Dissection of the role of CD3gamma chains in profound but reversible T-cell receptor down-regulation

Increasing LFA-1 Expression Enhances Immune Synapse Architecture and T Cell Receptor Signaling in Jurkat E6.1 Cells

The Jurkat E6.1 clone has been extensively used as a powerful tool for the genetic and biochemical dissection of the TCR signaling pathway. More recently, these cells have been exploited in imaging studies to identify key players in immunological synapse (IS) assembly in superantigen-specific conjugates and to track the dynamics of signaling molecules on glass surfaces coated with activating anti-CD3 antibodies. By comparison, Jurkat cells have been used only scantily for imaging on supported lipid bilayers (SLBs) incorporating laterally mobile TCR and integrin ligands, which allow to study synaptic rearrangements of surface molecules and the fine architecture of the mature IS, likely due to limitations in the assembly of immune synapses with well-defined architecture. Here we have explored whether upregulating the low levels of endogenous LFA-1 expression on Jurkat E6.1 cells through transduction with CD11a- and CD18-encoding lentiviruses can improve IS architecture. We show that, while forced LFA-1 expression did not affect TCR recruitment to the IS, E6.1 LFA-1 high cells assembled better structured synapses, with a tighter distribution of signaling-competent TCRs at the center of the IS. LFA-1 upregulation enhanced protein phosphotyrosine signaling on SLBs but not at the IS formed in conjugates with SEE-pulsed APCs, and led to the constitutive formation of an intracellular phosphotyrosine pool co-localizing with endosomal CD3ζ. This was paralleled by an increase in the levels of p-ZAP-70 and p-Erk both under basal conditions and following activation, and in enhanced Ca2+ mobilization from intracellular stores. The enhancement in early signaling E6.1 LFA-1 high cells did not affect expression of the early activation marker CD69 but led to an increase in IL-2 expression. Our results highlight a new role for LFA-1 in the core architecture of the IS that can be exploited to study the spatiotemporal redistribution of surface receptors on SLBs, thereby extending the potential of E6.1 cells and their derivatives for fine-scale imaging studies.

Separate signaling events control TCR downregulation and T cell activation in primary human T cells

Introduction

T‐cell antigen receptor (TCR) interaction with cognate peptide:MHC complexes trigger clustering of TCR:CD3 complexes and signal transduction. Triggered TCR:CD3 complexes are rapidly internalized and degraded in a process called ligand‐induced TCR downregulation. Classic studies in immortalized T‐cell lines have revealed a major role for the Src family kinase Lck in TCR downregulation. However, to what extent a similar mechanism operates in primary human T cells remains unclear.

Methods

Here, we developed an anti‐CD3‐mediated TCR downregulation assay, in which T‐cell gene expression in primary human T cells can be knocked down by microRNA constructs. In parallel, we used CRISPR/Cas9‐mediated knockout in Jurkat cells for validation experiments.

Results

We efficiently knocked down the expression of tyrosine kinases Lck, Fyn, and ZAP70, and found that, whereas this impaired T cell activation and effector function, TCR downregulation was not affected. Although TCR downregulation was marginally inhibited by the simultaneous knockdown of Lck and Fyn, its full abrogation required broad‐acting tyrosine kinase inhibitors.

Conclusions

These data suggest that there is substantial redundancy in the contribution of individual tyrosine kinases to TCR downregulation in primary human T cells. Our results highlight that TCR downregulation and T cell activation are controlled by different signaling events and illustrate the need for further research to untangle these processes.

Activation of intra-epithelial lymphocytes; their morphology, marker expression and ultimate fate

Intraepithelial lymphocytes (IELs) have been considered to play a key role in the defense system of the small intestine. Its mechanism has not been made sufficiently clear. Studies on IELs have been extremely limited to functions of αβ T-cell receptor (αβTCR) IELs (αβ-IELs). Since, in the mouse duodenum and jejunum, γδ-IELs consist 75 % of IELs, it thus would be inappropriate to argue the mechanism without extensive discussions over the functions of γδ-IELs. In previous studies, we found that the anti-CD3 monoclonal antibody (mAb) injection induced DNA fragmentation in intestinal epithelial cells (IECs) and DNA repair immediately after, that these responses were reproduced by anti-γδTCR mAb not by anti-αβTCR mAb and that the DNA fragmentation was induced by Granzyme B secreted by IELs, totally independent of Perforin. To further explore the functions of IELs in situ, we undertook experiments exclusively focused on IELs, on their changes and ultimate fate after the stimulation in mouse in vivo system. The current study demonstrated that the injected anti-CD3 mAb bound to CD3 on IELs, that the mAb activated γδ-IELs, leading to their degranulation, that changes occurred irreversibly in IELs and finally that activated IELs died in situ. γδ-IELs could be considered to respond to various stimulations most likely without the need of accessory cells ("always ready for rapid response"), to die in situ ("disposable") and thus to respond to the stimulation only once ("a one-shot responder"). These characteristics of γδ-IELs are important to further elucidate the functions of γδ-IELs in the intestinal defense system.

