Chemoattractant receptors and lymphocyte egress from extralymphoid tissue: changing requirements during the course of inflammation

Lymphatic vessel transit seeds cytotoxic resident memory T cells in skin draining lymph nodes

Lymphatic transport shapes the homeostatic immune repertoire of lymph nodes (LNs). LN-resident memory T cells (TRMs) play an important role in site-specific immune memory, yet how LN TRMs form de novo after viral infection remains unclear. Here, we tracked the anatomical distribution of antiviral CD8+ T cells as they seeded skin and LN TRMs using a model of vaccinia virus-induced skin infection. LN TRMs localized to the draining LNs (dLNs) of infected skin, and their formation depended on the lymphatic egress of effector CD8+ T cells from the skin, already poised for residence. Effector CD8+ T cell transit through skin was required to populate LN TRMs in dLNs, a process reinforced by antigen encounter in skin. Furthermore, LN TRMs were protective against viral rechallenge in the absence of circulating memory T cells. These data suggest that a subset of tissue-infiltrating CD8+ T cells egress from tissues during viral clearance and establish a layer of regional protection in the dLN basin.

Lymphatic vessels in the age of cancer immunotherapy

Lymphatic transport maintains homeostatic health and is necessary for immune surveillance, and yet lymphatic growth is often associated with solid tumour development and dissemination. Although tumour-associated lymphatic remodelling and growth were initially presumed to simply expand a passive route for regional metastasis, emerging research puts lymphatic vessels and their active transport at the interface of metastasis, tumour-associated inflammation and systemic immune surveillance. Here, we discuss active mechanisms through which lymphatic vessels shape their transport function to influence peripheral tissue immunity and the current understanding of how tumour-associated lymphatic vessels may both augment and disrupt antitumour immune surveillance. We end by looking forward to emerging areas of interest in the field of cancer immunotherapy in which lymphatic vessels and their transport function are likely key players: the formation of tertiary lymphoid structures, immune surveillance in the central nervous system, the microbiome, obesity and ageing. The lessons learnt support a working framework that defines the lymphatic system as a key determinant of both local and systemic inflammatory networks and thereby a crucial player in the response to cancer immunotherapy.

Thrombospondins in the tumor microenvironment

Many cancers begin with the formation of a small nest of transformed cells that can remain dormant for years. Thrombospondin-1 (TSP-1) initially promotes dormancy by suppressing angiogenesis, a key early step in tumor progression. Over time, increases in drivers of angiogenesis predominate, and vascular cells, immune cells, and fibroblasts are recruited to the tumor mass forming a complex tissue, designated the tumor microenvironment. Numerous factors, including growth factors, chemokine/cytokine, and extracellular matrix, participate in the desmoplastic response that in many ways mimics wound healing. Vascular and lymphatic endothelial cells, and cancer-associated pericytes, fibroblasts, macrophages and immune cells are recruited to the tumor microenvironment, where multiple members of the TSP gene family promote their proliferation, migration and invasion. The TSPs also affect the immune signature of tumor tissue and the phenotype of tumor-associated macrophages. Consistent with these observations, expression of some TSPs has been established to correlate with poor outcomes in specific types of cancer.

Lymphatic vessel transit seeds precursors to cytotoxic resident memory T cells in skin draining lymph nodes

Resident memory T cells (TRM) provide rapid, localized protection in peripheral tissues to pathogens and cancer. While TRM are also found in lymph nodes (LN), how they develop during primary infection and their functional significance remains largely unknown. Here, we track the anatomical distribution of anti-viral CD8+ T cells as they simultaneously seed skin and LN TRM using a model of skin infection with restricted antigen distribution. We find exquisite localization of LN TRM to the draining LN of infected skin. LN TRM formation depends on lymphatic transport and specifically egress of effector CD8+ T cells that appear poised for residence as early as 12 days post infection. Effector CD8+ T cell transit through skin is necessary and sufficient to populate LN TRM in draining LNs, a process reinforced by antigen encounter in skin. Importantly, we demonstrate that LN TRM are sufficient to provide protection against pathogenic rechallenge. These data support a model whereby a subset of tissue infiltrating CD8+ T cells egress during viral clearance, and establish regional protection in the draining lymphatic basin as a mechanism to prevent pathogen spread.

Get me out of here: Sphingosine 1-phosphate signaling and T cell exit from tissues during an immune response

During an immune response, the duration of T cell residence in lymphoid and non-lymphoid tissues likely affects T cell activation, differentiation, and memory development. The factors that govern T cell transit through inflamed tissues remain incompletely understood, but one important determinant of T cell exit from tissues is sphingosine 1-phosphate (S1P) signaling. In homeostasis, S1P levels are high in blood and lymph compared to lymphoid organs, and lymphocytes follow S1P gradients out of tissues into circulation using varying combinations of five G-protein coupled S1P receptors. During an immune response, both the shape of S1P gradients and the expression of S1P receptors are dynamically regulated. Here we review what is known, and key questions that remain unanswered, about how S1P signaling is regulated in inflammation and in turn how S1P shapes immune responses.

LFA-1/ICAM-1 Adhesion Pathway Mediates the Homeostatic Migration of Lymphocytes from Peripheral Tissues into Lymph Nodes through Lymphatic Vessels

Lymphocyte function-associated antigen-1 (LFA-1) and its endothelial ligand intercellular adhesion molecule-1 (ICAM-1) are important for the migration of lymphocytes from blood vessels into lymph nodes. However, it is largely unknown whether these molecules mediate the homeostatic migration of lymphocytes from peripheral tissues into lymph nodes through lymphatic vessels. In this study, we find that, in naive mice, ICAM-1 is expressed on the sinus endothelia of lymph nodes, but not on the lymphatic vessels of peripheral tissues. In in vivo lymphocyte migration assays, memory CD4+ T cells migrated to lymph nodes from peripheral tissues much more efficiently than from blood vessels, as compared to naive CD4+ T cells. Moreover, ICAM-1 deficiency in host mice significantly inhibited the migration of adoptively transferred wild-type donor lymphocytes from peripheral tissues, but not from blood vessels, into lymph nodes. The migration of LFA-1-deficient donor lymphocytes from peripheral tissues into the lymph nodes of wild-type host mice was also significantly reduced as compared to wild-type donor lymphocytes. Furthermore, the number of memory T cells in lymph nodes was significantly reduced in the absence of ICAM-1 or LFA-1. Thus, our study extends the functions of the LFA-1/ICAM-1 adhesion pathway, indicating its novel role in controlling the homeostatic migration of lymphocytes from peripheral tissues into lymph nodes and maintaining memory T cellularity in lymph nodes.

