Molecular mechanisms and functional implications of polarized actin remodeling at the T cell immunological synapse

T cell-based cancer immunotherapy: opportunities and challenges

T cells play a central role in the cancer immunity cycle. The therapeutic outcomes of T cell-based intervention strategies are determined by multiple factors at various stages of the cycle. Here, we summarize and discuss recent advances in T cell immunotherapy and potential barriers to it within the framework of the cancer immunity cycle, including T-cell recognition of tumor antigens for activation, T cell trafficking and infiltration into tumors, and killing of target cells. Moreover, we discuss the key factors influencing T cell differentiation and functionality, including TCR stimulation, costimulatory signals, cytokines, metabolic reprogramming, and mechanistic forces. We also highlight the key transcription factors dictating T cell differentiation and discuss how metabolic circuits and specific metabolites shape the epigenetic program of tumor-infiltrating T cells. We conclude that a better understanding of T cell fate decision will help design novel strategies to overcome the barriers to effective cancer immunity.

Formin-like 1β phosphorylation at S1086 is necessary for secretory polarized traffic of exosomes at the immune synapse in Jurkat T lymphocytes

We analyzed here how formin-like 1 β (FMNL1β), an actin cytoskeleton-regulatory protein, regulates microtubule-organizing center (MTOC) and multivesicular bodies (MVB) polarization and exosome secretion at an immune synapse (IS) model in a phosphorylation-dependent manner. IS formation was associated with transient recruitment of FMNL1β to the IS, which was independent of protein kinase C δ (PKCδ). Simultaneous RNA interference of all FMNL1 isoforms prevented MTOC/MVB polarization and exosome secretion, which were restored by FMNL1βWT expression. However, expression of the non-phosphorylatable mutant FMNL1βS1086A did not restore neither MTOC/MVB polarization nor exosome secretion to control levels, supporting the crucial role of S1086 phosphorylation in MTOC/MVB polarization and exosome secretion. In contrast, the phosphomimetic mutant, FMNL1βS1086D, restored MTOC/MVB polarization and exosome secretion. Conversely, FMNL1βS1086D mutant did not recover the deficient MTOC/MVB polarization occurring in PKCδ-interfered clones, indicating that S1086 FMNL1β phosphorylation alone is not sufficient for MTOC/MVB polarization and exosome secretion. FMNL1 interference inhibited the depletion of F-actin at the central region of the immune synapse (cIS), which is necessary for MTOC/MVB polarization. FMNL1βWT and FMNL1βS1086D, but not FMNL1βS1086A expression, restored F-actin depletion at the cIS. Thus, actin cytoskeleton reorganization at the IS underlies the effects of all these FMNL1β variants on polarized secretory traffic. FMNL1 was found in the IS made by primary T lymphocytes, both in T cell receptor (TCR) and chimeric antigen receptor (CAR)-evoked synapses. Taken together, these results point out a crucial role of S1086 phosphorylation in FMNL1β activation, leading to cortical actin reorganization and subsequent control of MTOC/MVB polarization and exosome secretion.

Calcineurin is an adaptor required for assembly of the TCR signaling complex

The serine/threonine phosphatase calcineurin is a component of the T cell receptor (TCR) signalosome, where it promotes T cell activation by dephosphorylating LckS59. Using small interfering RNA (siRNA)-mediated knockdown and CRISPR-Cas9-targeted genetic disruption of the calcineurin A chain α and β isoforms, we find that calcineurin also functions as an adaptor in TCR-signaled human T cells. Unlike inhibition of its phosphatase activity, in the absence of calcineurin A, TCR signaling results in attenuated actin rearrangement, markedly reduced TCR-Lck microcluster formation and recruitment of the adaptor RhoH, and diminished phosphorylation of critical targets downstream of Lck such as TCRζ and ZAP-70. Reconstitution of deficient T cells with either calcineurin Aα or Aβ restores TCR microcluster formation and signaling, as does reconstitution with a phosphatase-inactive Aα chain. These results assign a non-enzymatic adaptor function to calcineurin in the formation and stabilization of a functional TCR signaling complex.

Morphodynamics of T-lymphocytes: Scanning to spreading

Binding of the T cell receptor complex to its ligand, the subsequent molecular rearrangement, and the concomitant cell-scale shape changes represent the very first steps of adaptive immune recognition. The first minutes of the interaction of T cells and antigen presenting cells have been extensively scrutinized; yet, gaps remain in our understanding of how the biophysical properties of the environment may impact the sequence of events. In particular, many pioneering experiments were done on immobilized ligands and gave major insights into the process of T cell activation, whereas later experiments have indicated that ligand mobility was of paramount importance, especially to enable the formation of T cell receptor clusters. Systematic experiments to compare and reconcile the two schools are still lacking. Furthermore, recent investigations using compliant substrates have elucidated other intriguing aspects of T cell mechanics. Here we review experiments on interaction of T cells with planar artificial antigen presenting cells to explore the impact of mechanics on adhesion and actin morphodynamics during the spreading process. We enumerate a sequence tracing first contact to final spread state that is consistent with current understanding. Finally, we interpret the presented experimental results in light of a mechanical model that captures all the different morphodynamic states.

Imaging the immune synapse: Three-dimensional analysis of the immune synapse

T cell receptor (TCR) stimulation of T lymphocytes by antigen bound to the major histocompatibility complex (MHC) of an antigen-presenting cell (APC), together with the interaction of accessory molecules, induces the formation of the immunological synapse (IS), the convergence of secretion vesicles toward the centrosome, and the polarization of the centrosome to the IS. Upon IS formation, an initial increase in cortical filamentous actin (F-actin) at the IS takes place, followed by a decrease in F-actin density at the central region of the IS, which contains the secretory domain. These reversible, cortical actin cytoskeleton reorganization processes that characterize a mature IS occur during lytic granule secretion in cytotoxic T lymphocytes (CTL) and natural killer (NK) cells and cytokine-containing vesicle secretion in T-helper (Th) lymphocytes. Besides, IS formation constitutes the basis of a signaling platform that integrates signals and coordinates molecular interactions that are necessary for an appropriate antigen-specific immune response. In this chapter we deal with the three-dimensional (3D) analysis of the synaptic interface architecture, as well as the analysis of the localization of different markers at the IS.

