Lymphatic function is regulated by a coordinated expression of lymphangiogenic and anti-lymphangiogenic cytokines

Lymphangiogenic cytokines such as vascular endothelial growth factor-C (VEGF-C) are critically required for lymphatic regeneration; however, in some circumstances, lymphatic function is impaired despite normal or elevated levels of these cytokines. The recent identification of anti-lymphangiogenic molecules such as interferon-γ (IFN-γ), transforming growth factor-β1, and endostatin has led us to hypothesize that impaired lymphatic function may represent a dysregulated balance in the expression of pro/anti-lymphangiogenic stimuli. We observed that nude mice have significantly improved lymphatic function compared with wild-type mice in a tail model of lymphedema. We show that gradients of lymphatic fluid stasis regulate the expression of lymphangiogenic cytokines (VEGF-A, VEGF-C, and hepatocyte growth factor) and that paradoxically the expression of these molecules is increased in wild-type mice. More importantly, we show that as a consequence of T-cell-mediated inflammation, these same gradients also regulate expression patterns of anti-lymphangiogenic molecules corresponding temporally and spatially with impaired lymphatic function in wild-type mice. We show that neutralization of IFN-γ significantly increases inflammatory lymph node lymphangiogenesis independently of changes in VEGF-A or VEGF-C expression, suggesting that alterations in the balance of pro- and anti-lymphangiogenic cytokine expression can regulate lymphatic vessel formation. In conclusion, we show that gradients of lymphatic fluid stasis regulate not only the expression of pro-lymphangiogenic cytokines but also potent suppressors of lymphangiogenesis as a consequence of T-cell inflammation and that modulation of the balance between these stimuli can regulate lymphatic function.     

Nerve Growth Factor Regulates Neurolymphatic Remodeling during Corneal Inflammation and Resolution

The cellular and physiologic mechanisms that regulate the resolution of inflammation remain poorly defined despite their widespread importance in improving inflammatory disease outcomes. We studied the resolution of two cardinal signs of inflammation–pain and swelling–by investigating molecular mechanisms that regulate neural and lymphatic vessel remodeling during the resolution of corneal inflammation. A mouse model of corneal inflammation and wound recovery was developed to study this process in vivo. Administration of nerve growth factor (NGF) increased pain sensation and inhibited neural remodeling and lymphatic vessel regression processes during wound recovery. A complementary in vivo approach, the corneal micropocket assay, revealed that NGF-laden pellets stimulated lymphangiogenesis and increased protein levels of VEGF-C. Adult human dermal lymphatic endothelial cells did not express canonical NGF receptors TrkA and p75^NTR or activate downstream MAPK- or Akt-pathway effectors in the presence of NGF, although NGF treatment increased their migratory and tubulogenesis capacities in vitro. Blockade of the VEGF-R2/R3 signaling pathway ablated NGF-mediated lymphangiogenesis in vivo. These findings suggest a hierarchical relationship with NGF functioning upstream of the VEGF family members, particularly VEGF-C, to stimulate lymphangiogenesis. Taken together, these studies show that NGF stimulates lymphangiogenesis and that NGF may act as a pathogenic factor that negatively regulates the normal neural and lymphatic vascular remodeling events that accompany wound recovery.     

Th2 Cytokines Inhibit Lymphangiogenesis

Lymphangiogenesis is the process by which new lymphatic vessels grow in response to pathologic stimuli such as wound healing, inflammation, and tumor metastasis. It is well-recognized that growth factors and cytokines regulate lymphangiogenesis by promoting or inhibiting lymphatic endothelial cell (LEC) proliferation, migration and differentiation. Our group has shown that the expression of T-helper 2 (Th2) cytokines is markedly increased in lymphedema, and that these cytokines inhibit lymphatic function by increasing fibrosis and promoting changes in the extracellular matrix. However, while the evidence supporting a role for T cells and Th2 cytokines as negative regulators of lymphatic function is clear, the direct effects of Th2 cytokines on isolated LECs remains poorly understood. Using in vitro and in vivo studies, we show that physiologic doses of interleukin-4 (IL-4) and interleukin-13 (IL-13) have profound anti-lymphangiogenic effects and potently impair LEC survival, proliferation, migration, and tubule formation. Inhibition of these cytokines with targeted monoclonal antibodies in the cornea suture model specifically increases inflammatory lymphangiogenesis without concomitant changes in angiogenesis. These findings suggest that manipulation of anti-lymphangiogenic pathways may represent a novel and potent means of improving lymphangiogenesis.     

