Is defective lymphatic drainage a trigger for lymphoid neogenesis?

Lymph Nodes and Cancer Metastasis: New Perspectives on the Role of Intranodal Lymphatic Sinuses

The lymphatic system is essential for transporting interstitial fluid, soluble antigen, and immune cells from peripheral tissues to lymph nodes (LNs). Functional integrity of LNs is dependent on intact lymphatics and effective lymph drainage. Molecular mechanisms that facilitate interactions between tumor cells and lymphatic endothelial cells (LECs) during tumor progression still remain to be identified. The cellular and molecular structures of LNs are optimized to trigger a rapid and efficient immune response, and to participate in the process of tumor metastasis by stimulating lymphangiogenesis and establishing a premetastatic niche in LNs. Several molecules, e.g., S1P, CCR7-CCL19/CCL21, CXCL12/CXCR4, IL-7, IFN-γ, TGF-β, and integrin α4β1 play an important role in controlling the activity of LN stromal cells including LECs, fibroblastic reticular cells (FRCs) and follicular dendritic cells (DCs). The functional stromal cells are critical for reconstruction and remodeling of the LN that creates a unique microenvironment of tumor cells and LECs for cancer metastasis. LN metastasis is a major determinant for the prognosis of most human cancers and clinical management. Ongoing work to elucidate the function and molecular regulation of LN lymphatic sinuses will provide insight into cancer development mechanisms and improve therapeutic approaches for human malignancy.

Lymphoid Neogenesis and Tertiary Lymphoid Organs in Transplanted Organs

The progressive organization of immune effectors into functional ectopic lymphoid structures, named tertiary lymphoid organs (TLO), has been observed in many conditions in which target antigens fail to be eliminated by the immune system. Not surprisingly, TLO have been recurrently identified in chronically rejected allografts. Although significant progress has been made over the last decades in understanding the molecular mechanisms involved in TLO development (a process named lymphoid neogenesis), the role of intragraft TLO (if any) in chronic rejection remains elusive. The prevailing dogma is that TLO contribute to graft rejection by generating and propagating local humoral and cellular alloimmune responses. However, TLO have been recently observed in long-term accepting allografts, suggesting that they might also be able to regulate alloimmune responses. In this review, we discuss our current understanding of how TLO are induced and propose a unified model in which TLO can play deleterious or regulatory roles and therefore actively modulate the kinetics of chronic rejection.

The formation and function of tertiary lymphoid follicles in chronic pulmonary inflammation

Tertiary lymphoid follicles (TLFs) can develop in the respiratory tract in response to infections or chronic inflammation. However, their functional relevance remains unclear because they are implicated in both protective and pathological responses. In contrast to homeostatic conditions, external antigens and damage to the lung tissue may drive TLF formation in inflamed lungs, and once established, the presence of pulmonary TLFs may signal the progression of chronic lung disease. This novel concept will be discussed in light of recent work in chronic obstructive pulmonary disease and how changes in the pulmonary microbiota may drive and direct TLF formation and function. We will also discuss the cellularity of TLFs at the pulmonary mucosa, with emphasis on the potential roles of lymphoid tissue inducer cells, and B- and T-cell aggregates, and will examine the function of key chemokines and cytokines including CXCL13 and interleukin-17, in the formation and maintenance of pulmonary TLFs.

Lymphoid Aggregates Remodel Lymphatic Collecting Vessels that Serve Mesenteric Lymph Nodes in Crohn Disease

Early pathological descriptions of Crohn disease (CD) argued for a potential defect in lymph transport; however, this concept has not been thoroughly investigated. In mice, poor healing in response to infection-induced tissue damage can cause hyperpermeable lymphatic collecting vessels in mesenteric adipose tissue that impair antigen and immune cell access to mesenteric lymph nodes (LNs), which normally sustain appropriate immunity. To investigate whether analogous changes might occur in human intestinal disease, we established a three-dimensional imaging approach to characterize the lymphatic vasculature in mesenteric tissue from controls or patients with CD. In CD specimens, B-cell-rich aggregates resembling tertiary lymphoid organs (TLOs) impinged on lymphatic collecting vessels that enter and exit LNs. In areas of creeping fat, which characterizes inflammation-affected areas of the bowel in CD, we observed B cells and apparent innate lymphoid cells that had invaded the lymphatic vessel wall, suggesting these cells may be mediators of lymphatic remodeling. Although TLOs have been described in many chronic inflammatory states, their anatomical relationship to preestablished LNs has never been revealed. Our data indicate that, at least in the CD-affected mesentery, TLOs are positioned along collecting lymphatic vessels in a manner expected to affect delivery of lymph to LNs.

