Tissue targeting of T cells by DCs and microenvironments

Location versus ID: what matters to lung-resident memory T cells?

Tissue-resident memory T cells (TRM cells) are vital for the promotion of barrier immunity. The lung, a tissue constantly exposed to foreign pathogenic or non-pathogenic antigens, is not devoid of these cells. Lung TRM cells have been considered major players in either the protection against respiratory viral infections or the pathogenesis of lung allergies. Establishment of lung TRM cells rely on intrinsic and extrinsic factors. Among the extrinsic regulators of lung TRM cells, the magnitude of the impact of factors such as the route of antigen entry or the antigen natural tropism for the lung is not entirely clear. In this perspective, we provide a summary of the literature covering this subject and present some preliminary results on this potential dichotomy between antigen location versus antigen type. Finally, we propose a hypothesis to synthesize the potential contributions of these two variables for lung TRM cell development.

Intradermal vaccination with a Pseudomonas aeruginosa vaccine adjuvanted with a mutant bacterial ADP-ribosylating enterotoxin protects against acute pneumonia

Respiratory infections are a leading cause of morbidity and mortality globally. This is partially due to a lack of effective vaccines and a clear understanding of how vaccination route and formulation influence protective immunity in mucosal tissues such as the lung. Pseudomonas aeruginosa is an opportunistic pathogen capable of causing acute pulmonary infections and is a leading cause of hospital-acquired and ventilator-associated pneumonia. With multidrug-resistant P. aeruginosa infections on the rise, the need for a vaccine against this pathogen is critical. Growing evidence suggests that a successful P. aeruginosa vaccine may require mucosal antibody and Th1- and Th17-type CD4⁺ T cells to prevent pulmonary infection. Intradermal immunization with adjuvants, such as the bacterial ADP-Ribosylating Enterotoxin Adjuvant (BARE) double mutant of E. coli heat-labile toxin (dmLT), can direct protective immune responses to mucosal tissues, including the lungs. We reasoned that intradermal immunization with P. aeruginosa outer membrane proteins (OMPs) adjuvanted with dmLT could drive neutralizing antibodies and migration of CD4⁺ T cells to the lungs and protect against P. aeruginosa pneumonia in a murine model. Here we show that mice immunized with OMPs and dmLT had significantly more antigen-specific IgG and Th1- and Th17-type CD4⁺ memory T cells in the pulmonary environment compared to control groups of mice. Furthermore, OMPs and dmLT immunized mice were significantly protected against an otherwise lethal lung infection. Protection was associated with early IFN-γ and IL-17 production in the lungs of immunized mice. These results indicate that intradermal immunization with dmLT can drive protective immunity to the lung mucosa and may be a viable vaccination strategy for a multitude of respiratory pathogens.

The role of the innate immune system in allergic contact dermatitis

Abstract. Allergic contact dermatitis is a Tcell mediated inflammatory skin disease that is caused by low molecular weight chemicals and metal ions. These contact allergens induce skin inflammation, an essential element of the sensitization process. Our understanding of the molecular mechanisms that underlie chemical-induced inflammation has improved significantly over the last years. The emerging picture shows that contact allergens activate known innate immune and stress responses that play a role in immune responses to infections. Contact allergens use innate immune receptors such as the Toll-like receptors TLR2 and TLR4 and the NOD-like receptor NLRP3 as part of the inflammasome as well as the induction of oxidative stress to induce skin inflammation. The detailed identification of the relevant signaling pathways and the mechanisms of their activation by contact allergens will most likely lead to more targeted therapeutic approaches by interference with these pathways. Moreover, this will help to refine existing, and to develop new in vitro assays for the identification of contact allergens, an important step to replace animal testing e.g. for ingredients of cosmetics which has been prohibited now by EU legislation.

Mechanisms of T cell organotropism

Protective immunity relies upon T cell differentiation and subsequent migration to target tissues. Similarly, immune homeostasis requires the localization of regulatory T cells (Tregs) to the sites where immunity takes place. While naïve T lymphocytes recirculate predominantly in secondary lymphoid tissue, primed T cells and activated Tregs must traffic to the antigen rich non-lymphoid tissue to exert effector and regulatory responses, respectively. Following priming in draining lymph nodes, T cells acquire the 'homing receptors' to facilitate their access to specific tissues and organs. An additional level of topographic specificity is provided by T cells receptor recognition of antigen displayed by the endothelium. Furthermore, co-stimulatory signals (such as those induced by CD28) have been shown not only to regulate T cell activation and differentiation, but also to orchestrate the anatomy of the ensuing T cell response. We here review the molecular mechanisms supporting trafficking of both effector and regulatory T cells to specific antigen-rich tissues.

