Claudio Fiocchi

Inflammatory bowel disease: Etiology and pathogenesis

The field of gastroenterology offers many challenges to both the clinician and the investigator, but few are as complex and enigmatic as inflammatory bowel disease (IBD). Crohn’s disease (CD) and ulcerative colitis (UC) have been known for well over one half a century, but why affected individuals spend their lives with a chronic inflammatory process that relentlessly destroys their bowel remains a mystery. Advances, although unequal in different areas, have been plentiful in the last decade when the combined power of cellular and molecular biology began to unveil the enigmas of IBD. No single agent or distinct mechanism is the sine qua non motive that explains all aspects of IBD, and several distinguishing factors are likely necessary to result in either CD or UC (Table 1). This review will attempt to discuss those that currently appear important.

Epithelial-derived IL-33 and its receptor ST2 are dysregulated in ulcerative colitis and in experimental Th1/Th2 driven enteritis.

IL-33 is a novel member of the IL-1 family and ligand for the IL-1 receptor-related protein, ST2. Recent evidence suggests that the IL-33/ST2 axis plays a critical role in several autoimmune and inflammatory disorders; however, its role in inflammatory bowel disease (IBD) has not been clearly defined. We characterized IL-33 and ST2 expression and modulation after conventional anti-TNF therapy in Crohn’s disease and ulcerative colitis (UC) patients and investigated the role of IL-33 in SAMP1/YitFc (SAMP) mice, a mixed Th1/Th2 model of IBD. Our results showed a specific increase of mucosal IL-33 in active UC, localized primarily to intestinal epithelial cells (IEC) and colonic inflammatory infiltrates. Importantly, increased expression of full-length IL-33, representing the most bioactive form, was detected in UC epithelium, whereas elevated levels of cleaved IL-33 were present in IBD serum. ST2 isoforms were differentially modulated in UC epithelium, and sST2, a soluble decoy receptor with anti-inflammatory properties, was also elevated in IBD serum. Infliximab (anti-TNF) treatment of UC decreased circulating IL-33 and increased sST2, whereas stimulation of HT-29 IEC confirmed IL-33 and sST2 regulation by TNF. Similarly, IL-33 significantly increased and correlated with disease severity, and potently induced IL-5, IL-6, and IL-17 from mucosal immune cells in SAMP mice. Taken together, the IL-33/ST2 system plays an important role in IBD and experimental colitis, is modulated by anti-TNF therapy, and may represent a specific biomarker for active UC.

Immunopathogenesis of IBD: current state of the art

IBD is a chronic inflammatory condition of the gastrointestinal tract encompassing two main clinical entities: Crohns disease and ulcerative colitis. Although Crohns disease and ulcerative colitis have historically been studied together because they share common features (such as symptoms, structural damage and therapy), it is now clear that they represent two distinct pathophysiological entities. Both Crohns disease and ulcerative colitis are associated with multiple pathogenic factors including environmental changes, an array of susceptibility gene variants, a qualitatively and quantitatively abnormal gut microbiota and a broadly dysregulated immune response. In spite of this realization and the identification of seemingly pertinent environmental, genetic, microbial and immune factors, a full understanding of IBD pathogenesis is still out of reach and, consequently, treatment is far from optimal. An important reason for this unsatisfactory situation is the currently limited comprehension of what are the truly relevant components of IBD immunopathogenesis. This article will comprehensively review current knowledge of the classic immune components and will expand the concept of IBD immunopathogenesis to include various cells, mediators and pathways that have not been traditionally associated with disease mechanisms, but that profoundly affect the overall intestinal inflammatory process.