Bimodal Expansion of the Lymphatic Vessels Is Regulated by the Sequential Expression of IL-7 and Lymphotoxin α1β2 in Newly Formed Tertiary Lymphoid Structures

Lymphangiogenesis associated with tertiary lymphoid structure (TLS) has been reported in numerous studies. However, the kinetics and dynamic changes occurring to the lymphatic vascular network during TLS development have not been studied. Using a viral-induced, resolving model of TLS formation in the salivary glands of adult mice we demonstrate that the expansion of the lymphatic vascular network is tightly regulated. Lymphatic vessel expansion occurs in two distinct phases. The first wave of expansion is dependent on IL-7. The second phase, responsible for leukocyte exit from the glands, is regulated by lymphotoxin (LT)βR signaling. These findings, while highlighting the tight regulation of the lymphatic response to inflammation, suggest that targeting the LTα₁β₂/LTβR pathway in TLS-associated pathologies might impair a natural proresolving mechanism for lymphocyte exit from the tissues and account for the failure of therapeutic strategies that target these molecules in diseases such as rheumatoid arthritis.

Inducible Bronchus-Associated Lymphoid Tissue: Taming Inflammation in the Lung

Following pulmonary inflammation, leukocytes that infiltrate the lung often assemble into structures known as inducible Bronchus-Associated Lymphoid Tissue (iBALT). Like conventional lymphoid organs, areas of iBALT have segregated B and T cell areas, specialized stromal cells, high endothelial venules, and lymphatic vessels. After inflammation is resolved, iBALT is maintained for months, independently of inflammation. Once iBALT is formed, it participates in immune responses to pulmonary antigens, including those that are unrelated to the iBALT-initiating antigen, and often alters the clinical course of disease. However, the mechanisms that govern immune responses in iBALT and determine how iBALT impacts local and systemic immunity are poorly understood. Here, we review our current understanding of iBALT formation and discuss how iBALT participates in pulmonary immunity.

The Role of Lymphoid Neogenesis in Allografts

De novo induction of organized lymphoid aggregates at nonlymphoid sites has been observed in many chronic inflammatory conditions where foreign antigens such as infectious agents, autoantigens or alloantigens, persist. The prevailing opinion in the field of transplantation is that lymphoid neogenesis within allografts is detrimental to the establishment of immune tolerance. These structures, commonly referred to as tertiary lymphoid organs (TLOs), are thought to contribute to graft rejection by generating and propagating local alloimmune responses. However, recent studies have shown that TLOs rich in regulatory Foxp3(+) cells are present in long-term accepting allografts. The notion that TLOs can contribute to the local downregulation of immune responses has been corroborated in other chronic inflammation models. These findings suggest that contrary to previous suggestions that the induction of TLOs in allografts is necessarily harmful, the induction of "tolerogenic" TLOs may prove advantageous. In this review, we discuss our current understanding of how TLOs are induced and how they regulate immune responses with a particular focus on alloimmunity.

Therapeutic lymphangiogenesis ameliorates established acute lung allograft rejection

Lung transplantation is the only viable option for patients suffering from otherwise incurable end-stage pulmonary diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. Despite aggressive immunosuppression, acute rejection of the lung allograft occurs in over half of transplant recipients, and the factors that promote lung acceptance are poorly understood. The contribution of lymphatic vessels to transplant pathophysiology remains controversial, and data that directly address the exact roles of lymphatic vessels in lung allograft function and survival are limited. Here, we have shown that there is a marked decline in the density of lymphatic vessels, accompanied by accumulation of low-MW hyaluronan (HA) in mouse orthotopic allografts undergoing rejection. We found that stimulation of lymphangiogenesis with VEGF-C156S, a mutant form of VEGF-C with selective VEGFR-3 binding, alleviates an established rejection response and improves clearance of HA from the lung allograft. Longitudinal analysis of transbronchial biopsies from human lung transplant recipients demonstrated an association between resolution of acute lung rejection and decreased HA in the graft tissue. Taken together, these results indicate that lymphatic vessel formation after lung transplantation mediates HA drainage and suggest that treatments to stimulate lymphangiogenesis have promise for improving graft outcomes.

[Meningeal tertiary lymphoid organs: Major actors in intrathecal autoimmunity]

Multiple sclerosis (MS) is characterized by an intrathecal synthesis of immunoglobulins synthesized by B-cell clones and by a brain infiltrate of clonal T-cells. The clonal maturation of these lymphocytes takes place in tertiary lymphoid organs (TLO) developed in the intrathecal compartment. TLO are acquired lymphoid organs able to develop in the vicinity of the inflammatory sites, where they mount a complete antigen-driven immune response. We here review TLO pathophysiology in animal models of MS and human MS. Several pieces of evidence suggest that intrathecal TLO may play a major role in the clinical impairment. Potential therapeutic applications are examined.

Host immune response to infection and cancer: unexpected commonalities

Both microbes and tumors activate innate resistance, tissue repair, and adaptive immunity. Unlike acute infection, tumor growth is initially unapparent; however, inflammation and immunity affect all phases of tumor growth from initiation to progression and dissemination. Here, we discuss the shared features involved in the immune response to infection and cancer including modulation by commensal microbiota, reactive hematopoiesis, chronic immune responses and regulatory mechanisms to prevent collateral tissue damage. This comparative analysis of immunity to infection and cancer furthers our understanding of the basic mechanisms underlying innate resistance and adaptive immunity and their translational application to the design of new therapeutic approaches.

