An essential role for the MAL protein in targeting Lck to the plasma membrane of human T lymphocytes

Chimeric Antigen Receptor T Cell Therapy Targeting Epithelial Cell Adhesion Molecule in Gastric Cancer: Mechanisms of Tumor Resistance

Epithelial cell adhesion molecule (EpCAM) is a tumor-associated antigen that is frequently overexpressed in various carcinomas. We have developed chimeric antigen receptor (CAR) T cells specifically targeting EpCAM for the treatment of gastric cancer. This study sought to unravel the precise mechanisms by which tumors evade immune surveillance and develop resistance to CAR T cell therapy. Through a combination of whole-body CAR T cell imaging and single-cell multiomic analyses, we uncovered intricate interactions between tumors and tumor-infiltrating lymphocytes (TILs). In a gastric cancer model, tumor-infiltrating CD8 T cells exhibited both cytotoxic and exhausted phenotypes, while CD4 T cells were mainly regulatory T cells. A T cell receptor (TCR) clonal analysis provided evidence of CAR T cell proliferation and clonal expansion within resistant tumors, which was substantiated by whole-body CAR T cell imaging. Furthermore, single-cell transcriptomics showed that tumor cells in mice with refractory or relapsing outcomes were enriched for genes involved in major histocompatibility complex (MHC) and antigen presentation pathways, interferon-γ and interferon-α responses, mitochondrial activities, and a set of genes (e.g., CD74, IDO1, IFI27) linked to tumor progression and unfavorable disease prognoses. This research highlights an approach that combines imaging and multiomic methodologies to concurrently characterize the evolution of tumors and the differentiation of CAR T cells.

Clostridium perfringens Epsilon Toxin Binds to and Kills Primary Human Lymphocytes

Clostridium perfringens epsilon toxin (ETX) is the third most lethal bacterial toxin and has been suggested to be an environmental trigger of multiple sclerosis, an immune-mediated disease of the human central nervous system. However, ETX cytotoxicity on primary human cells has not been investigated. In this article, we demonstrate that ETX preferentially binds to and kills human lymphocytes expressing increased levels of the myelin and lymphocyte protein MAL. Using flow cytometry, ETX binding was determined to be time and dose dependent and was highest for CD4+ cells, followed by CD8+ and then CD19+ cells. Similar results were seen with ETX-induced cytotoxicity. To determine if ETX preference for CD4+ cells was related to MAL expression, MAL gene expression was determined by RT-qPCR. CD4+ cells had the highest amount of Mal gene expression followed by CD8+ and CD19+ cells. These data indicate that primary human cells are susceptible to ETX and support the hypothesis that MAL is a main receptor for ETX. Interestingly, ETX bindings to human lymphocytes suggest that ETX may influence immune response in multiple sclerosis.

The MAL Family of Proteins: Normal Function, Expression in Cancer, and Potential Use as Cancer Biomarkers

The MAL family of integral membrane proteins consists of MAL, MAL2, MALL, PLLP, CMTM8, MYADM, and MYADML2. The best characterized members are elements of the machinery that controls specialized pathways of membrane traffic and cell signaling. This review aims to help answer the following questions about the MAL-family genes: (i) is their expression regulated in cancer and, if so, how? (ii) What role do they play in cancer? (iii) Might they have biomedical applications? Analysis of large-scale gene expression datasets indicated altered levels of MAL-family transcripts in specific cancer types. A comprehensive literature search provides evidence of MAL-family gene dysregulation and protein function repurposing in cancer. For MAL, and probably for other genes of the family, dysregulation is primarily a consequence of gene methylation, although copy number alterations also contribute to varying degrees. The scrutiny of the two sources of information, datasets and published studies, reveals potential prognostic applications of MAL-family members as cancer biomarkers-for instance, MAL2 in breast cancer, MAL2 and MALL in pancreatic cancer, and MAL and MYADM in lung cancer-and other biomedical uses. The availability of validated antibodies to some MAL-family proteins sanctions their use as cancer biomarkers in routine clinical practice.

Directed growth and fusion of membrane-wall microdomains requires CASP-mediated inhibition and displacement of secretory foci

Casparian strips (CS) are aligned bands of lignin-impregnated cell walls, building an extracellular diffusion barrier in roots. Their structure profoundly differs from tight junctions (TJ), analogous structures in animals. Nonetheless, CS membrane domain (CSD) proteins 1-5 (CASP1-5) are homologues of occludins, TJ components. CASP-marked membranes display cell wall (matrix) adhesion and membrane protein exclusion. A full CASP knock-out now reveals CASPs are not needed for localized lignification, since correctly positioned lignin microdomains still form in the mutant. Ultra-structurally, however, these microdomains are disorganized, showing excessive cell wall growth, lack of exclusion zone and matrix adhesion, and impaired exocyst dynamics. Proximity-labelling identifies a Rab-GTPase subfamily, known exocyst activators, as potential CASP-interactors and demonstrate their localization and function at the CSD. We propose that CASP microdomains displace initial secretory foci by excluding vesicle tethering factors, thereby ensuring rapid fusion of microdomains into a membrane-cell wall band that seals the extracellular space.

Rapid increase in transferrin receptor recycling promotes adhesion during T cell activation

Background

T cell activation leads to increased expression of the receptor for the iron transporter transferrin (TfR) to provide iron required for the cell differentiation and clonal expansion that takes place during the days after encounter with a cognate antigen. However, T cells mobilise TfR to their surface within minutes after activation, although the reason and mechanism driving this process remain unclear.

Results

Here we show that T cells transiently increase endocytic uptake and recycling of TfR upon activation, thereby boosting their capacity to import iron. We demonstrate that increased TfR recycling is powered by a fast endocytic sorting pathway relying on the membrane proteins flotillins, Rab5- and Rab11a-positive endosomes. Our data further reveal that iron import is required for a non-canonical signalling pathway involving the kinases Zap70 and PAK, which controls adhesion of the integrin LFA-1 and eventually leads to conjugation with antigen-presenting cells.

Conclusions

Altogether, our data suggest that T cells boost their iron importing capacity immediately upon activation to promote adhesion to antigen-presenting cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01386-0.