Integrins in Health and Disease-Suitable Targets for Treatment?

Integrin receptors are heterodimeric surface receptors that play multiple roles regarding cell-cell communication, signaling, and migration. The four members of the β2 integrin subfamily are composed of an alternative α (CD11a-d) subunit, which determines the specific receptor properties, and a constant β (CD18) subunit. This review aims to present insight into the multiple immunological roles of integrin receptors, with a focus on β2 integrins that are specifically expressed by leukocytes. The pathophysiological role of β2 integrins is confirmed by the drastic phenotype of patients suffering from leukocyte adhesion deficiencies, most often resulting in severe recurrent infections and, at the same time, a predisposition for autoimmune diseases. So far, studies on the role of β2 integrins in vivo employed mice with a constitutive knockout of all β2 integrins or either family member, respectively, which complicated the differentiation between the direct and indirect effects of β2 integrin deficiency for distinct cell types. The recent generation and characterization of transgenic mice with a cell-type-specific knockdown of β2 integrins by our group has enabled the dissection of cell-specific roles of β2 integrins. Further, integrin receptors have been recognized as target receptors for the treatment of inflammatory diseases as well as tumor therapy. However, whereas both agonistic and antagonistic agents yielded beneficial effects in animal models, the success of clinical trials was limited in most cases and was associated with unwanted side effects. This unfavorable outcome is most probably related to the systemic effects of the used compounds on all leukocytes, thereby emphasizing the need to develop formulations that target distinct types of leukocytes to modulate β2 integrin activity for therapeutic applications.

Extracellular vesicles and microvilli in the immune synapse

T cell receptor (TCR) binding to cognate antigen on the plasma membrane of an antigen-presenting cell (APC) triggers the immune synapse (IS) formation. The IS constitutes a dedicated contact region between different cells that comprises a signaling platform where several cues evoked by TCR and accessory molecules are integrated, ultimately leading to an effective TCR signal transmission that guarantees intercellular message communication. This eventually leads to T lymphocyte activation and the efficient execution of different T lymphocyte effector tasks, including cytotoxicity and subsequent target cell death. Recent evidence demonstrates that the transmission of information between immune cells forming synapses is produced, to a significant extent, by the generation and secretion of distinct extracellular vesicles (EV) from both the effector T lymphocyte and the APC. These EV carry biologically active molecules that transfer cues among immune cells leading to a broad range of biological responses in the recipient cells. Included among these bioactive molecules are regulatory miRNAs, pro-apoptotic molecules implicated in target cell apoptosis, or molecules triggering cell activation. In this study we deal with the different EV classes detected at the IS, placing emphasis on the most recent findings on microvilli/lamellipodium-produced EV. The signals leading to polarized secretion of EV at the synaptic cleft will be discussed, showing that the IS architecture fulfills a fundamental task during this route.

Mechanobiological Strategies to Augment Cancer Treatment

Cancer cells exhibit aberrant extracellular matrix mechanosensing due to the altered expression of mechanosensory cytoskeletal proteins. Such aberrant mechanosensing of the tumor microenvironment (TME) by cancer cells is associated with disease development and progression. In addition, recent studies show that such mechanosensing changes the mechanobiological properties of cells, and in turn cells become susceptible to mechanical perturbations. Due to an increasing understanding of cell biomechanics and cellular machinery, several approaches have emerged to target the mechanobiological properties of cancer cells and cancer-associated cells to inhibit cancer growth and progression. In this Perspective, we summarize the progress in developing mechano-based approaches to target cancer by interfering with the cellular mechanosensing machinery and overall TME.

Mechano-modulation of T cells for cancer immunotherapy

Immunotherapy, despite its promise for future anti-cancer approach, faces significant challenges, such as off-tumor side effects, innate or acquired resistance, and limited infiltration of immune cells into stiffened extracellular matrix (ECM). Recent studies have highlighted the importance of mechano-modulation/-activation of immune cells (mainly T cells) for effective caner immunotherapy. Immune cells are highly sensitive to the applied physical forces and matrix mechanics, and reciprocally shape the tumor microenvironment. Engineering T cells with tuned properties of materials (e.g., chemistry, topography, and stiffness) can improve their expansion and activation ex vivo, and their ability to mechano-sensing the tumor specific ECM in vivo where they perform cytotoxic effects. T cells can also be exploited to secrete enzymes that soften ECM, thus increasing tumor infiltration and cellular therapies. Furthermore, T cells, such as chimeric antigen receptor (CAR)-T cells, genomic engineered to be spatiotemporally controllable by physical stimuli (e.g., ultrasound, heat, or light), can mitigate adverse off-tumor effects. In this review, we communicate these recent cutting-edge endeavors devoted to mechano-modulating/-activating T cells for effective cancer immunotherapy, and discuss future prospects and challenges in this field.

Analysis of centrosomal area actin reorganization and centrosome polarization upon lymphocyte activation at the immunological synapse

T cell receptor (TCR) and B cell receptor (BCR) stimulation of T and B lymphocytes, by antigen presented on an antigen-presenting cell (APC) induces the formation of the immunological synapse (IS). IS formation is associated with an initial increase in cortical filamentous actin (F-actin) at the IS, followed by a decrease in F-actin density at the central region of the IS, which contains the secretory domain. This is followed by the convergence of secretion vesicles towards the centrosome, and the polarization of the centrosome to the IS. These reversible, cortical actin cytoskeleton reorganization processes occur during lytic granule secretion in cytotoxic T lymphocytes (CTL) and natural killer (NK) cells, proteolytic granules secretion in B lymphocytes and during cytokine-containing vesicle secretion in T-helper (Th) lymphocytes. In addition, several findings obtained in T and B lymphocytes forming IS show that actin cytoskeleton reorganization also occurs at the centrosomal area. F-actin reduction at the centrosomal area appears to be associated with centrosome polarization. In this chapter we deal with the analysis of centrosomal area F-actin reorganization, as well as the centrosome polarization analysis toward the IS.

