What is the function of the dendritic cell side of the immunological synapse?

Reprogramming dendritic cells through the immunological synapse: A two-way street

Dendritic cells (DCs) bridge innate and adaptive immunity. Their main function is to present antigens to prime T cells and initiate and shape adaptive responses. Antigen presentation takes place through intimate contacts between the two cells, termed immune synapses (IS). During the formation of IS, information travels towards the T-cell side to induce and tune its activation; but it also travels in reverse via engagement of membrane receptors and within extracellular vesicles transferred to the DC. Such reverse information transfer and its consequences on DC fate have been largely neglected. Here, we review the events and effects of IS-mediated antigen presentation on DCs. In addition, we discuss novel technological advancements that enable monitoring DCs interactions with T lymphocytes, the main effects of DCs undergoing productive IS (postsynaptic DCs, or psDCs), and how reverse information transfer could be harnessed to modulate immune responses for therapeutic intervention.

The Actin Cytoskeleton at the Immunological Synapse of Dendritic Cells

Dendritic cells (DCs) are considered the most potent antigen-presenting cells. DCs control the activation of T cells (TCs) in the lymph nodes. This process involves forming a specialized superstructure at the DC-TC contact zone called the immunological synapse (IS). For the sake of clarity, we call IS(DC) and IS(TC) the DC and TC sides of the IS, respectively. The IS(DC) and IS(TC) seem to organize as multicentric signaling hubs consisting of surface proteins, including adhesion and costimulatory molecules, associated with cytoplasmic components, which comprise cytoskeletal proteins and signaling molecules. Most of the studies on the IS have focused on the IS(TC), and the information on the IS(DC) is still sparse. However, the data available suggest that both IS sides are involved in the control of TC activation. The IS(DC) may govern activities of DCs that confer them the ability to activate the TCs. One key component of the IS(DC) is the actin cytoskeleton. Herein, we discuss experimental data that support the concept that actin polarized at the IS(DC) is essential to maintaining IS stability necessary to induce TC activation.

The Scribble Complex PDZ Proteins in Immune Cell Polarities

hScrib and hDlg belong to the PDZ family of proteins. Since the identification of these highly phylogenetically conserved scaffolds, an increasing amount of experiments has elucidated the roles of hScrib and hDlg in a variety of cell functions. Remarkably, their participation during the establishment of polarity in epithelial cells is well documented. Although the role of both proteins in the immune system is scantly known, it has become a growing field of investigation. Here, we summarize the interactions and functions of hScrib and hDlg1, which participate in diverse functions involving cell polarization in immune cells, and discuss their relevance in the immune cell biology. The fundamental role of hScrib and hDlg1 during the establishment of the immunological synapse, hence T cell activation, and the recently described role of hScrib in reactive oxygen species production in macrophages and of hDlg1 in cytokine production by dendritic cells highlight the importance of both proteins in immune cell biology. The expression of these proteins in other leukocytes can be anticipated and needs to be confirmed. Due to their multiple interaction domains, there is a wide range of possible interactions of hScrib and hDlg1 that remains to be explored in the immune system.

T Cell Reprogramming Against Cancer

Advances in academic and clinical studies during the last several years have resulted in practical outcomes in adoptive immune therapy of cancer. Immune cells can be programmed with molecular modules that increase their therapeutic potency and specificity. It has become obvious that successful immunotherapy must take into account the full complexity of the immune system and, when possible, include the use of multifactor cell reprogramming that allows fast adjustment during the treatment. Today, practically all immune cells can be stably or transiently reprogrammed against cancer. Here, we review works related to T cell reprogramming, as the most developed field in immunotherapy. We discuss factors that determine the specific roles of αβ and γδ T cells in the immune system and the structure and function of T cell receptors in relation to other structures involved in T cell target recognition and immune response. We also discuss the aspects of T cell engineering, specifically the construction of synthetic T cell receptors (synTCRs) and chimeric antigen receptors (CARs) and the use of engineered T cells in integrative multifactor therapy of cancer.

Immunological Synapse Formation Induces Mitochondrial Clustering and Mitophagy in Dendritic Cells

The immunological synapse (IS) is a superstructure formed during T cell activation at the zone of contact between T cells and dendritic cells (DCs). The IS includes specific molecular components in the T cell and DCs sides that may result in different functionality. Most of the studies on the IS have focused on the T cell side of this structure and, in contrast, the information available on the IS of DCs is sparse. Autophagy is a cellular process involved in the clearance of damaged proteins and organelles via lysosomal degradation. Mitophagy is the selective autophagy of damaged mitochondria. In this study, it is shown that IS formation induces clustering of mitochondria in the IS of DCs and partial depolarization of these organelles. At the IS of the DCs also accumulate autophagy and mitophagy markers, even when the kinase complex mTORC1, an inhibitor of the autophagy, is active. Together the results presented indicate that IS formation induces local clustering of mitochondria and mitophagy, which could be a homeostatic mechanism to control the quality of mitochondria in this region. The data underline the complexity of the regulatory mechanisms operating in the IS of DCs.

