Interplay between TCR signalling and actin cytoskeleton dynamics

Current state of cancer immunity cycle: new strategies and challenges of using precision hydrogels to treat breast cancer

The cancer-immunity cycle provides a framework for a series of events in anti-cancer immune responses, initiated by T cell-mediated tumor cell killing, which leads to antigen presentation and T cell stimulation. Current immunomodulatory therapies for breast cancer are often associated with short duration, poor targeting to sites of action, and severe side effects. Hydrogels, with their extracellular matrix-mimicking properties, tunable characteristics, and diverse bioactivities, have garnered significant attention for their ability to locally deliver immunomodulators and cells, providing an immunomodulatory microenvironment to recruit, activate, and expand host immune cells. This review focuses on the design considerations of hydrogel platforms, including polymer backbone, crosslinking mechanisms, physicochemical properties, and immunomodulatory components. The immunomodulatory effects and therapeutic outcomes of various hydrogel systems in breast cancer treatment and tissue regeneration are highlighted, encompassing hydrogel depots for immunomodulator delivery, hydrogel scaffolds for cell delivery, and immunomodulatory hydrogels dependent on inherent material properties. Finally, the challenges that persist in current systems and future directions for immunomodulatory hydrogels are discussed.

New Therapeutic Approaches Using Hydrogen Sulfide Donors in Inflammation and Immune Response

Significance: Inflammation and immune response are associated with many pathological disorders, including rheumatoid arthritis, lupus, heart failure, and cancer(s). In recent times, important roles of hydrogen sulfide (H₂S) have been evidenced by researchers in inflammatory responses, as well as immunomodulatory effects in several disease models. Recent Advances: Numerous biological targets, including cytochrome c oxidase, various kinases, enzymes involved in epigenetic changes, transcription factors, namely nuclear factor kappa B and nuclear factor erythroid 2-related factor 2, and several membrane ion channels, are shown to be sensitive to H₂S and have been widely investigated in various preclinical models. Critical Issues: A complete understanding of the effects of H₂S in inflammatory and immune response is vital in the development of novel H₂S generating therapeutics. In this review, the biological effects and pharmacological properties of H₂S in inflammation and immune response are addressed. The review also covers some of the novel H₂S releasing prodrugs developed in recent years as tools to study this fascinating molecule. Future Directions: H₂S plays important roles in inflammation and immunity-related processes. Future researches are needed to further assess the immunomodulatory effects of H₂S and to assist in the design of more efficient H₂S carrier systems, or drug formulations, for the management of immune-related conditions in humans. Antioxid. Redox Signal. 35, 341-356.

Cofilin1-driven actin dynamics controls migration of thymocytes and is essential for positive selection in the thymus

Actin dynamics is essential for T-cell development. We show here that cofilin1 is the key molecule for controlling actin filament turnover in this process. Mice with specific depletion of cofilin1 in thymocytes showed increased steady-state levels of actin filaments, and associated alterations in the pattern of thymocyte migration and adhesion. Our data suggest that cofilin1 is controlling oscillatory F-actin changes, a parameter that influences the migration pattern in a 3-D environment. In a collagen matrix, cofilin1 controls the speed and resting intervals of migrating thymocytes. Cofilin1 was not involved in thymocyte proliferation, cell survival, apoptosis or surface receptor trafficking. However, in cofilin1 mutant mice, impaired adhesion and migration resulted in a specific block of thymocyte differentiation from CD4/CD8 double-positive thymocytes towards CD4 and CD8 single-positive cells. Our data suggest that tuning of the dwelling time of thymocytes in the thymic niches is tightly controlled by cofilin1 and essential for positive selection during T-cell differentiation. We describe a novel role of cofilin1 in the physiological context of migration-dependent cell differentiation.

T cell immunoengineering with advanced biomaterials

Recent advances in biomaterials design offer the potential to actively control immune cell activation and behaviour. Many human diseases, such as infections, cancer, and autoimmune disorders, are partly mediated by inappropriate or insufficient activation of the immune system. T cells play a central role in the host immune response to these diseases, and so constitute a promising cell type for manipulation. In vivo, T cells are stimulated by antigen presenting cells (APC), therefore to design immunoengineering biomaterials that control T cell behaviour, artificial interfaces that mimic the natural APC-T cell interaction are required. This review draws together research in the design and fabrication of such biomaterial interfaces, and highlights efforts to elucidate key parameters in T cell activation, such as substrate mechanical properties and spatial organization of receptors, illustrating how they can be manipulated by bioengineering approaches to alter T cell function.

Lower Affinity T Cells are Critical Components and Active Participants of the Immune Response

Kinetic and biophysical parameters of T cell receptor (TCR) and peptide:MHC (pMHC) interaction define intrinsic factors required for T cell activation and differentiation. Although receptor ligand kinetics are somewhat cumbersome to assess experimentally, TCR:pMHC affinity has been shown to predict peripheral T cell functionality and potential for forming memory. Multimeric forms of pMHC monomers have often been used to provide an indirect readout of higher affinity T cells due to their availability and ease of use while allowing simultaneous definition of other functional and phenotypic characteristics. However, multimeric pMHC reagents have introduced a bias that underestimates the lower affinity components contained in the highly diverse TCR repertoires of all polyclonal T cell responses. Advances in the identification of lower affinity cells have led to the examination of these cells and their contribution to the immune response. In this review, we discuss the identification of high- vs. low-affinity T cells as well as their attributed signaling and functional differences. Lastly, mechanisms are discussed that maintain a diverse range of low- and high-affinity T cells.