T Lymphocyte and CAR-T Cell-Derived Extracellular Vesicles and Their Applications in Cancer Therapy

Extracellular vesicles (EV) are a very diverse group of cell-derived vesicles released by almost all kind of living cells. EV are involved in intercellular exchange, both nearby and systemically, since they induce signals and transmit their cargo (proteins, lipids, miRNAs) to other cells, which subsequently trigger a wide variety of biological responses in the target cells. However, cell surface receptor-induced EV release is limited to cells from the immune system, including T lymphocytes. T cell receptor activation of T lymphocytes induces secretion of EV containing T cell receptors for antigen and several bioactive molecules, including proapoptotic proteins. These EV are specific for antigen-bearing cells, which make them ideal candidates for a cell-free, EV-dependent cancer therapy. In this review we examine the generation of EV by T lymphocytes and CAR-T cells and some potential therapeutic approaches of these EV.

Lymphocyte Polarization During Immune Synapse Assembly: Centrosomal Actin Joins the Game

Interactions among immune cells are essential for the development of adaptive immune responses. The immunological synapse (IS) provides a specialized platform for integration of signals and intercellular communication between T lymphocytes and antigen presenting cells (APCs). In the T cell the reorganization of surface molecules at the synaptic interface is initiated by T cell receptor binding to a cognate peptide-major histocompatibility complex on the APC surface and is accompanied by a polarized remodelling of the cytoskeleton and centrosome reorientation to a subsynaptic position. Although there is a general agreement on polarizing signals and mechanisms driving centrosome reorientation during IS assembly, the primary events that prepare for centrosome repositioning remain largely unexplored. It has been recently shown that in resting lymphocytes a local polymerization of filamentous actin (F-actin) at the centrosome contributes to anchoring this organelle to the nucleus. During early stages of IS formation centrosomal F-actin undergoes depletion, allowing for centrosome detachment from the nucleus and its polarization towards the synaptic membrane. We recently demonstrated that in CD4⁺ T cells the reduction in centrosomal F-actin relies on the activity of a centrosome-associated proteasome and implicated the ciliopathy-related Bardet-Biedl syndrome 1 protein in the dynein-dependent recruitment of the proteasome 19S regulatory subunit to the centrosome. In this short review we will feature our recent findings that collectively provide a new function for BBS proteins and the proteasome in actin dynamics, centrosome polarization and T cell activation.

A Murine Model of X-Linked Moesin-Associated Immunodeficiency (X-MAID) Reveals Defects in T Cell Homeostasis and Migration

X-linked moesin associated immunodeficiency (X-MAID) is a primary immunodeficiency disease in which patients suffer from profound lymphopenia leading to recurrent infections. The disease is caused by a single point mutation leading to a R171W amino acid change in the protein moesin (moesinR171W). Moesin is a member of the ERM family of proteins, which reversibly link the cortical actin cytoskeleton to the plasma membrane. Here, we describe a novel mouse model with global expression of moesinR171W that recapitulates multiple facets of patient disease, including severe lymphopenia. Further analysis reveals that these mice have diminished numbers of thymocytes and bone marrow precursors. X-MAID mice also exhibit systemic inflammation that is ameliorated by elimination of mature lymphocytes through breeding to a Rag1-deficient background. The few T cells in the periphery of X-MAID mice are highly activated and have mostly lost moesinR171W expression. In contrast, single-positive (SP) thymocytes do not appear activated and retain high expression levels of moesinR171W. Analysis of ex vivo CD4 SP thymocytes reveals defects in chemotactic responses and reduced migration on integrin ligands. While chemokine signaling appears intact, CD4 SP thymocytes from X-MAID mice are unable to polarize and rearrange cytoskeletal elements. This mouse model will be a valuable tool for teasing apart the complexity of the immunodeficiency caused by moesinR171W, and will provide new insights into how the actin cortex regulates lymphocyte function.

LFA-1 and kindlin-3 enable the collaborative transport of SLP-76 microclusters by myosin and dynein motors

Integrin engagement within the immune synapse enhances T cell activation, but our understanding of this process is incomplete. In response to T cell receptor (TCR) ligation, SLP-76 (LCP2), ADAP (FYB1) and SKAP55 (SKAP1) are recruited into microclusters and activate integrins via the effectors talin-1 and kindlin-3 (FERMT3). We postulated that integrins influence the centripetal transport and signaling of SLP-76 microclusters via these linkages. We show that contractile myosin filaments surround and are co-transported with SLP-76 microclusters, and that TCR ligand density governs the centripetal movement of both structures. Centripetal transport requires formin activity, actomyosin contraction, microtubule integrity and dynein motor function. Although immobilized VLA-4 (α4β1 integrin) and LFA-1 (αLβ2 integrin) ligands arrest the centripetal movement of SLP-76 microclusters and myosin filaments, VLA-4 acts distally, while LFA-1 acts in the lamellum. Integrin β2, kindlin-3 and zyxin are required for complete centripetal transport, while integrin β1 and talin-1 are not. CD69 upregulation is similarly dependent on integrin β2, kindlin-3 and zyxin, but not talin-1. These findings highlight the integration of cytoskeletal systems within the immune synapse and reveal extracellular ligand-independent roles for LFA-1 and kindlin-3. This article has an associated First Person interview with the first author of the paper.

Antibody-Functionalized Nanowires: A Tuner for the Activation of T Cells

T cells sense both chemical cues delivered by antigen molecules and physical cues delivered by the environmental elasticity and topography; yet, it is still largely unclear how these cues cumulatively regulate the immune activity of T cells. Here, we engineered a nanoscale platform for ex vivo stimulation of T cells based on antigen-functionalized nanowires. The nanowire topography and elasticity, as well as the immobilized antigens, deliver the physical and chemical cues, respectively, enabling the systematic study of the integrated effect of these cues on a T cell's immune response. We found that T cells sense both the topography and bending modulus of the nanowires and modulate their signaling, degranulation, and cytotoxicity with the variation in these physical features. Our study provides an important insight into the physical mechanism of T cell activation and paves the way to novel nanomaterials for the controlled ex vivo activation of T cells in immunotherapy.

Cytoskeleton Dynamics in Peripheral T Cell Lymphomas: An Intricate Network Sustaining Lymphomagenesis

Defects in cytoskeleton functions support tumorigenesis fostering an aberrant proliferation and promoting inappropriate migratory and invasive features. The link between cytoskeleton and tumor features has been extensively investigated in solid tumors. However, the emerging genetic and molecular landscape of peripheral T cell lymphomas (PTCL) has unveiled several alterations targeting structure and function of the cytoskeleton, highlighting its role in cell shape changes and the aberrant cell division of malignant T cells. In this review, we summarize the most recent evidence about the role of cytoskeleton in PTCLs development and progression. We also discuss how aberrant signaling pathways, like JAK/STAT3, NPM-ALK, RhoGTPase, and Aurora Kinase, can contribute to lymphomagenesis by modifying the structure and the signaling properties of cytoskeleton.