PDZ proteins are expressed and regulated in antigen-presenting cells and are targets of influenza A virus

In this work, we identified the expression, regulation, and viral targeting of Scribble and Dlg1 in antigen-presenting cells. Scribble and Dlg1 belong to the family of PDZ (postsynaptic density (PSD95), disc large (Dlg), and zonula occludens (ZO-1)) proteins involved in cell polarity. The relevance of PDZ proteins in cellular functions is reinforced by the fact that many viruses interfere with host PDZ-dependent interactions affecting cellular mechanisms thus favoring viral replication. The functions of Scribble and Dlg have been widely studied in polarized cells such as epithelial and neuron cells. However, within the cells of the immune system, their functions have been described only in T and B lymphocytes. Here we demonstrated that Scribble and Dlg1 are differentially expressed during antigen-presenting cell differentiation and dendritic cell maturation. While both Scribble and Dlg1 seem to participate in distinct dendritic cell functions, both are targeted by the viral protein NS1 of influenza A in a PDZ-dependent manner in dendritic cells. Our findings suggest that these proteins might be involved in the mechanisms of innate immunity and/or antigen processing and presentation that can be hijacked by viral pathogens.

Biophysical Properties and Motility of Human Mature Dendritic Cells Deteriorated by Vascular Endothelial Growth Factor through Cytoskeleton Remodeling

Dendritic cells (DCs), the most potent antigen-presenting cells, play a central role in the initiation, regulation, and maintenance of the immune responses. Vascular endothelial growth factor (VEGF) is one of the important cytokines in the tumor microenvironment (TME) and can inhibit the differentiation and functional maturation of DCs. To elucidate the potential mechanisms of DC dysfunction induced by VEGF, the effects of VEGF on the biophysical characteristics and motility of human mature DCs (mDCs) were investigated. The results showed that VEGF had a negative influence on the biophysical properties, including electrophoretic mobility, osmotic fragility, viscoelasticity, and transmigration. Further cytoskeleton structure analysis by confocal microscope and gene expression profile analyses by gene microarray and real-time PCR indicated that the abnormal remodeling of F-actin cytoskeleton may be the main reason for the deterioration of biophysical properties, motility, and stimulatory capability of VEGF-treated mDCs. This is significant for understanding the biological behavior of DCs and the immune escape mechanism of tumors. Simultaneously, the therapeutic efficacies may be improved by blocking the signaling pathway of VEGF in an appropriate manner before the deployment of DC-based vaccinations against tumors.

The Dendritic Cell Synapse: A Life Dedicated to T Cell Activation

T-cell activation within immunological synapses is a complex process whereby different types of signals are transmitted from antigen-presenting cells to T cells. The molecular strategies developed by T cells to interpret and integrate these signals have been systematically dissected in recent years and are now in large part understood. On the other side of the immune synapse, dendritic cells (DCs) participate actively in synapse formation and maintenance by remodeling of membrane receptors and intracellular content. However, the details of such changes have been only partially characterized. The DCs actin cytoskeleton has been one of the first systems to be identified as playing an important role in T-cell priming and some of the underlying mechanisms have been elucidated. Similarly, the DCs microtubule cytoskeleton undergoes major spatial changes during synapse formation that favor polarization of the DCs subcellular space toward the interacting T cell. Recently, we have begun to investigate the trafficking machinery that controls polarized delivery of endosomal vesicles at the DC–T immune synapse with the aim of understanding the functional relevance of polarized secretion of soluble factors during T-cell priming. Here, we will review the current knowledge of events occurring in DCs during synapse formation and discuss the open questions that still remain unanswered.