Increased T-cell reactivity and elevated levels of CD8+ memory T-cells in Alzheimer's disease-patients and T-cell hyporeactivity in an Alzheimer's disease-mouse model: implications for immunotherapy

Neuroinflammation is observed in neurodegenerative diseases like Alzheimer's disease (AD). However, a little is known about the mechanisms of neural-immune interactions. The involvement of peripheral T-cell function in AD is still far from clear, though it plays an important role in immunotherapy. The aim of this study was to determine peripheral T-cell reactivity in AD patients and in an AD mouse model. Mitogenic activation via ligation of the T-cell receptor (TCR) with PHA-L was measured in T lymphocytes from AD patients and Thy1(APP 751SL) x HMG(PS1 M146L)-transgenic mice (APP x PS1). In order to uncover failures in TCR signaling, the TCR was also bypassed by PMA and ionomycin treatment. All patients were sporadic late onset cases and the transgenic mice expressed no mutant APP in lymphocytes, so that direct interactions of mutant APP on T-cell function can be excluded. CD4+ and CD8+ T-cell showed increased reactivity (tyrosine phosphorylation, CD69 expression, and proliferation) in AD, while APP x PS1 transgenic mice displayed hyporeactive CD8+ T-cells after TCR ligation. Increased levels of CD8+ T memory cells and down regulation of CD8 receptor were found in AD and the animal model. Anergic TCR uncoupling was associated with loss of MAPK signaling (p38, ERK1 and ERK2) in APP x PS1. Our data implicate the generation of reactive memory T-cell in AD and of anergic memory T-cells in transgenic mice and should be taken into concern when designing immunotherapy.

T cells undergo rapid ON/OFF but not ON/OFF/ON cycling of cytokine production in response to antigen

Inflammatory cytokines such as IFN-gamma and TNF produced by Ag-stimulated CD4(+) and CD8(+) T cells are important in defense against microbial infection. However, production of these cytokines must be tightly regulated to prevent immunopathology. Previous studies, conducted with BALB/c mice, have suggested that 1) CD8(+) T cells maintain IFN-gamma production but transiently produce TNF in the continued presence of Ag and 2) lymphocytic choriomeningitis virus-specific and in vitro-propagated effector CD8(+) T cells could rapidly cycle IFN-gamma production ON/OFF/ON in response to Ag exposure, removal, and re-exposure. In contrast with CD8(+) T cells, our results show that Listeria monocytogenes-specific CD4(+) T cells from C57BL/6 mice rapidly initiate (ON cycling) and maintain production of both IFN-gamma and TNF in the continued presence of Ag. Upon Ag removal, production of both cytokines rapidly ceases (OFF cycling). However, if the initial stimulation was maximal, Ag-specific CD4(+) T cells were unable to reinitiate cytokine production after a second Ag exposure. Furthermore, L. monocytogenes-specific CD8(+) T cells in the same mice and lymphocytic choriomeningitis virus-specific CD8(+) T cells in BALB/c mice also underwent ON/OFF cycling, but if the initial Ag stimulus was maximal, they could not produce IFN-gamma after Ag re-exposure. As the initial Ag dose was reduced, the number of cells producing cytokine in response to the second Ag exposure exhibited a corresponding increase. However, T cells that were marked for IFN-gamma secretion during the first stimulation did not contribute cytokine production during the second stimulation. Thus, T cells are not able to undergo rapid ON/OFF/ON cytokine cycling in vitro in response to Ag.

Constitutive and Ligand-Induced TCR Degradation

Modulation of TCR expression levels is a central event during T cell development and activation, and it probably plays an important role in adjusting T cell responsiveness. Conflicting data have been published on down-regulation and degradation rates of the individual TCR subunits, and several divergent models for TCR down-regulation and degradation have been suggested. The aims of this study were to determine the rate constants for constitutive and ligand-induced TCR degradation and to determine whether the TCR subunits segregate or are processed as an intact unit during TCR down-regulation and degradation. We found that the TCR subunits in nonstimulated Jurkat cells were degraded with rate constants of approximately 0.0011 min(-1), resulting in a half-life of approximately 10.5 h. Triggering of the TCR by anti-TCR Abs resulted in a 3-fold increase in the degradation rate constants to approximately 0.0033 min(-1), resulting in a half-life of approximately 3.5 h. The subunits of the TCR complex were down-regulated from the cell surface and degraded with identical kinetics, and most likely remained associated during the passage throughout the endocytic pathway from the cell surface to the lysosomes. Similar results were obtained in studies of primary human Vbeta8+ T cells stimulated with superantigen. Based on these results, the simplest model for TCR internalization, sorting, and degradation is proposed.