Inflammatory triggers of lymphangiogenesis

Inflammation is the common denominator to the postnatal events that overlap with lymphatic vessel growth, or lymphangiogenesis. Undoubtedly, inflammation and accompanying fluid overload are cardinal factors in wound healing, lymphedema, the pathogenesis of some forms of lymphangiomatosis, and solid tumor lymphangiogenesis. The assertion that inflammation actually triggers lymphangiogenesis lies in the evidence set forth below that inflammation is the usual precursor to tissue repair and regeneration. Moreover, the panel of pro-inflammatory and anti-inflammatory molecules that orchestrates the inflammatory response abounds with cytokines and chemokines that foster survival, migration, and proliferation of lymphatic endothelial cells. Finally, both interstitial fluid overload and increased demand for removal of leukocytes can benefit from lymphangiogenesis, although the mechanisms controlling the exit of leukocytes from tissues via the lymphatics are practically unknown. The pertinent question actually is how and why inflammation presents with formation of new lymph vessels in liver fibrosis but not in rheumatoid arthritis. One possible explanation is that organ-specific histological and functional properties of the lymphatic endothelium gauge their response to death, survival, and proliferative factors. Alternatively, the decision to remain quiescent, proliferate or regress resides within the stroma microenvironment.     

Th17-related cytokines in inflammatory bowel diseases: friends or foes?

T helper (Th)17 cells and other interleukin (IL)-17-producing cells are supposed to play critical roles in several human immune-mediated diseases, including Crohn's disease (CD) and ulcerative colitis (UC), the main forms of inflammatory bowel diseases (IBD) in man. Th17 cells infiltrate massively the inflamed intestine of IBD patients and in vitro and in vivo studies have shown that Th17-type cytokines may trigger and amplify multiple inflammatory pathways. Nonetheless, some Th17-related cytokines, such as interleukin (IL)-17A and IL-22, may target gut epithelial cells and promote the activation of counter-regulatory mechanisms. This observation together with the demonstration that Th17 cells are not stable and can be converted into either regulatory T cells or Th1 cells if stimulated by immune-suppressive (e.g. TGF-β1) or inflammatory (e.g. IL-12, IL-23) cytokines have contributed to advance our understanding of mechanisms that regulate mucosal homeostasis and inflammation in the gut.     

白介素23 及Th17 细胞在炎症性肠病的作用

炎症性肠病(Inflammatory Bowel Diseases,IBD),是一组病因未明的累及胃肠道的慢性炎症性疾病,一般指克罗恩病(Crohn’sdisease,CD)和溃疡性结肠炎(ulcerative colitis,UC)。目前认为它是由多种因素相互作用所致的一种自身免疫性疾病,主要包括免疫、环境以及遗传等因素,其中免疫在IBD的发生过程中起着极其重要的作用。以往研究认为与T辅助细胞(T Helper cells)Th1或Th2细胞反应的增强或减弱有关。然而最近研究发现一类新细胞亚群,称为Th17细胞,与之相关的细胞因子可导致包括肠道在内的多脏器病变。Th17细胞分化过程中又需要IL-23的参与,因此IL-23/Th17细胞在炎症性肠病患者肠道内过度表达可以解释肠组织损伤的新途径,并为制定新的治疗策略提出依据。本文就IL-23/Th17轴在炎症性肠病中的作用的研究进展作一综述。     

Vascular development in early human embryos and in teratomas derived from human embryonic stem cells

During early human embryonic development, blood vessels are stimulated to grow, branch, and invade developing tissues and organs. Pluripotent human embryonic stem cells (hESCs) are endowed with the capacity to differentiate into cells of blood and lymphatic vessels. The present study aimed to follow vasculogenesis during the early stages of developing human vasculature and to examine whether human neovasculogenesis within teratomas generated in SCID mice from hESCs follows a similar course and can be used as a model for the development of human vasculature. Markers and gene profiling of smooth muscle cells and endothelial cells of blood and lymphatic vessels were used to follow neovasculogenesis and lymphangiogenesis in early developing human embryos (4-8 weeks) and in teratomas generated from hESCs. The involvement of vascular smooth muscle cells in the early stages of developing human embryonic blood vessels is demonstrated, as well as the remodeling kinetics of the developing human embryonic blood and lymphatic vasculature. In teratomas, human vascular cells were demonstrated to be associated with developing blood vessels. Processes of intensive remodeling of blood vessels during the early stages of human development are indicated by the upregulation of angiogenic factors and specific structural proteins. At the same time, evidence for lymphatic sprouting and moderate activation of lymphangiogenesis is demonstrated during these developmental stages. In the teratomas induced by hESCs, human angiogenesis and lymphangiogenesis are relatively insignificant. The main source of blood vessels developing within the teratomas is provided by the murine host. We conclude that the teratoma model has only limited value as a model to study human neovasculogenesis and that other in vitro methods for spontaneous and guided differentiation of hESCs may prove more useful.     