Interleukin-10 modulates antigen presentation by dendritic cells through regulation of NLRP3 inflammasome assembly during Chlamydia infection

Interleukin-10 (IL-10) has been implicated in susceptibility to genital chlamydial infection and the development of tubal pathologies. IL-10 limitation also resulted in the rapid elicitation of immune responses against Chlamydia, and decreased levels of IL-10 correlated with protective anti-Chlamydia immunity. To investigate the molecular basis for these effects, we compared the reproductive pathologies and fertility rates in Chlamydia-infected wild-type (WT) and IL-10-knockout (IL-10(-/-)) mice; we also analyzed the expression of the Toll-like receptor (TLR)/interleukin-1 receptor (IL-1R) superfamily, IL-1β production, NLRP3 inflammasome assembly and activation, and the immunostimulatory capacity and apoptotic predilection of Chlamydia-exposed dendritic cells (DCs) from WT and IL-10(-/-) mice. Our results revealed that, in addition to the rapid clearance of infection, genitally infected IL-10(-/-) mice were protected from tubal pathologies and infertility, whereas WT (IL-10(+/+)) mice were not. Chlamydia-pulsed IL-10(-/-) DCs expressed larger numbers of TLR4/IL-1R molecules and had enhanced IL-1β production. In addition, NLRP3 inflammasome assembly was suppressed in IL-10(-/-) DCs through the inhibition of the P2X purinoceptor 7 (P2X7) receptor (P2X7R), an ATP-gated ion channel, and a decrease in intracellular Ca(2+) levels, which inhibited DC apoptosis. Thus, the potent immunostimulatory capacity of IL-10-deficient DCs is due, at least in part, to the suppression of the intracellular inflammasome assembly, which prevents DC apoptosis, allowing efficient antigen presentation. The results indicate that IL-10 deficiency enables efficient antigen presentation by DCs for rapid and enhanced immune activation against Chlamydia, which results in rapid microbial clearance, which prevents tubal pathologies during infection. Our finding has important implications for the induction of protective immunity against Chlamydia and other infectious and noninfectious diseases by vaccines.

Plasma membrane-associated superstructure: Have we overlooked a new type of organelle in eukaryotic cells?

A variety of intriguing plasma membrane-associated regions, including focal adhesions, adherens junctions, tight junctions, immunological synapses, neuromuscular junctions and the primary cilia, among many others, have been described in eukaryotic cells. Emphasizing their importance, alteration in their molecular structures induces or correlates with different pathologies. These regions display surface proteins connected to intracellular molecules, including cytoskeletal component, which maintain their cytoarchitecture, and signalling proteins, which regulate their organization and functions. Based on the molecular similarities and other common features observed, we suggest that, despite differences in external appearances, all these regions are just the same superstructure that appears in different locations and cells. We hypothesize that this superstructure represents an overlooked new type of organelle that we call plasma membrane-associated superstructure (PMAS). Therefore, we suggest that eukaryotic cells include classical organelles (e.g. mitochondria, Golgi and others) and also PMAS. We speculate that this new type of organelle might be an innovation associated to the emergence of eukaryotes. Finally we discuss the implications of the hypothesis proposed.

Co-ordination of incoming and outgoing traffic in antigen-presenting cells by pattern recognition receptors and T cells

Dendritic cells are innate sentinels of the immune system and potent activators of naÏve T cells. Mechanisms must exist to enable these cells to achieve maximal activation of T cells specific for microbial antigens, while avoiding activation of T cells specific for self-antigens. Here we discuss how a combination of signals from pattern recognition receptors and T cells co-ordinates subcellular trafficking of antigen with both major histocompatibility complex class I and class II molecules and T-cell costimulatory molecules, resulting in the preferential presentation of microbial peptides within a stimulatory context.

Autoimmune therapies targeting costimulation and emerging trends in multivalent therapeutics

Proteins participating in immunological signaling have emerged as important targets for controlling the immune response. A multitude of receptor-ligand pairs that regulate signaling pathways of the immune response have been identified. In the complex milieu of immune signaling, therapeutic agents targeting mediators of cellular signaling often either activate an inflammatory immune response or induce tolerance. This review is primarily focused on therapeutics that inhibit the inflammatory immune response by targeting membrane-bound proteins regulating costimulation or mediating immune-cell adhesion. Many of these signals participate in larger, organized structures such as the immunological synapse. Receptor clustering and arrangement into organized structures is also reviewed and emerging trends implicating a potential role for multivalent therapeutics is posited.

Imaging techniques for assaying lymphocyte activation in action

Imaging techniques have greatly improved our understanding of lymphocyte activation. Technical advances in spatial and temporal resolution and new labelling tools have enabled researchers to directly observe the activation process. Consequently, research using imaging approaches to study lymphocyte activation has expanded, providing an unprecedented level of cellular and molecular detail in the field. As a result, certain models of lymphocyte activation have been verified, others have been revised and yet others have been replaced with new concepts. In this article, we review the current imaging techniques that are used to assess lymphocyte activation in different contexts, from whole animals to single molecules, and discuss the advantages and potential limitations of these methods.