Activated platelets are the source of elevated levels of soluble CD40 ligand in the circulation of inflammatory bowel disease patients

Background: The CD40/CD40L system, a key regulator and amplifier of immune reactivity, is activated in inflammatory bowel disease (IBD) mucosa. Aims: To determine whether plasma levels of sCD40L are elevated in Crohn’s disease (CD) and ulcerative colitis (UC) patients compared with normal controls, to investigate the cellular source of sCD40L, and to explore CD40L induction mechanisms. Patients: CD, UC, and normal control subjects were studied. Methods: The concentration of sCD40L in plasma and supernatants of freshly isolated platelets and autologous peripheral blood T cells (PBT) was measured by ELISA. Surface CD40L expression level was measured by flow cytometry in resting and thrombin activated platelets, and unstimulated and CD3/CD28 stimulated PBT before and after coculture with human intestinal microvascular endothelial cells (HIMEC). Results: Compared with normal controls, plasma sCD40L levels were significantly higher in both CD and UC patients and proportional to the extent of mucosal inflammation. Platelets from IBD patients displayed a significantly higher surface CD40L expression than those from control subjects, and released greater amounts of sCD40L than autologous PBT. Contact with IL-1β activated HIMEC induced significant upregulation of CD40L surface expression and release by platelets. Conclusions: Elevated levels of sCD40L in the circulation of IBD patients reflect enhanced surface expression and release of CD40L by platelets. This phenomenon translates to an increased platelet activation state apparently induced by passage through an inflamed mucosal microvascular bed, a conclusion supported by the positive correlation of plasma sCD40L levels with the extent of anatomical involvement by IBD. These results suggest that platelet-endothelial interactions critically contribute to activation of the CD40 pathway in IBD.

Intestinal inflammation: a complex interplay of immune and nonimmune cell interactions

Intestinal inflammation has traditionally been viewed as a process in which effector immune cells cause the destruction of other mucosal cells that behave as passive bystander targets. Progress in understanding the process of intestinal inflammation has led to a much broader and more integrated picture of the various mucosal components, a picture in which cytokines, growth factors, adhesion molecules, and the process of apoptosis act as functional mediators. Essentially all cellular and acellular components can exert immunelike activities, modifying the classical concept of selected immune cells acting on all other cells that has been the dogma of immunologically mediated tissue damage for decades. The existence of specialized communication pathways between epithelial cells and T cells is well documented, including abnormal epithelial cell-mediated T cell activation during inflammation. Mesenchymal cells contribute to fibrosis in the inflamed gut but are also responsible for retention and survival of leukocytes in the mucosa. In chronically inflamed intestine the local microvasculature displays leukocyte hyperadhesiveness, a phenomenon that probably contributes to persistence of inflammation. The extracellular matrix regulates the number, location, and activation of leukocytes, while metalloproteinases regulate the quantity and type of deposited matrix proteins. This evidence from the intestinal system, consolidated with the use of data from other organs and systems, reveals a rich network of reciprocal and finely orchestrated interactions among immune, epithelial, endothelial, mesenchymal, and nerve cells and the extracellular matrix. Although these interactions occur under normal conditions, the dysfunction of any component of this highly integrated mucosal system may lead to a disruption in communication and result in pathological inflammation.

VEGF-A links angiogenesis and inflammation in inflammatory bowel disease pathogenesis.

BACKGROUND & AIMS Vascular endothelial growth factor A (VEGF-A) mediates angiogenesis and might also have a role in inflammation and immunity. We examined whether VEGF-A signaling has a role in inflammatory bowel disease (IBD). METHODS Expression levels of VEGF-A, and its receptors VEGFR-1 and VEGFR-2, were examined in samples from patients with IBD and compared with those of controls. The capacity of VEGF-A to induce angiogenesis was tested in human intestinal microvascular endothelial cells using cell-migration and matrigel tubule-formation assays. Levels of vascular cellular adhesion molecule-1 and intercellular adhesion molecule were measured by flow cytometry to determine induction of inflammation; neutrophil adhesion was also assayed. Expression patterns were determined in tissues from mice with dextran sulfate sodium (DSS)-induced colitis; the effects of VEGF-A overexpression and blockade were assessed in these mice by adenoviral transfer of VEGF-A and soluble VEGFR-1. Intestinal angiogenesis was measured by quantitative CD31 staining and leukocyte adhesion in vivo by intravital microscopy. RESULTS Levels of VEGF-A and VEGFR-2 increased in samples from patients with IBD and colitic mice. VEGF-A induced angiogenesis of human intestinal microvascular endothelial cells in vitro as well as an inflammatory phenotype and adherence of neutrophils to intestinal endothelium. Overexpression of VEGF-A in mice with DSS-induced colitis worsened their condition, whereas overexpression of soluble VEGFR-1 had the opposite effect. Furthermore, overexpression of VEGF-A increased mucosal angiogenesis and stimulated leukocyte adhesion in vivo. CONCLUSIONS VEGF-A appears to be a novel mediator of IBD by promoting intestinal angiogenesis and inflammation. Agents that block VEGF-A signaling might reduce intestinal inflammation in patients with IBD.