HIF-1α coordinates lymphangiogenesis during wound healing and in response to inflammation

This study aimed to investigate the mechanisms that coordinate lymphangiogenesis. Using mouse models of lymphatic regeneration and inflammatory lymphangiogenesis, we explored the hypothesis that hypoxia inducible factor-α (HIF-1α) is a central regulator of lymphangiogenesis. We show that HIF-1α inhibition by small molecule inhibitors (YC-1 and 2-methyoxyestradiol) results in delayed lymphatic repair, decreased local vascular endothelial growth factor-C (VEGF-C) expression, reduced numbers of VEGF-C(+) cells, and reductions in inflammatory lymphangiogenesis. Using transgenic HIF-1α/luciferase mice to image HIF-1α expression in real time in addition to Western blot analysis and pimonidazole staining for cellular hypoxia, we demonstrate that hypoxia stabilizes HIF-1α during initial stages of wound repair (1-2 wk); whereas inflammation secondary to gradients of lymphatic fluid stasis stabilizes HIF-1α thereafter (3-6 wk). In addition, we show that CD4(+) cell-mediated inflammation is necessary for this response and regulates HIF-1α expression by macrophages, as CD4-deficient or CD4-depleted mice demonstrate 2-fold reductions in HIF-1α expression as compared to wild-types. In summary, we show that HIF-1α is a critical coordinator of lymphangiogenesis by regulating the expression of lymphangiogenic cytokines as part of an early response mechanism to hypoxia, inflammation, and lymphatic fluid stasis.     

Granulomatous Inflammation in Tuberculosis and Sarcoidosis: Does the Lymphatic System Contribute to Disease?

A striking and unexplained feature of granulomatous inflammation is its anatomical association with the lymphatic system. Accumulating evidence suggests that lymphatic tracks and granulomas may alter the function of each other. The formation of new lymphatics, or lymphangiogenesis, is an adaptive response to tumor formation, infection, and wound healing. Granulomas also may induce lymphangiogenesis which, through a variety of mechanisms, could contribute to disease outcomes in tuberculosis and sarcoidosis. On the other hand, alterations in lymph node function and lymphatic draining may be primary events which attenuate the risk and severity of granulomatous inflammation. This review begins with an introduction of granulomatous inflammation and the lymphatic system. A role of the lymphatic system in tuberculosis and sarcoidosis is then hypothesized. With a focus on lymphangiogenesis in these diseases, and on the potential for this process to promote dissemination, parallels are established with the well‐established role of lymphangiogenesis in tumor biology.     

Molecular control of lymphangiogenesis

The lymphatic vasculature plays a critical role in the regulation of body fluid volume and immune function. Extensive research into the molecular mechanisms that control blood vessel growth has led to identification of molecules that also regulate development and growth of the lymphatic vessels. This is generating a great deal of interest in the molecular control of the lymphatics in the context of embryogenesis, lymphatic disorders and tumor metastasis. Studies in animal models carried out over the past three years have shown that the soluble protein growth factors, vascular endothelial growth factor (VEGF)-C and VEGF-D, and their cognate receptor tyrosine kinase, VEGF receptor-3 (VEGFR-3), are critical regulators of lymphangiogenesis. Furthermore, disfunction of VEGFR-3 has recently been shown to cause lymphedema. The capacity to induce lymphangiogenesis by manipulation of the VEGF-C/VEGF-D/VEGFR-3 signaling pathway offers new opportunities to understand the function of the lymphatic system and to develop novel treatments for lymphatic disorders.     