Identification of novel genes associated with renal tertiary lymphoid organ formation in aging mice

A hallmark of aging-related organ deterioration is a dysregulated immune response characterized by pathologic leukocyte infiltration of affected tissues. Mechanisms and genes involved are as yet unknown. To identify genes associated with aging-related renal infiltration, we analyzed kidneys from aged mice (≥20 strains) for infiltrating leukocytes followed by Haplotype Association Mapping (HAM) analysis. Immunohistochemistry revealed CD45⁺ cell clusters (predominantly T and B cells) in perivascular areas coinciding with PNAd⁺ high endothelial venules and podoplanin⁺ lymph vessels indicative of tertiary lymphoid organs. Cumulative cluster size increased with age (analyzed at 6, 12 and 20 months). Based on the presence or absence of clusters in male and female mice at 20 months, HAM analysis revealed significant associations with loci on Chr1, Chr2, Chr8 and Chr14 in male mice, and with loci on Chr4, Chr7, Chr13 and Chr14 in female mice. Wisp2 (Chr2) showed the strongest association (P = 5.00×10⁻¹³⁷) in male mice; Ctnnbip1 (P = 6.42×10⁻²⁶⁷) and Tnfrsf8 (P = 5.42×10⁻²⁴⁵) (both on Chr4) showed the strongest association in female mice. Both Wisp2 and Ctnnbip1 are part of the Wnt-signaling pathway and the encoded proteins were expressed within the tertiary lymphoid organs. In conclusion, this study revealed differential lymphocytic infiltration and tertiary lymphoid organ formation in aged mouse kidneys across different inbred mouse strains. HAM analysis identified candidate genes involved in the Wnt-signaling pathway that may be causally linked to tertiary lymphoid organ formation.

Preferential lymphatic growth in bronchus-associated lymphoid tissue in sustained lung inflammation

Lymphatics proliferate, become enlarged, or regress in multiple inflammatory lung diseases in humans. Lymphatic growth and remodeling is known to occur in the mouse trachea in sustained inflammation, but whether intrapulmonary lymphatics exhibit similar plasticity is unknown. We examined the time course, distribution, and dependence on vascular endothelial growth factor receptor (VEGFR)-2/VEGFR-3 signaling of lung lymphatics in sustained inflammation. Lymphatics in mouse lungs were examined under baseline conditions and 3 to 28 days after Mycoplasma pulmonis infection, using prospero heomeobox 1-enhanced green fluorescence protein and VEGFR-3 as markers. Sprouting lymphangiogenesis was evident at 7 days. Lymphatic growth was restricted to regions of bronchus-associated lymphoid tissue (BALT), where VEGF-C-producing cells were scattered in T-cell zones. Expansion of lung lymphatics after infection was reduced 68% by blocking VEGFR-2, 83% by blocking VEGFR-3, and 99% by blocking both receptors. Inhibition of VEGFR-2/VEGFR-3 did not prevent the formation of BALT. Treatment of established infection with oxytetracycline caused BALT, but not the lymphatics, to regress. We conclude that robust lymphangiogenesis occurs in mouse lungs after M. pulmonis infection through a mechanism involving signaling of both VEGFR-2 and VEGFR-3. Expansion of the lymphatic network is restricted to regions of BALT, but lymphatics do not regress when BALT regresses after antibiotic treatment. The lung lymphatic network can thus expand in sustained inflammation, but the expansion is not as reversible as the accompanying inflammation.

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.

Inflammation-induced lymphangiogenesis and lymphatic dysfunction

The lymphatic system is intimately linked to tissue fluid homeostasis and immune cell trafficking. These functions are paramount in the establishment and development of an inflammatory response. In the past decade, an increasing number of reports has revealed that marked changes, such as lymphangiogenesis and lymphatic contractile dysfunction occur in both vascular and nodal parts of the lymphatic system during inflammation, as well as other disease processes. This review provides a critical update on the role of the lymphatic system in disease process such as chronic inflammation and cancer and examines the changes in lymphatic functions the diseases cause and the influence these changes have on the progression of the diseases.

The role of non-hematopoietic stromal cells in the persistence of inflammation

Inflammation results from the complex interaction between hematopoietic and stromal cells and growing evidence supports a key role for the stroma in driving the switch from acute resolving to persistence in chronic inflammatory diseases. Stromal cells have also been shown to play a critical role in cancer biology, being involved in cancer growth, dissemination, and inhibition of the autologous immune response, ultimately favoring persistence and metastatic spread. Similarly, blood and lymphatic endothelial cells contribute to tissue homeostasis during physiological inflammation but also lead to discorded leukocyte and tumor cell accumulation in pathological inflammation and cancer. This review aims to summarize the role that pathogenic stroma plays in the pathogenesis of diseases such as cancer and chronic inflammation.