CD47-dependent regulation of H₂S biosynthesis and signaling in T cells

Pharmacological concentrations of H2S donors inhibit some T cell functions by inhibiting mitochondrial function, but evidence is also emerging that H2S at physiological concentrations produced via chemical sources and endogenously is a positive physiological mediator of T cell function. Expression of the H2S biosynthetic enzymes cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS) is induced in response to T cell receptor signaling. Inhibiting the induction of these enzymes limits T cell activation and proliferation, which can be overcome by exposure to exogenous H2S at submicromolar concentrations. Exogenous H2S at physiological concentrations increases the ability of T cells to form an immunological synapse by altering cytoskeletal actin dynamics and increasing the reorientation of the microtubule-organizing center. Downstream, H2S enhances T cell receptor-dependent induction of CD69, CD25, and Interleukin-2 (IL-2) gene expression. The T cell stimulatory activity of H2S is enhanced under hypoxic conditions that limit its oxidative metabolism by mitochondrial and nonenzymatic processes. Studies of the receptor CD47 have revealed the first endogenous inhibitory signaling pathway that regulates H2S signaling in T cells. Binding of the secreted protein thrombospondin-1 to CD47 elicits signals that block the stimulatory activity of exogenous H2S on T cell activation and limit the induction of CSE and CBS gene expression. CD47 signaling thereby inhibits T cell receptor-mediated T cell activation.

Hydrogen sulfide is an endogenous potentiator of T cell activation

H(2)S is an endogenous signaling molecule that may act via protein sulfhydrylation to regulate various physiological functions. H(2)S is also a byproduct of dietary sulfate metabolism by gut bacteria. Inflammatory bowel diseases such as ulcerative colitis are associated with an increase in the colonization of the intestine by sulfate reducing bacteria along with an increase in H(2)S production. Consistent with its increased production, H(2)S is implicated as a mediator of ulcerative colitis both in its genesis or maintenance. As T cells are well established mediators of inflammatory bowel disease, we investigated the effect of H(2)S exposure on T cell activation. Using primary mouse T lymphocytes (CD3+), OT-II CD4+ T cells, and the human Jurkat T cell line, we show that physiological levels of H(2)S potentiate TCR-induced activation. Nanomolar levels of H(2)S (50-500 nM) enhance T cell activation assessed by CD69 expression, interleukin-2 expression, and CD25 levels. Exposure of T cells to H(2)S dose-dependently enhances TCR-stimulated proliferation with a maximum at 300 nM (30% increase, p < 0.01). Furthermore, activation increases the capacity of T cells to make H(2)S via increased expression of cystathionine γ-lyase and cystathionine β-synthase. Disrupting this response by silencing these H(2)S producing enzymes impairs T cell activation, and proliferation and can be rescued by the addition of 300 nM H(2)S. Thus, H(2)S represents a novel autocrine immunomodulatory molecule in T cells.

Signaling clusters in the cell membrane

Large-scale molecular assemblies, or signaling clusters, at the cell membrane are emerging as important regulators of cell signaling. Here, we review new findings and describe shared characteristics common to signaling clusters from a diverse set of cellular systems. The well-known T cell receptor cluster serves as our paradigmatic model. Specifically, each cluster initiates recruitment of hundreds of molecules to the membrane, interacts with the actin cytoskeleton, and contains a significant fraction of the entire signaling process. Probed by recent advancements in patterning and microscopy techniques, the signaling clusters display functional outcomes that are not readily predictable from the individual components.

Acute endotoxin-induced thymic atrophy is characterized by intrathymic inflammatory and wound healing responses

Background

Productive thymopoiesis is essential for a robust and healthy immune system. Thymus unfortunately is acutely sensitive to stress resulting in involution and decreased T cell production. Thymic involution is a complication of many clinical settings, including infection, malnutrition, starvation, and irradiation or immunosuppressive therapies. Systemic rises in glucocorticoids and inflammatory cytokines are known to contribute to thymic atrophy. Little is known, however, about intrathymic mechanisms that may actively contribute to thymus atrophy or initiate thymic recovery following stress events.

Methodology/Principal Findings

Phenotypic, histologic and transcriptome/pathway analysis of murine thymic tissue during the early stages of endotoxemia-induced thymic involution was performed to identify putative mechanisms that drive thymic involution during stress. Thymus atrophy in this murine model was confirmed by down-regulation of genes involved in T cell development, cell activation, and cell cycle progression, correlating with observed phenotypic and histologic thymus involution. Significant gene changes support the hypothesis that multiple key intrathymic pathways are differentially activated during stress-induced thymic involution. These included direct activation of thymus tissue by LPS through TLR signaling, local expression of inflammatory cytokines, inhibition of T cell signaling, and induction of wound healing/tissue remodeling.

Conclusions/Significance

Taken together, these observations demonstrated that in addition to the classic systemic response, a direct intrathymic response to endotoxin challenge concurrently contributes to thymic involution during endotoxemia. These findings are a substantial advancement over current understanding of thymus response to stress and may lead to the development of novel therapeutic approaches to ameliorate immune deficiency associated with stress events.

HIV-1 envelope, integrins and co-receptor use in mucosal transmission of HIV

It is well established that HIV-1 infection typically involves an interaction between the viral envelope protein gp120/41 and the CD4 molecule followed by a second interaction with a chemokine receptor, usually CCR5 or CXCR4. In the early stages of an HIV-1 infection CCR5 using viruses (R5 viruses) predominate. In some viral subtypes there is a propensity to switch to CXCR4 usage (X4 viruses). The receptor switch occurs in ~ 40% of the infected individuals and is associated with faster disease progression. This holds for subtypes B and D, but occurs less frequently in subtypes A and C. There are several hypotheses to explain the preferential transmission of R5 viruses and the mechanisms that lead to switching of co-receptor usage; however, there is no definitive explanation for either. One important consideration regarding transmission is that signaling by R5 gp120 may facilitate transmission of R5 viruses by inducing a permissive environment for HIV replication. In the case of sexual transmission, infection by HIV requires the virus to breach the mucosal barrier to gain access to the immune cell targets that it infects; however, the immediate events that follow HIV exposure at genital mucosal sites are not well understood. Upon transmission, the HIV quasispecies that is replicating in an infected donor contracts through a “genetic bottleneck”, and often infection results from a single infectious event. Many details surrounding this initial infection remain unresolved. In mucosal tissues, CD4⁺ T cells express high levels of CCR5, and a subset of these CD4⁺/CCR5^high cells express the integrin α₄β₇, the gut homing receptor. CD4⁺/CCR5^high/ α4β7^high T cells are highly susceptible to infection by HIV-1 and are ideal targets for an efficient productive infection at the point of transmission. In this context we have demonstrated that the HIV-1 envelope protein gp120 binds to α₄β₇ on CD4⁺ T cells. On CD4⁺/CCR5^high/ α4β7^high T cells, α₄β₇ is closely associated with CD4 and CCR5. Furthermore, α₄β₇ is ~3 times the size of CD4 on the cell surface, that makes it a prominent receptor for an efficient virus capture. gp120-α₄β₇ interactions mediate the activation of the adhesion-associated integrin LFA-1. LFA-1 facilitates the formation of virological synapses and cell-to-cell spread of HIV-1. gp120 binding to α₄β₇ is mediated by a tripeptide located in the V1/V2 domain of gp120. Of note, the V1/V2 domain of gp120 has been linked to variations in transmission fitness among viral isolates raising the intriguing possibility that gp120-α₄β₇ interactions may be linked to transmission fitness. Although many details remain unresolved, we hypothesize that gp120-α₄β₇ interactions play an important role in the very early events following sexual transmission of HIV and may have important implication in the design of vaccine strategies for the prevention of acquisition of HIV infection