The opposing roles of IL-21 and TGFβ1 in chronic inflammatory bowel disease

There are large numbers of T-cells in the mucosa of the intestine in healthy individuals. The stimulus for their presence is the normal gut microbiota. For unknown reasons, in patients with IBD (inflammatory bowel disease), there is inappropriate and chronic activation of mucosal T-cells which leads to gut damage and severe morbidity. In one form of IBD, namely Crohn's disease, the T-cells are probably responding to the microbiota. T-cell survival in the gut wall is dependent on pro-inflammatory cytokines and antibody-mediated inhibition of one of these cytokines, TNFα (tumour necrosis factor α), has shown efficacy in patients, thus encouraging investigations of other ways to control mucosal T-cell responses. In the present paper, we give a brief review of T-cell immunology in IBD and then discuss how two particular cytokines, namely IL-21 (interleukin 21), which is generally pro-inflammatory and important in gut T-cell survival and in maintaining Th17 cells, and TGFβ1 (transforming growth factor β1), which is generally immunosuppressive, play opposing roles in gut inflammation.     

Lymphatic pump function in the inflamed gut

The role of the lymphatic circulation to actively remove fluid, cells, proteins, and other particles from the interstitium to prevent mounting edema is well appreciated, but whether and how this function is compromised during inflammation has been scarcely investigated. We discuss here the mechanisms of lymphatic pumping and their modulation in inflammatory conditions or by inflammatory mediators in the context of inflammatory bowel disease (IBD), an ensemble of disorders typically described with abnormal or dysfunctional intestinal or mesenteric lymphatic vessels. We report our findings showing impaired mesenteric lymphatic contractile activity in an animal model of intestinal inflammation that recapitulates some features of IBD and suggests a role for prostanoids in this dysfunction. With the knowledge that prostaglandin E(2) and prostacyclin are implicated in IBD pathogenesis and induce a potent inhibition of lymphatic pumping, we established the pharmacological profile for these prostaglandin receptors in mesenteric lymphatic vessels and their respective role in pumping inhibition. Inhibition of mesenteric lymphatic pumping during inflammation may be a cause of edema, compromised immune response, and granuloma associated with IBD.     

Regulation of lymphangiogenesis by extracellular vesicles in cancer metastasis

Metastasis is not only one of the hallmarks of cancer but, unfortunately, it also is the most accurate biomarker for poor prognosis. Cancer cells metastasize through two different but eventually merged routes, the vasculature and lymphatic systems. The processes of cancer metastasis through blood vessel have been extensively studied and are well documented in the literature. In contrast, metastasis through the lymphatic system is less studied. Most people believe that cancer cells metastasize through lymphatic vessel are passive because the lymphatic system is thought to be a sewage draining system that collects whatever appears in the tissue fluid. It was recently found that cancer cells disseminated from lymphatic vessels are protected from being destroyed by our body’s defense system. Furthermore, some cancer cells or cancer-associated immune cells secrete lymphangiogenic factors to recruit lymphatic vessel infiltration to the tumor region, a process known as lymphangiogenesis. To ensure the efficiency of lymphangiogenesis, the lymphangiogenic mediators are carried or packed by nanometer-sized particles named extracellular vesicles. Extracellular vesicles are lipid bilayer particles released from eventually every single cell, including bacterium, with diameters ranging from 30 nm (exosome) to several micrometers (apoptotic body). Components carried by extracellular vesicles include but are not limited to DNA, RNA, protein, fatty acid, and other metabolites. Recent studies suggest that cancer cells not only secrete more extracellular vesicles but also upload critical mediators required for lymphatic metastasis onto extracellular vesicles. This review will summarize recent advances in cancer lymphatic metastasis and how cancer cells regulate this process via extracellular vesicle-dependent lymphangiogenesis.     

IBD disease-modifying therapies: insights from emerging therapeutics

Inflammatory bowel disease (IBD) pathogenesis is associated with gut mucosal inflammation, epithelial damage, and dysbiosis leading to a dysregulated gut mucosal barrier. However, the extent and underlying mechanisms remain largely unknown. Current treatment regimens have focused mainly on treating IBD symptoms; however, such treatment strategies do not address mucosal epithelial repair, barrier homeostasis, or intestinal dysbiosis. Although attempts have been made to identify new therapeutic modalities to enhance gut barrier functions, these are at an early developmental stage and have not been wholly successful. We review conventional therapies, the possible relevant role of gut barrier-protecting agents, and biomaterial strategies relating to combination therapies that may pave the way towards developing new therapeutic approaches for IBD.     