Interstitial Fluid and Lymph Formation and Transport: Physiological Regulation and Roles in Inflammation and Cancer

The interstitium describes the fluid, proteins, solutes, and the extracellular matrix (ECM) that comprise the cellular microenvironment in tissues. Its alterations are fundamental to changes in cell function in inflammation, pathogenesis, and cancer. Interstitial fluid (IF) is created by transcapillary filtration and cleared by lymphatic vessels. Herein we discuss the biophysical, biomechanical, and functional implications of IF in normal and pathological tissue states from both fluid balance and cell function perspectives. We also discuss analysis methods to access IF, which enables quantification of the cellular microenvironment; such methods have demonstrated, for example, that there can be dramatic gradients from tissue to plasma during inflammation and that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma. Accumulated recent data show that IF and its convection through the interstitium and delivery to the lymph nodes have many and diverse biological effects, including in ECM reorganization, cell migration, and capillary morphogenesis as well as in immunity and peripheral tolerance. This review integrates the biophysical, biomechanical, and biological aspects of interstitial and lymph fluid and its transport in tissue physiology, pathophysiology, and immune regulation.

Lymphatic and interstitial flow in the tumour microenvironment: linking mechanobiology with immunity

Tumours often engage the lymphatic system in order to invade and metastasize. The tumour-draining lymph node may be an immune-privileged site that protects the tumour from host immunity, and lymph flow that drains tumours is often increased, enhancing communication between the tumour and the sentinel node. In addition to increasing the transport of tumour antigens and regulatory cytokines to the lymph node, increased lymph flow in the tumour margin causes mechanical stress-induced changes in stromal cells that stiffen the matrix and alter the immune microenvironment of the tumour. We propose that synergies between lymphatic drainage and flow-induced mechanotransduction in the stroma promote tumour immune escape by appropriating lymphatic mechanisms of peripheral tolerance.

Characterization of chemokines and adhesion molecules associated with T cell presence in tertiary lymphoid structures in human lung cancer

De novo formation of tertiary lymphoid structures (TLS) has been described in lung cancers. Intratumoral TLS seem to be functional and are associated with a long-term survival for lung cancer patients, suggesting that they represent an activation site for tumor-specific T cells. Here, we characterized T-cell recruitment to TLS in human lung cancer to identify the adhesion molecules and chemoattractants orchestrating this migration. We found that most TLS T cells were CD62L+ and mainly of CD4+ memory phenotype, but naive T cells were highly enriched in these structures as compared with the rest of the tumor. A specific gene expression signature associated with T cell presence was identified in TLS, which included chemokines (CCL19, CCL21, CXCL13, CCL17, CCL22, and IL16), adhesion molecules (ICAM-2, ICAM-3, VCAM-1, and MAdCAM-1) and integrins (alphaL, alpha4, and alphaD). The presence of the corresponding receptors on TLS T cells was confirmed. Intratumoral PNAd+ high endothelial venules also were exclusively associated with TLS and colocalized with CD62L+ lymphocytes. Together, these data bring new insights into the T-cell recruitment to intratumoral TLS and suggest that blood T cell enter into TLS via high endothelial venules, which represent a new gateway for T cells to the tumor. Findings identify the molecules that mediate migration of tumor-specific T cells into TLS where T cell priming occurs, suggesting new strategies to enhance the efficacy of cancer immunotherapies.

Tertiary lymphoid organs in lymphatic malformations

BACKGROUND

Examine lymphatic malformation lymphoid aggregates for the expression of tertiary lymphoid organ markers. Determine how lymphoid aggregate density relates to lymphatic malformation clinical features.

METHODS AND RESULTS

Retrospective cohort study. Tissue and clinical data were reviewed from 29 patients in the Vascular Anomaly Database who represented the spectrum of head and neck lymphatic malformations and had >5 years of follow-up. Archived formalin-fixed, paraffin-embedded lymphatic malformation tissue was immunohistochemically stained with antibodies for tertiary lymphoid organ markers, which included follicular and mature myeloid dendritic cells, high endothelial venules, segregated B and T-cells, lymphatic endothelial cells, and lymphoid homing chemokines (CXCL13, CCL21). Lymphoid aggregate density (count/mm(2)) was quantified by 2 independent, blinded reviewers. Lymphoid aggregate density and lymphatic malformation clinical features were characterized using analysis of variance. Larger lymphatic malformation tissue lymphoid aggregates stained consistently for tertiary lymphoid organ markers. In oral cavity and neck specimens from the same patients (n = 9), there were more tertiary lymphoid organ in oral cavity than in neck specimens (p = 0.0235). In lymphatic malformation neck tissue, de Serres stage 4 lymphatic malformations displayed the highest tertiary lymphoid organ density. No significant association was seen between tertiary lymphoid organ density and other clinical features.

CONCLUSION

This study demonstrates that some lymphoid aggregates within lymphatic malformations represent tertiary lymphoid organs. There was an association between tertiary lymphoid organ density and lymphatic malformation location. Further study is required to define the role of lymphoid neogenesis and tertiary lymphoid organ formation in lymphatic malformation pathogenesis.