A Novel C-C Chemoattractant Cytokine (Chemokine) Receptor 6 (CCR6) Antagonist (PF-07054894) Distinguishes between Homologous Chemokine Receptors, Increases Basal Circulating CCR6+ T Cells, and Ameliorates Interleukin-23-Induced Skin Inflammation

Blocking chemokine receptor C-C chemoattractant cytokine (chemokine) receptor (CCR) 6-dependent T cell migration has therapeutic promise in inflammatory diseases. PF-07054894 is a novel CCR6 antagonist that blocked only CCR6, CCR7, and C-X-C chemoattractant cytokine (chemokine) receptor (CXCR) 2 in a β-arrestin assay panel of 168 G protein-coupled receptors. Inhibition of CCR6-mediated human T cell chemotaxis by (R)-4-((2-(((1,4-Dimethyl-1H-pyrazol-3-yl)(1-methylcyclopentyl)methyl)amino)-3,4-dioxocyclobut-1-en-1-yl)amino)-3-hydroxy-N,N-dimethylpicolinamide (PF-07054894) was insurmountable by CCR6 ligand, C-C motif ligand (CCL) 20. In contrast, blockade of CCR7-dependent chemotaxis in human T cells and CXCR2-dependent chemotaxis in human neutrophils by PF-07054894 were surmountable by CCL19 and C-X-C motif ligand 1, respectively. [3H]-PF-07054894 showed a slower dissociation rate for CCR6 than for CCR7 and CXCR2 suggesting that differences in chemotaxis patterns of inhibition could be attributable to offset kinetics. Consistent with this notion, an analog of PF-07054894 with fast dissociation rate showed surmountable inhibition of CCL20/CCR6 chemotaxis. Furthermore, pre-equilibration of T cells with PF-07054894 increased its inhibitory potency in CCL20/CCR6 chemotaxis by 10-fold. The functional selectivity of PF-07054894 for inhibition of CCR6 relative to CCR7 and CXCR2 is estimated to be at least 50- and 150-fold, respectively. When administered orally to naïve cynomolgus monkeys, PF-07054894 increased the frequency of CCR6+ peripheral blood T cells, suggesting that blockade of CCR6 inhibited homeostatic migration of T cells from blood to tissues. PF-07054894 inhibited interleukin-23-induced mouse skin ear swelling to a similar extent as genetic ablation of CCR6. PF-07054894 caused an increase in cell surface CCR6 in mouse and monkey B cells, which was recapitulated in mouse splenocytes in vitro. In conclusion, PF-07054894 is a potent and functionally selective CCR6 antagonist that blocks CCR6-mediated chemotaxis in vitro and in vivo. SIGNIFICANCE STATEMENT: The chemokine receptor, C-C chemoattractant cytokine (chemokine) receptor 6 (CCR6) plays a key role in the migration of pathogenic lymphocytes and dendritic cells into sites of inflammation. (R)-4-((2-(((1,4-Dimethyl-1H-pyrazol-3-yl)(1-methylcyclopentyl)methyl)amino)-3,4-dioxocyclobut-1-en-1-yl)amino)-3-hydroxy-N,N-dimethylpicolinamide (PF-07054894) is a novel CCR6 small molecule antagonist that illustrates the importance of binding kinetics in achieving pharmacological potency and selectivity. Orally administered PF-07054894 blocks homeostatic and pathogenic functions of CCR6, suggesting that it is a promising therapeutic agent for the treatment of a variety of autoimmune and inflammatory diseases.

The Lymphatic System in Kidney Disease

The high-capacity vessels of the lymphatic system drain extravasated fluid and macromolecules from nearly every part of the body. However, far from merely a passive conduit for fluid removal, the lymphatic system also plays a critical and active role in immune surveillance and immune response modulation through the presentation of fluid, macromolecules, and trafficking immune cells to surveillance cells in regional draining lymph nodes before their return to the systemic circulation. The potential effect of this system in numerous disease states both within and outside of the kidney is increasingly being explored for their therapeutic potential. In the kidneys, the lymphatics play a critical role in both fluid and macromolecule removal to maintain oncotic and hydrostatic pressure gradients for normal kidney function, as well as in shaping kidney immunity, and potentially in balancing physiological pathways that promote healthy organ maintenance and responses to injury. In many states of kidney disease, including AKI, the demand on the preexisting lymphatic network increases for clearance of injury-related tissue edema and inflammatory infiltrates. Lymphangiogenesis, stimulated by macrophages, injured resident cells, and other drivers in kidney tissue, is highly prevalent in settings of AKI, CKD, and transplantation. Accumulating evidence points toward lymphangiogenesis being possibly harmful in AKI and kidney allograft rejection, which would potentially position lymphatics as another target for novel therapies to improve outcomes. However, the extent to which lymphangiogenesis is protective rather than maladaptive in the kidney in various settings remains poorly understood and thus an area of active research.

T cell egress via lymphatic vessels is tuned by antigen encounter and limits tumor control

Antigen-specific CD8+ T cell accumulation in tumors is a prerequisite for effective immunotherapy, and yet the mechanisms of lymphocyte transit are not well defined. Here we show that tumor-associated lymphatic vessels control T cell exit from tumors via the chemokine CXCL12, and intratumoral antigen encounter tunes CXCR4 expression by effector CD8+ T cells. Only high-affinity antigen downregulates CXCR4 and upregulates the CXCL12 decoy receptor, ACKR3, thereby reducing CXCL12 sensitivity and promoting T cell retention. A diverse repertoire of functional tumor-specific CD8+ T cells, therefore, exit the tumor, which limits the pool of CD8+ T cells available to exert tumor control. CXCR4 inhibition or loss of lymphatic-specific CXCL12 boosts T cell retention and enhances tumor control. These data indicate that strategies to limit T cell egress might be an approach to boost the quantity and quality of intratumoral T cells and thereby response to immunotherapy.

Barriers to immune cell infiltration in tumors

Increased immune cell infiltration into tumors is associated with improved patient survival and predicts response to immune therapies. Thus, identification of factors that determine the extent of immune infiltration is crucial, so that methods to intervene on these targets can be developed. T cells enter tumor tissues through the vasculature, and under control of interactions between homing receptors on the T cells and homing receptor ligands (HRLs) expressed by tumor vascular endothelium and tumor cell nests. HRLs are often deficient in tumors, and there also may be active barriers to infiltration. These remain understudied but may be crucial for enhancing immune-mediated cancer control. Multiple intratumoral and systemic therapeutic approaches show promise to enhance T cell infiltration, including both approved therapies and experimental therapies. This review highlights the intracellular and extracellular determinants of immune cell infiltration into tumors, barriers to infiltration, and approaches for intervention to enhance infiltration and response to immune therapies.

PD-1 and ICOS coexpression identifies tumor-reactive CD4+ T cells in human solid tumors

CD4⁺ Th cells play a key role in orchestrating immune responses, but the identity of the CD4⁺ Th cells involved in the antitumor immune response remains to be defined. We analyzed the immune cell infiltrates of head and neck squamous cell carcinoma and colorectal cancers and identified a subset of CD4⁺ Th cells distinct from FOXP3⁺ Tregs that coexpressed programmed cell death 1 (PD-1) and ICOS. These tumor-infiltrating lymphocyte CD4⁺ Th cells (CD4⁺ Th TILs) had a tissue-resident memory phenotype, were present in MHC class II–rich areas, and proliferated in the tumor, suggesting local antigen recognition. The T cell receptor repertoire of the PD-1⁺ICOS⁺ CD4⁺ Th TILs was oligoclonal, with T cell clones expanded in the tumor, but present at low frequencies in the periphery. Finally, these PD-1⁺ICOS⁺ CD4⁺ Th TILs were shown to recognize both tumor-associated antigens and tumor-specific neoantigens. Our findings provide an approach for isolating tumor-reactive CD4⁺ Th TILs directly ex vivo that will help define their role in the antitumor immune response and potentially improve future adoptive T cell therapy approaches.