Vascular and lymphatic regulation of gastrointestinal function and disease risk

The vascular and lymphatic systems in the gut regulate lipid transport while restricting transfer of commensal gut microbiota and directing immune cell trafficking. Increased permeability of the endothelial systems in the intestine associates with passage of antigens and microbiota from the gut into the bloodstream leading to tissue inflammation, the release of pro-inflammatory mediators and ultimately to abnormalities of systemic metabolism. Recent studies show that lipid metabolism maintains homeostasis and function of intestinal blood and lymphatic endothelial cells, BECs and LECs, respectively. This review highlights recent progress in this area, and information related to the contribution of the lipid transporter CD36, abundant in BECs and LECs, to gastrointestinal barrier integrity, inflammation, and to gut regulation of whole body metabolism. The potential role of endothelial lipid delivery in epithelial tissue renewal after injury and consequently in the risk of gastric and intestinal diseases is also discussed.     

Macrophages define dermal lymphatic vessel calibre during development by regulating lymphatic endothelial cell proliferation

Macrophages have been suggested to stimulate neo-lymphangiogenesis in settings of inflammation via two potential mechanisms: (1) acting as a source of lymphatic endothelial progenitor cells via the ability to transdifferentiate into lymphatic endothelial cells and be incorporated into growing lymphatic vessels; and (2) providing a crucial source of pro-lymphangiogenic growth factors and proteases. We set out to establish whether cells of the myeloid lineage are important for development of the lymphatic vasculature through either of these mechanisms. Here, we provide lineage tracing evidence to demonstrate that lymphatic endothelial cells arise independently of the myeloid lineage during both embryogenesis and tumour-stimulated lymphangiogenesis in the mouse, thus excluding macrophages as a source of lymphatic endothelial progenitor cells in these settings. In addition, we demonstrate that the dermal lymphatic vasculature of PU.1(-/-) and Csf1r(-/-) macrophage-deficient mouse embryos is hyperplastic owing to elevated lymphatic endothelial cell proliferation, suggesting that cells of the myeloid lineage provide signals that act to restrain lymphatic vessel calibre in the skin during development. In contrast to what has been demonstrated in settings of inflammation, macrophages do not comprise the principal source of pro-lymphangiogenic growth factors, including VEGFC and VEGFD, in the embryonic dermal microenvironment, illustrating that the sources of patterning and proliferative signals driving embryonic and disease-stimulated lymphangiogenesis are likely to be distinct.     

Lymphangiogenesis is required for pancreatic islet inflammation and diabetes

Lymphangiogenesis is a common phenomenon observed during inflammation and engraftment of transplants, but its precise role in the immune response and underlying mechanisms of regulation remain poorly defined. Here we showed that in response to injury and autoimmunity, lymphangiogenesis occurred around islets and played a key role in the islet inflammation in mice. Vascular endothelial growth factors receptor 3 (VEGFR3) is specifically involved in lymphangiogenesis, and blockade of VEGFR3 potently inhibited lymphangiogenesis in both islets and the draining LN during multiple low-dose streptozotocin (MLDS) induced autoimmune insulitis, which resulted in less T cell infiltration, preservation of islets and prevention of the onset of diabetes. In addition to their well-known conduit function, lymphatic endothelial cells (LEC) also produced chemokines in response to inflammation. These LEC attracted two distinct CX3CR1(hi) and LYVE-1(+) macrophage subsets to the inflamed islets and CX3CR1(hi) cells were influenced by LEC to differentiate into LYVE-1(+) cells closely associated with lymphatic vessels. These observations indicate a linkage among lymphangiogenesis and myeloid cell inflammation during insulitis. Thus, inhibition of lymphangiogenesis holds potential for treating insulitis and autoimmune diabetes.     

CD4+ Cells Regulate Fibrosis and Lymphangiogenesis in Response to Lymphatic Fluid Stasis

Introduction

Lymphedema is a chronic disorder that occurs commonly after lymph node removal for cancer treatment and is characterized by swelling, fibrosis, inflammation, and adipose deposition. Although previous histological studies have investigated inflammatory changes that occur in lymphedema, the precise cellular make up of the inflammatory infiltrate remains unknown. It is also unclear if this inflammatory response plays a causal role in the pathology of lymphedema. The purpose of this study was therefore to characterize the inflammatory response to lymphatic stasis and determine if these responses are necessary for the pathological changes that occur in lymphedema.