Age-related defects in the cytoskeleton signaling pathways of CD4 T cells

It has been postulated that the cytoskeleton controls many aspects of T cell function, including activation, proliferation and apoptosis. Recent advances in our understanding of F-actin polymerization and the Ezrin-Radixin-Moesin (ERM) family of cytoskeleton signal proteins have provided new insights into immunological synapse formation during T cell activation. During aging there is a significant decline of T cell function largely attributable to declines in activation of CD4 T cells and defects in the formation of the immunological synapse. Here we discuss recent progress in the understanding of how aging alters F-actin and ERM proteins in mouse CD4 T cells, and the implications of these changes for the T cell activation process.

Signaling microdomains in T cells

Sub-micron scale signaling domains induced in the plasma membrane of cells are thought to play important roles in signal transduction. In T cells, agonist MHC-peptide complexes induce small diffraction-limited domains enriched in T cell receptor (TCR) and signaling molecules. These microclusters serve as transient platforms for signal initiation and are required for sustained signaling in T cells, although each microcluster functions for only a couple of minutes. How they are formed, and what mechanisms promote and regulate signaling within TCR microclusters is largely unknown, although it is clear that TCR engagement and dynamic reorganization of cortical actin are involved. Here, we review current understanding of signaling within microclusters in T cells, and speculate on how these structures may form, initiate biochemical signals, and serve as sites of both signal integration and amplification, while also facilitating appropriate termination of TCR and related signaling.

The role of multiple toll-like receptor signalling cascades on interactions between biomedical polymers and dendritic cells

Biomaterials are used in several health-related applications ranging from tissue regeneration to antigen-delivery systems. Yet, biomaterials often cause inflammatory reactions suggesting that they profoundly alter the homeostasis of host immune cells such as dendritic cells (DCs). Thus, there is a major need to understand how biomaterials affect the function of these cells. In this study, we have analysed the influence of chemically and physically diverse biomaterials on DCs using several murine knockouts. DCs can sense biomedical polymers through a mechanism, which involves multiple TLR/MyD88-dependent signalling pathways, in particular TLR2, TLR4 and TLR6. TLR-biomaterial interactions induce the expression of activation markers and pro-inflammatory cytokines and are sufficient to confer on DCs the ability to activate antigen-specific T cells. This happens through a direct biomaterial-DC interaction although, for degradable biomaterials, soluble polymer molecules can also alter DC function. Finally, the engagement of TLRs by biomaterials profoundly alters DC adhesive properties. Our findings could be useful for designing structure-function studies aimed at developing more bioinert materials. Moreover, they could also be exploited to generate biomaterials for studying the molecular mechanisms of TLR signalling and DC activation aiming at fine-tuning desired and pre-determined immune responses.

Seven kinds of intermediate filament networks in the cytoplasm of polarized cells: structure and function

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

Inhibition of T-cell receptor-induced actin remodeling and relocalization of Lck are evolutionarily conserved activities of lentiviral Nef proteins

Nef, an important pathogenicity factor of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), elevates virus replication in vivo. Among other activities, Nef affects T-cell receptor (TCR) signaling via several mechanisms. For HIV-1 Nef these include alteration of the organization and function of the immunological synapse (IS) such as relocalization of the Lck kinase, as well as early inhibition of TCR/CD3 complex (TCR-CD3)-mediated actin rearrangements and tyrosine phosphorylation. Although most SIV and HIV-2 Nef alleles (group 2) potently downregulate cell surface TCR-CD3, this activity was lost in the viral lineage that gave rise to HIV-1 and its SIV counterparts (group 1). To address the contribution of TCR-CD3 downregulation to Nef effects on TCR signal initiation, we compared the activities of 18 group 1 and group 2 Nef proteins, as well as SIV Nef mutants with defects in TCR-CD3 downmodulation. We found that alteration of Lck's subcellular localization is largely conserved and occurs independently of actin remodeling inhibition or TCR-CD3 downregulation. Surprisingly, Nef proteins of both groups also strongly reduced TCR-induced actin remodeling and tyrosine phosphorylation on TCR-stimulatory surfaces and TCR-CD3 downmodulation competence by group 2 Nef proteins only slightly elevated these effects. Furthermore, Nef proteins from HIV-1 and SIV reduced conjugation between infected primary human T lymphocytes and Raji B cells and potently prevented F-actin polarization at the IS independently of their ability to downmodulate TCR-CD3. These results establish alterations of early TCR signaling events at the IS, including F-actin remodeling and relocalization of Lck, as evolutionary conserved activities of highly divergent lentiviral Nef proteins.