Efficiency of dendritic cell vaccination against B16 melanoma depends on the immunization route

Dendritic cells (DC) presenting tumor antigens are crucial to induce potent T cell-mediated anti-tumor immune responses. Therefore DC-based cancer vaccines have been established for therapy, however clinical outcomes are often poor and need improvement. Using a mouse model of B16 melanoma, we found that the route of preventive DC vaccination critically determined tumor control. While repeated DC vaccination did not show an impact of the route of DC application on the prevention of tumor growth, a single DC vaccination revealed that both the imprinting of skin homing receptors and an enhanced proliferation state of effector T cells was seen only upon intracutaneous but not intravenous or intraperitoneal immunization. Tumor growth was prevented only by intracutaneous DC vaccination. Our results indicate that under suboptimal conditions the route of DC vaccination crucially determines the efficiency of tumor defense. DC-based strategies for immunotherapy of cancer should take into account the immunization route in order to optimize tissue targeting of tumor antigen specific T cells.

Tissue specific heterogeneity in effector immune cell response

Post pathogen invasion, migration of effector T-cell subsets to specific tissue locations is of prime importance for generation of robust immune response. Effector T cells are imprinted with distinct “homing codes” (adhesion molecules and chemokine receptors) during activation which regulate their targeted trafficking to specific tissues. Internal cues in the lymph node microenvironment along with external stimuli from food (vitamin A) and sunlight (vitamin D₃) prime dendritic cells, imprinting them to play centre stage in the induction of tissue tropism in effector T cells. B cells as well, in a manner similar to effector T cells, exhibit tissue-tropic migration. In this review, we have focused on the factors regulating the generation and migration of effector T cells to various tissues along with giving an overview of tissue tropism in B cells.

Peripheral tissue homing receptors enable T cell entry into lymph nodes and affect the anatomical distribution of memory cells

Peripheral tissue homing receptors enable T cells to access inflamed nonlymphoid tissues. In this study, we show that two such molecules, E-selectin ligand and α4β1 integrin, enable activated and memory T cells to enter lymph nodes (LN) as well. This affects the quantitative and qualitative distribution of these cells among regional LN beds. CD8 memory T cells in LN that express these molecules were mostly CD62L(lo) and would normally be classified as effector memory cells. However, similar to central memory cells, they expanded upon Ag re-encounter. This led to differences in the magnitude of the recall response that depended on the route of immunization. These novel cells share properties of both central and effector memory cells and reside in LN based on previously undescribed mechanisms of entry.

Costimulatory molecules on immunogenic versus tolerogenic human dendritic cells

Dendritic cells (DC) are sentinels of immunity, essential for homeostasis of T cell-dependent immune responses. Both functions of DC, initiation of antigen-specific T cell immunity and maintenance of tissue-specific tolerance originate from distinct stages of differentiation, immunogenic versus tolerogenic. Dependent on local micro milieu and inflammatory stimuli, tissue resident immature DC with functional plasticity differentiate into tolerogenic or immunogenic DC with stable phenotypes. They efficiently link innate and adaptive immunity and are ideally positioned to modify T cell-mediated immune responses. Since the T cell stimulatory properties of DC are significantly influenced by their expression of signal II ligands, it is critical to understand the impact of distinct costimulatory pathways on DC function. This review gives an overview of functional different human DC subsets with unique profiles of costimulatory molecules and outlines how different costimulatory pathways together with the immunosuppressive cytokine IL-10 bias immunogenic versus tolerogenic DC functions. Furthermore, we exemplarily describe protocols for the generation of two well-defined monocyte-derived DC subsets for their clinical use, immunogenic versus tolerogenic.

Immunobiology of beta-cell destruction

Type 1 diabetes is a chronic disease characterized by severe insulin deficiency and hyperglycemia, due to autoimmune destruction of pancreatic islets of Langerhans. A susceptible genetic background is necessary, but not sufficient, for the development of the disease. Epidemiological and clinical observations underscore the importance of environmental factors as triggers of type 1 diabetes, currently under investigation. Islet-specific autoantibodies precede clinical onset by months to years and are established tools for risk prediction, yet minor players in the pathogenesis of the disease. Many efforts have been made to elucidate disease-relevant defects in the key immune effectors of islet destruction, from the early failure of specific tolerance to the vicious circle of destructive insulitis. However, the events triggering islet autoimmunity as well as the transition to overt diabetes are still largely unknown, making prevention and treatment strategies still a challenge.

Chemokines shape the immune responses to tuberculosis

Mycobacterium tuberculosis (Mtb) is the intracellular pathogen that causes the disease, tuberculosis. Chemokines and chemokine receptors are key regulators in immune cell recruitment to sites of infection and inflammation. This review highlights our recent advances in understanding the role of chemokines and chemokine receptors in cellular recruitment of immune cells to the lung, role in granuloma formation and host defense against Mtb infection.