Methods

We used mouse-tail lymphedema and axillary lymph node dissection (ANLD) models in order to study tissue inflammatory changes. Single cell suspensions were created and analyzed using multi-color flow cytometry to identify individual cell types. We utilized antibody depletion techniques to analyze the causal role of CD4+, CD8+, and CD25+ cells in the regulation of inflammation, fibrosis, adipose deposition, and lymphangiogenesis.

Results

Lymphedema in the mouse-tail resulted in a mixed inflammatory cell response with significant increases in T-helper, T-regulatory, neutrophils, macrophages, and dendritic cell populations. Interestingly, we found that ALND resulted in significant increases in T-helper cells suggesting that these adaptive immune responses precede changes in macrophage and dendritic cell infiltration. In support of this we found that depletion of CD4+, but not CD8 or CD25+ cells, significantly decreased tail lymphedema, inflammation, fibrosis, and adipose deposition. In addition, depletion of CD4+ cells significantly increased lymphangiogenesis both in our tail model and also in an inflammatory lymphangiogenesis model.

Conclusions

Lymphedema and lymphatic stasis result in CD4+ cell inflammation and infiltration of mature T-helper cells. Loss of CD4+ but not CD8+ or CD25+ cell inflammation markedly decreases the pathological changes associated with lymphedema. In addition, CD4+ cells regulate lymphangiogenesis during wound repair and inflammatory lymphangiogenesis.     

The resolution of lymphedema by interstitial flow in the mouse tail skin

Lymphangiogenesis is considered a promising approach for increasing fluid drainage during secondary lymphedema. However, organization of lymphatics into functional capillaries may be dependent upon interstitial flow (IF). The present study was undertaken to determine the importance of lymphangiogenesis for lymphedema resolution. We created a lymphatic obstruction that produces lymphedema in mouse tail skin. The relatively scar-free skin regeneration that occurred across the obstruction allowed the progression of lymphangiogenesis to be observed and compared with the evolution of lymphedema. The role of vascular endothelial growth factor-C (VEGF-C)/VEGF receptor (VEGFR)-3 signaling in lymphedema resolution was investigated by exogenous administration of VEGF-C or neutralizing antibodies against VEGFR-3. VEGF-C protein improved lymphedema at 15 days [reducing dermal thickness from 742 +/- 105 to 559 +/- 141 microm with 95% confidence intervals (CIs), P < 0.05] without increasing lymphatic capillary coverage (11.6 +/- 6.4% following VEGF-C treatment relative to 9.6 +/- 6.2% with 95% CIs, P > 0.50). Blocking VEGFR-3 signaling did not inhibit lymphedema resolution at 25 days (dermal thickness of 462 +/- 127 microm following VEGFR-3 inhibition relative to 502 +/- 87 microm with 95% CIs) or inhibit IF, although VEGFR-3 blocking prevented lymphangiogenesis (reducing lymphatic coverage to 0.2 +/- 0.7% relative to 8.7 +/- 7.3% with 95% CIs, P < 0.005). A second mouse tail lymphedema model was employed to investigate the ability of VEGF-C to increase fluid drainage across a scar. We found that neither neutralization of VEGFR-3 nor administration of VEGF-C affected the course of skin swelling over 25 days. These findings suggest that resolution of lymphedema in the mouse tail skin may be more dependent upon IF and regeneration of the extracellular matrix across the obstruction than lymphatic capillary regeneration.     

Role of reactive metabolites of oxygen and nitrogen in inflammatory bowel disease

The inflammatory bowel diseases (IBD; Crohn's disease, ulcerative colitis) are a collection of chronic idiopathic inflammatory disorders of the intestine and/or colon. Although the pathophysiology of IBD is not known with certainty, a growing body of experimental and clinical data suggests that chronic gut inflammation may result from a dysregulated immune response to normal bacterial antigens. This uncontrolled immune system activation results in the sustained overproduction of reactive metabolites of oxygen and nitrogen. It is thought that some of the intestinal and/or colonic injury and dysfunction observed in IBD is due to elaboration of these reactive species. This review summarizes the current state-of-knowledge of the role of reactive oxygen species and nitric oxide in the pathophysiology of IBD.