T-Cell Adhesion in Healthy and Inflamed Skin

The diverse populations of tissue-resident and transitory T cells present in the skin share a common functional need to enter, traverse, and interact with their environment. These processes are largely dependent on the regulated expression of adhesion molecules, such as selectins and integrins, which mediate bidirectional interactions between immune cells and skin stroma. Dysregulation and engagement of adhesion pathways contribute to ectopic T-cell activity in tissues, leading to the initiation and/or exacerbation of chronic inflammation. In this paper, we review how the molecular interactions supported by adhesion pathways contribute to T-cell dynamics and function in the skin. A comprehensive understanding of the molecular mechanisms underpinning T-cell adhesion in inflammatory skin disorders will facilitate the development of novel tissue-specific therapeutic strategies.

Structure and Immune Function of Afferent Lymphatics and Their Mechanistic Contribution to Dendritic Cell and T Cell Trafficking

Afferent lymphatic vessels (LVs) mediate the transport of antigen and leukocytes to draining lymph nodes (dLNs), thereby serving as immunologic communication highways between peripheral tissues and LNs. The main cell types migrating via this route are antigen-presenting dendritic cells (DCs) and antigen-experienced T cells. While DC migration is important for maintenance of tolerance and for induction of protective immunity, T cell migration through afferent LVs contributes to immune surveillance. In recent years, great progress has been made in elucidating the mechanisms of lymphatic migration. Specifically, time-lapse imaging has revealed that, upon entry into capillaries, both DCs and T cells are not simply flushed away with the lymph flow, but actively crawl and patrol and even interact with each other in this compartment. Detachment and passive transport to the dLN only takes place once the cells have reached the downstream, contracting collecting vessel segments. In this review, we describe how the anatomy of the lymphatic network supports leukocyte trafficking and provide updated knowledge regarding the cellular and molecular mechanisms responsible for lymphatic migration of DCs and T cells. In addition, we discuss the relevance of DC and T cell migration through afferent LVs and its presumed implications on immunity.

Afferent Lymphatic Transport and Peripheral Tissue Immunity

Lymphatic vessels provide an anatomical framework for immune surveillance and adaptive immune responses. Although appreciated as the route for Ag and dendritic cell transport, peripheral lymphatic vessels are often not considered active players in immune surveillance. Lymphatic vessels, however, integrate contextual cues that directly regulate transport, including changes in intrinsic pumping and capillary remodeling, and express a dynamic repertoire of inflammatory chemokines and adhesion molecules that facilitates leukocyte egress out of inflamed tissue. These mechanisms together contribute to the course of peripheral tissue immunity. In this review, we focus on context-dependent mechanisms that regulate fluid and cellular transport out of peripheral nonlymphoid tissues to provide a framework for understanding the effects of afferent lymphatic transport on immune surveillance, peripheral tissue inflammation, and adaptive immunity.

In Sickness and in Health: The Immunological Roles of the Lymphatic System

The lymphatic system plays crucial roles in immunity far beyond those of simply providing conduits for leukocytes and antigens in lymph fluid. Endothelial cells within this vasculature are distinct and highly specialized to perform roles based upon their location. Afferent lymphatic capillaries have unique intercellular junctions for efficient uptake of fluid and macromolecules, while expressing chemotactic and adhesion molecules that permit selective trafficking of specific immune cell subsets. Moreover, in response to events within peripheral tissue such as inflammation or infection, soluble factors from lymphatic endothelial cells exert “remote control” to modulate leukocyte migration across high endothelial venules from the blood to lymph nodes draining the tissue. These immune hubs are highly organized and perfectly arrayed to survey antigens from peripheral tissue while optimizing encounters between antigen-presenting cells and cognate lymphocytes. Furthermore, subsets of lymphatic endothelial cells exhibit differences in gene expression relating to specific functions and locality within the lymph node, facilitating both innate and acquired immune responses through antigen presentation, lymph node remodeling and regulation of leukocyte entry and exit. This review details the immune cell subsets in afferent and efferent lymph, and explores the mechanisms by which endothelial cells of the lymphatic system regulate such trafficking, for immune surveillance and tolerance during steady-state conditions, and in response to infection, acute and chronic inflammation, and subsequent resolution.

Differential Expression of CD49a and CD49b Determines Localization and Function of Tumor-Infiltrating CD8+ T Cells

CD8⁺ T-cell infiltration and effector activity in tumors are correlated with better overall survival of patients, suggesting that the ability of T cells to enter and remain in contact with tumor cells supports tumor control. CD8⁺ T cells express the collagen-binding integrins CD49a and CD49b, but little is known about their function or how their expression is regulated in the tumor microenvironment (TME). Here, we found that tumor-infiltrating CD8⁺ T cells initially expressed CD49b, gained CD49a, and then lost CD49b over the course of tumor outgrowth. This differentiation sequence was driven by antigen-independent elements in the TME, although T-cell receptor (TCR) stimulation further increased CD49a expression. Expression of exhaustion markers and CD49a associated temporally but not mechanistically. Intratumoral CD49a-expressing CD8⁺ T cells failed to upregulate TCR-dependent Nur77 expression, whereas CD69 was constitutively expressed, consistent with both a lack of productive antigen engagement and a tissue-resident memory-like phenotype. Imaging T cells in live tumor slices revealed that CD49a increased their motility, especially of those in close proximity to tumor cells, suggesting that it may interfere with T-cell recognition of tumor cells by distracting them from productive engagement, although we were not able to augment productive engagement by short-term CD49a blockade. CD49b also promoted relocalization of T cells at a greater distance from tumor cells. Thus, our results demonstrate that expression of these integrins affects T-cell trafficking and localization in tumors via distinct mechanisms, and suggests a new way in which the TME, and likely collagen, could promote tumor-infiltrating CD8⁺ T-cell dysfunction.

T helper cell trafficking in autoimmune kidney diseases

CD4⁺ T cells are key drivers of autoimmune diseases, including crescentic GN. Many effector mechanisms employed by T cells to mediate renal damage and repair, such as local cytokine production, depend on their presence at the site of inflammation. Therefore, the mechanisms regulating the renal CD4⁺ T cell infiltrate are of central importance. From a conceptual point of view, there are four distinct factors that can regulate the abundance of T cells in the kidney: (1) T cell infiltration, (2) T cell proliferation, (3) T cell death and (4) T cell retention/egress. While a substantial amount of data on the recruitment of T cells to the kidneys in crescentic GN have accumulated over the last decade, the roles of T cell proliferation and death in the kidney in crescentic GN is less well characterized. However, the findings from the data available so far do not indicate a major role of these processes. More importantly, the molecular mechanisms underlying both egress and retention of T cells from/in peripheral tissues, such as the kidney, are unknown. Here, we review the current knowledge of mechanisms and functions of T cell migration in renal autoimmune diseases with a special focus on chemokines and their receptors.

HIV-Captured DCs Regulate T Cell Migration and Cell-Cell Contact Dynamics to Enhance Viral Spread

Trafficking of cell-associated HIV-1 from the genital mucosa to lymphoid organs represents a critical first step toward systemic infection. Mature DCs capture and transmit HIV-1 to T cells, but insights into DC-to-T cell viral spread dynamics within a 3-dimensional environment is lacking. Using live-cell imaging, we show that mature DCs rapidly compartmentalize HIV-1 within surface-accessible invaginations near the uropod. HIV-1 capture did not interfere with DC migration toward lymph node homing chemo-attractants and their ability to enter lymphatic vessels. However, HIV-captured DCs engaged in prolonged contacts with autologous CD4+ T cells, which led to high T cell infection. Interestingly, we show that surface bound, virion-associated Env induced signal transduction in motile T cells that facilitated prolonged DC:T cell interactions, partially through high-affinity LFA-1 expression. Together, we describe a mechanism by which surface bound HIV-1 particles function as signaling receptors that regulate T cell motility, cell-cell contact dynamics, and productive infection.

