Antigen archiving by lymph node stroma: A novel function for the lymphatic endothelium

Secondary lymphoid stroma performs far more functions than simple structural support for lymphoid tissues, providing a host of soluble and membrane‐bound cues to trafficking leukocytes during inflammation and homeostasis. More recently it has become clear that stromal cells can manipulate T‐cell responses, either through direct antigen‐mediated stimulation of T cells or more indirectly through the retention and management of antigen after viral infection or vaccination. In light of recent data, this review provides an overview of stromal cell subsets and functions during the progression of an adaptive immune response with particular emphasis on antigen capture and retention by follicular dendritic cells as well as the recently described “antigen archiving” function of lymphatic endothelial cells (LECs). Given its impact on the maintenance of protective immune memory, we conclude by discussing the most pressing questions pertaining to LEC antigen capture, archiving and exchange with hematopoetically derived antigen‐presenting cells.     

The role of chemokines in the recruitment of lymphocytes to the liver

Chemokines direct leukocyte trafficking and positioning within tissues. They thus play critical roles in regulating immune responses and inflammation. The chemokine system is complex involving interactions between multiple chemokines and their receptors that operate in combinatorial cascades with adhesion molecules. The involvement of multiple chemokines and chemokine receptors in these processes brings flexibility and specificity to recruitment. The hepatic vascular bed is a unique low flow environment through which leukocyte are recruited to the liver during homeostatic immune surveillance and in response to infection or injury. The rate of leukocyte recruitment and the nature of cells recruited through the sinusoids in response to inflammatory signals will shape the severity of disease. At one end of the spectrum fulminant liver failure results from a rapid recruitment of leukocytes that leads to hepatocyte destruction and liver failure at the other diseases such as chronic hepatitis C infection may progress over many years from hepatitis to fibrosis and cirrhosis. Chronic hepatitis is charactezised by a T lymphocyte rich infiltrate and the nature and outcome of hepatitis will depend on the T cell subsets recruited, their activation and function within the liver. Different subsets of effector T cells have been described based on their secretion of cytokines and specific functions. These include Th1 and Th2 cells and more recently Th17 and Th9 cells which are associated with different types of immune response and which express distinct patterns of chemokine receptors that promote their recruitment under particular conditions. The effector function of these cells is balanced by the recruitment of regulatory T cells that are able to suppress antigen-specific effectors to allow resolution of immune responses and restoration of immune homeostasis. Understanding the signals that are responsible for recruiting different lymphocyte subsets to the liver will elucidate disease pathogenesis and open up new therapeutic approaches to modulate recruitment in favour of resolution rather than injury.     

Interstitial leukocyte migration and immune function

The trafficking of leukocytes into and within lymphoid and peripheral tissues is central to immune cell development, immunosurveillance and effector function. Interstitial leukocyte trafficking is the result of amoeboid polarization and migration, guided by soluble or tissue-bound chemoattractant signals for positioning and local arrest. In contrast to other migration modes, amoeboid movement is particularly suited for scanning cellular networks and tissues. Here, we review mechanisms of leukocyte migration and sensing involved in diapedesis, tissue-based interstitial migration and egress, immune cell positioning in inflammation, and emerging therapeutic interference strategies.     

Orchestrating the orchestrators: chemokines in control of T cell traffic

The understanding of how chemokines orchestrate the trafficking and activity of immune cells has increased considerably. So far, over 50 chemokines and 20 chemokine receptors have been identified. Detailed analyses have demonstrated the function of chemokine receptors on T cell subsets, the temporal and spatial expression patterns of chemokines in vivo and the phenotypes of animals genetically deficient in one component or several components of the chemokine-chemokine receptor system. New microscopy modalities for studying the influence of chemokines on the migratory activity of T cells in the lymph node have also brought new insights. Here we review such advances with particular emphasis on control of the migration of T cell subsets in lymph nodes and in peripheral tissues in homeostasis and inflammation.     