The Amazing Power of Cancer Cells to Recapitulate Extraembryonic Functions: The Cuckoo's Tricks

Inflammation is implicated in tumor development, invasion, and metastasis. Hence, it has been suggested that common cellular and molecular mechanisms are activated in wound repair and in cancer development. In addition, it has been previously proposed that the inflammatory response, which is associated with the wound healing process, could recapitulate ontogeny through the reexpression of the extraembryonic, that is, amniotic and vitelline, functions in the interstitial space of the injured tissue. If so, the use of inflammation by the cancer-initiating cell can also be supported in the ability to reacquire extraembryonic functional axes for tumor development, invasion, and metastasis. Thus, the diverse components of the tumor microenvironment could represent the overlapping reexpression of amniotic and vitelline functions. These functions would favor a gastrulation-like process, that is, the creation of a reactive stroma in which fibrogenesis and angiogenesis stand out.

Increased lymphatic vessel density and lymphangiogenesis in inflammatory bowel disease

BACKGROUND

Involvement of the lymphatic system in inflammatory bowel disease (IBD) has been suggested.

AIMS

To examine the density and distribution of lymphatic vessels (LV) within inflamed and non-inflamed wall sections of IBD patients compared with controls, and to evaluate expression of major lymphangiogenic factors.

METHODS

Ileal and colon specimens of 22 patients with Crohn's disease (CD), 16 patients with ulcerative colitis (UC) and 11 controls were studied. Quantification of LV was performed using immunohistochemistry with podoplanin and D2-40 antibodies on seven randomly selected fields. Mucosal expression of podoplanin and lymphangiogenic factor mRNA was measured using PCR.

RESULTS

In CD patients, lymphatic density was significantly increased in non-inflamed and inflamed ileal (P < 0.01 and P < 0.001) and colonic (P < 0.01 and P < 0.001) mucosa compared to controls. Podoplanin mRNA levels were similar in non-inflamed mucosal areas and controls, whereas a four- and sixfold increase was seen in inflamed ileal and colonic areas (P < 0.05). In UC, lymphatic density increased fourfold in non-inflamed (P < 0.001) and fivefold in inflamed colonic mucosa (P < 0.001) compared with controls. An increase in podoplanin mRNA levels was seen in both non-inflamed and inflamed areas (P < 0.01) compared with controls. In CD and UC, lymphatics were found throughout the inflamed mucosa, including the upper half of the lamina propria. Expression of lymphangiogenic factors was similar in patients and controls.

CONCLUSIONS

Increased density of lymphatic vessels is a constant feature of IBD and is present in non-inflamed areas. It is transmural in CD and confined to the mucosa in UC. Its origin remains unclear.

Role of the lymphatic system in the pathogenesis of Crohn's disease

PURPOSE OF REVIEW

Intestinal lymph containing interstitial fluid, proteins, immune cells, and digested lipids is actively transported back to the blood stream thanks to rhythmical contractions of the mesenteric lymphatic vessels. During this process, lymph flows through several lymph nodes, allowing antigens to be sampled by the immune system. Abnormalities in lymphatic drainage have been noted in the original descriptions of Crohn's disease, but essentially ignored since. The lymphatic system is re-emerging as a critical player in inflammatory and immune processes and the purpose of this review is to present and discuss new concepts related to the involvement of the lymphatic system in the development of inflammatory bowel diseases (IBDs) and more specifically Crohn's disease.

RECENT FINDINGS

Recent studies reporting lymphangitis, lymphangiogenesis, bacterial infiltration and lymph node infection, immune cell trafficking, and fat-wrapping in Crohn's disease suggest altered lymph drainage and lymphatic pumping, implicating the lymphatic system as a likely player in inflammatory disorders and IBDs.

SUMMARY

Improved knowledge and appreciation of the roles that the lymphatic system plays in immune cell trafficking, infection, fat transport, distribution and metabolism and, of course, edema resolution is necessary to better understand the pathogenesis of chronic inflammatory conditions such as Crohn's disease and may provide the basis for new therapeutic strategies.

Bronchus-associated lymphoid tissue (BALT) structure and function

Bronchus-associated lymphoid tissue (BALT) is a constitutive mucosal lymphoid tissue adjacent to major airways in some mammalian species, including rats and rabbits, but not humans or mice. A related tissue, inducible BALT (iBALT), is an ectopic lymphoid tissue that is formed upon inflammation or infection in both mice and humans and can be found throughout the lung. Both BALT and iBALT acquire antigens from the airways and initiate local immune responses and maintain memory cells in the lungs. Here, we discuss the development and function of BALT and iBALT in the context of pulmonary immunity to infectious agents, tumors, and allergens as well as autoimmunity and inflammatory diseases of the lung.

Salivary glands act as mucosal inductive sites via the formation of ectopic germinal centers after site-restricted MCMV infection

We investigated the hypothesis that salivary gland inoculation stimulates formation of ectopic germinal centers (GCs), transforming the gland into a mucosal inductive site. Intraglandular infection of mice with murine cytomegalovirus (MCMV; control: UV-inactivated MCMV) induces salivary gland ectopic follicles comprising cognate interactions between CD4(+) and B220(+) lymphocytes, IgM(+) and isotype-switched IgG(+) and IgA(+) B cells, antigen presenting cells, and follicular dendritic cells. B cells coexpressed the GC markers GCT (57%) and GL7 (52%), and bound the lectin peanut agglutinin. Lymphoid follicles were characterized by a 2- to 3-fold increase in mRNA for CXCL13 (lymphoid neogenesis), syndecan-1 (plasma cells), Blimp-1 (plasma cell development/differentiation), and a 2- to 6-fold increase for activation-induced cytidine deaminase, PAX5, and the nonexcised rearranged DNA of an IgA class-switch event, supporting somatic hypermutation and class-switch recombination within the salivary follicles. Intraglandular inoculation also provided protection against a systemic MCMV challenge, as evidenced by decreased viral titers (10(5) plaque-forming units to undetectable), and restoration of normal salivary flow rates from a 6-fold decrease. Therefore, these features suggest that the salivary gland participates in oral mucosal immunity via generation of ectopic GCs, which function as ectopic mucosal inductive sites.