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Mature DCs compartmentalize HIV particles near the uropodia via Siglec-1 receptor

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HIV-captured DCs respond to lymph node-homing chemokines and access lymphatics

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Prolonged contacts between HIV-captured DCs and CD4 T cells facilitate virus transfer

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Surface-accessible HIV particles can induce T cell signaling via Env:CD4 engagement

Finding a Way Out: S1P Signaling and Immune Cell Migration

The signaling lipid sphingosine 1-phosphate (S1P) plays critical roles in an immune response. Drugs targeting S1P signaling have been remarkably successful in treatment of multiple sclerosis, and they have shown promise in clinical trials for colitis and psoriasis. One mechanism of these drugs is to block lymphocyte exit from lymph nodes, where lymphocytes are initially activated, into circulation, from which lymphocytes can reach sites of inflammation. Indeed, S1P can be considered a circulation marker, signaling to immune cells to help them find blood and lymphatic vessels, and to endothelial cells to stabilize the vasculature. That said, S1P plays pleiotropic roles in the immune response, and it will be important to build an integrated view of how S1P shapes inflammation. S1P can function so effectively because its distribution is exquisitely tightly controlled. Here we review how S1P gradients regulate immune cell exit from tissues, with particular attention to key outstanding questions in the field.

Approaches to overcome flow cytometry limitations in the analysis of cells from veterinary relevant species

Background

Flow cytometry is a powerful tool for the multiparameter analysis of leukocyte subsets on the single cell level. Recent advances have greatly increased the number of fluorochrome-labeled antibodies in flow cytometry. In particular, an increase in available fluorochromes with distinct excitation and emission spectra combined with novel multicolor flow cytometers with several lasers have enhanced the generation of multidimensional expression data for leukocytes and other cell types. However, these advances have mainly benefited the analysis of human or mouse cell samples given the lack of reagents for most animal species. The flow cytometric analysis of important veterinary, agricultural, wildlife, and other animal species is still hampered by several technical limitations, even though animal species other than the mouse can serve as more accurate models of specific human physiology and diseases.

Results

Here we present time-tested approaches that our laboratory regularly uses in the multiparameter flow cytometric analysis of ovine leukocytes. The discussed approaches will be applicable to the analysis of cells from most animal species and include direct modification of antibodies by covalent conjugation or Fc-directed labeling (Zenon™ technology), labeled secondary antibodies and other second step reagents, labeled receptor ligands, and antibodies with species cross-reactivity.

Conclusions

Using refined technical approaches, the number of parameters analyzed by flow cytometry per cell sample can be greatly increased, enabling multidimensional analysis of rare samples and giving critical insight into veterinary and other less commonly analyzed species. By maximizing information from each cell sample, multicolor flow cytometry can reduce the required number of animals used in a study.

Secrets and lyase: Control of sphingosine 1-phosphate distribution

The signaling lipid sphingosine 1-phosphate (S1P) plays key roles in many physiological processes. In the immune system, S1P's best-described function is to draw cells out of tissues into circulation. Here, we will review models of S1P distribution in the thymus, lymph nodes, spleen, and nonlymphoid tissues. These models have been challenging to construct, because of the lack of tools to map lipid gradients. Nonetheless, evidence to date suggests that S1P distribution is exquisitely tightly controlled, and that concentrations of signaling-available S1P cannot be predicted by standard rules of thumb. The fine regulation of S1P gradients may explain how S1P can simultaneously direct multiple cell movements both between tissues and circulation and within tissues. It may also make it feasible to develop drugs that enable spatially specific modulation of S1P signaling.

IgM Plasma Cells Reside in Healthy Skin and Accumulate with Chronic Inflammation

Antibodies are key to cutaneous host defense and inflammation. Despite their importance, the mechanisms by which skin antibodies are sustained are poorly described. Here, we identified that, in addition to antibody production in lymphoid tissues, plasma cells reside in healthy mouse and human skin. In naïve mice, IgM was the predominant isotype produced in skin. Skin plasma cells developed independently of T cells and microbiota. Importantly, chronic skin inflammation promoted the massive accumulation of IgM-secreting cells, and cutaneous immunization directed both T cell-dependent and -independent antigen-specific IgM-secreting cells into skin. Unlike their counterparts in lymphoid tissues, cutaneous IgM-secreting cells were completely dependent on survival factors such as a proliferation-inducing ligand or B cell-activating factor, which were constitutively expressed and upregulated during inflammation in skin. Our data support a model in which skin plasma cells supply natural and adaptive IgM to the cutaneous environment, thereby supporting homeostatic skin barrier functions and providing defense against pathogen intrusion. Our results are also of potential relevance for manipulation of cutaneous plasma cells in inflammatory skin diseases or cutaneous plasma cell malignancies.

Lymphatic Migration of Immune Cells

Lymphatic vessels collect interstitial fluid that has extravasated from blood vessels and return it to the circulatory system. Another important function of the lymphatic network is to facilitate immune cell migration and antigen transport from the periphery to draining lymph nodes. This migration plays a crucial role in immune surveillance, initiation of immune responses and tolerance. Here we discuss the significance and mechanisms of lymphatic migration of innate and adaptive immune cells in homeostasis, inflammation and cancer.

Leucocyte Trafficking via the Lymphatic Vasculature- Mechanisms and Consequences

The lymphatics fulfill a vital physiological function as the conduits through which leucocytes traffic between the tissues and draining lymph nodes for the initiation and modulation of immune responses. However, until recently many of the molecular mechanisms controlling such migration have been unclear. As a result of careful research, it is now apparent that the process is regulated at multiple stages from initial leucocyte entry and intraluminal crawling in peripheral tissue lymphatics, through to leucocyte exit in draining lymph nodes where the migrating cells either participate in immune responses or return to the circulation via efferent lymph. Furthermore, it is increasingly evident that most if not all leucocyte populations migrate in lymph and that such migration is not only important for immune modulation, but also for the timely repair and resolution of tissue inflammation. In this article, I review the latest research findings in these areas, arising from new insights into the distinctive ultrastructure of lymphatic capillaries and lymph node sinuses. Accordingly, I highlight the emerging importance of the leucocyte glycocalyx and its novel interactions with the endothelial receptor LYVE-1, the intricacies of endothelial chemokine secretion and sequestration that direct leucocyte trafficking and the significance of the process for normal immune function and pathology.

Non-hematopoietic Control of Peripheral Tissue T Cell Responses: Implications for Solid Tumors

In response to pathological challenge, the host generates rapid, protective adaptive immune responses while simultaneously maintaining tolerance to self and limiting immune pathology. Peripheral tissues (e.g., skin, gut, lung) are simultaneously the first site of pathogen-encounter and also the location of effector function, and mounting evidence indicates that tissues act as scaffolds to facilitate initiation, maintenance, and resolution of local responses. Just as both effector and memory T cells must adapt to their new interstitial environment upon infiltration, tissues are also remodeled in the context of acute inflammation and disease. In this review, we present the biochemical and biophysical mechanisms by which non-hematopoietic stromal cells and extracellular matrix molecules collaborate to regulate T cell behavior in peripheral tissue. Finally, we discuss how tissue remodeling in the context of tumor microenvironments impairs T cell accumulation and function contributing to immune escape and tumor progression.