Uncoordinated 119 protein controls trafficking of Lck via the Rab11 endosome and is critical for immunological synapse formation

The activation of T cells through the TCR is essential for development of the adaptive immune response. TCR does not have any enzymatic activity and relies on the plasma membrane-associated lymphocyte-specific protein tyrosine kinase (Lck) for initiation of signaling. Here we uncover a mechanism that is responsible for plasma membrane targeting of Lck. We show that Lck is transported to the membrane via a specific endosomal compartment. The transport depends on the adaptor protein Uncoordinated 119 (Unc119), on the GTPase rat brain 11 (Rab11), and on the actin cytoskeleton. Unc119 regulates the activation of Rab11. Consequently, Unc119 orchestrates the recruitment of the actin-based motor protein, myosin 5B, and the organization of multiprotein complexes on endosomes. The Unc119-regulated pathway is essential for immunological synapse formation and T cell activation.

A novel flow cytometric high throughput assay for a systematic study on molecular mechanisms underlying T cell receptor-mediated integrin activation

Lymphocyte function-associated antigen 1 (LFA-1), a member of β2-integrin family, exerts multiple roles in host T cell immunity and has been identified as a useful drug-development target for inflammatory and autoimmune diseases. Applying the findings that primary resting T cells absorb nanometric membrane vesicles derived from antigen presenting cells (APC) via dual receptor/ligand interactions of T cell receptor (TCR) with cognate peptide-major histocompatibility complex (MHC) complex (pMHC) and LFA-1 with its ligand, intercellular adhesion molecule-1 (ICAM-1), and that signaling cascades triggered by TCR/pMHC interaction take a part in the vesicle-absorption, we established a cell-based high throughput assay for systematic investigation, via isolation of small molecules modulating the level of vesicle-absorption, of molecular mechanisms underlying the T cell absorption of APC-derived vesicles, i.e., structural basis of TCR/pMHC and LFA-1/ICAM-1 interactions and TCR-mediated LFA-1 activation. As primary T cells along with physiological ligands expressed in biological membrane are used and also individual cells in assay samples are analyzed by flow cytometry, results obtained using the assay system hold superior physiological and therapeutic relevance as well as statistical precision.

HIV-1 at the immunological and T-lymphocytic virological synapse

Cell-cell transmission of human immunodeficiency virus type 1 (HIV-1) is considered the most effective mode of viral spread in T-lymphocyte cultures. Evidence has accumulated that HIV-1 assembles polarized synaptic-like structures, referred to as virological synapses, as specialized sites of viral transfer. Interestingly, it was recently also discovered that HIV-1 impairs the formation of the structurally similar immunological synapse, thereby modulating exogenous T-lymphocyte stimulation to yield an optimal activation state for productive HIV-1 infection. The careful dissection of these opposing effects will contribute to our understanding of retroviral spread and cellular signal transduction machineries.

T-cell receptor- and CD28-induced Vav1 activity is required for the accumulation of primed T cells into antigenic tissue

Localization of primed T cells to antigenic tissue is essential for the development of effective immunity. Together with tissue-selective homing molecules, T-cell receptor (TCR)- and CD28-mediated signals have been shown to promote transendothelial migration of specific T cells into nonlymphoid antigen-rich tissue. However, the cellular and molecular requirements for T-cell accumulation to target tissue following their recruitment are largely undefined. The guanine nucleotide exchange factor (GEF) Vav1 has an integral role in coupling TCR and CD28 to signaling pathways that regulate T-cell activation and migration. Here, we have investigated the contribution of TCR- and CD28-induced Vav1 activity to the trafficking and localization of primed HY-specific CD4(+) T cells to antigenic sites. Severe migratory defects displayed by Vav1(-/-) T cells in vitro were fully compensated by a combination of shear flow and chemokines, leading to normal recruitment of Vav1(-/-) T cells in vivo. In contrast, Vav1(-/-) T-cell retention into antigen-rich tissue was severely impaired, reflecting T cells' inability to engage in sustained TCR- and CD28-mediated interactions with tissue-resident antigen-presenting cells (APCs). This novel function of APC-induced, and TCR- and CD28-mediated Vav1 activity in the regulation of effector T-cell immunity highlights its potential as a therapeutic target in T cell-mediated tissue damage.

Functional analysis of FOXP3

Tolerance to self antigens is established in two ways: first in the thymus through the deletion of thymocytes expressing self-reactive T cell receptors; and second, in the periphery through multiple mechanisms involving deletion, anergy, and suppression. Dominant tolerance to self antigens in the periphery is primarily the function of the CD4(+)CD25(+)FOXP3(+) subset of T cells, which have the capability of suppressing autoreactive T cells that have escaped deletion during thymic selection. The essential role of the transcription factor FOXP3 in the development and function of these cells has been well documented. However, the underlying mechanisms by which FOXP3 controls this process are less well understood. This review will focus on the role of FOXP3 in regulating CD4 T cell function in both humans and mice, with an emphasis on recent work in human systems.

T cell receptor-dependent tyrosine phosphorylation of beta2-chimaerin modulates its Rac-GAP function in T cells

The actin cytoskeleton has an important role in the organization and function of the immune synapse during antigen recognition. Dynamic rearrangement of the actin cytoskeleton in response to T cell receptor (TCR) triggering requires the coordinated activation of Rho family GTPases that cycle between active and inactive conformations. This is controlled by GTPase-activating proteins (GAP), which regulate inactivation of Rho GTPases, and guanine exchange factors, which mediate their activation. Whereas much attention has centered on guanine exchange factors for Rho GTPases in T cell activation, the identity and functional roles of the GAP in this process are largely unknown. We previously reported beta2-chimaerin as a diacylglycerol-regulated Rac-GAP that is expressed in T cells. We now demonstrate Lck-dependent phosphorylation of beta2-chimaerin in response to TCR triggering. We identify Tyr-153 as the Lck-dependent phosphorylation residue and show that its phosphorylation negatively regulates membrane stabilization of beta2-chimaerin, decreasing its GAP activity to Rac. This study establishes the existence of TCR-dependent regulation of beta2-chimaerin and identifies a novel mechanism for its inactivation.