Nanowire Based Guidance of the Morphology and Cytotoxic Activity of Natural Killer Cells

The cytotoxic activity of natural killer (NK) cells is regulated by many chemical and physical cues, whose integration mechanism is still obscure. Here, a multifunctional platform is engineered for NK cell stimulation, to study the effect of the signal integration and spatial heterogeneity on NK cell function. The platform is based on nanowires, whose mechanical compliance and site-selective tip functionalization with antigens produce the physical and chemical stimuli, respectively. The nanowires are confined to micron-sized islands, which induce a splitting of the NK cells into two subpopulations with distinct morphologies and immune responses: NK cells atop the nanowire islands display symmetrical spreading and enhanced activation, whereas cells lying in the straits between the islands develop elongated profiles and show lower activation levels. The demonstrated tunability of NK cell cytotoxicity provides an important insight into the mechanism of their immune function and introduces a novel technological route for the ex vivo shaping of cytotoxic lymphocytes in immunotherapy.

Role of Actin Cytoskeleton Reorganization in Polarized Secretory Traffic at the Immunological Synapse

T cell receptor (TCR) and B cell receptor (BCR) stimulation by antigen presented on an antigen-presenting cell (APC) induces the formation of the immune synapse (IS), the convergence of secretory vesicles from T and B lymphocytes toward the centrosome, and the polarization of the centrosome to the immune synapse. Immune synapse formation is associated with an initial increase in cortical F-actin at the synapse, followed by a decrease in F-actin density at the central region of the immune synapse, which contains the secretory domain. These reversible, actin cytoskeleton reorganization processes occur during lytic granule degranulation in cytotoxic T lymphocytes (CTL) and cytokine-containing vesicle secretion in T-helper (Th) lymphocytes. Recent evidences obtained in T and B lymphocytes forming synapses show that F-actin reorganization also occurs at the centrosomal area. F-actin reduction at the centrosomal area appears to be involved in centrosome polarization. In this review we deal with the biological significance of both cortical and centrosomal area F-actin reorganization and some of the derived biological consequences.

Remodeling the epigenome and (epi)cytoskeleton: a new paradigm for co-regulation by methylation

The epigenome determines heritable patterns of gene expression in the absence of changes in DNA sequence. The result is programming of different cellular-, tissue- and organ-specific phenotypes from a single organismic genome. Epigenetic marks that comprise the epigenome (e.g. methylation) are placed upon or removed from chromatin (histones and DNA) to direct the activity of effectors that regulate gene expression and chromatin structure. Recently, the cytoskeleton has been identified as a second target for the cell's epigenetic machinery. Several epigenetic 'readers, writers and erasers' that remodel chromatin have been discovered to also remodel the cytoskeleton, regulating structure and function of microtubules and actin filaments. This points to an emerging paradigm for dual-function remodelers with 'chromatocytoskeletal' activity that can integrate cytoplasmic and nuclear functions. For example, the SET domain-containing 2 methyltransferase (SETD2) has chromatocytoskeletal activity, methylating both histones and microtubules. The SETD2 methyl mark on chromatin is required for efficient DNA repair, and its microtubule methyl mark is required for proper chromosome segregation during mitosis. This unexpected convergence of SETD2 activity on histones and microtubules to maintain genomic stability suggests the intriguing possibility of an expanded role in the cell for chromatocytoskeletal proteins that read, write and erase methyl marks on the cytoskeleton as well as chromatin. Coordinated use of methyl marks to remodel both the epigenome and the (epi)cytoskeleton opens the possibility for integrated regulation (which we refer to as 'epiregulation') of other higher-level functions, such as muscle contraction or learning and memory, and could even have evolutionary implications.

Artificial Biosystem for Modulation of Interactions between Antigen-Presenting Cells and T Cells

T cell activation is triggered by signal molecules on the surface of antigen-presenting cells (APC) and subsequent exertion of cellular forces. Deciphering the biomechanical and biochemical signals in this complex process is of interest and will contribute to an improvement in immunotherapy strategies. To address underlying questions, coculture and biomimetic models are established. Mature dendritic cells (mDC) are first treated with cytochalasin B (CytoB), a cytoskeletal disruption agent known to lower apparent cellular stiffness and reduction in T cell proliferation is observed. It is attempted to mimic mDC and T cell interactions using polyacrylamide (PA) gels with defined stiffness corresponding to mDC (0.2-25 kPa). Different ratios of anti-CD3 (aCD3) and anti-CD28 (aCD28) antibodies are immobilized onto PA gels. The results show T cell proliferation is triggered by both aCD3 and aCD28 in a stiffness-dependent manner. Cells cultured on aCD3 immobilized on gels has significantly enhanced proliferation and IL-2 secretion, compared to aCD28. Furthermore, ZAP70 phosphorylation is enhanced in stiffer substrate a in a aCD3-dependent manner. The biosystem provides an approach to study the reduction of T cell proliferation observed on CytoB-treated mDC. Overall, the biosystem allows distinguishing the impact of biophysical and biochemical signals of APC and T cell interactions in vitro.

Inducible Polarized Secretion of Exosomes in T and B Lymphocytes

Exosomes are extracellular vesicles (EV) of endosomal origin (multivesicular bodies, MVB) constitutively released by many different eukaryotic cells by fusion of MVB to the plasma membrane. However, inducible exosome secretion controlled by cell surface receptors is restricted to very few cell types and a limited number of cell surface receptors. Among these, exosome secretion is induced in T lymphocytes and B lymphocytes when stimulated at the immune synapse (IS) via T-cell receptors (TCR) and B-cell receptors (BCR), respectively. IS formation by T and B lymphocytes constitutes a crucial event involved in antigen-specific, cellular, and humoral immune responses. Upon IS formation by T and B lymphocytes with antigen-presenting cells (APC), the convergence of MVB towards the microtubule organization center (MTOC), and MTOC polarization to the IS, are involved in polarized exosome secretion at the synaptic cleft. This specialized mechanism provides the immune system with a finely-tuned strategy to increase the specificity and efficiency of crucial secretory effector functions of B and T lymphocytes. As inducible exosome secretion by antigen-receptors is a critical and unique feature of the immune system this review considers the study of the traffic events leading to polarized exosome secretion at the IS and some of their biological consequences.