Transcriptional dynamics and epigenetic regulation of E and ID protein encoding genes during human T cell development

T cells are generated from hematopoietic stem cells through a highly organized developmental process, in which stage-specific molecular events drive maturation towards αβ and γδ T cells. Although many of the mechanisms that control αβ- and γδ-lineage differentiation are shared between human and mouse, important differences have also been observed. Here, we studied the regulatory dynamics of the E and ID protein encoding genes during pediatric human T cell development by evaluating changes in chromatin accessibility, histone modifications and bulk and single cell gene expression. We profiled patterns of ID/E protein activity and identified up- and downstream regulators and targets, respectively. In addition, we compared transcription of E and ID protein encoding genes in human versus mouse to predict both shared and unique activities in these species, and in prenatal versus pediatric human T cell differentiation to identify regulatory changes during development. This analysis showed a putative involvement of TCF3/E2A in the development of γδ T cells. In contrast, in αβ T cell precursors a pivotal pre-TCR-driven population with high ID gene expression and low predicted E protein activity was identified. Finally, in prenatal but not postnatal thymocytes, high HEB/TCF12 levels were found to counteract high ID levels to sustain thymic development. In summary, we uncovered novel insights in the regulation of E and ID proteins on a cross-species and cross-developmental level.

MALL, a membrane-tetra-spanning proteolipid overexpressed in cancer, is present in membraneless nuclear biomolecular condensates

Proteolipids are proteins with unusual lipid-like properties. It has long been established that PLP and plasmolipin, which are two unrelated membrane-tetra-spanning myelin proteolipids, can be converted in vitro into a water-soluble form with a distinct conformation, raising the question of whether these, or other similar proteolipids, can adopt two different conformations in the cell to adapt their structure to distinct environments. Here, we show that MALL, another proteolipid with a membrane-tetra-spanning structure, distributes in membranes outside the nucleus and, within the nucleus, in membrane-less, liquid-like PML body biomolecular condensates. Detection of MALL in one or other environment was strictly dependent on the method of cell fixation used, suggesting that MALL adopts different conformations depending on its physical environment —lipidic or aqueous— in the cell. The acquisition of the condensate-compatible conformation requires PML expression. Excess MALL perturbed the distribution of the inner nuclear membrane proteins emerin and LAP2β, and that of the DNA-binding protein BAF, leading to the formation of aberrant nuclei. This effect, which is consistent with studies identifying overexpressed MALL as an unfavorable prognostic factor in cancer, could contribute to cell malignancy. Our study establishes a link between proteolipids, membranes and biomolecular condensates, with potential biomedical implications.

CNS endothelial derived extracellular vesicles are biomarkers of active disease in multiple sclerosis

Background

Multiple sclerosis (MS) is a complex, heterogenous disease characterized by inflammation, demyelination, and blood–brain barrier (BBB) permeability. Currently, active disease is determined by physician confirmed relapse or detection of contrast enhancing lesions via MRI indicative of BBB permeability. However, clinical confirmation of active disease can be cumbersome. As such, disease monitoring in MS could benefit from identification of an easily accessible biomarker of active disease. We believe extracellular vesicles (EV) isolated from plasma are excellent candidates to fulfill this need. Because of the critical role BBB permeability plays in MS pathogenesis and identification of active disease, we sought to identify EV originating from central nervous system (CNS) endothelial as biomarkers of active MS. Because endothelial cells secrete more EV when stimulated or injured, we hypothesized that circulating concentrations of CNS endothelial derived EV will be increased in MS patients with active disease.

Methods

To test this, we developed a novel method to identify EV originating from CNS endothelial cells isolated from patient plasma using flow cytometry. Endothelial derived EV were identified by the absence of lymphocyte or platelet markers CD3 and CD41, respectively, and positive expression of pan-endothelial markers CD31, CD105, or CD144. To determine if endothelial derived EV originated from CNS endothelial cells, EV expressing CD31, CD105, or CD144 were evaluated for expression of the myelin and lymphocyte protein MAL, a protein specifically expressed by CNS endothelial cells compared to endothelial cells of peripheral organs.

Results

Quality control experiments indicate that EV detected using our flow cytometry method are 0.2 to 1 micron in size. Flow cytometry analysis of EV isolated from 20 healthy controls, 16 relapsing–remitting MS (RRMS) patients with active disease not receiving disease modifying therapy, 14 RRMS patients with stable disease not receiving disease modifying therapy, 17 relapsing-RRMS patients with stable disease receiving natalizumab, and 14 RRMS patients with stable disease receiving ocrelizumab revealed a significant increase in the plasma concentration of CNS endothelial derived EV in patients with active disease compared to all other groups (p = 0.001). Conclusions: For the first time, we have identified a method to identify CNS endothelial derived EV in circulation from human blood samples. Results from our pilot study indicate that increased levels of CNS endothelial derived EV may be a biomarker of BBB permeability and active disease in MS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00299-4.

Plasmolipin regulates basolateral-to-apical transcytosis of ICAM-1 and leukocyte adhesion in polarized hepatic epithelial cells

Apical localization of Intercellular Adhesion Receptor (ICAM)-1 regulates the adhesion and guidance of leukocytes across polarized epithelial barriers. Here, we investigate the molecular mechanisms that determine ICAM-1 localization into apical membrane domains of polarized hepatic epithelial cells, and their effect on lymphocyte-hepatic epithelial cell interaction. We had previously shown that segregation of ICAM-1 into apical membrane domains, which form bile canaliculi and bile ducts in hepatic epithelial cells, requires basolateral-to-apical transcytosis. Searching for protein machinery potentially involved in ICAM-1 polarization we found that the SNARE-associated protein plasmolipin (PLLP) is expressed in the subapical compartment of hepatic epithelial cells in vitro and in vivo. BioID analysis of ICAM-1 revealed proximal interaction between this adhesion receptor and PLLP. ICAM-1 colocalized and interacted with PLLP during the transcytosis of the receptor. PLLP gene editing and silencing increased the basolateral localization and reduced the apical confinement of ICAM-1 without affecting apicobasal polarity of hepatic epithelial cells, indicating that ICAM-1 transcytosis is specifically impaired in the absence of PLLP. Importantly, PLLP depletion was sufficient to increase T-cell adhesion to hepatic epithelial cells. Such an increase depended on the epithelial cell polarity and ICAM-1 expression, showing that the epithelial transcytotic machinery regulates the adhesion of lymphocytes to polarized epithelial cells. Our findings strongly suggest that the polarized intracellular transport of adhesion receptors constitutes a new regulatory layer of the epithelial inflammatory response.

Transcriptomic analysis revealed increased expression of genes involved in keratinization in the tears of COVID-19 patients

Recent studies have focused their attention on conjunctivitis as one of the symptoms of coronavirus disease 2019 (COVID-19). Therefore, tear samples were taken from COVID-19 patients and the presence of SARS-CoV-2 was evidenced using Real Time reverse transcription polymerase chain reaction. The main aim of this study was to analyze mRNA expression in the tears of patients with COVID-19 compared with healthy subjects using Next Generation Sequencing (NGS). The functional evaluation of the transcriptome highlighted 25 genes that differ statistically between healthy individuals and patients affected by COVID-19. In particular, the NGS analysis identified the presence of several genes involved in B cell signaling and keratinization. In particular, the genes involved in B cell signaling were downregulated in the tears of COVID-19 patients, while those involved in keratinization were upregulated. The results indicated that SARS-CoV-2 may induce a process of ocular keratinization and a defective B cell response.