Regulatory T cell migration during an immune response

CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells migrate into both inflammatory sites and draining lymph nodes (LNs) during an immune response, and have unique and overlaping functions in each location. Current studies suggest that Treg cells in draining LNs and inflamatory sites may not simply be a division of labor, but rather Treg cells migrate in a coordinated fashion between peripheral tissues and draining LNs. Trafficking between inflammatory sites and draining LNs is not only crucial for Treg cells to act, but also for them to acquire optimal immune regulatory activities. Furthermore, recent work has revealed that T helper (Th)1, Th2 and Th17 cell master transcription factors control Treg cell function by regulating genes important for Treg cell migration and suppression, and consequently affect disease pathogenesis.

Neonatal T-cell maturation and homing receptor responses to Toll-like receptor ligands differ from those of adult naive T cells: relationship to prematurity

INTRODUCTION

Inflammation and infection are associated with premature birth and with activation of the fetal immune system. We hypothesized that exposure to microbial Toll-like receptor (TLR) ligands plays an important role in neonatal T-cell maturation and that early exposure to microbial products may result in early T-cell maturation and a tendency for these matured effector cells to change their homing receptor patterns.

RESULTS

Expression of the CD45RO marker was induced in term neonatal T cells after in vitro exposure to TLR ligands for 7 days. Interestingly, naive T cells from adult blood were unaffected by TLR ligand exposure. In addition, neonatal T cells had more cells with decreased expression of the α4β7 integrins and increased expression of CCR4 after in vitro exposure of TLR ligands-similar to the expression of these molecules in adult naive T cells.

DISCUSSION

These findings are relevant for the understanding of neonatal T-cell maturation and may contribute to our understanding of multiorgan inflammatory complications of prematurity.

METHODS

Cord blood was obtained from term and preterm infants. Using flow cytometry, we identified a mature (CD45RO(+)) phenotype in preterm infant cord blood (CB) T cells that had decreased expression of the α4β7 integrins and increased expression of the C-C chemokine receptor 4 (CCR4) as compared with term infant CB.

Adjuvants and delivery systems in veterinary vaccinology: current state and future developments

Modern adjuvants should induce strong and balanced immune responses, and it is often desirable to induce specific types of immunity. As an example, efficient Th1-immunity-inducing adjuvants are highly in demand. Such adjuvants promote good cell-mediated immunity against subunit vaccines that have low immunogenicity themselves. The development of such adjuvants may take advantage of the increased knowledge of the molecular mechanisms and factors controlling these responses. However, knowledge of such molecular details of immune mechanisms is relatively scarce for species other than humans and laboratory rodents, and in addition, there are special considerations pertaining to the use of adjuvants in veterinary animals, such as production and companion animals. With a focus on veterinary animals, this review highlights a number of approaches being pursued, including cytokines, CpG oligonucleotides, microparticles and liposomes.

Lack of the purinergic receptor P2X(7) results in resistance to contact hypersensitivity

Engagement of P2X₇ on mouse dendritic cells, presumably by ATP released in response to contact allergen, is needed for IL-1β production and the sensitization phase of contact hypersensitivity.

Sensitization to contact allergens requires activation of the innate immune system by endogenous danger signals. However, the mechanisms through which contact allergens activate innate signaling pathways are incompletely understood. In this study, we demonstrate that mice lacking the adenosine triphosphate (ATP) receptor P2X₇ are resistant to contact hypersensitivity (CHS). P2X₇-deficient dendritic cells fail to induce sensitization to contact allergens and do not release IL-1β in response to lipopolysaccharide (LPS) and ATP. These defects are restored by pretreatment with LPS and alum in an NLRP3- and ASC-dependent manner. Whereas pretreatment of wild-type mice with P2X₇ antagonists, the ATP-degrading enzyme apyrase or IL-1 receptor antagonist, prevents CHS, IL-1β injection restores CHS in P2X₇-deficient mice. Thus, P2X₇ is a crucial receptor for extracellular ATP released in skin in response to contact allergens. The lack of P2X₇ triggering prevents IL-1β release, which is an essential step in the sensitization process. Interference with P2X₇ signaling may be a promising strategy for the prevention of allergic contact dermatitis.

