be rigorously tested. Together, these data provide a framework for how Akt mediates T cell growth and survival downstream of CD Although the proline motifs in the tail of CD28 are required for CDmediated proliferation and IL-2 production, these motifs are dispensable for Bcl-xl upregulation This function appears to be more reliant upon the proximal YMNM p85 binding site Thus, CD28 can differentially regulate proliferation and survival in activated T cells.

Another more recently described function of CD28 is induction of arginine methylation. Following CD28 ligation, protein arginine methyltransferase activity increases, and arginine methylation of multiple proteins, including Vav1, is induced. Vav1 arginine methylation appears to occur in the nucleus and correlates with IL-2 production While the precise biologic significance of this posttranslational modification is unknown, this pathway may provide yet another mechanism by which CD28 regulates TCR signaling.

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Life Sci. Bertin, S. Mucosal Immunol. Konicek, B. Regulation of mouse colony-stimulating factor-1 gene promoter activity by AP1 and cellular nucleic acid-binding protein. Interaction of the TCR with peptide-MHC in the absence of co-stimulation switches the T cells off, so they do not respond inappropriately. Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines.

Each one of these cells performs a specific task in the tissue and in developing further immune responses. The resulting cell population moves out to the site of the infection or inflammation in order to deal with the pathogen. Other cells present at the tissue site of inflammation— such as neutrophils, mast cells, and epithelial cells — can also release cytokines, chemokines, short peptides and other molecules which induce further activation and proliferation of the T cells.

Register Log in. T-cell activation Download T-cell activation. Signal One T cells are generated in the T hymus and are programmed to be specific for one particular foreign particle antigen.