TCR comodulation of nonengaged TCR takes place by a protein kinase C and CD3 gamma di-leucine-based motif-dependent mechanism

One of the earliest events following TCR triggering is TCR down-regulation. However, the mechanisms behind TCR down-regulation are still not fully known. Some studies have suggested that only directly triggered TCR are internalized, whereas others studies have indicated that, in addition to triggered receptors, nonengaged TCR are also internalized (comodulated). In this study, we used transfected T cells expressing two different TCR to analyze whether comodulation took place. We show that TCR triggering by anti-TCR mAb and peptide-MHC complexes clearly induced internalization of nonengaged TCR. By using a panel of mAb against the Ti beta chain, we demonstrate that the comodulation kinetics depended on the affinity of the ligand. Thus, high-affinity mAb (K(D) = 2.3 nM) induced a rapid but reversible comodulation, whereas low-affinity mAb (K(D) = 6200 nM) induced a slower but more permanent type of comodulation. Like internalization of engaged TCR, comodulation was dependent on protein tyrosine kinase activity. Finally, we found that in contrast to internalization of engaged TCR, comodulation was highly dependent on protein kinase C activity and the CD3 gamma di-leucine-based motif. Based on these observations, a physiological role of comodulation is proposed and the plausibility of the TCR serial triggering model is discussed.

TCR dynamics in human mature T lymphocytes lacking CD3 gamma

The contribution of CD3gamma to the surface expression, internalization, and intracellular trafficking of the TCR/CD3 complex (TCR) has not been completely defined. However, CD3gamma is believed to be crucial for constitutive as well as for phorbol ester-induced internalization. We have explored TCR dynamics in resting and stimulated mature T lymphocytes derived from two unrelated human congenital CD3gamma-deficient (gamma(-)) individuals. In contrast to gamma(-) mutants of the human T cell line Jurkat, which were selected for their lack of membrane TCR and are therefore constitutively surface TCR negative, these natural gamma(-) T cells constitutively expressed surface TCR, mainly through biosynthesis of new chains other than CD3gamma. However, surface (but not intracellular) TCR expression in these cells was less than wild-type cells, and normal surface expression was clearly CD3gamma-dependent, as it was restored by retroviral transduction of CD3gamma. The reduced surface TCR expression was likely caused by an impaired assembly or membrane transport step during recycling, whereas constitutive internalization and degradation were apparently normal. Ab binding to the mutant TCR, but not phorbol ester treatment, caused its down-modulation from the cell surface, albeit at a slower rate than in normal controls. Kinetic confocal analysis indicated that early ligand-induced endocytosis was impaired. After its complete down-modulation, TCR re-expression was also delayed. The results suggest that CD3gamma contributes to, but is not absolutely required for, the regulation of TCR trafficking in resting and Ag-stimulated mature T lymphocytes. The results also indicate that TCR internalization is regulated differently in each case.

On the role of CD3delta chains in TCRgammadelta/CD3 complexes during assembly and membrane expression

The present study was performed in order to analyze whether T-cell receptor (TCR)/CD3 assembly, intracellular transport and surface expression are carried in a similar way in alphabeta-and gammadelta-T cells. By means of optimal immunoprecipitation conditions with 35S-methionine/cysteine- or biotin-labelled TCR/CD3 proteins from alphabeta- or gammadelta-T-lymphoma-cell lines, as well as TCRgammadelta cDNA transfectants, it was found that CD3delta chains associate less strongly with TCRgammadelta heterodimers compared to TCRalphabeta heterodimers. This preferential reactivity of CD3delta chains appears to be structural and not owing to differences in gammadelta- versus alphabeta-T-cell intracellular environments. Our results are in accordance firstly, with data from CD3delta-deficient mice, which have gammadelta-T cells but no alphabeta-T cells, secondly with the suggested role of CD3delta chains in the positive selection of alphabeta-T cells, a process apparently not followed by gammadelta-T cells, and lastly with the differential roles of CD3delta chains versus CD3gamma chains, explaining the maintenance of two CD3delta and CD3gamma genes after the duplication from a CD3delta/gamma gene present in avians. The impaired reactivity of CD3delta chains with TCRgammadelta heterodimers seems to be owing to a less efficient association with TCRgamma chains. In contrast, CD3delta chains interact as strongly with TCRdelta chains as do CD3gamma chains with both TCRgamma and TCRdelta chains. These data may explain, at the molecular levels, why surface TCR/CD3 expression levels are impaired in gammadelta-T cells from CD3gamma-deficient mice but not from CD3delta-deficient mice.