CD45 down-regulates Lck-mediated CD44 signaling and modulates actin rearrangement in T cells

The tyrosine phosphatase CD45 dephosphorylates the negative regulatory tyrosine of the Src family kinase Lck and plays a positive role in TCR signaling. In this study we demonstrate a negative regulatory role for CD45 in CD44 signaling leading to actin rearrangement and cell spreading in activated thymocytes and T cells. In BW5147 T cells, CD44 ligation led to CD44 and Lck clustering, which generated a reduced tyrosine phosphorylation signal in CD45(+) T cells and a more sustained, robust tyrosine phosphorylation signal in CD45(-) T cells. This signal resulted in F-actin ring formation and round spreading in the CD45(+) cells and polarized, elongated cell spreading in CD45(-) cells. The enhanced signal in the CD45(-) cells was consistent with enhanced Lck Y394 phosphorylation compared with the CD45(+) cells where CD45 was recruited to the CD44 clusters. This enhanced Src family kinase-dependent activity in the CD45(-) cells led to PI3K and phospholipase C activation, both of which were required for elongated cell spreading. We conclude that CD45 induces the dephosphorylation of Lck at Y394, thereby preventing sustained Lck activation and propose that the amplitude of the Src family kinase-dependent signal regulates the outcome of CD44-mediated signaling to the actin cytoskeleton and T cell spreading.

Tumor resistance to specific lysis: a major hurdle for successful immunotherapy of cancer

Research over the past decade in tumor immunology has shown that immune reactivity to tumor antigens can decrease tumor growth in experimental models. These observations have been translated into clinical studies involving both passive and active forms of immunotherapy. Immunotherapy, an alternative treatment for cancer, is confronted to a major hurdle: tumor escape of specific lysis. Cancer antigen-specific cytotoxic T lymphocytes (CTL) are the major effectors used in immunotherapy against cancer cells. However, large established tumors are usually not fully controlled by CTL. These effector cells could indeed have a dual activity, which allow cancer cells to escape destruction. In this review, we will focus on the essential role of the p53 tumor suppressor gene in the dynamic regulation of tumor cell death induced by cytotoxic T lymphocytes and the involving of structural changes of cytoskeleton in the acquisition of tumor resistance.

Coronin-1A links cytoskeleton dynamics to TCR alpha beta-induced cell signaling

Actin polymerization plays a critical role in activated T lymphocytes both in regulating T cell receptor (TCR)-induced immunological synapse (IS) formation and signaling. Using gene targeting, we demonstrate that the hematopoietic specific, actin- and Arp2/3 complex-binding protein coronin-1A contributes to both processes. Coronin-1A-deficient mice specifically showed alterations in terminal development and the survival of αβT cells, together with defects in cell activation and cytokine production following TCR triggering. The mutant T cells further displayed excessive accumulation yet reduced dynamics of F-actin and the WASP-Arp2/3 machinery at the IS, correlating with extended cell-cell contact. Cell signaling was also affected with the basal activation of the stress kinases sAPK/JNK1/2; and deficits in TCR-induced Ca²⁺ influx and phosphorylation and degradation of the inhibitor of NF-κB (IκB). Coronin-1A therefore links cytoskeleton plasticity with the functioning of discrete TCR signaling components. This function may be required to adjust TCR responses to selecting ligands accounting in part for the homeostasis defect that impacts αβT cells in coronin-1A deficient mice, with the exclusion of other lympho/hematopoietic lineages.

Cholera toxin B accelerates disease progression in lupus-prone mice by promoting lipid raft aggregation

Infectious agents, including bacteria and viruses, are thought to provide triggers for the development or exacerbation of autoimmune diseases such as systemic lupus erythematosus in the genetically predisposed individual. Molecular mimicry and engagement of TLRs have been assigned limited roles that link infection to autoimmunity, but additional mechanisms are suspected to be involved. In this study we show that T cells from lupus-prone mice display aggregated lipid rafts that harbor signaling, costimulatory, inflammatory, adhesion, and TLR molecules. The percentage of T cells with clustered lipid rafts increases with age and peaks before the development of lupus pathology. We show that cholera toxin B, a component of Vibrio cholerae, promotes autoantibody production and glomerulonephritis in lupus-prone mice by enhancing lipid raft aggregation in T cells. In contrast, disruption of lipid raft aggregation results in delay of disease pathology. Our results demonstrate that lipid rafts contribute significantly to the pathogenesis of lupus and provide a novel mechanism whereby aggregated lipid rafts represent a potential link between infection and autoimmunity.

R5 and X4 HIV viruses differentially modulate host gene expression in resting CD4+ T cells

During HIV-1 infection, distinct biological phenotypes are observed between R5 and X4 HIV-1 strains with respect to pathogenicity and tropism. In this study, temporal changes of the expression levels of the complete human transcriptome, representing 47,000 well-characterized human transcripts, were monitored in the first 24 h during HIV-1 R5 and X4 exposition in resting primary CD4(+) T cells. We provide evidence that R5 viruses modulate, to a greater extent than X4 viruses, the level of mRNA of the resting CD4(+) T cells. Indeed, modulation of the TCR signaling and the actin organization involving the WAVE/ABI complex and the ARP2/3 complex appeared to be associated with R5 exposition. The data suggest that the ability of R5 viruses to modulate TCR-mediated actin polymerization and signaling creates a favorable environment for CD4(+) T cell activation after TCR stimulation and may partly explain why R5 is the primary strain observed early in the natural infection process.

Regulation of T-cell migration by co-stimulatory molecules

Migration of primed T-cells to the antigenic site is an essential event in the development of effective immunity. This process is tightly regulated in order to ensure efficient and specific responses. Most studies have focused on non-specific mediators of T-cell migration, including integrins and chemokines. However, recent studies have highlighted the key role of the T-cell receptor and co-stimulatory molecules in guiding T-cell access to antigenic tissue. Here, we review the experimental evidence for an essential contribution of co-stimulation-mediated molecular interactions regulating T-cell migration in the development of T-cell immunity and tolerance.