The MAL Protein, an Integral Component of Specialized Membranes, in Normal Cells and Cancer

The MAL gene encodes a 17-kDa protein containing four putative transmembrane segments whose expression is restricted to human T cells, polarized epithelial cells and myelin-forming cells. The MAL protein has two unusual biochemical features. First, it has lipid-like properties that qualify it as a member of the group of proteolipid proteins. Second, it partitions selectively into detergent-insoluble membranes, which are known to be enriched in condensed cell membranes, consistent with MAL being distributed in highly ordered membranes in the cell. Since its original description more than thirty years ago, a large body of evidence has accumulated supporting a role of MAL in specialized membranes in all the cell types in which it is expressed. Here, we review the structure, expression and biochemical characteristics of MAL, and discuss the association of MAL with raft membranes and the function of MAL in polarized epithelial cells, T lymphocytes, and myelin-forming cells. The evidence that MAL is a putative receptor of the epsilon toxin of Clostridium perfringens, the expression of MAL in lymphomas, the hypermethylation of the MAL gene and subsequent loss of MAL expression in carcinomas are also presented. We propose a model of MAL as the organizer of specialized condensed membranes to make them functional, discuss the role of MAL as a tumor suppressor in carcinomas, consider its potential use as a cancer biomarker, and summarize the directions for future research.

Differentiation and activation of human CD4 T cells is associated with a gradual loss of myelin and lymphocyte protein

Upon generation of monoclonal antibodies to the T cell antigen receptor/CD3 (TCR/CD3) complex, we isolated mAb MT3, whose reactivity correlates inversely with the production of IFN‐γ by human peripheral blood T lymphocytes. Using eukaryotic expression cloning, we identified the MT3 antigen as myelin‐and‐lymphocyte (MAL) protein. Flow cytometry analysis demonstrates high surface expression of MAL on all naïve CD4⁺ T cells whereas MAL expression is diminished on central memory‐ and almost lost on effector memory T cells. MAL^– T cells proliferate strongly in response to stimulation with CD3/CD28 antibodies, corroborating that MAL⁺ T cells are naïve and MAL^– T cells memory subtypes. Further, resting MAL^– T cells harbor a larger pool of Ser59‐ and Tyr394‐ double phosphorylated lymphocyte‐specific kinase (Lck), which is rapidly increased upon in vitro restimulation. Previously, lack of MAL was reported to prevent transport of Lck, the key protein tyrosine kinase of TCR/CD3 signaling to the cell membrane, and to result in strongly impaired human T cell activation. Here, we show that knocking out MAL did not significantly affect Lck membrane localization and immune synapse recruitment, or transcriptional T cell activation. Collectively, our results indicate that loss of MAL is associated with activation‐induced differentiation of human T cells but not with impaired membrane localization of Lck or TCR signaling capacity.

A Model for Primary Cilium Biogenesis by Polarized Epithelial Cells: Role of the Midbody Remnant and Associated Specialized Membranes

Primary cilia are solitary, microtubule-based protrusions surrounded by a ciliary membrane equipped with selected receptors that orchestrate important signaling pathways that control cell growth, differentiation, development and homeostasis. Depending on the cell type, primary cilium assembly takes place intracellularly or at the cell surface. The intracellular route has been the focus of research on primary cilium biogenesis, whereas the route that occurs at the cell surface, which we call the "alternative" route, has been much less thoroughly characterized. In this review, based on recent experimental evidence, we present a model of primary ciliogenesis by the alternative route in which the remnant of the midbody generated upon cytokinesis acquires compact membranes, that are involved in compartmentalization of biological membranes. The midbody remnant delivers part of those membranes to the centrosome in order to assemble the ciliary membrane, thereby licensing primary cilium formation. The midbody remnant's involvement in primary cilium formation, the regulation of its inheritance by the ESCRT machinery, and the assembly of the ciliary membrane from the membranes originally associated with the remnant are discussed in the context of the literature concerning the ciliary membrane, the emerging roles of the midbody remnant, the regulation of cytokinesis, and the role of membrane compartmentalization. We also present a model of cilium emergence during evolution, and summarize the directions for future research.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Maternal Influenza A Virus Infection Restricts Fetal and Placental Growth and Adversely Affects the Fetal Thymic Transcriptome

Maternal influenza A viral infections in humans are associated with low birth weight, increased risk of pre-term birth, stillbirth and congenital defects. To examine the effect of maternal influenza virus infection on placental and fetal growth, pregnant C57BL/6 mice were inoculated intranasally with influenza A virus A/CA/07/2009 pandemic H1N1 or phosphate-buffered saline (PBS) at E3.5, E7.5 or E12.5, and the placentae and fetuses collected and weighed at E18.5. Fetal thymuses were pooled from each litter. Placentae were examined histologically, stained by immunohistochemistry (IHC) for CD34 (hematopoietic progenitor cell antigen) and vascular channels quantified. RNA from E7.5 and E12.5 placentae and E7.5 fetal thymuses was subjected to RNA sequencing and pathway analysis. Placental weights were decreased in litters inoculated with influenza at E3.5 and E7.5. Placentae from E7.5 and E12.5 inoculated litters exhibited decreased labyrinth development and the transmembrane protein 150A gene was upregulated in E7.5 placentae. Fetal weights were decreased in litters inoculated at E7.5 and E12.5 compared to controls. RNA sequencing of E7.5 thymuses indicated that 957 genes were downregulated ≥2-fold including Mal, which is associated with Toll-like receptor signaling and T cell differentiation. There were 28 upregulated genes. It is concluded that maternal influenza A virus infection impairs fetal thymic gene expression as well as restricting placental and fetal growth.

Spatiotemporal Regulation of Signaling: Focus on T Cell Activation and the Immunological Synapse

In a signaling network, not only the functions of molecules are important but when (temporal) and where (spatial) those functions are exerted and orchestrated is what defines the signaling output. To temporally and spatially modulate signaling events, cells generate specialized functional domains with variable lifetime and size that concentrate signaling molecules, enhancing their transduction potential. The plasma membrane is a key in this regulation, as it constitutes a primary signaling hub that integrates signals within and across the membrane. Here, we examine some of the mechanisms that cells exhibit to spatiotemporally regulate signal transduction, focusing on the early events of T cell activation from triggering of T cell receptor to formation and maturation of the immunological synapse.