Generation, persistence and plasticity of CD4 T-cell memories

The development of immune memory mediated by T lymphocytes is central to durable, long-lasting protective immunity. A key issue in the field is how to direct the generation and persistence of memory T cells to elicit the appropriate secondary response to provide protection to a specific pathogen. Two prevailing views have emerged; that cellular and molecular regulators control the lineage fate and functional capacities of memory T cells early after priming, or alternatively, that populations of memory T cells are inherently plastic and subject to alterations in function and/or survival at many stages during their long-term maintenance. Here, we will review current findings in CD4 T-cell memory that suggest inherent plasticity in populations of memory CD4 T cells at all stages of their development--originating with their generation from multiple types of primed CD4 T cells, during their persistence and homeostatic turnover in response to T-cell receptor signals, and also following secondary challenge. These multiple aspects of memory CD4 T-cell flexibility contrast the more defined lineages and functions ascribed to memory CD8 T cells, suggesting a dynamic nature to memory CD4 T-cell populations and responses. The flexible attributes of CD4 T-cell memory suggest opportunities and mechanisms for therapeutic manipulation at all phases of immune memory development, maintenance and recall.

Immunology of beta-cell destruction

The pancreatic islet beta-cells are the target for an autoimmune process that eventually results in an inability to control blood glucose due to the lack of insulin. The different steps that eventually lead to the complete loss of the beta-cells are reviewed to include the very first step of a triggering event that initiates the development of beta-cell autoimmunity to the last step of appearance of islet-cell autoantibodies, which may mark that insulitis is about to form. The observations that the initial beta-cell destruction by virus or other environmental factors triggers islet autoimmunity not in the islets but in the draining pancreatic lymph nodes are reviewed along with possible basic mechanisms of loss of tolerance to islet autoantigens. Once islet autoimmunity is established the question is how beta-cells are progressively killed by autoreactive lymphocytes which eventually results in chronic insulitis. Many of these series of events have been dissected in spontaneously diabetic mice or rats, but controlled clinical trials have shown that rodent observations are not always translated into mechanisms in humans. Attempts are therefore needed to clarify the step 1 triggering mechanisms and the step to chronic autoimmune insulitis to develop evidence-based treatment approaches to prevent type 1 diabetes.

Lymph node stromal cells support dendritic cell-induced gut-homing of T cells

T cells are imprinted to express tissue-specific homing receptors upon activation in tissue-draining lymph nodes, resulting in their migration to the site of Ag entry. Expression of gut-homing molecules alpha(4)beta(7) and CCR9 is induced by retinoic acid, a vitamin A metabolite produced by retinal dehydrogenases, which are specifically expressed in dendritic cells as well as stromal cells in mucosa-draining lymph nodes. In this study, we demonstrate that mesenteric lymph node stromal cell-derived retinoic acid can directly induce the expression of gut-homing molecules on proliferating T cells, a process strongly enhanced by bone marrow-derived dendritic cells in vitro. Therefore, cooperation of sessile lymph node stromal cells with mobile dendritic cells warrants the imprinting of tissue specific homing receptors on activated T cells.

Influence of the tissue microenvironment on Toll-like receptor expression by CD11c+ antigen-presenting cells isolated from mucosal tissues

It is recognized that functional activities of antigen-presenting cells (APCs) in mucosal tissue sites differ from those of systemic APCs; however, it is unknown whether there are further differences between APC populations residing in different mucosal sites. In this study, we directly compared murine CD11c+ APCs isolated from colon, lung, and spleen and found that APCs isolated from these tissues differ considerably in Toll-like receptor (TLR) expression and responses to in vitro TLR ligand stimulation. We also provide evidence that tissue microenvironments dictate distinct patterns of TLR expression by CD11c+ APCs in different mucosal tissues. Moreover, CD11c+ cells isolated from different tissues have varied capacities to induce the development of T helper 1 (Th1), Th2, or regulatory CD4+ T cells. Thus, unique tissue microenvironments have a significant influence on determining TLR expression by CD11c+ cells that migrate to and reside in each mucosal tissue and are likely to modulate their functional activities.

Modulation of cutaneous inflammation induced by ticks in contrasting phenotypes of infestation in bovines