Dendritic cell migration in inflammation and immunity

Dendritic cells (DCs) are the key link between innate immunity and adaptive immunity and play crucial roles in both the promotion of immune defense and the maintenance of immune tolerance. The trafficking of distinct DC subsets across lymphoid and nonlymphoid tissues is essential for DC-dependent activation and regulation of inflammation and immunity. DC chemotaxis and migration are triggered by interactions between chemokines and their receptors and regulated by multiple intracellular mechanisms, such as protein modification, epigenetic reprogramming, metabolic remodeling, and cytoskeletal rearrangement, in a tissue-specific manner. Dysregulation of DC migration may lead to abnormal positioning or activation of DCs, resulting in an imbalance of immune responses and even immune pathologies, including autoimmune responses, infectious diseases, allergic diseases and tumors. New strategies targeting the migration of distinct DC subsets are being explored for the treatment of inflammatory and infectious diseases and the development of novel DC-based vaccines. In this review, we will discuss the migratory routes and immunological consequences of distinct DC subsets, the molecular basis and regulatory mechanisms of migratory signaling in DCs, and the association of DC migration with the pathogenesis of autoimmune and infectious diseases.     

Preferential chemotaxis of activated human CD4+ T cells by extracellular cyclophilin A

The recruitment and trafficking of leukocytes are essential aspects of the inflammatory process. Although chemokines are thought to be the main regulators of cell trafficking, extracellular cyclophilins have been shown recently to have potent chemoattracting properties for human leukocytes. Cyclophilins are secreted by a variety of cell types and are detected at high levels in tissues with ongoing inflammation. CD147 has been identified as the main signaling receptor for cyclophilin A (CypA) on human leukocytes. It is interesting that the expression of CD147 is elevated on leukocytes from inflamed tissue, suggesting a correlation among the presence of extracellular cyclophilins, CD147 expression, and inflammatory responses. Thus, cyclophilin-CD147 interactions may contribute directly to the recruitment of leukocytes into inflamed tissues. In the current studies, we show that activated human T lymphocytes express elevated levels of CD147, compared with resting T cells and that these activated T cells migrate more readily to CypA than resting cells. Furthermore, we show that unlike resting CD4+ T cells, the cyclophilin-mediated migration of activated T cells does not require interaction with heparan sulfate receptors but instead, is dependent on CD147 interaction alone. Such findings suggest that cyclophilin-CD147 interactions will be most potent when leukocytes are in an activated state, for example, during inflammatory responses. Thus, targeting cyclophilin-CD147 interactions may provide a novel approach for alleviating tissue inflammation.     

The multiple faces of leukocyte interstitial migration

Spatiotemporal control of leukocyte dynamics within tissues is critical for successful innate and adaptive immune responses. Homeostatic trafficking and coordinated infiltration into and within sites of inflammation and infection rely on signaling in response to extracellular cues that in turn controls a variety of intracellular protein networks regulating leukocyte motility, migration, chemotaxis, positioning, and cell–cell interaction. In contrast to mesenchymal cells, leukocytes migrate in an amoeboid fashion by rapid cycles of actin polymerization and actomyosin contraction, and their migration in tissues is generally referred to as low adhesive and nonproteolytic. The interplay of actin network expansion, contraction, and adhesion shapes the exact mode of amoeboid migration, and in this review, we explore how leukocyte subsets potentially harness the same basic biomechanical mechanisms in a cell-type-specific manner. Most of our detailed understanding of these processes derives from in vitro migration studies in three-dimensional gels and confined spaces that mimic geometrical aspects of physiological tissues. We summarize these in vitro results and then critically compare them to data from intravital imaging of leukocyte interstitial migration in mouse tissues. We outline the technical challenges of obtaining conclusive mechanistic results from intravital studies, discuss leukocyte migration strategies in vivo, and present examples of mode switching during physiological interstitial migration. These findings are also placed in the context of leukocyte migration defects in primary immunodeficiencies. This overview of both in vitro and in vivo studies highlights recent progress in understanding the molecular and biophysical mechanisms that shape robust leukocyte migration responses in physiologically complex and heterogeneous environments.     

Chemokines in rapid leukocyte adhesion triggering and migration

Leukocyte subsets are recruited from the blood to lymphoid and non-lymphoid tissues via a multi-step process that involves distinct adhesive and activation steps. Chemokines, a family of chemotactic cytokines that signal through G-protein-coupled receptors, play critical roles in regulating the leukocyte recruitment cascade. Chemokines can be transported and immobilized on the surface of vascular endothelial cells, where they activate leukocyte subsets expressing specific receptors. Activation signals induce firm adhesion of rolling leukocytes by rapidly upregulating integrin affinity and/or avidity. Chemokines can also direct migration of adherent cells across the endothelium, and control segregation of cells into specific microenvironments within tissues. The regulated expression of chemokines and their receptors is a critical determinant for homing of specialized lymphocyte subsets, and controls both tissue and inflammation-specific immune processes.     