β2 Integrins-Multi-Functional Leukocyte Receptors in Health and Disease

β2 integrins are heterodimeric surface receptors composed of a variable α (CD11a-CD11d) and a constant β (CD18) subunit and are specifically expressed by leukocytes. The α subunit defines the individual functional properties of the corresponding β2 integrin, but all β2 integrins show functional overlap. They mediate adhesion to other cells and to components of the extracellular matrix (ECM), orchestrate uptake of extracellular material like complement-opsonized pathogens, control cytoskeletal organization, and modulate cell signaling. This review aims to delineate the tremendous role of β2 integrins for immune functions as exemplified by the phenotype of LAD-I (leukocyte adhesion deficiency 1) patients that suffer from strong recurrent infections. These immune defects have been largely attributed to impaired migratory and phagocytic properties of polymorphonuclear granulocytes. The molecular base for this inherited disease is a functional impairment of β2 integrins due to mutations within the CD18 gene. LAD-I patients are also predisposed for autoimmune diseases. In agreement, polymorphisms within the CD11b gene have been associated with autoimmunity. Consequently, β2 integrins have received growing interest as targets in the treatment of autoimmune diseases. Moreover, β2 integrin activity on leukocytes has been implicated in tumor development.

A flow cytometry-based method to screen for modulators of tumor-specific T cell cytotoxicity

Cancer immunotherapy relies on the ability of immune cells to kill malignant cells. The cytotoxic T lymphocyte (CTL) response is perhaps the functional measure that best reflects cell-mediated immunity against cancer. Straightforward methods that facilitate quantitative evaluation of the potency of compounds that can modulate T cell-mediated, tumor antigen-specific immune responses are central in the screening cascade when searching for new immunotherapeutic agents for cancer. Here we describe a simple, sensitive method, based on flow cytometry analyses, to quantitatively measure cytotoxicity in vitro. We provide examples that validate its feasibility and specificity using CD8⁺ T lymphocytes specific for a surrogate tumor antigen, and a blocking antibody for the inhibitory PD-1/PD-L1 axis. This method can nonetheless be applied to the screening of virtually any cytotoxicity modulatory compound, including antibodies and small molecules or T cell-based therapies, and can be scaled up for high-throughput workflow and automation.

Microparticle traction force microscopy reveals subcellular force exertion patterns in immune cell-target interactions

Force exertion is an integral part of cellular behavior. Traction force microscopy (TFM) has been instrumental for studying such forces, providing spatial force measurements at subcellular resolution. However, the applications of classical TFM are restricted by the typical planar geometry. Here, we develop a particle-based force sensing strategy for studying cellular interactions. We establish a straightforward batch approach for synthesizing uniform, deformable and tuneable hydrogel particles, which can also be easily derivatized. The 3D shape of such particles can be resolved with superresolution (<50 nm) accuracy using conventional confocal microscopy. We introduce a reference-free computational method allowing inference of traction forces with high sensitivity directly from the particle shape. We illustrate the potential of this approach by revealing subcellular force patterns throughout phagocytic engulfment and force dynamics in the cytotoxic T-cell immunological synapse. This strategy can readily be adapted for studying cellular forces in a wide range of applications.

Primary immunodeficiencies caused by mutations in actin regulatory proteins

The identification of patients with monogenic gene defects have illuminated the function of different proteins in the immune system, including proteins that regulate the actin cytoskeleton. Many of these actin regulatory proteins are exclusively expressed in leukocytes and regulate the formation and branching of actin filaments. Their absence or abnormal function leads to defects in immune cell shape, cellular projections, migration, and signaling. Through the study of patients' mutations and generation of mouse models that recapitulate the patients' phenotypes, our laboratory and others have gained a better understanding of the role these proteins play in cell biology and the underlying pathogenesis of immunodeficiencies and immune dysregulatory syndromes.

Loss of ARPC1B impairs cytotoxic T lymphocyte maintenance and cytolytic activity

CD8 cytotoxic T lymphocytes (CTLs) rely on rapid reorganization of the branched F-actin network to drive the polarized secretion of lytic granules, initiating target cell death during the adaptive immune response. Branched F-actin is generated by the nucleation factor actin-related protein 2/3 (Arp2/3) complex. Patients with mutations in the actin-related protein complex 1B (ARPC1B) subunit of Arp2/3 show combined immunodeficiency, with symptoms of immune dysregulation, including recurrent viral infections and reduced CD8+ T cell count. Here, we show that loss of ARPC1B led to loss of CTL cytotoxicity, with the defect arising at 2 different levels. First, ARPC1B is required for lamellipodia formation, cell migration, and actin reorganization across the immune synapse. Second, we found that ARPC1B is indispensable for the maintenance of TCR, CD8, and GLUT1 membrane proteins at the plasma membrane of CTLs, as recycling via the retromer and WASH complexes was impaired in the absence of ARPC1B. Loss of TCR, CD8, and GLUT1 gave rise to defects in T cell signaling and proliferation upon antigen stimulation of ARPC1B-deficient CTLs, leading to a progressive loss of CD8+ T cells. This triggered an activation-induced immunodeficiency of CTL activity in ARPC1B-deficient patients, which could explain the susceptibility to severe and prolonged viral infections.