Tick saliva contains molecules that are inoculated at the site of attachment on their hosts in order to modulate local immune responses and facilitate a successful blood meal. Bovines express heritable, contrasting phenotypes of infestations with the cattle tick, Rhipicephalus (Boophilus) microplus: breeds of Bos taurus indicus are significantly more resistant than those of Bos taurus taurus. Tick saliva may contain molecules that interfere with adhesion of leukocytes to endothelium and resistant hosts may mount an inflammatory profile that is more efficient to hamper the tick's blood meal. We show in vitro that adhesion of peripheral blood mononuclear cells to monolayers of cytokine-activated bovine umbilical endothelial cells was significantly inhibited by tick saliva. The inflammatory response to bites of adults of R. microplus mounted by genetically resistant and susceptible bovine hosts managed in the same pasture was investigated in vivo. The inflammatory infiltrates and levels of message coding for adhesion molecules were measured in biopsies of tick-bitten and control skin taken when animals of both breeds were exposed to low and high tick infestations. Histological studies reveal that cutaneous reactions of resistant hosts to bites of adult ticks contained significantly more basophils and eosinophils compared with reactions of the susceptible breed. Expression of the adhesion molecules - intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1) and P-selectin - was higher in adult-infested skin of susceptible hosts undergoing low infestations compared to resistant hosts; when host was exposed to high infestations expression of these adhesion molecules was down-regulated in both phenotypes of infestations. Expression of leukocyte adhesion glycoprotein-1 (LFA-1) was higher in skin from susceptible hosts undergoing low or high infestations compared to resistant hosts. Conversely, higher levels of E-selectin, which promotes adhesion of memory T cells, were expressed in skin of resistant animals. This finding may explain the resistant host's ability to mount more rapid and efficient secondary responses that limit hematophagy and infestations. The expression profiles observed for adhesion molecules indicate that there are differences in the kinetics of the inflammatory reactions mounted by resistant and susceptible hosts and the balance between tick and host is affected by the number of tick bites a host receives. We show that the contrasting phenotypes of infestations seen in bovines infested with R. microplus are correlated with differences in the cellular and molecular composition of inflammatory infiltrates elicited by bites with adult ticks.

Dynamic development of homing receptor expression and memory cell differentiation of infant CD4+CD25high regulatory T cells

Migration of CD4+CD25+FOXP3+ regulatory T cells (Treg) is important for suppressing immune responses in different tissues. Previous studies show that the majority of Treg at birth express gut homing receptor alpha(4)beta(7) and that only few express CCR4, while the reverse pattern is found in adults. The age at which homing receptor switch occurs in vivo is not known. In this study, we show, in a prospective study of human infants from birth to 3 years of age, that homing receptor switch from alpha(4)beta(7) to CCR4 commences between 1 1/2 and 3 years of age and that Treg at that age also had started their switch to a memory phenotype. The majority of naive Treg express alpha(4)beta(7) in infants but not in adults, while the majority of memory Treg express CCR4 both infants and adults. The homing receptor expression on Treg corresponds to their actual migration properties, because Treg from cord blood migrate foremost toward the gut-associated chemokine CCL25. CD4+FOXP3+ T cell numbers increase rapidly in the circulation during the first days of life indicating conversion to suppressive Treg from CD25(high) Treg precursors. These findings suggest that the gut is the primary site of Treg stimulation to exogenous Ags during the first 18 mo of life and that homing receptor switch toward a more extra-intestinal phenotype occurs thereafter.

Cutting edge: instructive role of peripheral tissue cells in the imprinting of T cell homing receptor patterns

Tissue-specific homing of effector and memory T cells to skin and small intestine requires the imprinting of specific combinations of adhesion molecules and chemokine receptors by dendritic cells in the draining lymph nodes. In this study, we demonstrate that CD8(+) T cells activated by Ag-pulsed bone marrow-derived dendritic cells were induced to express the small intestine homing receptors alpha(4)beta(7) integrin and chemokine receptor CCR9 in coculture with small intestinal epithelial cells. In contrast, in coculture with dermal fibroblasts the skin-homing receptor E-selectin ligand was induced. Interestingly, the imprinting of gut homing receptors on anti-CD3/anti-CD28 stimulated T cells was induced by soluble factors produced by small intestinal epithelial cells. Retinoic acid was identified as a crucial factor. These findings show that peripheral tissue cells directly produce homing receptor imprinting factors and suggest that dendritic cells can acquire their imprinting potential already in the peripheral tissue of origin.

Organ-derived dendritic cells have differential effects on alloreactive T cells

Dendritic cells (DCs) are considered critical for the induction of graft-versus-host disease (GVHD) after bone marrow transplantation (BMT). In addition to their priming function, dendritic cells have been shown to induce organ-tropism through induction of specific homing molecules on T cells. Using adoptive transfer of CFSE-labeled cells, we first demonstrated that alloreactive T cells differentially up-regulate specific homing molecules in vivo. Host-type dendritic cells from the GVHD target organs liver and spleen or skin- and gut-draining lymph nodes effectively primed naive allogeneic T cells in vitro with the exception of liver-derived dendritic cells, which showed less stimulatory capacity. Gut-derived dendritic cells induced alloreactive donor T cells with a gut-homing phenotype that caused increased GVHD mortality and morbidity compared with T cells stimulated with dendritic cells from spleen, liver, and peripheral lymph nodes in an MHC-mismatched murine BMT model. However, in vivo analysis demonstrated that the in vitro imprinting of homing molecules on alloreactive T cells was only transient. In conclusion, organ-derived dendritic cells can efficiently induce specific homing molecules on alloreactive T cells. A gut-homing phenotype correlates with increased GVHD mortality and morbidity after murine BMT, underlining the importance of the gut in the pathophysiology of GVHD.