Natural killer cell-directed therapies: moving from unexpected results to successful strategies

Natural killer (NK) cells influence innate and adaptive immune host defenses. Existing data indicate that manipulating the balance between inhibitory and activating NK receptor signals, the sensitivity of target cells to NK cell-mediated apoptosis, and NK cell cross-talk with dendritic cells might hold therapeutic promise. Efforts to modulate NK cell trafficking into inflamed tissues and/or lymph nodes, and to counteract NK cell suppressors, might also prove fruitful in the clinic. However, deeper investigation into the benefits of combination therapy, greater understanding of the functional distinctions between NK cell subsets, and design of new tools to monitor NK cell activity are needed to strengthen our ability to harness the power of NK cells for therapeutic aims.     

Chemokines

Motility is a hallmark of leukocytes, and breakdown in the control of migration contributes to many inflammatory diseases. Chemotactic migration of leukocytes largely depends on adhesive interaction with the substratum and recognition of a chemoattractant gradient. Chemokines are secreted proteins and have emerged as key controllers of integrin function and cell locomotion. Numerous distinct chemokines exist that target all types of leukocytes, including hematopoietic precursors, leukocytes of the innate immune system, as well as naive memory, and effector lymphocytes. The combinatorial diversity in responsiveness to chemokines ensures the proper tissue distribution of distinct leukocyte subsets under normal and pathological conditions. Inflammatory chemokines are readily detected in lesional tissue and local cellular infiltrates carry corresponding chemokine receptors. Blocking of inflammatory chemokines represents a promising strategy for the development of novel anti-inflammatory therapeutics.This review focuses on a separate class of chemokines, termed homeostatic chemokines, with steady-state production at diverse sites, including primary and secondary lymphoid tissues as well as peripheral (extralymphoid) tissues. More precisely, we discuss the chemokines involved in T-cell traffic during the initiation of adaptive immunity and compare the distinct migration properties of short-lived effector T cells and long-lived memory T cells. Memory T cells are currently classified according to the presence of the lymph node-homing receptor CCR7 into CCR7+ central memory T (T(CM)) cells and CCR7- effector memory T (T(EM)) cells. For better understanding memory T-cell function, we propose the distinction of a third category, termed peripheral immune surveillance T (T(PS)) cells, which typically reside in healthy peripheral tissues, such as skin, lung, and gastrointestinal tract.Localization and relocation of memory T cells is strictly related to their function in recall responses. Therefore, detailed knowledge of their generation and tissue distribution may help to design better vaccination strategies.     

Migration of naive, effector and memory T cells: implications for the regulation of immune responses

Summary: T cells play an important role in protective immune responses and in the pathogenesis of many diseases. Understanding the mechanisms regulating their distribution in vivo may therefore be of therapeutic value. Reviewing studies that have followed the migration of labelled naive, effector and memory T cells in healthy animals reveals that all T-cell subsets enter all organs investigated. Within the tissue, two principally different migration patterns can be identified. First, naive and memory T cells accumulate in lymphoid organs for about 48 h after injection, as the time needed for migration through lymphoid organs is longer than through non-lymphoid organs. During this time, surface molecule expression is temporarily modified. These changes are reversed before leaving the lymphoid organs and entering the blood to start a new cycle of migration. Second, effector T cells are evenly distributed throughout the body, and most die in the tissues within 24 h. However, depending on the presence of cytokines, some are able to survive and to proliferate, and thereby accumulate in defined microenvironments of the body. Analysing the principles regulating T-cell migration and survival within the tissue may lead to the development of new options for the treatment of disease.     

γδ T cells: novel initiators of adaptive immunity

Summary:  In this review, we discuss the potential role of human γδ T cells in the control of adaptive immunity. Our latest findings emerged as a consequence of our working hypothesis, which predicts a close relationship between the migration control in leukocytes and their function in immune processes as diverse as hematopoiesis, initiation of adaptive immunity, and immune surveillance in peripheral tissues. Leukocyte migration control is defined by the combination of migration and adhesion receptors on their surface and the tissue distribution of the corresponding ligands. According to our hypothesis, leukocytes featuring migration receptors for homing to lymph nodes (LNs) will also display activities that preferentially take place within LNs. Following this line of thought, by showing LN-homing properties in a subset of human γδ T cells, we speculated that γδ T cells influence the initiation of T- and B-cell responses. Here, we summarize our recent data, showing that LN-homing γδ T cells have potent antigen-presenting cell characteristics. This unexpected finding is discussed with regards to microbial sensing by human γδ T cells and a possible role for these cells in anti-microbial immunity.     