Loss of ARPC1B impairs cytotoxic T lymphocyte maintenance and cytolytic activity

CD8 cytotoxic T lymphocytes (CTLs) rely on rapid reorganization of the branched F-actin network to drive the polarized secretion of lytic granules, initiating target cell death during the adaptive immune response. Branched F-actin is generated by the nucleation factor actin-related protein 2/3 (Arp2/3) complex. Patients with mutations in the actin-related protein complex 1B (ARPC1B) subunit of Arp2/3 show combined immunodeficiency, with symptoms of immune dysregulation, including recurrent viral infections and reduced CD8+ T cell count. Here, we show that loss of ARPC1B led to loss of CTL cytotoxicity, with the defect arising at 2 different levels. First, ARPC1B is required for lamellipodia formation, cell migration, and actin reorganization across the immune synapse. Second, we found that ARPC1B is indispensable for the maintenance of TCR, CD8, and GLUT1 membrane proteins at the plasma membrane of CTLs, as recycling via the retromer and WASH complexes was impaired in the absence of ARPC1B. Loss of TCR, CD8, and GLUT1 gave rise to defects in T cell signaling and proliferation upon antigen stimulation of ARPC1B-deficient CTLs, leading to a progressive loss of CD8+ T cells. This triggered an activation-induced immunodeficiency of CTL activity in ARPC1B-deficient patients, which could explain the susceptibility to severe and prolonged viral infections.

Protein Kinase C δ Regulates the Depletion of Actin at the Immunological Synapse Required for Polarized Exosome Secretion by T Cells

Multivesicular bodies (MVB) are endocytic compartments that enclose intraluminal vesicles (ILVs) formed by inward budding from the limiting membrane of endosomes. In T lymphocytes, ILVs are secreted as Fas ligand-bearing, pro-apoptotic exosomes following T cell receptor (TCR)-induced fusion of MVB with the plasma membrane at the immune synapse (IS). In this study we show that protein kinase C δ (PKCδ), a novel PKC isotype activated by diacylglycerol (DAG), regulates TCR-controlled MVB polarization toward the IS and exosome secretion. Concomitantly, we demonstrate that PKCδ-interfered T lymphocytes are defective in activation-induced cell death. Using a DAG sensor based on the C1 DAG-binding domain of PKCδ and a GFP-PKCδ chimera, we reveal that T lymphocyte activation enhances DAG levels at the MVB endomembranes which mediates the association of PKCδ to MVB. Spatiotemporal reorganization of F-actin at the IS is inhibited in PKCδ-interfered T lymphocytes. Therefore, we propose PKCδ as a DAG effector that regulates the actin reorganization necessary for MVB traffic and exosome secretion.

Origin, Organization, Dynamics, and Function of Actin and Actomyosin Networks at the T Cell Immunological Synapse

The engagement of a T cell with an antigen-presenting cell (APC) or activating surface results in the formation within the T cell of several distinct actin and actomyosin networks. These networks reside largely within a narrow zone immediately under the T cell's plasma membrane at its site of contact with the APC or activating surface, i.e., at the immunological synapse. Here we review the origin, organization, dynamics, and function of these synapse-associated actin and actomyosin networks. Importantly, recent insights into the nature of these actin-based cytoskeletal structures were made possible in several cases by advances in light microscopy.

The role of WASp in T cells and B cells

Wiskott-Aldrich syndrome (WAS) is a form of primary immunodeficiency (PIDs) resulting from mutations of the gene that encodes Wiskott-Aldrich syndrome protein (WASp). WASp is the first identified and most widely studied protein belonging to the actin nucleation-promoting factor family and plays significant role in integrating and transforming signals from critical receptors on the cell surface to actin remodeling. WASp functions in immune defense and homeostasis through the regulation of actin cytoskeleton-dependent cellular processes as well as processes uncoupled with actin polymerization like nuclear transcription programs. In this article, we review the mechanisms of WASp activation through an understanding of its structure. We further discuss the role of WASp in adaptive immunity, paying special attention to some recent findings on the crucial role of WASp in the formation of immunological synapse, the regulation of T follicular helper (Tfh) cells and in the prevention of autoimmunity.

Polarity of CD4+ T cells towards the antigen presenting cell is regulated by the Lck adapter TSAd

Polarization of T cells towards the antigen presenting cell (APC) is critically important for appropriate activation and differentiation of the naïve T cell. Here we used imaging flow cytometry (IFC) and show that the activation induced Lck and Itk adapter T cell specific adapter protein (TSAd), encoded by SH2D2A, modulates polarization of T cells towards the APC. Upon exposure to APC presenting the cognate antigen Id, Sh2d2a−/− CD4+ T cells expressing Id-specific transgenic T cell receptor (TCR), displayed impaired polarization of F-actin and TCR to the immunological synapse (IS). Sh2d2a−/− T-cells that did polarize F-actin and TCR still displayed impaired polarization of PKCξ, PAR3 and the microtubule-organizing center (MTOC). In vitro differentiation of activated Sh2d2a−/− T cells was skewed towards an effector memory (Tem) rather than a central memory (Tcm) phenotype. A similar trend was observed for Id-specific TCR Sh2d2a−/− T cells stimulated with APC and cognate antigen. Taken together our data suggest that TSAd modulates differentiation of experienced T cells possibly through polarization of CD4+ T cells towards the APC.

Binding of intercellular adhesion molecule 1 to β2-integrin regulates distinct cell adhesion processes on hepatic and cerebral endothelium

Flowing polymorphonuclear neutrophils (PMNs) are forced to recruit toward inflamed tissue and adhere to vascular endothelial cells, which is primarily mediated by the binding of β₂-integrins to ICAM-1. This process is distinct among different organs such as liver and brain; however, the underlying kinetic and mechanical mechanisms regulating tissue-specific recruitment of PMNs remain unclear. Here, binding kinetics measurement showed that ICAM-1 on murine hepatic sinusoidal endothelial cells (LSECs) bound to lymphocyte function-associated antigen-1 (LFA-1) with higher on- and off-rates but lower effective affinity compared with macrophage-1 antigen (Mac-1), whereas ICAM-1 on cerebral endothelial cells (BMECs or bEnd.3 cells) bound to LFA-1 with higher on-rates, similar off-rates, and higher effective affinity compared with Mac-1. Physiologically, free crawling tests of PMN onto LSEC, BMEC, or bEnd.3 monolayers were consistent with those kinetics differences between two β₂-integrins interacting with hepatic sinusoid or cerebral endothelium. Numerical calculations and Monte Carlo simulations validated tissue-specific contributions of β₂-integrin-ICAM-1 kinetics to PMN crawling on hepatic sinusoid or cerebral endothelium. Thus, this work first quantified the biophysical regulation of PMN adhesion in hepatic sinusoids compared with cerebral endothelium.