Bcl10 controls TCR- and FcgammaR-induced actin polymerization

Bcl10 plays an essential role in the adaptive immune response, because Bcl10-deficient lymphocytes show impaired Ag receptor-induced NF-kappaB activation and cytokine production. Bcl10 is a phosphoprotein, but the physiological relevance of this posttranslational modification remains poorly defined. In this study, we report that Bcl10 is rapidly phosphorylated upon activation of human T cells by PMA/ionomycin- or anti-CD3 treatment, and identify Ser(138) as a key residue necessary for Bcl10 phosphorylation. We also show that a phosphorylation-deficient Ser(138)/Ala mutant specifically inhibits TCR-induced actin polymerization yet does not affect NF-kappaB activation. Moreover, silencing of Bcl10, but not of caspase recruitment domain-containing MAGUK protein-1 (Carma1) induces a clear defect in TCR-induced F-actin formation, cell spreading, and conjugate formation. Remarkably, Bcl10 silencing also impairs FcgammaR-induced actin polymerization and phagocytosis in human monocytes. These results point to a key role of Bcl10 in F-actin-dependent immune responses of T cells and monocytes/macrophages.

FOXP3 modifies the phenotypic and functional properties of regulatory T cells

In the periphery, tolerance to self antigens is mainly mediated by the CD4(+)CD25(+)FOXP3(+) subset of regulatory T cells, which can suppress the activity of autoreactive T cells that have escaped deletion in the thymus. The essential role of the transcription factor FOXP3 (forkhead box P3) in the development and function of these regulatory T cells has been well documented. It is also clear that regulatory T cells and effector T cells respond differently to T-cell receptor stimulation. In this Opinion article, we propose that these differences in responses are mediated by FOXP3, and are manifested by alterations in biochemical signalling pathways, patterns of gene expression and the appearance of cell-surface homing receptors.

Global analysis of alternative splicing during T-cell activation

The role of alternative splicing (AS) in eliciting immune responses is poorly understood. We used quantitative AS microarray profiling to survey changes in AS during activation of Jurkat cells, a leukemia-derived T-cell line. Our results indicate that approximately 10-15% of the profiled alternative exons undergo a >10% change in inclusion level during activation. The majority of the genes displaying differential AS levels are distinct from the set of genes displaying differential transcript levels. These two gene sets also have overlapping yet distinct functional roles. For example, genes that show differential AS patterns during T-cell activation are often closely associated with cell-cycle regulation, whereas genes with differential transcript levels are highly enriched in functions associated more directly with immune defense and cytoskeletal architecture. Previously unknown AS events were detected in genes that have important roles in T-cell activation, and these AS level changes were also observed during the activation of normal human peripheral CD4+ and CD8+ lymphocytes. In summary, by using AS microarray profiling, we have discovered many new AS changes associated with T-cell activation. Our results suggest an extensive role for AS in the regulation of the mammalian immune response.

Impaired T cell receptor signaling in Foxp3+ CD4 T cells

Dominant tolerance to autoantigens is primarily achieved through the action of the CD4(+)CD25(+)Foxp3(+) subset of T cells, which have the capability of suppressing autoreactive T cells that have escaped deletion during thymic selection. The essential role of the forkhead/winged-helix transcription factor Foxp3 in the development and function of these cells has been well documented. What is less clear is the role of Foxp3 in the altered TCR signaling that is seen in Tregs. We have used a Foxp3 transgenic mouse line to demonstrate that Foxp3 expression correlates with attenuated TCR signaling, and that the deficit in Foxp3-transgenic CD4 T cells, as well as in CD4(+)CD25(+) Tregs, affects multiple biochemical pathways.

Gelsolin overexpression alters actin dynamics and tyrosine phosphorylation of lipid raft-associated proteins in Jurkat T cells

Upon T cell receptor engagement, both the actin cytoskeleton and substrates of tyrosine phosphorylation are remodeled to create a signaling complex at the interface of the antigen-presenting cell and responding T cell. While T cell signaling has been shown to regulate actin reorganization, the mechanisms by which changes in actin dynamics affect early T cell signaling have not been fully explored. Using gelsolin, an actin-binding protein with capping and severing activities, and latrunculin, an actin-depolymerizing agent, we have further investigated the interplay between actin dynamics and the regulation of T cell signaling. Overexpression of gelsolin altered actin dynamics in Jurkat T cells, and alteration of actin dynamics correlated with dysregulation of tyrosine phosphorylation of raft-associated substrates. This perturbation of tyrosine phosphorylation was correlated with inhibition of activation-dependent signaling pathways regulating Erk-1/2 phosphorylation, NF-AT transcriptional activation and IL-2 production. Modification of actin by the depolymerizing agent latrunculin also altered the tyrosine phosphorylation patterns of proteins associated with lipid rafts, and pre-treatment with latrunculin inhibited anti-CD3 mAb-mediated NF-AT activation. Thus, our data indicate that actin cytoskeletal dynamics modulate the tyrosine phosphorylation of raft-associated proteins and subsequent downstream signal transduction.

Cell death and cytotoxic effects in YAC-1 lymphoma cells following exposure to various forms of mercury

The effects of 1 min-4 h exposures to four Hg compounds (mercuric chloride [HgCl2], methyl mercuric chloride [CH3HgCl], p-chloromercuribenzoate [p-CMB] and thimerosal [TMS; ethylmercurithiosalicylate]) on cell death, microtubules, actin, CD3 receptor expression, protein tyrosine phosphorylation (PTyr-P) and intracellular calcium ([Ca2+]i) levels were investigated in YAC-1 lymphoma cells using flow cytometry. YOPRO-1 (YP) and propidium iodide (PI) dye uptake indicated all forms of Hg tested were toxic at concentrations ranging from 25.8-48.4 microM, with two distinct patterns of effects. Early apoptosis was prolonged for CH3HgCl- and TMS-treated cells, with more than 50% remaining YP+/PI- after 4h. Both CH3HgCl and TMS induced complete loss of beta-tubulin fluorescence, indicative of microtubule depolymerization and inhibition of tubulin synthesis and/or beta-tubulin degradation, while F-actin fluorescence diminished to a lesser degree and only after loss beta-tubulin. CH3HgCl and TMS induced an almost immediate two-fold increase in CD3 fluorescence, with levels returning to baseline within minutes. With continued exposure, CD3 fluorescence was reduced to approximately 50% of baseline values. Both compounds also increased PTyr-P two- to three-fold immediately, with levels returning to baseline at 4h. Similarly, two- to three-fold increases in [Ca2+]i were noted after 1 min exposure. [Ca2+]i increased progressively, reaching levels five- to eight-fold greater than control values. In contrast, dye uptake was delayed with HgCl2 and p-CMB, although cell death proceeded rapidly, with almost all non-viable cells being late apoptotic (YP+/PI+) by 4h. p-CMB produced early reductions in F-actin, and after 4h, complete loss of F-actin with only partial reduction of total beta-tubulin was seen with both p-CMB and HgCl2. HgCl2 reduced CD3 expression and PTyr-P slightly within minutes, while p-CMB produced similar effects on CD3 only at 4h, at which time PTyr-P was increased two- to three-fold. Both compounds increased [Ca2+]i within minutes, though levels remained under twice the baseline concentration after 15 min exposure. With continued exposure, [Ca2+]i increased to levels two- to five-fold greater than control values. These findings indicate the two groups of Hg compounds may induce cell death by distinct pathways, reflecting interactions with different cellular targets leading to cell death.