Clostridium perfringens epsilon toxin induces blood brain barrier permeability via caveolae-dependent transcytosis and requires expression of MAL

Clostridium perfringens epsilon toxin (ETX) is responsible for causing the economically devastating disease, enterotoxaemia, in livestock. It is well accepted that ETX causes blood brain barrier (BBB) permeability, however the mechanisms involved in this process are not well understood. Using in vivo and in vitro methods, we determined that ETX causes BBB permeability in mice by increasing caveolae-dependent transcytosis in brain endothelial cells. When mice are intravenously injected with ETX, robust ETX binding is observed in the microvasculature of the central nervous system (CNS) with limited to no binding observed in the vasculature of peripheral organs, indicating that ETX specifically targets CNS endothelial cells. ETX binding to CNS microvasculature is dependent on MAL expression, as ETX binding to CNS microvasculature of MAL-deficient mice was not detected. ETX treatment also induces extravasation of molecular tracers including 376Da fluorescein salt, 60kDA serum albumin, 70kDa dextran, and 155kDA IgG. Importantly, ETX-induced BBB permeability requires expression of both MAL and caveolin-1, as mice deficient in MAL or caveolin-1 did not exhibit ETX-induced BBB permeability. Examination of primary murine brain endothelial cells revealed an increase in caveolae in ETX-treated cells, resulting in dynamin and lipid raft-dependent vacuolation without cell death. ETX-treatment also results in a rapid loss of EEA1 positive early endosomes and accumulation of large, RAB7-positive late endosomes and multivesicular bodies. Based on these results, we hypothesize that ETX binds to MAL on the apical surface of brain endothelial cells, causing recruitment of caveolin-1, triggering caveolae formation and internalization. Internalized caveolae fuse with early endosomes which traffic to late endosomes and multivesicular bodies. We believe that these multivesicular bodies fuse basally, releasing their contents into the brain parenchyma.

Clostridium perfringens epsilon toxin (ETX) is an extremely lethal bacterial toxin known to cause a devastating disease in livestock animals and may be a possible cause of multiple sclerosis in humans. ETX is well known to cause disruption of the blood-brain barrier (BBB), a critical structure necessary for proper brain function. Deterioration of this barrier allows entry of toxic blood-borne material to enter the brain. Although ETX-induced BBB dysfunction is well accepted, how this happens is unknown. Here, we demonstrate that ETX causes BBB permeability by inducing formation of cell-surface invaginations called caveolae in endothelial cells, the cells that line blood vessels. Importantly, only endothelial cells from the brain and other central nervous system organs appear to be a target of ETX, as the toxin only binds to blood vessels in these organs and not blood vessels from other organs. These ETX-induced caveolae fuse with other caveolae and specialized intracellular vesicles called endosomes. We predict that these endosomes engulf blood-borne material during their internalization, allowing material to travel from the blood, through the cell, and into brain tissue. We also show that expression of the protein MAL and caveolin-1 is necessary for ETX-induced BBB permeability.

A Fluorescent Kinase Inhibitor that Exhibits Diagnostic Changes in Emission upon Binding

The development of a fluorescent LCK inhibitor that exhibits favourable solvatochromic properties upon binding the kinase is described. Fluorescent properties were realised through the inclusion of a prodan-derived fluorophore into the pharmacophore of an ATP-competitive kinase inhibitor. Fluorescence titration experiments demonstrate the solvatochromic properties of the inhibitor, in which dramatic increase in emission intensity and hypsochromic shift in emission maxima are clearly observed upon binding LCK. Microscopy experiments in cellular contexts together with flow cytometry show that the fluorescence intensity of the inhibitor correlates with the LCK concentration. Furthermore, multiphoton microscopy experiments demonstrate both the rapid cellular uptake of the inhibitor and that the two-photon cross section of the inhibitor is amenable for excitation at 700 nm.

Orchestration of Immunological Synapse Assembly by Vesicular Trafficking

Ligation of the T-cell antigen receptor (TCR) by cognate peptide bound to the Major Histocompatibility Complex on the surface of an antigen-presenting cell (APC) leads to the spatial reorganization of the TCR and accessory receptors to form a specialized area of intimate contact between T cell and APC, known as the immunological synapse (IS), where signals are deciphered, coordinated, and integrated to promote T cell activation. With the discovery that an endosomal TCR pool contributes to IS assembly and function by undergoing polarized recycling to the IS, recent years have witnessed a shift from a plasma membrane-centric view of the IS to the vesicular trafficking events that occur at this location following the TCR-dependent translocation of the centrosome toward the synaptic membrane. Here we will summarize our current understanding of the trafficking pathways that are responsible for the steady delivery of endosomal TCRs, kinases, and adapters to the IS to sustain signaling, as well as of the endocytic pathways responsible for signal termination. We will also discuss recent evidence highlighting a role for endosomes in sustaining TCR signaling after its internalization at the IS and identifying the IS as a site of formation and release of extracellular vesicles that allow for transcellular communication with the APC.

On The Role of Myelin and Lymphocyte Protein (MAL) In Cancer: A Puzzle With Two Faces

Myelin and lymphocyte protein (MAL) is an integral membrane protein constituent of lipid rafts, and it is implicated in apical transport of proteins in polarized epithelial cells. However, beyond the involvement of MAL in apical sorting and as its function as a raft stabilizer, it is still not totally clear how MAL participates in cell proliferating processes. More controversial and interesting is the fact that MAL has been implicated in carcinogenesis in two opposite ways. First, this protein is overexpressed in ovarian cancer and some kinds of lymphomas where it seems to favor cancer progression. Conversely, it has been reported that downregulation of the MAL gene by promoter hypermethylation is a hallmark of several adenocarcinomas. So far, there is not enough experimental evidence to help us understand this phenomenon, and no MAL mutations or MAL isoforms have been associated with these opposite functions. This review provides an updated summary of the structure and functions of MAL, and we will discuss the possible mechanisms underlying its roles as a tumor suppressor and a tumor progression factor.

HIV-1 Nef Hijacks Lck and Rac1 Endosomal Traffic To Dually Modulate Signaling-Mediated and Actin Cytoskeleton-Mediated T Cell Functions

Endosomal traffic of TCR and signaling molecules regulates immunological synapse formation and T cell activation. We recently showed that Rab11 endosomes regulate the subcellular localization of the tyrosine kinase Lck and of the GTPase Rac1 and control their functions in TCR signaling and actin cytoskeleton remodeling. HIV-1 infection of T cells alters their endosomal traffic, activation capacity, and actin cytoskeleton organization. The viral protein Nef is pivotal for these modifications. We hypothesized that HIV-1 Nef could jointly alter Lck and Rac1 endosomal traffic and concomitantly modulate their functions. In this study, we show that HIV-1 infection of human T cells sequesters both Lck and Rac1 in a pericentrosomal compartment in an Nef-dependent manner. Strikingly, the Nef-induced Lck compartment contains signaling-competent forms (phosphorylated on key Tyr residues) of Lck and some of its downstream effectors, TCRζ, ZAP70, SLP76, and Vav1, avoiding the proximal LAT adaptor. Importantly, Nef-induced concentration of signaling molecules was concomitant with the upregulation of several early and late T cell activation genes. Moreover, preventing the concentration of the Nef-induced Lck compartment by depleting the Rab11 effector FIP3 counteracted Nef-induced gene expression upregulation. In addition, Nef extensively sequesters Rac1 and downregulates Rac1-dependent actin cytoskeleton remodeling, thus reducing T cell spreading. Therefore, by modifying their endosomal traffic, Nef hijacks signaling and actin cytoskeleton regulators to dually modulate their functional outputs. Our data shed new light into the molecular mechanisms that modify T cell physiology during HIV-1 infection.