The Rho regulator Myosin IXb enables nonlymphoid tissue seeding of protective CD8+ T cells

Moalli et al. combine in vitro CD8⁺ T cell motility analysis with intravital imaging of mouse tissues to identify the actomyosin regulator Myo9b as a central player for nonlymphoid tissue infiltration during adaptive immune responses by facilitating crossing of tissue barriers.

T cells are actively scanning pMHC-presenting cells in lymphoid organs and nonlymphoid tissues (NLTs) with divergent topologies and confinement. How the T cell actomyosin cytoskeleton facilitates this task in distinct environments is incompletely understood. Here, we show that lack of Myosin IXb (Myo9b), a negative regulator of the small GTPase Rho, led to increased Rho-GTP levels and cell surface stiffness in primary T cells. Nonetheless, intravital imaging revealed robust motility of Myo9b^−/− CD8⁺ T cells in lymphoid tissue and similar expansion and differentiation during immune responses. In contrast, accumulation of Myo9b^−/− CD8⁺ T cells in NLTs was strongly impaired. Specifically, Myo9b was required for T cell crossing of basement membranes, such as those which are present between dermis and epidermis. As consequence, Myo9b^−/− CD8⁺ T cells showed impaired control of skin infections. In sum, we show that Myo9b is critical for the CD8⁺ T cell adaptation from lymphoid to NLT surveillance and the establishment of protective tissue–resident T cell populations.

Compartmentalized Cyclic AMP Production by the Bordetella pertussis and Bacillus anthracis Adenylate Cyclase Toxins Differentially Affects the Immune Synapse in T Lymphocytes

A central feature of the immune synapse (IS) is the tight compartmentalization of membrane receptors and signaling mediators that is functional for its ability to coordinate T cell activation. Second messengers centrally implicated in this process, such as Ca²⁺ and diacyl glycerol, also undergo compartmentalization at the IS. Current evidence suggests a more complex scenario for cyclic AMP (cAMP), which acts both as positive and as negative regulator of T-cell antigen receptor (TCR) signaling and which, as such, must be subjected to a tight spatiotemporal control to allow for signaling at the IS. Here, we have used two bacterial adenylate cyclase toxins that produce cAMP at different subcellular localizations as the result of their distinct routes of cell invasion, namely Bordetella pertussis CyaA and Bacillus anthracis edema toxin (ET), to address the ability of the T cell to confine cAMP to the site of production and to address the impact of compartmentalized cAMP production on IS assembly and function. We show that CyaA, which produces cAMP close to the synaptic membrane, affects IS stability by modulating not only the distribution of LFA-1 and its partners talin and L-plastin, as previously partly reported but also by promoting the sustained synaptic accumulation of the A-kinase adaptor protein ezrin and protein kinase A while suppressing the β-arrestin-mediated recruitment of phosphodiesterase 4B. These effects are dependent on the catalytic activity of the toxin and can be reproduced by treatment with a non-hydrolyzable cAMP analog. Remarkably, none of these effects are elicited by ET, which produces cAMP at a perinuclear localization, despite its ability to suppress TCR signaling and T cell activation through its cAMP-elevating activity. These results show that the IS responds solely to local elevations of cAMP and provide evidence that potent compartmentalization mechanisms are operational in T cells to contain cAMP close to the site of production, even when produced at supraphysiological levels.

Immunology: Is Actin at the Lytic Synapse a Friend or a Foe?

Cytotoxic T cells and natural killer cells defend us against disease by secreting lytic granules. Whether actin facilitates or thwarts lytic granule secretion has been an open question. Recent results now indicate that the answer depends on the maturation stage of the immune cell-target cell contact.

Asymmetric PI3K Activity in Lymphocytes Organized by a PI3K-Mediated Polarity Pathway

Unequal transmission of nutritive signaling during cell division establishes fate disparity between sibling lymphocytes, but how asymmetric signaling becomes organized is not understood. We show that receptor-associated class I phosphatidylinositol 3-kinase (PI3K) signaling activity, indexed by phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) staining, is spatially restricted to the microtubule-organizing center and subsequently to one pole of the mitotic spindle in activated T and B lymphocytes. Asymmetric PI3K activity co-localizes with polarization of antigen receptor components implicated in class I PI3K signaling and with facultative glucose transporters whose trafficking is PI3K dependent and whose abundance marks cells destined for differentiation. Perturbation of class I PI3K activity disrupts asymmetry of upstream antigen receptors and downstream glucose transporter traffic. The roles of PI3K signaling in nutrient utilization, proliferation, and gene expression may have converged with the conserved role of PI3K signaling in cellular symmetry breaking to form a logic for regenerative lymphocyte divisions.

Integrins Modulate T Cell Receptor Signaling by Constraining Actin Flow at the Immunological Synapse

Full T cell activation requires coordination of signals from multiple receptor–ligand pairs that interact in parallel at a specialized cell–cell contact site termed the immunological synapse (IS). Signaling at the IS is intimately associated with actin dynamics; T cell receptor (TCR) engagement induces centripetal flow of the T cell actin network, which in turn enhances the function of ligand-bound integrins by promoting conformational change. Here, we have investigated the effects of integrin engagement on actin flow, and on associated signaling events downstream of the TCR. We show that integrin engagement significantly decelerates centripetal flow of the actin network. In primary CD4⁺ T cells, engagement of either LFA-1 or VLA-4 by their respective ligands ICAM-1 and VCAM-1 slows actin flow. Slowing is greatest when T cells interact with low mobility integrin ligands, supporting a predominately drag-based mechanism. Using integrin ligands presented on patterned surfaces, we demonstrate that the effects of localized integrin engagement are distributed across the actin network, and that focal adhesion proteins, such as talin, vinculin, and paxillin, are recruited to sites of integrin engagement. Further analysis shows that talin and vinculin are interdependent upon one another for recruitment, and that ongoing actin flow is required. Suppression of vinculin or talin partially relieves integrin-dependent slowing of actin flow, indicating that these proteins serve as molecular clutches that couple engaged integrins to the dynamic actin network. Finally, we found that integrin-dependent slowing of actin flow is associated with reduction in tyrosine phosphorylation downstream of the TCR, and that this modulation of TCR signaling depends on expression of talin and vinculin. More generally, we found that integrin-dependent effects on actin retrograde flow were strongly correlated with effects on TCR signaling. Taken together, these studies support a model in which ligand-bound integrins engage the actin cytoskeletal network via talin and vinculin, and tune TCR signaling events by modulating actin dynamics at the IS.