Dendritic cell subsets in lymph nodes are characterized by the specific draining area and influence the phenotype and fate of primed T cells

Dendritic cells (DC) are important in differential T-cell priming. Little is known about the local priming by DC in the microenvironment of different lymph nodes and about the fate of the imprinted T cells. Therefore, freshly isolated rat DC from mesenteric lymph nodes (mLN) and axillary lymph nodes (axLN) were phenotyped and cultured with blood T cells in the presence of the superantigen Mycoplasma arthritidis mitogen (MAM). The phenotype, proliferation and apoptosis of the primed T cells were analysed. Our data show that a common DC population exists in both mLN and axLN. In addition, region-specific DC with an organotypical marker expression imprinted by the drained area were found. Coculture of T cells with DC from mLN or axLN resulted in a distinct shift in the CD4 and CD8 expression of T cells and their phenotype. Furthermore, when these differentially primed mLN and axLN T cells were injected into recipients, mLN-primed T cells survived longer in other lymphoid organs. The results show that the region-specific DC have a unique phenotype and an impact on the ratio of CD4 : CD8 T cells during an immune response in vivo.

Distinctive role of CCR7 in migration and functional activity of naive- and effector/memory-like Treg subsets

Foxp3+CD25+CD4+ Treg play a fundamental role in the maintenance of self tolerance and the control of inflammatory reactions. Previous data demonstrated a division of labor between naive- and effector/memory-like Treg subsets, which is largely based on their lymph node-recirculating and inflammation-seeking migration behavior, respectively. The chemokine receptor CCR7 is expressed on both types of Treg subsets, albeit at different levels. Whether it fulfills similar or distinct roles in these subsets has not been studied so far. We here show that the recirculation of naive-like Treg through LN and, to some extent, the gut is dependent on CCR7. Lack of CCR7 not only prevents recirculation, but also almost completely abolishes the ability of naive-like Treg to control the priming phase of an immune response. In contrast, CCR7 deficiency in effector/memory-like Treg promotes their accumulation in inflamed sites, compatible with a role of CCR7 for exit from the tissue. Local Treg accumulation was accompanied by an enhanced suppression of inflammation. Together, our findings provide conclusive evidence that CCR7 expression on Treg differentially controls in vivo function of the naive- and effector/memory-like subsets.

Induction of organ-selective CD4+ regulatory T cell homing

Compelling evidence suggests that Foxp3(+)CD25(+)CD4(+) Treg play a fundamental role in immunoregulation. We have previously demonstrated that Treg have to enter peripheral tissues to suppress ongoing inflammation. However, relatively little is known about how Treg acquire the expression of homing receptors required for tissue- or inflammation-specific migration. Migratory properties of conventional naïve T cells are shaped by the tissue microenvironment and organ-specific dendritic cells during priming. Here, we show that this basic concept also holds true for CD25(+)CD4(+) Treg: Priming of Treg within peripheral LN led to the expression of selectin ligands, which facilitate migration into inflamed skin, whereas activation within mesenteric LN led to induction of the integrin alpha(4)beta(7), which is required for migration into mucosal tissues. Furthermore, we could establish in vitro culture systems containing either dendritic cells from mesenteric and peripheral LN, or retinoic acid and IL-12 as polarizing compounds to induce mucosa- and skin-seeking Treg, respectively. Together, our results demonstrate that Treg, similarly to conventional T cells, can be configured with organ-selective homing properties allowing efficient targeting into distinct tissues.

Dendritic cell immunization route determines integrin expression and lymphoid and nonlymphoid tissue distribution of CD8 T cells

Exogenous dendritic cells (bone marrow-derived dendritic cell (BMDC)) display restricted trafficking in vivo after injection into mice, but the route(s) by which they generate gut-homing effector cells is unclear. Mesenteric lymph nodes (LN) and spleen were differentially targeted by i.p. and i.v. administration of BMDC, respectively, whereas mediastinal LN were targeted by both routes. BMDC injected by either route activated CD8(+) T cells to up-regulate both alpha(4)beta(1) and alpha(4)beta(7) integrins. However, the lymphoid compartment in which activation occurred determined their expression kinetics, magnitude, and population distribution. Only T cells activated in mesenteric LN after i.p. immunization expressed high levels of alpha(4)beta(7), which also correlated with localization to small intestine. These alpha(4)beta(7)(high) cells also redistributed to mediastinal LN in a manner sensitive to treatment with alpha(4)beta(7) blocking Abs, but not to mucosal addressin cell adhesion molecule-1 blocking Abs. Our results demonstrate the importance of lymphoid compartment, as dictated by immunization route, in determining integrin expression on activated T cells and their distribution in lymphoid and nonlymphoid tissues.