Taking the lymphatic route: dendritic cell migration to draining lymph nodes

In contrast to leukocyte migration through blood vessels, trafficking via lymphatic vessels (LVs) is much less well characterized. An important cell type migrating via this route is antigen-presenting dendritic cells (DCs), which are key for the induction of protective immunity as well as for the maintenance of immunological tolerance. In this review, we will summarize and discuss current knowledge of the cellular and molecular events that control DC migration from the skin towards, into, and within LVs, followed by DC arrival and migration in draining lymph nodes. Finally, we will discuss potential strategies to therapeutically target this migratory step to modulate immune responses.     

Imaging ocular immune responses by intravital microscopy

The eye offers excellent opportunities to observe cellular interactions in vivo. This applies especially to the immune response in which discrete events can be studied, including cell trafficking, transendothelial migration, adhesion, antigen presentation, and T cell activation. Intravital microscopy has allowed study of immune cell interactions in tissues such as the conjunctiva, the inflamed cornea, and the iris. Thus the realtime observation of presentation of antigen injected into the anterior chamber of the eye can be imaged using fluorescently labelled antigen and cells. Application of the scanning laser ophthalmoscope to the rat and mouse eye allows analysis of leukocyte-endothelial interactions in the retinal and choroidal circulations. These studies have provided important information on rolling and adhesion of leukocytes in real time in different microvascular beds that have not been manipulated in any way and has, for instance, provided quantitation to the effects of shear stress on leukocyte-endothelial adhesion. In addition, the model permits an accurate analysis of the timing of trafficking of T cells into the eye and the possibility of determining which cells, if any, may be responsible for antigen presentation in the tissues as opposed to the secondary lymphoid organs. Finally, these experimental methods are now being applied to the human eye and should prove valuable in determining the nature of tissue damage events in the eye as well as evaluating the response to treatments.     

IMAGING OCULAR IMMUNE RESPONSES BY INTRAVITAL MICROSCOPY

The eye offers excellent opportunities to observe cellular interactions in vivo. This applies especially to the immune response in which discrete events can be studied, including cell trafficking, transendothelial migration, adhesion, antigen presentation, and T cell activation. Intravital microscopy has allowed study of immune cell interactions in tissues such as the conjunctiva, the inflamed cornea, and the iris. Thus the real-time observation of presentation of antigen injected into the anterior chamber of the eye can be imaged using fluorescently labelled antigen and cells. Application of the scanning laser ophthalmoscope to the rat and mouse eye allows analysis of leukocyte-endothelial interactions in the retinal and choroidal circulations. These studies have provided important information on rolling and adhesion of leukocytes in real time in different microvascular beds that have not been manipulated in any way and has, for instance, provided quantitation to the effects of shear stress on leukocyte-endothelial adhesion. In addition, the model permits an accurate analysis of the timing of trafficking of T cells into the eye and the possibility of determining which cells, if any, may be responsible for antigen presentation in the tissues as opposed to the secondary lymphoid organs. Finally, these experimental methods are now being applied to the human eye and should prove valuable in determining the nature of tissue damage events in the eye as well as evaluating the response to treatments.     

Natural killer cell trafficking in vivo requires a dedicated sphingosine 1-phosphate receptor

Consistent with their function in immune surveillance, natural killer (NK) cells are distributed throughout lymphoid and nonlymphoid tissues. However, the mechanisms governing the steady-state trafficking of NK cells remain unknown. The lysophospholipid sphingosine 1-phosphate (S1P), by binding to its receptor S1P1, regulates the recirculation of T and B lymphocytes. In contrast, S1P5 is detected in the brain and regulates oligodendrocyte migration and survival in vitro. Here we show that S1P5 was also expressed in NK cells in mice and humans and that S1P5-deficient mice had aberrant NK cell homing during steady-state conditions. In addition, we found that S1P5 was required for the mobilization of NK cells to inflamed organs. Our data emphasize distinct mechanisms regulating the circulation of various lymphocyte subsets and raise the possibility that NK cell trafficking may be manipulated by therapies specifically targeting S1P5.     