ICAM-1-LFA-1 Dependent CD8+ T-Lymphocyte Aggregation in Tumor Tissue Prevents Recirculation to Draining Lymph Nodes

The quantity of T-lymphocytes reaching the draining lymph nodes from tumors is likely important to mount effective distant responses and for the establishment of long term systemic memory. Looking into mechanisms behind lymphocyte egress, we directed our attention to leukocyte adhesion mechanisms inside tumors. Here we demonstrate that activated T-cells form intra-tumor aggregates in a LFA-1-ICAM-1-dependent fashion in mouse models of melanoma and breast cancer. We also provide evidence of the presence of T-cell clusters in primary human melanoma. Disruption of LFA-1-ICAM-1 interactions, and thereby T-cell clustering, enhances the arrival of activated CD8+ T-cells to tumor draining lymph nodes in both transplanted and spontaneous cancer models. Interestingly, upon ICAM-1 blockade, the expression of the chemotactic receptor CCR7 augments in tumor infiltrating lymphocytes and in in-vitro de-clustered T cells, as well as their ability to transmigrate across lymphatic endothelial cells. We propose that ICAM-1-mediated homotypic T-lymphocyte aggregation may serve as a tumor-mediated immune retention mechanism entrapping activated CD8+ T cells in the tumor microenvironment. Modulation of T-cell adhesion may be of use to improve the transit of activated lymphocytes toward the lymph nodes and their subsequent recirculation.

Cellular traffic through afferent lymphatic vessels

The lymphatic system has long been known to serve as a highway for migrating leukocytes from peripheral tissue to draining lymph nodes (dLNs) and back to circulation, thereby contributing to the induction of adaptive immunity and immunesurveillance. Lymphatic vessels (LVs) present in peripheral tissues upstream of a first dLN are generally referred to as afferent LVs. In contrast to migration through blood vessels (BVs), the detailed molecular and cellular requirements of cellular traffic through afferent LVs have only recently started to be unraveled. Progress in our ability to track the migration of lymph-borne cell populations, in combination with cutting-edge imaging technologies, nowadays allows the investigation and visualization of lymphatic migration of endogenous leukocytes, both at the population and at the single-cell level. These studies have revealed that leukocyte trafficking through afferent LVs generally follows a step-wise migration pattern, relying on the active interplay of numerous molecules. In this review, we will summarize and discuss current knowledge of cellular traffic through afferent LVs. We will first outline how the structure of the afferent LV network supports leukocyte migration and highlight important molecules involved in the migration of dendritic cells (DCs), T cells and neutrophils, i.e. the most prominent cell types trafficking through afferent LVs. Additionally, we will describe how tumor cells hijack the lymphatic system for their dissemination to draining LNs. Finally, we will summarize and discuss our current understanding of the functional significance as well as the therapeutic implications of cell traffic through afferent LVs.

Accumulation of CD69+ tissue‑resident memory T cells in the nasal polyps of patients with chronic rhinosinusitis

In patients with chronic rhinosinusitis with nasal polyps (CRSwNP), a relative accumulation of cluster of differentiation (CD)8+ T cells over CD4+ T cells occurs in nasal polyps compared with the peripheral blood. Nasal CD8+ T cells and CD4+ T cells predominantly present an effector memory phenotype. Immunological studies have reported that memory T cells recirculate from the tissues to the peripheral blood and a high percentage of these T cells persist within the tissue. The aim of the present study was to characterize CD69+ sphingosine‑1‑phosphate receptor 1 (S1PR1)‑ tissue resident memory T cells (Trm) in the polyps of patients with CRSwNP. Tissue and blood samples were collected from 10 patients undergoing nasal sinus surgery. Expression of specific extra‑ and intracellular molecules were analyzed using multicolor flow cytometry. A significantly higher level of CD8+ T cells than CD4+ T cells was present in nasal polyps, while significantly more CD4+ T cells than CD8+ T cells were detected in the peripheral blood of patients with CRSwNP. The frequency of CD69+ T cells was significantly higher in CD8+ and CD4+ T cells in nasal polyps compared with the peripheral blood. The frequency of CD69+ S1PR1‑ Trm was also significantly higher in CD4+ and CD8+ T cells from nasal polyps compared with the peripheral blood. Within polyps, the frequency of CD69+ S1PR1‑ Trm was again significantly higher in CD8+ compared with CD4+ T cells. In summary, a significantly higher frequency of CD69+ S1PR1‑ T cells was observed in the nasal polyps compared with the peripheral blood in patients with CRSwNP. The results of the present study suggest that local regulation of the immune response occurs within nasal polyps. As such, Trm should be considered a potential stimulus in the pathogenesis of nasal polyps. However, the role of Trm in nasal polyps as a pathogenic trigger of the local inflammatory reaction requires further investigation.

Re-programming immunosurveillance in persistent non-infectious ocular inflammation

Ocular function depends on a high level of anatomical integrity. This is threatened by inflammation, which alters the local tissue over short and long time-scales. Uveitis due to autoimmune disease, especially when it involves the retina, leads to persistent changes in how the eye interacts with the immune system. The normal pattern of immune surveillance, which for immune privileged tissues is limited, is re-programmed. Many cell types, that are not usually present in the eye, become detectable. There are changes in the tissue homeostasis and integrity. In both human disease and mouse models, in the most extreme cases, immunopathological findings consistent with development of ectopic lymphoid-like structures and disrupted angiogenesis accompany severely impaired eye function. Understanding how the ocular environment is shaped by persistent inflammation is crucial to developing novel approaches to treatment.

T Cell Migration from Inflamed Skin to Draining Lymph Nodes Requires Intralymphatic Crawling Supported by ICAM-1/LFA-1 Interactions

T cells are the most abundant cell type found in afferent lymph, but their migration through lymphatic vessels (LVs) remains poorly understood. Performing intravital microscopy in the murine skin, we imaged T cell migration through afferent LVs in vivo. T cells entered into and actively migrated within lymphatic capillaries but were passively transported in contractile collecting vessels. Intralymphatic T cell number and motility were increased during contact-hypersensitivity-induced inflammation and dependent on ICAM-1/LFA-1 interactions. In vitro, blockade of endothelial cell-expressed ICAM-1 reduced T cell adhesion, crawling, and transmigration across lymphatic endothelium and decreased T cell advancement from capillaries into lymphatic collectors in skin explants. In vivo, T cell migration to draining lymph nodes was significantly reduced upon ICAM-1 or LFA-1 blockade. Our findings indicate that T cell migration through LVs occurs in distinct steps and reveal a key role for ICAM-1/LFA-1 interactions in this process.