Sailing to and Docking at the Immune Synapse: Role of Tubulin Dynamics and Molecular Motors

The different cytoskeleton systems and their connecting molecular motors move vesicles and intracellular organelles to shape cells. Polarized cells with specialized functions display an exquisite spatio-temporal regulation of both cytoskeletal and organelle arrangements that support their specific tasks. In particular, T cells rapidly change their shape and cellular function through the establishment of cell surface and intracellular polarity in response to a variety of cues. This review focuses on the contribution of the microtubule-based dynein/dynactin motor complex, the tubulin and actin cytoskeletons, and different organelles to the formation of the antigen-driven immune synapse.

Plasma membrane LAT activation precedes vesicular recruitment defining two phases of early T-cell activation

The relative importance of plasma membrane-localized LAT versus vesicular LAT for microcluster formation and T-cell receptor (TCR) activation is unclear. Here, we show the sequence of events in LAT microcluster formation and vesicle delivery, using lattice light sheet microscopy to image a T cell from the earliest point of activation. A kinetic lag occurs between LAT microcluster formation and vesicular pool recruitment to the synapse. Correlative 3D light and electron microscopy show an absence of vesicles at microclusters at early times, but an abundance of vesicles as activation proceeds. Using TIRF-SIM to look at the activated T-cell surface with high resolution, we capture directed vesicle movement between microclusters on microtubules. We propose a model in which cell surface LAT is recruited rapidly and phosphorylated at sites of T-cell activation, while the vesicular pool is subsequently recruited and dynamically interacts with microclusters.

Intrinsic properties and plasma membrane trafficking route of Src family kinase SH4 domains sensitive to retargeting by HIV-1 Nef

The HIV type 1 pathogenicity factor Nef enhances viral replication by modulating multiple host cell pathways, including tuning the activation state of infected CD4 T lymphocytes to optimize virus spread. For this, Nef inhibits anterograde transport of the Src family kinase (SFK) Lck toward the plasma membrane (PM). This leads to retargeting of the kinase to the trans-Golgi network, whereas the intracellular transport of a related SFK, Fyn, is unaffected by Nef. The 18-amino acid Src homology 4 (SH4) domain membrane anchor of Lck is necessary and sufficient for Nef-mediated retargeting, but other details of this process are not known. The goal of this study was therefore to identify characteristics of SH4 domains responsive to Nef and the transport machinery used. Screening a panel of SFK SH4 domains revealed two groups that were sensitive or insensitive for trans-Golgi network retargeting by Nef as well as the importance of the amino acid at position 8 for determining Nef sensitivity. Anterograde transport of Nef-sensitive domains was characterized by slower delivery to the PM and initial targeting to Golgi membranes, where transport was arrested in the presence of Nef. For Nef-sensitive SH4 domains, ectopic expression of the lipoprotein binding chaperone Unc119a or the GTPase Arl3 or reduction of their endogenous expression phenocopied the effect of Nef. Together, these results suggest that, analogous to K-Ras, Nef-sensitive SH4 domains are transported to the PM by a cycle of solubilization and membrane insertion and that intrinsic properties define SH4 domains as cargo of this Nef-sensitive lipoprotein binding chaperone-GTPase transport cycle.

Chimeric antigen receptor T cells form nonclassical and potent immune synapses driving rapid cytotoxicity

Davenport et al. discovered that the chimeric antigen receptor (CAR) immune synapse structure is different from the T cell receptor (TCR) synapse. The CAR immune synapse formed a disorganized pattern of Lck and more rapidly recruited lytic granules compared with the TCR. The differing immune synapse correlated with faster killing of tumor target cells and detachment from dying tumor cells by CAR-T cells. These findings provide a mechanism whereby CAR-T cells can effectively reduce large tumor burden in patients. This study will form a basis upon which to compare future receptor design to modulate signaling and programming of cytotoxic CAR-T cells to improve treatment of solid cancers.

Chimeric antigen receptor T (CAR-T) cells are effective serial killers with a faster off-rate from dying tumor cells than CAR-T cells binding target cells through their T cell receptor (TCR). Here we explored the functional consequences of CAR-mediated signaling using a dual-specific CAR-T cell, where the same cell was triggered via TCR (tcrCTL) or CAR (carCTL). The carCTL immune synapse lacked distinct LFA-1 adhesion rings and was less reliant on LFA to form stable conjugates with target cells. carCTL receptors associated with the synapse were found to be disrupted and formed a convoluted multifocal pattern of Lck microclusters. Both proximal and distal receptor signaling pathways were induced more rapidly and subsequently decreased more rapidly in carCTL than in tcrCTL. The functional consequence of this rapid signaling in carCTL cells included faster lytic granule recruitment to the immune synapse, correlating with faster detachment of the CTL from the target cell. This study provides a mechanism for how CAR-T cells can debulk large tumor burden quickly and may contribute to further refinement of CAR design for enhancing the quality of signaling and programming of the T cell.

Inverted formin 2 regulates intracellular trafficking, placentation, and pregnancy outcome

Healthy pregnancy depends on proper placentation-including proliferation, differentiation, and invasion of trophoblast cells-which, if impaired, causes placental ischemia resulting in intrauterine growth restriction and preeclampsia. Mechanisms regulating trophoblast invasion, however, are unknown. We report that reduction of Inverted formin 2 (INF2) alters intracellular trafficking and significantly impairs invasion in a model of human extravillous trophoblasts. Furthermore, global loss of Inf2 in mice recapitulates maternal and fetal phenotypes of placental insufficiency. Inf2^-/- dams have reduced spiral artery numbers and late gestational hypertension with resolution following delivery. Inf2^-/- fetuses are growth restricted and demonstrate changes in umbilical artery Doppler consistent with poor placental perfusion and fetal distress. Loss of Inf2 increases fetal vascular density in the placenta and dysregulates trophoblast expression of angiogenic factors. Our data support a critical regulatory role for INF2 in trophoblast invasion-a necessary process for placentation-representing a possible future target for improving placentation and fetal outcomes.