Remembrance of antigens past: new insights into memory T cells

Memory immune responses against foreign antigens protect the host from pathogens previously encountered via illness or vaccination, yet can also contribute to the pathology of autoimmune disease when generated against self-antigens. Memory immune responses are classically attributed to the reactivation of long-lived, antigen-specific T lymphocytes that arise directly from differentiated effector T cells and persist in a uniformly quiescent state. Recent findings in both humans and mice, using new biochemical, molecular and cellular approaches, have identified novel features of memory T cells providing new insight into models for memory cell development and differentiation. Biochemical and molecular studies on memory T cells have identified novel markers for memory T cells that may play integral roles in their generation and maintenance. Recent cellular immunological studies have uncovered remarkable heterogeneity amongst antigen-specific memory T cells. Memory cell heterogeneity in the expression of homing and chemokine receptor delineates functional subsets of memory T cells that differ in their proliferative capacity, differentiation potential, homing properties and protective abilities. These findings suggest that memory T cells with diverse properties residing in both lymphoid and nonlymphoid tissues may be necessary to elicit a rapid and effective protective recall immune response involving both cellular and humoral immunity.     

Evidence for recruitment of plasmacytoid dendritic cell precursors to inflamed lymph nodes through high endothelial venules

Recruitment of dendritic cells (DCs) to lymph nodes (LNs) is pivotal to the establishment of immune response. Whereas DCs have been proven to undergo afferent lymphatic pathway to enter LNs from peripheral tissues, a question remains if DCs also migrate into LNs directly from the circulation. Here we demonstrate that plasmacytoid DC (pDC) precursors can transmigrate across high endothelial venules (HEVs) of inflamed LNs in mice. Bacterial infection induces a significant number of pDC and myeloid DC (mDC) precursors into the circulation. Both subsets express a common set of chemokine receptors except CXCR3, display parallel mobilization into the blood, but show distinct trafficking pathway to the LNs. In a short-term homing assay, whereas mDC precursors migrate to peripheral tissues and subsequently to draining LNs, pDC precursors directly enter the LNs in a CXCL9 and E-selectin dependent manner. Tumor necrosis factor-alpha controls not only DC precursor mobilization into the blood but also chemokine up-regulation on LN HEVs. A similar trafficking pathway is observed also in viral infection, and CXCR3(-/-) mice-derived pDC precursors show defective trans-HEV migration. This study clarifies the inflammation-dependent, chemokine-driven distinct property of DC precursor trafficking.     

Chemokine receptors in allergic diseases

Under homeostatic conditions, as well as in various diseases, leukocyte migration is a crucial issue for the immune system that is mainly organized through the activation of bone marrow‐derived cells in various tissues. Immune cell trafficking is orchestrated by a family of small proteins called chemokines. Leukocytes express cell‐surface receptors that bind to chemokines and trigger transendothelial migration. Most allergic diseases, such as asthma, rhinitis, food allergies, and atopic dermatitis, are generally classified by the tissue rather than the type of inflammation, making the chemokine/chemokine receptor system a key point of the immune response. Moreover, because small antagonists can easily block such receptors, various molecules have been developed to suppress the recruitment of immune cells during allergic reactions, representing potential new drugs for allergies. We review the chemokines and chemokine receptors that are important in asthma, food allergies, and atopic dermatitis and their respectively developed antagonists.     

Active participation of endothelial cells in inflammation

Leukocyte migration from the blood into tissues is vital for immune surveillance and inflammation. During this diapedesis of leukocytes, the leukocytes bind to endothelial cell adhesion molecules and then migrate across the vascular endothelium. Endothelial cell adhesion molecules and their counter-receptors on leukocytes generate intracellular signals. This review focuses on the active function of endothelial cells during leukocyte-endothelial cell interactions. We include a discussion of the "outside-in" signals in endothelial cells, which are stimulated by antibody cross-linking or leukocyte binding to platelet-endothelial cell adhesion molecule-1, intercellular adhesion molecule-1, and vascular cell adhesion molecule-1. Some of these signals in endothelial cells have been demonstrated to actively participate in leukocyte migration. We suggest that some of the adhesion molecule signals, which have not been assigned a function, are consistent with signals that stimulate retraction of lateral junctions, stimulate endothelial cell basal surface adhesion, or induce gene expression.