Sphingosine-1-Phosphate Promotes the Persistence of Activated CD4 T Cells in Inflamed Sites

Inflammation can be protective or pathogenic depending on context and timeframe. Acute inflammation, including the accumulation of CD4 T cells, accompanies protective immune responses to pathogens, but the presence of activated CD4 T cells at sites of inflammation is associated with chronic inflammatory disease. While significant progress has been made in understanding the migration of CD4 T cells into inflamed sites, the signals that lead to their persistence are poorly characterized. Using a murine ear model of acute inflammation and intravital two-photon imaging, we have dissected the signals that mediate CD4 T cell persistence. We report the unexpected finding that the bioactive lipid, sphingosine-1-phosphate (S1P), is both necessary and sufficient for the persistence of activated CD4 T cells at peripheral tissues in acute inflammation. S1P mediated the enhanced motility of CD4 T cells at inflamed tissues but did not affect their migration to the downstream draining lymph node. We found that sphingosine kinase-1, which regulates S1P production is increased at inflamed sites in mice and in patients with the chronic inflammatory disease, rheumatoid arthritis. Together, these data suggest that S1P, or its regulators, may be key targets to promote or disrupt accumulation of CD4 T cells at inflamed tissues.

Chemoattractant-mediated leukocyte trafficking enables HIV dissemination from the genital mucosa

HIV vaginal transmission accounts for the majority of newly acquired heterosexual infections. However, the mechanism by which HIV spreads from the initial site of viral entry at the mucosal surface of the female genital tract to establish a systemic infection of lymphoid and peripheral tissues is not known. Once the virus exits the mucosa it rapidly spreads to all tissues, leading to CD4⁺ T cell depletion and the establishment of a viral reservoir that cannot be eliminated with current treatments. Understanding the molecular and cellular requirements for viral dissemination from the genital tract is therefore of great importance, as it could reveal new strategies to lengthen the window of opportunity to target the virus at its entry site in the mucosa where it is the most vulnerable and thus prevent systemic infection. Using HIV vaginal infection of humanized mice as a model of heterosexual transmission, we demonstrate that blocking the ability of leukocytes to respond to chemoattractants prevented HIV from leaving the female genital tract. Furthermore, blocking lymphocyte egress from lymph nodes prevented viremia and infection of the gut. Leukocyte trafficking therefore plays a major role in viral dissemination, and targeting the chemoattractant molecules involved can prevent the establishment of a systemic infection.

T Cell Trafficking through Lymphatic Vessels

T cell migration within and between peripheral tissues and secondary lymphoid organs is essential for proper functioning of adaptive immunity. While active T cell migration within a tissue is fairly slow, blood vessels and lymphatic vessels (LVs) serve as speedy highways that enable T cells to travel rapidly over long distances. The molecular and cellular mechanisms of T cell migration out of blood vessels have been intensively studied over the past 30 years. By contrast, less is known about T cell trafficking through the lymphatic vasculature. This migratory process occurs in one manner within lymph nodes (LNs), where recirculating T cells continuously exit into efferent lymphatics to return to the blood circulation. In another manner, T cell trafficking through lymphatics also occurs in peripheral tissues, where T cells exit the tissue by means of afferent lymphatics, to migrate to draining LNs and back into blood. In this review, we highlight how the anatomy of the lymphatic vasculature supports T cell trafficking and review current knowledge regarding the molecular and cellular requirements of T cell migration through LVs. Finally, we summarize and discuss recent insights regarding the presumed relevance of T cell trafficking through afferent lymphatics.

The Lymphatic System: Integral Roles in Immunity

The lymphatic vasculature is not considered a formal part of the immune system, but it is critical to immunity. One of its major roles is in the coordination of the trafficking of antigen and immune cells. However, other roles in immunity are emerging. Lymphatic endothelial cells, for example, directly present antigen or express factors that greatly influence the local environment. We cover these topics herein and discuss how other properties of the lymphatic vasculature, such as mechanisms of lymphatic contraction (which immunologists traditionally do not take into account), are nonetheless integral in the immune system. Much is yet unknown, and this nascent subject is ripe for exploration. We argue that to consider the impact of lymphatic biology in any given immunological interaction is a key step toward integrating immunology with organ physiology and ultimately many complex pathologies.

High Endothelial Venules and Lymphatic Vessels in Tertiary Lymphoid Organs: Characteristics, Functions, and Regulation

High endothelial venules (HEVs) and lymphatic vessels (LVs) are essential for the function of the immune system, by providing communication between the body and lymph nodes (LNs), specialized sites of antigen presentation and recognition. HEVs bring in naïve and central memory cells and LVs transport antigen, antigen-presenting cells, and lymphocytes in and out of LNs. Tertiary lymphoid organs (TLOs) are accumulations of lymphoid and stromal cells that arise and organize at ectopic sites in response to chronic inflammation in autoimmunity, microbial infection, graft rejection, and cancer. TLOs are distinguished from primary lymphoid organs – the thymus and bone marrow, and secondary lymphoid organs (SLOs) – the LNs, spleen, and Peyer’s patches, in that they arise in response to inflammatory signals, rather than in ontogeny. TLOs usually do not have a capsule but are rather contained within the confines of another organ. Their structure, cellular composition, chemokine expression, and vascular and stromal support resemble SLOs and are the defining aspects of TLOs. T and B cells, antigen-presenting cells, fibroblast reticular cells, and other stromal cells and vascular elements including HEVs and LVs are all typical components of TLOs. A key question is whether the HEVs and LVs play comparable roles and are regulated similarly to those in LNs. Data are presented that support this concept, especially with regard to TLO HEVs. Emerging data suggest that the functions and regulation of TLO LVs are also similar to those in LNs. These observations support the concept that TLOs are not merely cellular accumulations but are functional entities that provide sites to generate effector cells, and that their HEVs and LVs are crucial elements in those activities.

Intercellular Adhesion Molecule-1 and Vascular Cell Adhesion Molecule Are Induced by Ionizing Radiation on Lymphatic Endothelium

PURPOSE/OBJECTIVES

The goal of this study was to assess the effects of ionizing radiation on the expression of the integrin ligands ICAM-1 and VCAM that control leucocyte transit by lymphatic endothelial cells.

MATERIALS/METHODS

Confluent monolayers of primary human lymphatic endothelial cells (LEC) were irradiated with single dose of 2, 5, 10 or 20 Gy, with 6 MeV-x-rays using a Linear-Accelerator. ICAM-1 and VCAM expression was determined by flow cytometry. Human tissue specimens received a single dose of 20 Gy with 15 MeV-x-rays. MC38, B16-OVA or B16-VEGF-C tumors grown in C57BL/6 mice were irradiated with single dose of 20Gy using a Linear-Accelerator fitted with a 10mm Radiosurgery collimator. Clinical samples were obtained from patients previous and 4 weeks after complete standard radiotherapy. ICAM-1 and VCAM expression was detected in all tissue specimens by confocal microscopy. To understand the role of TGFβ in this process anti-TGFβ blocking mAb were injected i.p. 30min before radiotherapy. Cell adhesion to irradiated LEC was analyzed in adhesion experiments performed in the presence or in the absence of anti- TGFβ and /or anti-ICAM1 blocking mAb.

RESULTS

We demonstrate that lymphatic endothelial cells in tumor samples experience induction of surface ICAM-1 and VCAM when exposed to ionizing radiation in a dose- and time-dependent manner. These effects can be recapitulated in cultured LEC, and are in part mediated by TGFβ. These data are consistent with increases in ICAM-1 and VCAM expression on LYVE-1+ endothelial cells in freshly explanted human tumor tissue and in mouse transplanted tumors after radiotherapy. Finally, ICAM-1 and VCAM expression accounts for enhanced adherence of human T lymphocytes to irradiated LEC.