Inverted formins: A subfamily of atypical formins

Formins are a family of regulators of actin and microtubule dynamics that are present in almost all eukaryotes. These proteins are involved in many cellular processes, including cytokinesis, stress fiber formation, and cell polarization. Here we review one subfamily of formins, the inverted formins. Inverted formins as a group break several formin stereotypes, having atypical biochemical properties and domain organization, and they have been linked to kidney disease and neuropathy in humans. In this review, we will explore recent research on members of the inverted formin sub-family in mammals, zebrafish, fruit flies, and worms.

Rab11-FIP3 Regulation of Lck Endosomal Traffic Controls TCR Signal Transduction

The role of endosomes in receptor signal transduction is a long-standing question, which remains largely unanswered. The T cell Ag receptor and various components of its proximal signaling machinery are associated with distinct endosomal compartments, but how endosomal traffic affects T cell signaling remains ill-defined. In this article, we demonstrate in human T cells that the subcellular localization and function of the protein tyrosine kinase Lck depends on the Rab11 effector FIP3 (Rab11 family interacting protein-3). FIP3 overexpression or silencing and its ability to interact with Rab11 modify Lck subcellular localization and its delivery to the immunological synapse. Importantly, FIP3-dependent Lck localization controls early TCR signaling events, such as tyrosine phosphorylation of TCRζ, ZAP70, and LAT and intracellular calcium concentration, as well as IL-2 gene expression. Interestingly, FIP3 controls both steady-state and poststimulation phosphotyrosine and calcium levels. Finally, our findings indicate that FIP3 modulates TCR-CD3 cell surface expression via the regulation of steady-state Lck-mediated TCRζ phosphorylation, which in turn controls TCRζ protein levels. This may influence long-term T cell activation in response to TCR-CD3 stimulation. Therefore, our data underscore the importance of finely regulated endosomal traffic in TCR signal transduction and T cell activation leading to IL-2 production.

Imaging Vesicular Traffic at the Immune Synapse

Immunological synapse formation is the result of a profound T cell polarization process that involves the coordinated action of the actin and microtubule cytoskeleton, as well as intracellular vesicle traffic. Endosomal vesicle traffic ensures the targeting of the T cell receptor (TCR) and various signaling molecules to the synapse, being necessary for the generation of signaling complexes downstream of the TCR. Here we describe the microscopy imaging methods that we currently use to unveil how TCR and signaling molecules are associated with endosomal compartments and deliver their cargo to the immunological synapse.

Studying the Immune Synapse in HIV-1 Infection

T cells are the main cellular targets of the human immunodeficiency virus 1 (HIV-1). HIV-1 infection induces pleiotropic effects on the infected T cell that modify the T cell capacity to respond to antigen and facilitates virus replication. HIV-1 infection subverts the formation and function of the immunological synapse altering both actin cytoskeleton remodeling and intracellular vesicle traffic. We describe here our methods to unveil how HIV-1 and in particular its protein Nef modify vesicle traffic to the immunological synapse, perturbing the synapse activation capacity.

Aurora-A shines on T cell activation through the regulation of Lck

Different protein kinases control signaling emanating from the T cell receptor (TCR) during antigen-specific T cell activation. Mitotic kinases, e.g. Aurora-A, have been widely studied in the context of mitosis due to their role during microtubule (MT) nucleation, becoming critical regulators of cell cycle progression. We have recently described a specific role for Aurora-A kinase in antigenic T cell activation. Blockade of Aurora-A in T cells severely disrupts the dynamics of MTs and CD3ζ-bearing signaling vesicles during T cell activation. Furthermore, Aurora-A deletion impairs the activation of signaling molecules downstream of the TCR. Targeting Aurora-A disturbs the activation of Lck, which is one of the first signals that drive T cell activation in an antigen-dependent manner. This work describes possible models of regulation of Lck by Aurora-A during T cell activation. We also discuss possible roles for Aurora-A in other systems similar to the IS, and its putative functions in cell polarization.

HIV-1 Nef Impairs the Formation of Calcium Membrane Territories Controlling the Signaling Nanoarchitecture at the Immunological Synapse

The ability of HIV-1 to replicate and to establish long-term reservoirs is strongly influenced by T cell activation. Through the use of membrane-tethered, genetically encoded calcium (Ca²⁺) indicators, we were able to detect for the first time, to our knowledge, the formation of Ca²⁺ territories and determine their role in coordinating the functional signaling nanostructure of the synaptic membrane. Consequently, we report a previously unknown immune subversion mechanism involving HIV-1 exploitation, through its Nef accessory protein, of the interconnectivity among three evolutionarily conserved cellular processes: vesicle traffic, signaling compartmentalization, and the second messenger Ca²⁺ We found that HIV-1 Nef specifically associates with the traffic regulators MAL and Rab11b compelling the vesicular accumulation of Lck. Through its association with MAL and Rab11b, Nef co-opts Lck switchlike function driving the formation Ca²⁺ membrane territories, which, in turn, control the fusion of LAT-transporting Rab27 and Rab37 vesicles and the formation of LAT nanoclusters at the immunological synapse. Consequently, HIV-1 Nef disengages TCR triggering from the generation of p-LAT and p-SLP nanoclusters driving TCR signal amplification and diversification. Altogether our results indicate that HIV-1 exploits the interconnectivity among vesicle traffic, Ca²⁺ membrane territories, and signaling nanoclusters to modulate T cell signaling and function.

Biogenesis and Function of T Cell-Derived Exosomes

Exosomes are a particular type of extracellular vesicle, characterized by their endosomal origin as intraluminal vesicles present in large endosomes with a multivesicular structure. After these endosomes fuse with the plasma membrane, exosomes are secreted into the extracellular space. The ability of exosomes to carry and selectively deliver bioactive molecules (e.g., lipids, proteins, and nucleic acids) confers on them the capacity to modulate the activity of receptor cells, even if these cells are located in distant tissues or organs. Since exosomal cargo depends on cell type, a detailed understanding of the mechanisms that regulate the biochemical composition of exosomes is fundamental to a comprehensive view of exosome function. Here, we review the latest advances concerning exosome function and biogenesis in T cells, with particular focus on the mechanism of protein sorting at multivesicular endosomes. Exosomes secreted by specific T-cell subsets can modulate the activity of immune cells, including other T-cell subsets. Ceramide, tetraspanins and MAL have been revealed to be important in exosome biogenesis by T cells. These molecules, therefore, constitute potential molecular targets for artificially modulating exosome production and, hence, the immune response for therapeutic purposes.