The interstitial lymphatic peritoneal mesothelium axis in portal hypertensive ascites: when in danger, go back to the sea

Portal hypertension induces a splanchnic and systemic low-grade inflammatory response that could induce the expression of three phenotypes, named ischemia-reperfusion, leukocytic, and angiogenic phenotypes.During the splanchnic expression of these phenotypes, interstitial edema, increased lymph flow, and lymphangiogenesis are produced in the gastrointestinal tract. Associated liver disease increases intestinal bacterial translocation, splanchnic lymph flow, and induces ascites and hepatorenal syndrome. Extrahepatic cholestasis in the rat allows to study the worsening of the portal hypertensive syndrome when associated with chronic liver disease. The splanchnic interstitium, the mesenteric lymphatics, and the peritoneal mesothelium seem to create an inflammatory pathway that could have a key pathophysiological relevance in the production of the portal hypertension syndrome complications. The hypothetical comparison between the ascitic and the amniotic fluids allows for translational investigation. From a phylogenetic point of view, the ancestral mechanisms for amniotic fluid production were essential for animal survival out of the aquatic environment. However, their hypothetical appearance in the cirrhotic patient is considered pathological since ultimately they lead to ascites development. But, the adult human being would take advantage of the potential beneficial effects of this “amniotic-like fluid” to manage the interstitial fluids without adverse effects when chronic liver disease aggravates.

Role of lymphatic vessels in tumor immunity: passive conduits or active participants?

Research in lymphatic biology and cancer immunology may soon intersect as emerging evidence implicates the lymphatics in the progression of chronic inflammation and autoimmunity as well as in tumor metastasis and immune escape. Like the blood vasculature, the lymphatic system comprises a highly dynamic conduit system that regulates fluid homeostasis, antigen transport and immune cell trafficking, which all play important roles in the progression and resolution of inflammation, autoimmune diseases, and cancer. This review presents emerging evidence that lymphatic vessels are active modulators of immunity, perhaps fine-tuning the response to adjust the balance between peripheral tolerance and immunity. This suggests that the tumor-associated lymphatic vessels and draining lymph node may be important in tumor immunity which in turn governs metastasis.

Review article: lymphatic system and associated adipose tissue in the development of inflammatory bowel disease

BACKGROUND

The lymphatic system plays critical roles in tissue fluid homoeostasis, immune defence and metabolic maintenance. Lymphatic vessels transport lymph, proteins, immune cells and digested lipids, allowing fluid and proteins to be returned to the blood stream, lipids to be stored and metabolized and antigens to be sampled in lymph nodes. Lymphatic drainage is mainly driven by rhythmic constrictions intrinsic to the vessels and critically modulated by fluid pressure and inflammatory mediators.

AIM

To collect and discuss the compelling available information linking the lymphatic system, adiposity and inflammation.

METHODS

A literature search was performed through PubMed focusing on lymphatic system, inflammation, immune cells and fat transport and function in the context of IBD.

RESULTS

Evidence collected allows us to propose the following working model. Compromised lymph drainage, reported in IBD, leads to oedema, lymphangiogenesis, impaired immune cell trafficking and lymph leakage. Lymph factor(s) stimulate adipose tissue to proliferate and produce cytokines, which affect immune cell functions and exacerbate inflammation.

CONCLUSIONS

Understanding the lymphatic system's role in immune cell trafficking and immune responses, contribution to fat transport, distribution, metabolism and implication in the pathogenesis of chronic intestinal inflammation may provide the basis for new therapeutic strategies and improved quality-of life.

Immune responses elicited in tertiary lymphoid tissues display distinctive features

During chronic inflammation, immune effectors progressively organize themselves into a functional tertiary lymphoid tissue (TLT) within the targeted organ. TLT has been observed in a wide range of chronic inflammatory conditions but its pathophysiological significance remains unknown. We used the rat aortic interposition model in which a TLT has been evidenced in the adventitia of chronically rejected allografts one month after transplantation. The immune responses elicited in adventitial TLT and those taking place in spleen and draining lymph nodes (LN) were compared in terms of antibody production, T cell activation and repertoire perturbations. The anti-MHC humoral response was more intense and more diverse in TLT. This difference was associated with an increased percentage of activated CD4+ T cells and a symmetric reduction of regulatory T cell subsets. Moreover, TCR repertoire perturbations in TLT were not only increased and different from the common pattern observed in spleen and LN but also “stochastic,” since each recipient displayed a specific pattern. We propose that the abnormal activation of CD4+ T cells promotes the development of an exaggerated pathogenic immune humoral response in TLT. Preliminary findings suggest that this phenomenon i) is due to a defective immune regulation in this non-professional inflammatory-induced lymphoid tissue, and ii) also occurs in human chronically rejected grafts.