CONCLUSION

Our results show induction of ICAM-1 and VCAM on LVs in irradiated lesions and offer a starting point for elucidating the biological and therapeutic implications of targeting leukocyte traffic in combination to immunotherapy.

Exit Strategies: S1P Signaling and T Cell Migration

Whereas the role of sphingosine 1-phosphate receptor 1 (S1PR1) in T cell egress and the regulation of S1P gradients between lymphoid organs and circulatory fluids in homeostasis are increasingly well understood, much remains to be learned about S1P signaling and distribution during an immune response. Recent data suggest that the role of S1PR1 in directing cells from tissues into circulatory fluids is reprised again and again, particularly in guiding activated T cells from non-lymphoid tissues into lymphatics. Conversely, S1P receptor 2 (S1PR2), which antagonizes migration towards chemokines, confines cells within tissues. Here we review the current understanding of the roles of S1P signaling in activated T cell migration. In this context, we outline open questions, particularly regarding the shape of S1P gradients in different tissues in homeostasis and inflammation, and discuss recent strategies to measure S1P.

E-Selectin Mediates Immune Cell Trafficking in Corneal Transplantation

BACKGROUND

Immune rejection continues to threaten all tissue transplants. Here we sought to determine whether platelet (P)- and endothelial (E)-selectin mediate T cell recruitment in corneal transplantation and whether their blockade can reduce T cell graft infiltration and improve long-term corneal allograft survival.

METHODS

In a murine model of allogeneic corneal transplantation, we used PCR and immunohistochemistry to investigate expression of P- and E-selectin in rejected versus accepted allografts and lymph node flow cytometry to assess expression of selectin ligands by effector T cells. Using P- and E-selectin neutralizing antibodies, we evaluated the effect of blockade on CD4 T cell recruitment, as well as the effect of anti-E-selectin on long-term allograft survival.

RESULTS

The P- (93.3-fold, P < 0.05) and E-selectin (17.1-fold, P < 0.005) are upregulated in rejected versus accepted allogeneic transplants. Type 1 T helper cells from hosts with accepted and rejected grafts express high levels of P-selectin glycoprotein ligand 1 and glycosylated CD43. In vivo blockade of P (0.47 ± 0.03, P < 0.05) and E selectin (0.49 ± 0.1, P < 0.05) reduced the number of recruited T cells compared with IgG control (0.98 ± 0.1). Anti-E-selectin reduced the number of mature antigen-presenting cells trafficking to lymphoid tissue compared with control (6.96 ± 0.9 vs 12.67 ± 0.5, P < 0.05). Anti-E-selectin treatment delayed graft rejection and increased survival compared with control, although this difference did not reach statistical significance.

CONCLUSIONS

In a model of corneal transplantation, P- and E-selectin mediate T cell recruitment to the graft, E-selectin mediates APC trafficking to lymphoid tissue, and blockade of E-selectin has a modest effect on improving long-term graft survival.

Distinct mechanisms of B and T lymphocyte accumulation generate tumor-draining lymph node hypertrophy

Tumor-draining lymph nodes (TDLNs) often enlarge in human cancer patients and in murine tumor models, due to lymphocyte accumulation and lymphatic sinus growth. B lymphocytes within TDLNs can drive lymph node hypertrophy in response to tumor growth, however little is known about the mechanisms directing the preferential accumulation of B lymphocytes relative to T cells in enlarging TDLNs. To define why B and T lymphocytes accumulate in TDLNs, we quantified lymphocyte proliferation, apoptosis, entry, and exit in TDLNs versus contralateral non-TDLNs (NTDLNs) in a footpad B16-F10 melanoma mouse model. B and T lymphocyte proliferation and apoptosis were increased as the TDLNs enlarged, although relative rates were similar to those of NTDLNs. TDLN entry of B and T lymphocytes via high endothelial venules was also modestly increased in enlarged TDLNs. Strikingly, the egress of B cells was strongly reduced in TDLNs versus NTDLNs, while T cell egress was modestly decreased, indicating that regulation of lymphocyte exit from TDLNs is a major mechanism of preferential B lymphocyte accumulation. Surface sphingosine-1-phosphate receptor 1 (S1PR1) which binds S1P and signals lymphocyte egress, exhibited greater downregulation in B relative to T lymphocytes, consistent with preferential retention of B lymphocytes in TDLNs. TDLN lymphocytes did not activate surface CD69 expression, indicating a CD69-independent mechanism of downregulation of S1PR1. B and T cell trafficking via afferent lymphatics to enter TDLNs also increased, suggesting a pathway for accumulation of tumor-educated lymphocytes in TDLNs. These mechanisms regulating TDLN hypertrophy could provide new targets to manipulate lymphocyte responses to cancer.

IL-10+ Innate-like B Cells Are Part of the Skin Immune System and Require α4β1 Integrin To Migrate between the Peritoneum and Inflamed Skin

The skin is an important barrier organ and frequent target of autoimmunity and allergy. In this study, we found innate-like B cells that expressed the anti-inflammatory cytokine IL-10 in the skin of humans and mice. Unexpectedly, innate-like B1 and conventional B2 cells showed differential homing capacities with peritoneal B1 cells preferentially migrating into the inflamed skin of mice. Importantly, the skin-homing B1 cells included IL-10-secreting cells. B1 cell homing into the skin was independent of typical skin-homing trafficking receptors and instead required α4β1-integrin. Moreover, B1 cells constitutively expressed activated β1 integrin and relocated from the peritoneum to the inflamed skin and intestine upon innate stimulation, indicating an inherent propensity to extravasate into inflamed and barrier sites. We conclude that innate-like B cells migrate from central reservoirs into skin, adding an important cell type with regulatory and protective functions to the skin immune system.

AKAP9 regulates activation-induced retention of T lymphocytes at sites of inflammation

The mechanisms driving T cell homing to lymph nodes and migration to tissue are well described but little is known about factors that affect T cell egress from tissues. Here, we generate mice with a T cell-specific deletion of the scaffold protein A kinase anchoring protein 9 (AKAP9) and use models of inflammatory disease to demonstrate that AKAP9 is dispensable for T cell priming and migration into tissues and lymph nodes, but is required for T cell retention in tissues. AKAP9 deficiency results in increased T cell egress to draining lymph nodes, which is associated with impaired T cell re-activation in tissues and protection from organ damage. AKAP9-deficient T cells exhibit reduced microtubule-dependent recycling of TCRs back to the cell surface and this affects antigen-dependent activation, primarily by non-classical antigen-presenting cells. Thus, AKAP9-dependent TCR trafficking drives efficient T cell re-activation and extends their retention at sites of inflammation with implications for disease pathogenesis.

A-kinase anchoring protein 9 (AKAP9) is a scaffold protein that binds signalling proteins and regulates microtubules. Here the authors show that during inflammation AKAP9 in T cells is required for their reactivation and retention at the inflammation site and that its deletion protects from inflammation-induced organ damage.