Orchestrating Lymphocyte Polarity in Cognate Immune Cell-Cell Interactions

The immune synapse (IS) is a specialized structure established between different immune cells that fulfills several functions, including a role as a communication bridge. This intimate contact between a T cell and an antigen-presenting cell promotes the proliferation and differentiation of lymphocytes involved in the contact. T-cell activation requires the specific triggering of the T-cell receptor (TCR), which promotes the activation of different signaling pathways inducing the polarization of the T cell. During this process, different adhesion and signaling receptors reorganize at specialized membrane domains, concomitantly to the polarization of the tubulin and actin cytoskeletons, forming stable polarization platforms. The centrosome also moves toward the IS, driving the movement of different organelles, such as the biosynthetic, secretory, degrading machinery, and mitochondria, to sustain T-cell activation. A proper orchestration of all these events is essential for T-cell effector functions and the accomplishment of a complete immune response.

New Insights into How Trafficking Regulates T Cell Receptor Signaling

There is emerging evidence that exocytosis plays an important role in regulating T cell receptor (TCR) signaling. The trafficking molecules involved in lytic granule (LG) secretion in cytotoxic T lymphocytes (CTL) have been well-studied due to the immune disorder known as familial hemophagocytic lymphohistiocytosis (FHLH). However, the knowledge of trafficking machineries regulating the exocytosis of receptors and signaling molecules remains quite limited. In this review, we summarize the reported trafficking molecules involved in the transport of the TCR and downstream signaling molecules to the cell surface. By combining this information with the known knowledge of LG exocytosis and general exocytic trafficking machinery, we attempt to draw a more complete picture of how the TCR signaling network and exocytic trafficking matrix are interconnected to facilitate T cell activation. This also highlights how membrane compartmentalization facilitates the spatiotemporal organization of cellular responses that are essential for immune functions.

Rac1-Rab11-FIP3 regulatory hub coordinates vesicle traffic with actin remodeling and T-cell activation

The immunological synapse generation and function is the result of a T-cell polarization process that depends on the orchestrated action of the actin and microtubule cytoskeleton and of intracellular vesicle traffic. However, how these events are coordinated is ill defined. Since Rab and Rho families of GTPases control intracellular vesicle traffic and cytoskeleton reorganization, respectively, we investigated their possible interplay. We show here that a significant fraction of Rac1 is associated with Rab11-positive recycling endosomes. Moreover, the Rab11 effector FIP3 controls Rac1 intracellular localization and Rac1 targeting to the immunological synapse. FIP3 regulates, in a Rac1-dependent manner, key morphological events, like T-cell spreading and synapse symmetry. Finally, Rab11-/FIP3-mediated regulation is necessary for T-cell activation leading to cytokine production. Therefore, Rac1 endosomal traffic is key to regulate T-cell activation.

Aurora A drives early signalling and vesicle dynamics during T-cell activation

Aurora A is a serine/threonine kinase that contributes to the progression of mitosis by inducing microtubule nucleation. Here we have identified an unexpected role for Aurora A kinase in antigen-driven T-cell activation. We find that Aurora A is phosphorylated at the immunological synapse (IS) during TCR-driven cell contact. Inhibition of Aurora A with pharmacological agents or genetic deletion in human or mouse T cells severely disrupts the dynamics of microtubules and CD3ζ-bearing vesicles at the IS. The absence of Aurora A activity also impairs the activation of early signalling molecules downstream of the TCR and the expression of IL-2, CD25 and CD69. Aurora A inhibition causes delocalized clustering of Lck at the IS and decreases phosphorylation levels of tyrosine kinase Lck, thus indicating Aurora A is required for maintaining Lck active. These findings implicate Aurora A in the propagation of the TCR activation signal.

Aurora A is a protein kinase that contributes to the progression of mitosis by stimulating microtubule nucleation. Here the authors show that Aurora A also functions during T cell activation by maintaining TCR signaling through Lck activation.

Cutting Edge: Regulation of Exosome Secretion by the Integral MAL Protein in T Cells

Exosomes secreted by T cells play an important role in coordinating the immune response. HIV-1 Nef hijacks the route of exosome secretion of T cells to modulate the functioning of uninfected cells. Despite the importance of the process, the protein machinery involved in exosome biogenesis is yet to be identified. In this study, we show that MAL, a tetraspanning membrane protein expressed in human T cells, is present in endosomes that travel toward the plasma membrane for exosome secretion. In the absence of MAL, the release of exosome particles and markers was greatly impaired. This effect was accompanied by protein sorting defects at multivesicular endosomes that divert the exosomal marker CD63 to autophagic vacuoles. Exosome release induced by HIV-1 Nef was also dependent on MAL expression. Therefore, MAL is a critical element of the machinery for exosome secretion and may constitute a target for modulating exosome secretion by human T cells.

The Myelin and Lymphocyte Protein MAL Is Required for Binding and Activity of Clostridium perfringens ε-Toxin

Clostridium perfringens ε-toxin (ETX) is a potent pore-forming toxin responsible for a central nervous system (CNS) disease in ruminant animals with characteristics of blood-brain barrier (BBB) dysfunction and white matter injury. ETX has been proposed as a potential causative agent for Multiple Sclerosis (MS), a human disease that begins with BBB breakdown and injury to myelin forming cells of the CNS. The receptor for ETX is unknown. Here we show that both binding of ETX to mammalian cells and cytotoxicity requires the tetraspan proteolipid Myelin and Lymphocyte protein (MAL). While native Chinese Hamster Ovary (CHO) cells are resistant to ETX, exogenous expression of MAL in CHO cells confers both ETX binding and susceptibility to ETX-mediated cell death. Cells expressing rat MAL are ~100 times more sensitive to ETX than cells expressing similar levels of human MAL. Insertion of the FLAG sequence into the second extracellular loop of MAL abolishes ETX binding and cytotoxicity. ETX is known to bind specifically and with high affinity to intestinal epithelium, renal tubules, brain endothelial cells and myelin. We identify specific binding of ETX to these structures and additionally show binding to retinal microvasculature and the squamous epithelial cells of the sclera in wild-type mice. In contrast, there is a complete absence of ETX binding to tissues from MAL knockout (MAL-/-) mice. Furthermore, MAL-/- mice exhibit complete resistance to ETX at doses in excess of 1000 times the symptomatic dose for wild-type mice. We conclude that MAL is required for both ETX binding and cytotoxicity.

Regulation of vesicular traffic at the T cell immune synapse: lessons from the primary cilium

The signals that orchestrate the process of T cell activation are coordinated at the specialized interface that forms upon contact with an antigen presenting cell displaying a specific MHC-associated peptide ligand, known as the immune synapse. The central role of vesicular traffic in the assembly of the immune synapse has emerged only in recent years with the finding that sustained T-cell receptor (TCR) signaling involves delivery of TCR/CD3 complexes from an intracellular pool associated with recycling endosomes. A number of receptors as well as membrane-associated signaling mediators have since been demonstrated to exploit this process to localize to the immune synapse. Here, we will review our current understanding of the mechanisms responsible for TCR recycling, with a focus on the intraflagellar transport system, a multimolecular complex that is responsible for the assembly and function of the primary cilium which we have recently implicated in polarized endosome recycling to the immune synapse.