Chronic rejection triggers the development of an aggressive intragraft immune response through recapitulation of lymphoid organogenesis

The unwarranted persistence of the immunoinflammatory process turns this critical component of the body's natural defenses into a destructive mechanism, which is involved in a wide range of diseases, including chronic rejection. Performing a comprehensive analysis of human kidney grafts explanted because of terminal chronic rejection, we observed that the inflammatory infiltrate becomes organized into an ectopic lymphoid tissue, which harbors the maturation of a local humoral immune response. Interestingly, intragraft humoral immune response appeared uncoupled from the systemic response because the repertoires of locally produced and circulating alloantibodies only minimally overlapped. The organization of the immune effectors within adult human inflamed tissues recapitulates the biological program recently identified in murine embryos during the ontogeny of secondary lymphoid organs. When this recapitulation was incomplete, intragraft B cell maturation was impeded, limiting the aggressiveness of the local humoral response. Identification of the molecular checkpoints critical for completion of the lymphoid neogenesis program should help develop innovative therapeutic strategies to fight chronic inflammation.

Does plasminogen activator inhibitor-1 drive lymphangiogenesis?

The purpose of this study is to explore the function of plasminogen activator inhibitor-1 (PAI-1) during pathological lymphangiogenesis. PAI-1, the main physiological inhibitor of plasminogen activators is involved in pathological angiogenesis at least by controlling extracellular proteolysis and by regulating endothelial cell survival and migration. Protease system's role in lymphangiogenesis is unknown yet. Thus, based on its important pro-angiogenic effect, we hypothesized that PAI-1 may regulate lymphangiogenesis associated at least with metastatic dissemination of cancer cells. To address this issue, we studied the impact of PAI-1 deficiency in various murine models of tumoral lymphangiogenesis. Wild-type PAI-1 proficient mice were used as controls. We provide for the first time evidence that PAI-1 is dispensable for tumoral lymphangiogenesis associated with breast cancers either induced by mammary carcinoma cell injection or spontaneously appearing in transgenic mice expressing the polyomavirus middle T antigen (PymT) under the control of a mouse mammary tumor virus long-terminal repeat promoter (MMTV-LTR). We also investigated inflammation-related lymphatic vessel recruitment by using two inflammatory models. PAI-1 deficiency did neither affect the development of lymphangioma nor burn-induced corneal lymphangiogenesis. These novel data suggest that vascular remodelling associated with lymphangiogenesis and angiogenesis involve different molecular determinants. PAI-1 does not appear as a potential therapeutic target to counteract pathological lymphangiogenesis.

Differential expression of proteoglycans in tissue remodeling and lymphangiogenesis after experimental renal transplantation in rats

Background

Chronic transplant dysfunction explains the majority of late renal allograft loss and is accompanied by extensive tissue remodeling leading to transplant vasculopathy, glomerulosclerosis and interstitial fibrosis. Matrix proteoglycans mediate cell-cell and cell-matrix interactions and play key roles in tissue remodeling. The aim of this study was to characterize differential heparan sulfate proteoglycan and chondroitin sulfate proteoglycan expression in transplant vasculopathy, glomerulosclerosis and interstitial fibrosis in renal allografts with chronic transplant dysfunction.

Methods

Renal allografts were transplanted in the Dark Agouti-to-Wistar Furth rat strain combination. Dark Agouti-to-Dark Agouti isografts and non-transplanted Dark Agouti kidneys served as controls. Allograft and isograft recipients were sacrificed 66 and 81 days (mean) after transplantation, respectively. Heparan sulfate proteoglycan (collXVIII, perlecan and agrin) and chondroitin sulfate proteoglycan (versican) expression, as well as CD31 and LYVE-1 (vascular and lymphatic endothelium, respectively) expression were (semi-) quantitatively analyzed using immunofluorescence.

Findings

Arteries with transplant vasculopathy and sclerotic glomeruli in allografts displayed pronounced neo-expression of collXVIII and perlecan. In contrast, in interstitial fibrosis expression of the chondroitin sulfate proteoglycan versican dominated. In the cortical tubular basement membranes in both iso- and allografts, induction of collXVIII was detected. Allografts presented extensive lymphangiogenesis (p<0.01 compared to isografts and non-transplanted controls), which was associated with induced perlecan expression underneath the lymphatic endothelium (p<0.05 and p<0.01 compared to isografts and non-transplanted controls, respectively). Both the magnitude of lymphangiogenesis and perlecan expression correlated with severity of interstitial fibrosis and impaired graft function.

Interpretation

Our results reveal that changes in the extent of expression and the type of proteoglycans being expressed are tightly associated with tissue remodeling after renal transplantation. Therefore, proteoglycans might be potential targets for clinical intervention in renal chronic transplant dysfunction.

