Signal transduction through the T cell receptor is dynamically regulated by balancing kinase and phosphatase activities

Vitamin E reverses impaired linker for activation of T cells activation in T cells from aged C57BL/6 mice

Supplemental vitamin E alleviates age-related defects in interleukin (IL)-2 production, T cell proliferation, and immune synapse formation. Here, we evaluated the effect of in vitro supplementation with 46 mumol/L of vitamin E on T cell receptor-proximal signaling events of CD4(+) T cells from young (4-6 mo) and old (22-26 mo) C57BL mice. Aged murine CD4(+) T cells stimulated via CD3 and CD28, tyrosine 191 of the adaptor protein Linker for Activation of T cells (LAT), was hypo-phosphorylated. Supplementation with vitamin E eliminated this difference in the tyrosine phosphorylation of LAT. By using a flow cytometric assay, the age-related differences in the activation-induced phosphorylation of LAT were observed in both naïve and memory T cell subsets. In addition, supplementation with vitamin E eliminates the age-related differences in LAT phosphorylation in both T cell subsets. Neither age nor vitamin E supplementation altered the fraction of LAT entering the membrane compartment. Furthermore, neither age nor vitamin E influenced the phosphorylation of Lck and Zap70, indicating that associated changes in LAT phosphorylation were not caused by alterations in activation states of the upstream kinases Lck and Zap70.

Redox equilibrium in mucosal T cells tunes the intestinal TCR signaling threshold

Mucosal immune tolerance in the healthy intestine is typified by lamina propria T cell (LPT) functional hyporesponsiveness after TCR engagement when compared with peripheral blood T cell (PBT). When LPT from an inflamed intestine are activated through TCR cross-linking, their responsiveness is stronger. LPT are thus capable of switching from a tolerant to a reactive state, toggling between high and low thresholds of activation. We demonstrate that in normal LPT global tyrosine phosphorylation upon TCR cross-linking or an increase in intracellular H2O2, an inhibitor of protein tyrosine phosphatases, is muted. Thus, we propose that LPT have a greater reducing capacity than PBT, shifting the balance between kinases and protein tyrosine phosphatases in favor of the latter. Surface gamma-glutamyl transpeptidase, an indirect indicator of redox potential, and glutathione are significantly elevated in LPT compared with PBT, suggesting that elevated glutathione detoxifies TCR-induced reactive oxygen species. When glutathione is depleted, TCR-induced LPT tyrosine phosphorylation rises to PBT levels. Conversely, increasing glutathione in PBT attenuates tyrosine phosphorylation. In LPT isolated from inflamed mucosa, TCR cross-linking induces greater phosphorylation, and gamma-glutamyl transpeptidase levels are reduced compared with those from autologous noninflamed tissue. We conclude that the high TCR signaling threshold of mucosal T cells is tuned by intracellular redox equilibrium, whose dysregulation may mediate intestinal inflammation.

Tyrosine kinase activity and remodelling of the actin cytoskeleton are co-temporally required for degranulation by cytotoxic T lymphocytes

In this study, we examined the contribution of the actin cytoskeleton to T-cell receptor (TCR)-initiated signalling in cytotoxic T lymphocytes (CTLs). We demonstrate that cytoskeletal remodelling is required for sustaining TCR-stimulated signals that lead to degranulation by CTLs. Disruption of the actin cytoskeleton in CTLs already undergoing signalling responses results in an almost immediate loss of essentially all protein tyrosine phosphorylation. This signal reversal is not restricted to tyrosine phosphorylation, as disruption of the actin cytoskeleton also reverses the phosphorylation of the more downstream serine/threonine kinase extracellular signal regulated kinase (Erk). An intact cytoskeleton and cell spreading are not sufficient for maintaining signals, as stabilization of actin filaments, at a point when peak tyrosine phosphorylation is occurring, also leads to the rapid loss of protein tyrosine phosphorylation. Disruption of tyrosine kinase activity after TCR signals are maximally induced causes the immediate reversal of tyrosine phosphorylation as well as cytoskeletal disruption, as indicated by loss of cell spreading, adhesion and CTL degranulation. Taken together, our results indicate that actin remodelling occurs co-temporally with ongoing tyrosine kinase activity, leading to CTL degranulation. We hypothesize that continuous actin remodelling is important for sustaining productive signals, even after downstream signalling molecules such as Erk have been activated, and that the actin cytoskeleton is not solely required for initiating and maintaining the T cell in contact with its stimulus.

Chemical Inhibitors when Timing Is Critical: A Pharmacological Concept for the Maturation of T Cell Contacts

Cellular signal transduction proceeds through a complex network of molecular interactions and enzymatic activities. The timing of these molecular events is critical for the propagation of a signal and the generation of a specific cellular response. To define the timing of signalling events, we introduce the combination of high-resolution confocal microscopy with the application of small-molecule inhibitors at various stages of signal transduction in T cells. Inhibitors of Src-family tyrosine kinases and actin dynamics were employed to dissect the role of the lymphocyte-specific tyrosine kinase Lck in the formation and maintenance of T cell receptor/CD3-dependent contacts. Anti-CD3epsilon-coated coverslips served as a highly defined stimulus. The kinetics of the recruitment of the yellow fluorescent protein-tagged signalling protein ZAP-70 were detected by high-resolution confocal microscopy. The analysis revealed that at 5 min after receptor engagement, Lck activity was required for maintenance of contacts. In contrast, after 20 min of receptor engagement, the contacts were Lck-independent. The relevance of the timing of inhibitor application provides a pharmacological concept for the maturation of T cell-substrate contacts.

Phosphatases in concert with kinases set the gain for signal transduction through the T cell receptor

The 'tunable activation thresholds' model for signal transduction through the T cell receptor (TCR)/CD3 signaling complex proposes that rapid cycles of phosphorylation and dephosphorylation are integral to regulating the frequency of protein-protein interaction, thus having considerable influence over the activation of downstream signaling pathways. Co-temporal activation of kinases and phosphatases could serve to modulate the ongoing signaling response, depending on the relative balance of their opposing enzymatic activities. Although recent reports have addressed the mechanisms by which specific kinase/phosphatase pairs contribute to the initiation and termination of signaling, we sought a more global understanding of the ability of the kinase/phosphatase balance to regulate, or "tune", the very proximal steps of TCR signaling in primary human T cells. Herein, we provide biochemical evidence that phosphotyrosine induction via the TCR is subject to fine-tuning based on the overall activity of kinases and phosphatases relative to one another, leading to cycles of phosphorylation and dephosphorylation, with implications for developing the next generation of immunotherapeutic agents.