The late endosomal transporter CD222 directs the spatial distribution and activity of Lck

The spatial and temporal organization of T cell signaling molecules is increasingly accepted as a crucial step in controlling T cell activation. CD222, also known as the cation-independent mannose 6-phosphate/insulin-like growth factor 2 receptor, is the central component of endosomal transport pathways. In this study, we show that CD222 is a key regulator of the early T cell signaling cascade. Knockdown of CD222 hampers the effective progression of TCR-induced signaling and subsequent effector functions, which can be rescued via reconstitution of CD222 expression. We decipher that Lck is retained in the cytosol of CD222-deficient cells, which obstructs the recruitment of Lck to CD45 at the cell surface, resulting in an abundant inhibitory phosphorylation signature on Lck at the steady state. Hence, CD222 specifically controls the balance between active and inactive Lck in resting T cells, which guarantees operative T cell effector functions.

Orchestrating cytoskeleton and intracellular vesicle traffic to build functional immunological synapses

Immunological synapses are specialized cell-cell contacts formed between T lymphocytes and antigen-presenting cells. They are induced upon antigen recognition and are crucial for T-cell activation and effector functions. The generation and function of immunological synapses depend on an active T-cell polarization process, which results from a finely orchestrated crosstalk between the antigen receptor signal transduction machinery, the actin and microtubule cytoskeletons, and controlled vesicle traffic. Although we understand how some of these particular events are regulated, we still lack knowledge on how these multiple cellular elements are harmonized to ensure appropriate T-cell responses. We discuss here our view on how T-cell receptor signal transduction initially commands cytoskeletal and vesicle traffic polarization, which in turn sets the immunological synapse molecular design that regulates T-cell activation. We also discuss how the human immunodeficiency virus (HIV-1) hijacks some of these processes impairing immunological synapse generation and function.

Lck mediates signal transmission from CD59 to the TCR/CD3 pathway in Jurkat T cells

The glycosylphosphatidylinositol (GPI)-anchored molecule CD59 has been implicated in the modulation of T cell responses, but the underlying molecular mechanism of CD59 influencing T cell signaling remained unclear. Here we analyzed Jurkat T cells stimulated via anti-CD3ε- or anti-CD59-coated surfaces, using time-resolved single-cell Ca²⁺ imaging as a read-out for stimulation. This analysis revealed a heterogeneous Ca²⁺ response of the cell population in a stimulus-dependent manner. Further analysis of T cell receptor (TCR)/CD3 deficient or overexpressing cells showed that CD59-mediated signaling is strongly dependent on TCR/CD3 surface expression. In protein co-patterning and fluorescence recovery after photobleaching experiments no direct physical interaction was observed between CD59 and CD3 at the plasma membrane upon anti-CD59 stimulation. However, siRNA-mediated protein knock-downs of downstream signaling molecules revealed that the Src family kinase Lck and the adaptor molecule linker of activated T cells (LAT) are essential for both signaling pathways. Furthermore, flow cytometry measurements showed that knock-down of Lck accelerates CD3 re-expression at the cell surface after anti-CD59 stimulation similar to what has been observed upon direct TCR/CD3 stimulation. Finally, physically linking Lck to CD3ζ completely abolished CD59-triggered Ca²⁺ signaling, while signaling was still functional upon direct TCR/CD3 stimulation. Altogether, we demonstrate that Lck mediates signal transmission from CD59 to the TCR/CD3 pathway in Jurkat T cells, and propose that CD59 may act via Lck to modulate T cell responses.

The role of membrane rafts in Lck transport, regulation and signalling in T-cells

Tyrosine phosphorylation is one of the key covalent modifications that occur in multicellular organisms. Since its discovery more than 30 years ago, tyrosine phosphorylation has come to be understood as a fundamentally important mechanism of signal transduction and regulation in all eukaryotic cells. The tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) plays a crucial role in the T-cell response by transducing early activation signals triggered by TCR (T-cell receptor) engagement. These signals result in the phosphorylation of immunoreceptor tyrosine-based activation motifs present within the cytosolic tails of the TCR-associated CD3 subunits that, once phosphorylated, serve as scaffolds for the assembly of a large supramolecular signalling complex responsible for T-cell activation. The existence of membrane nano- or micro-domains or rafts as specialized platforms for protein transport and cell signalling has been proposed. The present review discusses the signals that target Lck to membrane rafts and the importance of these specialized membranes in the transport of Lck to the plasma membrane, the regulation of Lck activity and the phosphorylation of the TCR.

Regulated vesicle fusion generates signaling nanoterritories that control T cell activation at the immunological synapse

The assembly of signaling nanoterritories at the T cell immunological synapse is controlled by the coordinated trafficking and fusion of specific vesicles containing the signaling molecules Lck, LAT, and TCRζ.

How the vesicular traffic of signaling molecules contributes to T cell receptor (TCR) signal transduction at the immunological synapse remains poorly understood. In this study, we show that the protein tyrosine kinase Lck, the TCRζ subunit, and the adapter LAT traffic through distinct exocytic compartments, which are released at the immunological synapse in a differentially regulated manner. Lck vesicular release depends on MAL protein. Synaptic Lck, in turn, conditions the calcium- and synaptotagmin-7–dependent fusion of LAT and TCRζ containing vesicles. Fusion of vesicles containing TCRζ and LAT at the synaptic membrane determines not only the nanoscale organization of phosphorylated TCRζ, ZAP70, LAT, and SLP76 clusters but also the presence of phosphorylated LAT and SLP76 in interacting signaling nanoterritories. This mechanism is required for priming IL-2 and IFN-γ production and may contribute to fine-tuning T cell activation breadth in response to different stimulatory conditions.

Do signalling endosomes play a role in T cell activation?

Signalling endosomes represent a general mechanism for modulating and compartmentalizing cell signalling, which is achieved by delineating specific spatial environments and connecting the plasma membrane with intracellular events. The molecular composition of vesicles, together with their targeting mechanisms and endocytic routes, contributes to the outcome of signalling pathways that are initiated either at the plasma membrane or within endosomes themselves. In T cell signalling, it is now accepted that the spatial distribution of signalling proteins is central to T cell activation not only at the immunological synapse, but also in endosomes travelling to and from the plasma membrane. In addition, there is a global rearrangement of the endosome machinery upon T cell activation, and emerging experimental evidence suggests that vesicles in T cells contain key T cell signalling proteins. We review the various mechanisms by which endosomes contribute to signalling pathways and consider whether signalling endosomes play a role in T cell signalling.