Ectopic lymphoid tissues and local immunity

Ectopic or tertiary lymphoid tissues develop at sites of inflammation or infection in peripheral, non-lymphoid organs. These tissues are architecturally similar to conventional secondary lymphoid organs, with separated B and T cell areas, specialized populations of dendritic cells, well-differentiated stromal cells and high endothelial venules. Ectopic lymphoid tissues are often associated with the local pathology that results from chronic infection or chronic inflammation. However, there are also examples in which ectopic lymphoid tissues appear to contribute to local protective immune responses. Here we review how ectopic lymphoid structures develop and function in the context of local immunity and pathology.

Plasticity and heterogeneity of lymphoid organs. What are the criteria to call a lymphoid organ primary, secondary or tertiary?

Lymphoid organs are generally classified in a hierarchy with primary lymphoid organs such as the thymus and bone marrow for the production of receptor specific T and B lymphocytes, respectively, independent of antigens. In secondary lymphoid organs such as lymph nodes, spleen, and tonsils, the lymphocytes are expanded due to antigen exposure, producing memory T cells and effector B cells, resulting in plasma cells. Tertiary lymphoid tissues are often defined as aggregations of lymphoid cells in autoimmune diseases. It will be outlined that all these organs have a high plasticity and also the thymic medulla is included in the route of migrating mature T cells and the bone marrow, not only in the traffic of CD4+ but also of CD8+ lymphocytes. The mucosa-associated lymphoid organs depend to a much larger extent on microbial antigen and are much more diverse than often described. The role of structural elements as well as blood and lymphatic vessels as an entry and exit site of lymphocytes will be outlined. Using a precise terminology, taking account of the plasticity of these organs at different ages and considering species differences will reduce misunderstandings among immunologists.

How to control lymphangiogenesis: a novel role for rapamycin

Analysis of the lymphatic microvasculature has become possible only recently by the discovery of novel proteins specifically expressed in lymphatic endothelial cells only. Therapeutic manipulation of de novo lymphangiogenesis might become clinically relevant in the future in diverse situations, such as renal transplant rejection. In this issue Huber et al. demonstrate that rapamycin acts as an efficient inhibitor of lymphangiogenesis.

SPECT-CT: a valuable method to document the regeneration of lymphatics and autotransplanted lymph node fragments

PURPOSE

The transplantation of lymph node fragments and stimulation of lymph vessel regeneration seems to be a promising model to prevent lymphoedema in patients after mammary tumour excision combined with axillary lymph node dissection and irradiation. This study evaluated the advantages of SPECT-CT in studying the regeneration of lymph vessels and lymphoid tissue after autologous lymph node transplantation.

METHODS

Five minipigs underwent autologous lymph node transplantation in the left groin. The lymph node was excised, cut into six pieces and embedded into two newly created subcutaneous pouches on this side. The superficial lymph node of the right groin was removed as a control. Five months after surgery the lymph flow of both legs was investigated using conventional lymphoscintigraphy and SPECT-CT with 10 MBq(99m)Tc-nanocolloid in combination with Berlin Blue injected subcutaneously into the draining area.

RESULTS

The integration of the transplanted lymph node fragments was shown. The SPECT-CT results correlated with the in situ findings observed at dissection. Afferent and efferent lymph flow could be followed up to the lumbar trunks. The use of SPECT-CT allowed exact localisation of the lymph node fragments in three-dimensional space and the regeneration of the lymph node fragments was documented histologically.

CONCLUSION

SPECT-CT is a good method to evaluate lymphatic flow and document lymph node regeneration. The data suggest that autologous lymph node transplantation is a promising model for prevention of lymphoedema.

Topological determinants and consequences of adventitial responses to arterial wall injury

Arteries are composed of 3 concentric tissue layers which exhibit different structures and properties. Because arterial injury is generally initiated at the interface with circulating blood, most studies performed to unravel the mechanisms involved in injury-induced arterial responses have focused on the innermost layer (intima) rather than on the outermost adventitial layer. In the present review, we focus on the involvement of the adventitia in response to various types of arterial injury leading to vascular remodeling. Physiologically, soluble vascular mediators are centrifugally conveyed by mass transport toward the adventitia. Moreover, in pathological conditions, neomediators and antigens can be generated within the arterial wall, whose outward conveyance triggers different patterns of local adventitial response. Adventitial angiogenesis, immunoinflammation, and fibrosis sequentially interact and their net balance defines the participation of the adventitial response in arterial pathology. In the present review we discuss 4 pathological entities in which the adventitial response to arterial wall injury participates in arterial wall remodeling. Hence, the adventitial adaptive immune response predominates in chronic rejection. Inflammatory phagocytic cell recruitment and initiation of a shift from innate to adaptive immunity characterize the adventitial response to products of proteolysis in abdominal aortic aneurysm. Adventitial sprouting of neovessels, leading to intraplaque hemorrhages, predominates in atherothrombosis. Adventitial fibrosis characterizes the response to mechanical stress and is responsible for the constrictive remodeling of arterial segments and initiating interstitial fibrosis in perivascular tissues. These adventitial events, therefore, have an impact not only on the vessel wall biology but also on the surrounding tissue.