CD28 interaction with filamin-A controls lipid raft accumulation at the T-cell immunological synapse

During physiological T-cell stimulation by antigen presenting cells (APCs), a major T-cell membrane rearrangement is known to occur leading to the organization of 'supramolecular activation clusters' at the immunological synapse. A possible role for the synapse is the generation of membrane compartments where signalling may be organized and propagated. Thus, engagement of the costimulatory molecule CD28 at the immunological synapse promotes the organization of a signalling compartment by inducing cytoskeletal changes and lipid raft accumulation. We identified the actin-binding protein Filamin-A (FLNa) as a novel molecular partner of CD28. We found that, after physiological stimulation, CD28 associated with and recruited FLNa into the immunological synapse, where FLNa organized CD28 signalling. FLNa knockdown by short interfering RNA (siRNA) inhibited CD28-mediated raft accumulation at the immunological synapse and T-cell costimulation. Together, our data indicate that CD28 binding to FLNa is required to induce the T-cell cytoskeletal rearrangements leading to recruitment of lipid microdomains and signalling mediators into the immunological synapse.

Human immunodeficiency virus type-1 infection impairs the formation of the immunological synapse

HIV-1-infected lymphocytes improperly respond to T cell antigen receptor (TCR) stimulation. To document this phenomenon, we studied the capacity of HIV-1-infected lymphocytes to form immunological synapses. We show here that HIV-1-infected T cells poorly conjugated with antigen-presenting cells, and when they formed conjugates, the synapses were abnormal. TCR and Lck accumulated in the recycling endosomal compartment, and their clustering at the synapse was severely reduced. These phenomena were, to a large extent, caused by Nef, a viral protein affecting intracellular trafficking and signaling pathways. Concomitantly, in HIV-infected cells, tyrosine phosphorylation at the synapse and the patterns of tyrosine phosphorylated proteins were disturbed in a Nef-dependent manner. These findings underscore the importance of Lck and TCR endosomal trafficking in synapse formation and early T cell signaling. Alteration of endocytic and signaling networks at the immunological synapse likely impacts the function and fate of HIV-1-infected cells.

The HIV-1 pathogenicity factor Nef interferes with maturation of stimulatory T-lymphocyte contacts by modulation of N-Wasp activity

The Nef protein is a key determinant of human immunodeficiency virus (HIV) pathogenicity that, among other activities, sensitizes T-lymphocytes for optimal virus production. The initial events by which Nef modulates the T-cell receptor (TCR) cascade are poorly understood. TCR engagement triggers actin rearrangements that control receptor clustering for signal initiation and dynamic organization of signaling protein complexes to form an immunological synapse. Here we report that Nef potently interferes with cell spreading and formation of actin-rich circumferential rings in T-lymphocytes upon surface-supported TCR stimulation. These effects were conserved among Nef proteins from different lentiviruses and occurred in HIV-1-infected primary human T-lymphocytes. This novel Nef activity critically depended on its Src homology 3 domain binding motif and required efficient association with Pak2 activity. Notably, whereas overall signaling microcluster formation immediately following TCR engagement occurred normally in Nef-expressing cells, the viral protein inhibited the concomitant activation of the actin organizer N-Wasp. During the subsequent maturation phase of the stimulatory contact, Nef interfered with the translocation of N-Wasp to the cell periphery, the overall induction of tyrosine phosphorylation, and the selective recruitment of phosphorylated LAT to stimulatory contacts. Consistent with such a critical role of N-Wasp in this process, Nef also blocked morphological changes induced by the known N-Wasp regulators Rac1 and Cdc42. Together, our results demonstrate that Nef alters both the amount and composition of signaling microclusters. We propose modulation of actin dynamics as an important mechanism for Nef-induced alterations of TCR signaling.

Clinical impact of research on the podocyte slit diaphragm

This Review summarizes recent research on the podocyte slit diaphragm. A growing number of molecules that function at the slit diaphragm have been identified in patients with inherited and sporadic nephrotic syndromes. Genetic deletion of nearly all of these molecules results in proteinuria and effacement of foot processes. Nephrin, Neph1 and podocin seem to form a multifunctional receptor complex at the slit diaphragm. Most of the other components of the slit diaphragm interact directly with this complex, in many cases coupling slit diaphragm components to the podocyte's actin cytoskeleton. These molecular findings are being applied to patients with glomerular disease. Over the next decade, these data might help to improve disease classification and prediction of which patients will respond to immunosuppressive treatment.

Role for the Abi/wave protein complex in T cell receptor-mediated proliferation and cytoskeletal remodeling

BACKGROUND

The molecular reorganization of signaling molecules after T cell receptor (TCR) activation is accompanied by polymerization of actin at the site of contact between a T cell and an antigen-presenting cell (APC), as well as extension of actin-rich lamellipodia around the APC. Actin polymerization is critical for the fidelity and efficiency of the T cell response to antigen. The ability of T cells to polymerize actin is critical for several steps in T cell activation including TCR clustering, mature immunological synapse formation, calcium flux, IL-2 production, and proliferation. Activation of the Rac GTPase has been linked to regulation of actin polymerization after TCR stimulation. However, the molecules required for TCR-mediated actin polymerization downstream of activated Rac have remained elusive. Here we identify a novel role for the Abi/Wave protein complex, which signals downstream of activated Rac, in the regulation of actin polymerization and T cell activation in response to TCR stimulation.