Visualization and Quantitative Analysis of the Actin Cytoskeleton Upon B Cell Activation

The formation of the immunological synapse upon B cell activation critically depends on the rearrangement of the submembranous actin cytoskeleton. Polymerization of actin monomers into filaments provides the force required for B cell spreading on the antigen-presenting cell (APC). Interestingly, the actin network also participates in cellular signaling at multiple levels. Fluorescence microscopy plays a critical role in furthering our understanding of the various functions of the cytoskeleton, and has become an important tool in the studies on B cell activation. The actin cytoskeleton can be tracked in live cells with various fluorescent probes binding to actin, or in fixed cells typically with phalloidin staining. Here, we present the usage of TIRF microscopy and an image analysis workflow for studying the overall density and organization of the actin network upon B cell spreading on antigen-coated glass, a widely used model system for the formation of the immunological synapse.

Using Murine Models to Investigate Tumor-Lymphoid Interactions: Spotlight on Chronic Lymphocytic Leukemia and Angioimmunoblastic T-Cell Lymphoma

The role of the tumor microenvironment in leukemias and lymphomas is well established, yet the intricacies of how the malignant cells regulate and influence their non-malignant counterparts remain elusive. For example, chronic lymphocytic leukemia (CLL) is an expansion of malignant CD5⁺CD19⁺ B cells, yet the non-malignant T cells play just as large of a role in disease presentation and etiology. Herein, we review the dynamic tumor cell to lymphoid repertoire interactions found in two non-Hodgkin's lymphoma subtypes: CLL and angioimmunoblastic T-cell lymphoma. We aim to highlight the pivot work done in the murine models which recapitulate these diseases and explore the insights that can be gained from studying the immuno-oncological regulation of non-malignant lymphoid counterparts.

Analyzing Actin Dynamics at the Immunological Synapse

T cell signaling is inextricably linked to actin cytoskeletal dynamics at the immunological synapse (IS). This process can be imaged in living T cells expressing GFP actin or fluorescent F-actin binding proteins. Because of its planar nature, the IS provides a unique opportunity to image events as they happen, monitoring changes in actin retrograde flow in T cells interacting with different stimulatory surfaces or after pharmacological treatments. Here, we described the imaging methods and analytical procedures used to measure actin velocity across the IS in T cells spreading on planar stimulatory surfaces.

Mechanical Communication at the Immunological Synapse

T and B lymphocytes communicate by forming immunological synapses with antigen-presenting target cells. These highly dynamic contacts are characterized by continuous cytoskeletal remodeling events, which not only structure the interface but also exert a considerable amount of mechanical force. In recent years, it has become increasingly clear that synaptic forces influence information transfer both into and out of the lymphocyte. Here, we review our current understanding of synapse mechanics, focusing on its role as an avenue for intercellular communication.

Formin-generated actomyosin arcs propel T cell receptor microcluster movement at the immune synapse

Murugesan et al. report that actomyosin arcs at the T cell synapse are formin-generated structures that directly propel T cell receptor cluster movement. The authors reveal the origin, organization, and functions of a major cytoskeletal network during synapse maturation.

Actin assembly and inward flow in the plane of the immunological synapse (IS) drives the centralization of T cell receptor microclusters (TCR MCs) and the integrin leukocyte functional antigen 1 (LFA-1). Using structured-illumination microscopy (SIM), we show that actin arcs populating the medial, lamella-like region of the IS arise from linear actin filaments generated by one or more formins present at the IS distal edge. After traversing the outer, Arp2/3-generated, lamellipodia-like region of the IS, these linear filaments are organized by myosin II into antiparallel concentric arcs. Three-dimensional SIM shows that active LFA-1 often aligns with arcs, whereas TCR MCs commonly reside between arcs, and total internal reflection fluorescence SIM shows TCR MCs being swept inward by arcs. Consistently, disrupting actin arc formation via formin inhibition results in less centralized TCR MCs, missegregated integrin clusters, decreased T–B cell adhesion, and diminished TCR signaling. Together, our results define the origin, organization, and functional significance of a major actomyosin contractile structure at the IS that directly propels TCR MC transport.

Cytotoxic T Cells Use Mechanical Force to Potentiate Target Cell Killing

The immunological synapse formed between a cytotoxic T lymphocyte (CTL) and an infected or transformed target cell is a physically active structure capable of exerting mechanical force. Here, we investigated whether synaptic forces promote the destruction of target cells. CTLs kill by secreting toxic proteases and the pore forming protein perforin into the synapse. Biophysical experiments revealed a striking correlation between the magnitude of force exertion across the synapse and the speed of perforin pore formation on the target cell, implying that force potentiates cytotoxicity by enhancing perforin activity. Consistent with this interpretation, we found that increasing target cell tension augmented pore formation by perforin and killing by CTLs. Our data also indicate that CTLs coordinate perforin release and force exertion in space and time. These results reveal an unappreciated physical dimension to lymphocyte function and demonstrate that cells use mechanical forces to control the activity of outgoing chemical signals.

Dances with Membranes: Breakthroughs from Super-resolution Imaging

Biological membrane organization mediates numerous cellular functions and has also been connected with an immense number of human diseases. However, until recently, experimental methodologies have been unable to directly visualize the nanoscale details of biological membranes, particularly in intact living cells. Numerous models explaining membrane organization have been proposed, but testing those models has required indirect methods; the desire to directly image proteins and lipids in living cell membranes is a strong motivation for the advancement of technology. The development of super-resolution microscopy has provided powerful tools for quantification of membrane organization at the level of individual proteins and lipids, and many of these tools are compatible with living cells. Previously inaccessible questions are now being addressed, and the field of membrane biology is developing rapidly. This chapter discusses how the development of super-resolution microscopy has led to fundamental advances in the field of biological membrane organization. We summarize the history and some models explaining how proteins are organized in cell membranes, and give an overview of various super-resolution techniques and methods of quantifying super-resolution data. We discuss the application of super-resolution techniques to membrane biology in general, and also with specific reference to the fields of actin and actin-binding proteins, virus infection, mitochondria, immune cell biology, and phosphoinositide signaling. Finally, we present our hopes and expectations for the future of super-resolution microscopy in the field of membrane biology.