Migration and Tissue Tropism of Innate Lymphoid Cells

Innate lymphoid cell (ILCs) subsets differentially populate various barrier and non-barrier tissues, where they play important roles in tissue homeostasis and tissue-specific responses to pathogen attack. Recent findings have provided insight into the molecular mechanisms that guide ILC migration into peripheral tissues, revealing common features among different ILC subsets as well as important distinctions. Recent studies have also highlighted the impact of tissue-specific cues on ILC migration, and the importance of the local immunological milieu. We review these findings here and discuss how the migratory patterns and tissue tropism of different ILC subsets relate to the development and differentiation of these cells, and to ILC-mediated tissue-specific regulation of innate and adaptive immune responses. In this context we outline open questions and important areas of future research.

Lymphatic Vessels, Inflammation, and Immunity in Skin Cancer

Skin is a highly ordered immune organ that coordinates rapid responses to external insult while maintaining self-tolerance. In healthy tissue, lymphatic vessels drain fluid and coordinate local immune responses; however, environmental factors induce lymphatic vessel dysfunction, leading to lymph stasis and perturbed regional immunity. These same environmental factors drive the formation of local malignancies, which are also influenced by local inflammation. Herein, we discuss clinical and experimental evidence supporting the tenet that lymphatic vessels participate in regulation of cutaneous inflammation and immunity, and are important contributors to malignancy and potential biomarkers and targets for immunotherapy.

SIGNIFICANCE

The tumor microenvironment and tumor-associated inflammation are now appreciated not only for their role in cancer progression but also for their response to therapy. The lymphatic vasculature is a less-appreciated component of this microenvironment that coordinates local inflammation and immunity and thereby critically shapes local responses. A mechanistic understanding of the complexities of lymphatic vessel function in the unique context of skin provides a model to understand how regional immune dysfunction drives cutaneous malignancies, and as such lymphatic vessels represent a biomarker of cutaneous immunity that may provide insight into cancer prognosis and effective therapy.

Effector T Cell Egress via Afferent Lymph Modulates Local Tissue Inflammation

Memory/effector T cells recirculate through extralymphoid tissues by entering from blood and egressing via afferent lymph. Although T cell entry into effector sites is key to inflammation, the relevance of T cell egress to this process is unknown. In this study, we found that Ag recognition at the effector site reduced the tissue egress of proinflammatory Th1 cells in a mouse model of delayed hypersensitivity. Transgenic expression of "tissue exit receptor" CCR7 enhanced lymphatic egress of Ag-sequestered Th1 cells from the inflamed site and alleviated inflammation. In contrast, lack of CCR7 on Th1 cells diminished their tissue egress while enhancing inflammation. Lymph-borne Th1 and Th17 cells draining the inflamed skin of sheep migrated toward the CCR7 ligand CCL21, suggesting the CCR7-CCL21 axis as a physiological target in regulating inflammation. In conclusion, exit receptors can be targeted to modulate T cell dwell time and inflammation at effector sites, revealing T cell tissue egress as a novel control point of inflammation.

Chemokine receptor CCR7 regulates the intestinal TH1/TH17/Treg balance during Crohn's-like murine ileitis

The regulation of T cell and DC retention and lymphatic egress within and from the intestine is critical for intestinal immunosurveillance; however, the cellular processes that orchestrate this balance during IBD remain poorly defined. With the use of a mouse model of TNF-driven Crohn's-like ileitis (TNF(Δ) (ARE)), we examined the role of CCR7 in the control of intestinal T cell and DC retention/egress during experimental CD. We observed that the frequency of CCR7-expressing TH1/TH17 effector lymphocytes increased during active disease in TNF(Δ) (ARE) mice and that ΔARE/CCR7(-/-) mice developed exacerbated ileitis and multiorgan inflammation, with a marked polarization and ileal retention of TH1 effector CD4(+) T cells. Furthermore, adoptive transfer of ΔARE/CCR7(-/-) effector CD4(+) into lymphopenic hosts resulted in ileo-colitis, whereas those transferred with ΔARE/CCR7(+/+) CD4(+) T cells developed ileitis. ΔARE/CCR7(-/-) mice had an acellular draining MLN, decreased CD103(+) DC, and decreased expression of RALDH enzymes and of CD4(+)CD25(+)FoxP3(+) Tregs. Lastly, a mAb against CCR7 exacerbated ileitis in TNF(Δ) (ARE) mice, phenocopying the effects of congenital CCR7 deficiency. Our data underscore a critical role for the lymphoid chemokine receptor CCR7 in orchestrating immune cell traffic and TH1 versus TH17 bias during chronic murine ileitis.

CXCR4 is dispensable for T cell egress from chronically inflamed skin via the afferent lymph

T cell recirculation through extralymphoid tissues is essential to immune surveillance, host defense and inflammation. In this process, T cells enter the tissue from the blood and subsequently leave via the afferent lymph. In the absence of inflammation, T cells require CCR7 expression to egress from the skin or lung, which is consistent with the constitutive expression of the CCR7 ligand CCL21 on lymphatic endothelium. However, during chronic inflammation alternative chemoattractants come into play, allowing Ccr7-deficient (Ccr7^−/−) T cells to egress efficiently from affected skin. As T cell egress from inflamed sites is a potential control point of the inflammatory response, we aimed to determine alternative T cell exit receptors using a mouse and a sheep model. We show that CCR7⁺ and CCR7^– T cells exiting from the chronically inflamed skin were highly responsive to the CXCR4 ligand CXCL12, which was induced in the lymphatics in the inflamed site. Based on these findings, we hypothesized that CXCR4 mediates T cell egress from inflamed skin. However, pharmacological inhibition of CXCR4 did not affect the tissue egress of wildtype or Ccr7^−/− CD4 and CD8 T cells after adoptive transfer into chronically inflamed skin. Similarly, adoptively transferred Cxcr4^−/− Ccr7^−/− and Ccr7^−/− T cells egressed from the inflamed skin equally well. Based on these data, we conclude that, while CXCR4 might play an essential role for other cell types that enter the afferent lymphatics, it is dispensable for T cell egress from the chronically inflamed skin.

Lymphatic vessels and tertiary lymphoid organs

Tertiary lymphoid organs (TLOs) are accumulations of lymphoid cells in chronic inflammation that resemble LNs in their cellular content and organization, high endothelial venules, and lymphatic vessels (LVs). Although acute inflammation can result in defective LVs, TLO LVs appear to function normally in that they drain fluid and transport cells that respond to chemokines and sphingosine-1-phosphate (S1P) gradients. Molecular regulation of TLO LVs differs from lymphangiogenesis in ontogeny with a dependence on cytokines and hematopoietic cells. Ongoing work to elucidate the function and molecular regulation of LVs in TLOs is providing insight into therapies for conditions as diverse as lymphedema, autoimmunity, and cancer.

Chemokines and chemokine receptors: positioning cells for host defense and immunity

Chemokines are chemotactic cytokines that control the migratory patterns and positioning of all immune cells. Although chemokines were initially appreciated as important mediators of acute inflammation, we now know that this complex system of approximately 50 endogenous chemokine ligands and 20 G protein-coupled seven-transmembrane signaling receptors is also critical for the generation of primary and secondary adaptive cellular and humoral immune responses. Recent studies demonstrate important roles for the chemokine system in the priming of naive T cells, in cell fate decisions such as effector and memory cell differentiation, and in regulatory T cell function. In this review, we focus on recent advances in understanding how the chemokine system orchestrates immune cell migration and positioning at the organismic level in homeostasis, in acute inflammation, and during the generation and regulation of adoptive primary and secondary immune responses in the lymphoid system and peripheral nonlymphoid tissue.