RESULTS

Here we show that Abi and Wave rapidly translocate from the T cell cytoplasm to the T cell:B cell contact site in the presence of antigen. Abi and Wave colocalize with actin at the T cell:B cell conjugation site. Moreover, Wave and Abi are necessary for actin polymerization after T cell activation, and loss of Abi proteins in mice impairs TCR-induced cell proliferation and IL-2 production in primary T cells. Significantly, the impairment in actin polymerization in cells lacking Abi proteins is due to the inability of Wave proteins to localize to the T cell:B cell contact site in the presence of antigen, rather than the destabilization of the components of the Wave protein complex.

CONCLUSIONS

The Abi/Wave complex is a novel regulator of TCR-mediated actin dynamics, IL-2 production, and proliferation.

Identification of substrates of human protein-tyrosine phosphatase PTPN22

Stimulation of mature T cells activates a downstream signaling cascade involving temporally and spatially regulated phosphorylation and dephosphorylation events mediated by protein-tyrosine kinases and phosphatases, respectively. PTPN22 (Lyp), a non-receptor protein-tyrosine phosphatase, is expressed exclusively in cells of hematopoietic origin, notably in T cells where it represses signaling through the T cell receptor. We used substrate trapping coupled with mass spectrometry-based peptide identification in an unbiased approach to identify physiological substrates of PTPN22. Several potential substrates were identified in lysates from pervanadate-stimulated Jurkat cells using PTPN22-D195A/C227S, an optimized substrate trap mutant of PTPN22. These included three novel PTPN22 substrates (Vav, CD3epsilon, and valosin containing protein) and two known substrates of PEP, the mouse homolog of PTPN22 (Lck and Zap70). T cell antigen receptor (TCR) zeta was also identified as a potential substrate in Jurkat lysates by direct immunoblotting. In vitro experiments with purified recombinant proteins demonstrated that PTPN22-D195A/C227S interacted directly with activated Lck, Zap70, and TCRzeta, confirming the initial substrate trap results. Native PTPN22 dephosphorylated Lck and Zap70 at their activating tyrosine residues Tyr-394 and Tyr-493, respectively, but not at the regulatory tyrosines Tyr-505 (Lck) or Tyr-319 (Zap70). Native PTPN22 also dephosphorylated TCRzeta in vitro and in cells, and its substrate trap variant co-immunoprecipitated with TCRzeta when both were coexpressed in 293T cells, establishing TCRzeta as a direct substrate of PTPN22.

Identification of target actin content and polymerization status as a mechanism of tumor resistance after cytolytic T lymphocyte pressure

To investigate tumor resistance to T cell lysis, a resistant variant was selected after specific cytolytic T lymphocytes (CTL) selection pressure. Although the resistant variant triggered perforin and granzyme B transcription in specific CTLs, as well as their degranulation, it exhibited a dramatic resistance to cytotoxic T cell killing. It also displayed strong morphological changes with alterations of the actin cytoskeleton. Electron microscopy analysis revealed a loosen interaction between CTLs and the resistant variant despite the formation of apparently normal conjugates. Transcriptional profiling identified a gene expression signature that distinguished sensitive from resistant tumor targets. More notably, we found that actin-related genes ephrin-A1 and scinderin were overexpressed in resistant target. Silencing of these genes using RNA interference resulted in a restoration of normal cell morphology and a significant attenuation of variant resistance to CTL killing. Our present study shows that a shift in cytoskeletal organization can be used, by tumor cells, as a strategy to promote their resistance after CTL selection pressure.

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.

Acute in vivo elevation of intravascular triacylglycerol lipolysis impairs peripheral T cell activation in humans

BACKGROUND

Previous studies have shown suppressive effects of polyunsaturated fatty acids (PUFAs) on T cell proliferation, but the precise mechanism for this effect has not been fully investigated in vivo in humans.

OBJECTIVE

The objective was to determine whether this effect is the result of altered T cell membrane properties and impaired CD3- and CD28-mediated signaling in vivo in humans.

DESIGN

Peripheral T cells were isolated from healthy subjects before and 2 h after an intravenous infusion of heparin plus a PUFA-rich lipid emulsion during a euglycemic hyperinsulinemic clamp to induce a 2.5-fold elevation in plasma linoleic acid concentration without significant change in plasma total free fatty acid concentrations.

RESULTS

Intravenous infusion of heparin plus the lipid emulsion reduced peripheral T cell membrane fluidity and altered lipid raft organization, both of which were associated with reduced T cell proliferation after stimulation with CD3 plus CD28. Tyrosine phosphorylation of linker of activated T cells and activation of protein kinase B in T cells were also impaired without a reduction in T cell receptor expression. In addition, acute PUFA elevation was associated with a reduction in T cell membrane cholesterol exchange with the cellular milieu ex vivo.

CONCLUSIONS

A selective increase in plasma linoleic acid concentration and in intravascular lipolysis has a suppressive effect on peripheral T cell CD28-dependent activation, and this effect is associated with changes in plasma membrane properties. Our results have important implications for nutritional therapy in patients at high risk of septic complications and may also be of relevance to postprandial lipid metabolism disorders such as insulin resistance and type 2 diabetes.

Cytoskeletal regulation: rich in lipids

Phosphorylated derivatives of the phospholipid phosphatidylinositol, or phosphoinositides, are implicated in many aspects of cell function. Binding of phosphoinositides that are localized within cell membranes to soluble protein ligands allows spatially selective regulation at the cytoplasm-membrane interface. Recently, studies that relate phosphoinositide production to membrane domains are converging with those that show effects of these lipids on the assembly of cellular actin, and are therefore linking membrane and cytoskeletal structures in new ways.