Immunological principles of T-cell-mediated adverse drug reactions in skin

Drug hypersensitivity reactions in skin are an immune-mediated phenomenon associated with significant patient mortality and morbidity. Antigen-specific T cells, which have been isolated from the peripheral circulation and target organs of hypersensitive patients, are thought to propagate and regulate the development of clinical symptoms. The investigation of clinical cases with respect to the basic cellular and chemical mechanisms that underpin drug hypersensitivity has resulted in: i) the need to redress some aspects of present immunological dogma; and ii) additional fundamental immunological questions. Thus, the aim of this review article is to summarise present opinion on how and why drugs initiate a pathogenic T-cell response in a small section of the population and subsequently reflect on gaps in basic immunology and where future research might lead.

The essential role of chemokines in the selective regulation of lymphocyte homing

Knowledge of lymphocyte migration has become a major issue in our understanding of acquired immunity. The selective migration of naïve, effector, memory and regulatory T-cells is a multiple step process regulated by a specific arrangement of cytokines, chemokines and adhesion receptors that guide these cells to specific locations. Recent research has outlined two major pathways of lymphocyte trafficking under homeostatic and inflammatory conditions, one concerning tropism to cutaneous tissue and a second one related to mucosal-associated sites. In this article we will outline our present understanding of the role of cytokines and chemokines as regulators of lymphocyte migration through tissues.

Generation of gut-homing T cells and their localization to the small intestinal mucosa

The intestinal mucosa represents the largest body surface toward the external environment and harbors numerous T lymphocytes that take up resident within the intestinal epithelium or in the underlying lamina propria (LP). The intraepithelial lymphocytes include subsets of 'unconventional' T cells with unclear ontogeny and reactivity that localize to this site independently of antigen-specific activation in secondary lymphoid organs. In contrast, the majority of the 'conventional' gut T cells are recruited into the intestinal mucosa subsequent to their activation in intestinal inductive sites, including Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). T cells homing to the small intestine express a distinct pattern of homing molecules, allowing them to interact with and transmigrate across intestinal postcapillary endothelium. At least some of these homing molecules, including the integrin alpha(4)beta(7) and the chemokine receptor CCR9, are induced on T cells during their activation in PPs or MLNs. Mucosal dendritic cells (DCs) play a key role in this process, but not all intestinal DCs possess the ability to confer a gut-homing capacity to T cells. Instead, functionally specialized CD103(+) DCs derived from the small intestinal LP appear to selectively regulate T-cell homing to the small intestine.

The importance of regional lymph nodes for mucosal tolerance

The immune system is organized as a number of distinct lymphoid organs interconnected by recirculating lymphocytes. These organs, such as lymph nodes, spleen, and gut-associated Peyer's patches, are compartmentalized, providing separate niches for T and B cells. In addition, regional compartmentalization of lymphoid organs themselves exists, leading to the distinction between the mucosal and the systemic immune systems. This distinction not only reflects the anatomical localization but also is based on functional differences, with predominant tolerance induction via mucosal routes and immunity seen after systemic antigen exposure. These differences are associated with regional differences in the lymphoid organs and with environmental conditions of the tissues in which the immune system functions. Recirculation patterns of lymphocytes differ between mucosal and systemic lymphoid organs, and more insight into the mechanisms that imprint this behavior has been generated recently. Differences in dendritic cells have been observed between mucosal and systemic sites, and knowledge on how local factors contribute to the immune system is emerging. From our studies on mucosal tolerance in mouse models, it has become evident that regional lymph nodes draining the mucosa are important sites to direct immune responses. Here, we discuss the way regional lymph nodes contribute to the direction of immune responses and what is known about the local factors and cell behavior that form the basis for these differences.

From ubiquitous antigens to joint-specific inflammation: could local vascular permeability be the missing link?

The mechanisms underlying organ-specific autoimmune diseases are incompletely understood. In particular, it is still unclear how autoantibodies, often against ubiquitous antigens, can cause pathology preferentially in certain tissues. Recently, Binstadt and colleagues, using observations in the K/BxN murine arthritis model, suggested novel pathways underlying the site-specific localization of inflammation driven by immune complexes. In this commentary, we discuss these findings, the problems related to their general applicability and the implications for future research.

The good turned ugly: immunopathogenic basis for diabetogenic CD8+ T cells in NOD mice

Type 1 diabetes (T1D) in both humans and nonobese diabetic (NOD) mice is a T-cell-mediated autoimmune disease in which the insulin-producing pancreatic islet beta-cells are selectively eliminated. As a result, glucose metabolism cannot be regulated unless exogenous insulin is administered. Both the CD4(+) and the CD8(+) T-cell subsets are required for T1D development. Approximately 20 years ago, an association between certain class II major histocompatibility complex (MHC) alleles and susceptibility to T1D was reported. This finding led to enormous interest in the CD4(+) T cells participating in the development of T1D, while the CD8(+) subset was relatively ignored. However, the isolation of beta-cell-autoreactive CD8(+) T-cell clones from the islets of NOD mice helped to generate interest in the pathogenic role of this subset, as has accumulating evidence that certain class I MHC alleles are additional risk factors for T1D development in humans. Three distinct diabetogenic CD8(+) T-cell populations have now been characterized in NOD mice. Here, we review recent investigations exploring their selection, activation, trafficking, and antigenic specificities. As CD8(+) T cells are suspected contributors to beta-cell demise in humans, continued exploration of these critical areas could very possibly lead to tangible benefits for T1D patients and at-risk individuals.

Inducing P-selectin ligand formation in CD8 T cells: IL-2 and IL-12 are active in vitro but not required in vivo

In vitro studies have demonstrated that IL-2 and IL-12 can support formation of P-selectin ligands (P-SelL) in activated T cells, ligands that are variably required for efficient lymphocyte recruitment to sites of inflammation. To ascertain whether these cytokines were required for P-SelL formation in vivo, TCR transgenic CD8 T cells specific for male Ag (HY) were transferred into male mice under conditions in which either IL-2 and/or IL-15 or IL-12Rp40 were absent. P-SelL formation at day 2 was unperturbed in HY-TCR IL-2(null) CD8 T cells responding in doubly deficient IL-2(null)IL-12(null) or IL-2(null)IL-15(null) male recipients. HY-specific CD8 T cell proliferative responses detected in both spleen and peritoneum occurred vigorously, but only splenic CD8 T cells up-regulated P-SelL, demonstrating that in vivo induction of P-SelL is an active, nonprogrammed event following T cell activation and that despite the efficacy of IL-2 and IL-12 in supporting P-SelL formation in vitro, these cytokines appear to be dispensable for this purpose in vivo.

Dendritic cells govern induction and reprogramming of polarized tissue-selective homing receptor patterns of T cells: important roles for soluble factors and tissue microenvironments

Tissue-selective homing is established during naive T cell activation by the tissue microenvironment and tissue-specific dendritic cells (DC). The factors driving induction and maintenance of T cell homing patterns are still largely unknown. Here we show that soluble factors produced during the interaction of T cells with CD11c(+) DC isolated from skin- or small intestine-associated tissues differentially modulate expression of the corresponding tissue-selective homing receptors (E-selectin ligands and alpha4beta7 integrin/CCR9, respectively) on murine CD8(+) T cells. Injection of tissue-specific DC via different routes induces T cells with homing receptors characteristic of the corresponding local tissue microenvironment, independent of the origin of the DC. These data indicate an important role for signals delivered in trans. Moreover, DC can reprogram the homing receptor expression on T cells previously polarized in vitro for homing to skin or small intestine. Importantly, skin-homing memory T cells stimulated directly ex vivo can also be reprogrammed by intestinal DC to a gut-homing phenotype. Our results show that tissue-selective homing receptor expression on effector and memory T cells is governed by inductive as well as suppressive signals from both DC and tissue microenvironments.

A stromal address code defined by fibroblasts

To navigate into and within tissues, leukocytes require guidance cues that enable them to recognize which tissues to enter and which to avoid. Such cues are partly provided at the time of extravasation from blood by an endothelial address code on the luminal surface of the vascular endothelium. Here, we review the evidence that fibroblasts help define an additional stromal address code that directs leukocyte behaviour within tissues. We examine how this stromal code regulates site-specific leukocyte accumulation, differentiation and survival in a variety of physiological stromal niches, and how the aberrant expression of components of this code in the wrong tissue at the wrong time contributes to the persistence of chronic inflammatory diseases.

Liposomes as Protein Carriers in Immunology

Liposomes are excellent carriers for protein antigens since they can contain large amounts of antigen, potentially in association with adjuvants. Liposomes may be made to mimic the pathogens that stimulated the evolution of the immune system. As such, numerous mechanisms exist to promote their uptake by antigen presenting cells and exposure of encapsulated antigens to the lymphocytes of the immune system for the induction of responses. The review is intended to describe the 30 year history of the use of liposomes are carriers of protein antigens, notably from the perspective of what we have learned about the immune system using liposomes.