Immune Regulation by Microvascular Endothelial Cells: Directing Innate and Adaptive Immunity, Coagulation, and Inflammation

An effective immune response depends not only on the proper activation, regulation, and function of immune cells, but also on their distribution and retention in diverse tissue microenvironments where they encounter a number of stimuli and other cell types. These activities are mediated by endothelial cells, which form specialized microcirculatory networks used by immune cells under both physiological and pathological circumstances. Endothelial cells represent a highly heterogeneous population of cells with the ability to interact with and modulate the function of immune cells. This review is focused on the role of microvascular endothelial cells in innate and adaptive immunity, inflammation, coagulation, angiogenesis, and the therapeutic implications of targeting endothelial cells in selected autoimmune and chronic inflammatory disorders.

Platelets trigger a CD40-dependent inflammatory response in the microvasculature of inflammatory bowel disease patients

BACKGROUND & AIMS Platelets circulate in an activated state in patients with inflammatory bowel disease (IBD), but their role in the pathogenesis of IBD is unclear. The recent demonstration that activated platelets express CD40 ligand (L) provides a mechanism of interaction with CD40-positive endothelial cells, inducing them to produce proinflammatory mediators. We investigated whether platelets from patients with IBD express enhanced levels of CD40L and induce human intestinal microvascular endothelial cells (HIMEC) to up-regulate cell adhesion molecule (CAM) expression and secrete chemokines. METHODS CD40L expression was assessed in resting and thrombin-activated platelets by flow cytometry and in mucosal microthrombi by confocal microscopy. Platelet-HIMEC cocultures were used to study CAM up-regulation, and interleukin (IL)-8 and RANTES production by HIMEC. RESULTS IBD platelets expressed significantly higher CD40L levels than those of healthy subjects, and CD40L-positive platelets were detected in IBD-involved mucosa. Activated platelets up-regulated expression of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 as well as production of interleukin 8 by HIMEC in a CD40-dependent fashion. High levels of RANTES were present in platelet-HIMEC cocultures and platelets were identified as the source of this chemokine, which mediated T-cell adhesion to HIMEC. CONCLUSIONS These results show that platelets can actively contribute to mucosal inflammation and represent a previously unrecognized component of IBD pathogenesis.

Intestinal fibrosis in IBD—a dynamic, multifactorial process

Intestinal fibrosis is a common and potentially serious complication of IBD that results from the reaction of intestinal tissue to the damage inflicted by chronic inflammation. The traditional view that fibrosis is inevitable or irreversible in patients with IBD is progressively changing in light of improved understanding of the cellular and molecular mechanisms that underlie the pathogenesis of fibrosis in general, and, in particular, intestinal fibrosis. These mechanisms are complex and dynamic, and involve multiple cell types, interconnected cellular events and a large number of soluble factors. In addition, owing to a breakdown of the epithelial barrier during inflammation of the gut, luminal bacterial products induce an innate immune response, which is triggered by activation of immune and nonimmune cells alike. Comprehension of the mechanisms of intestinal fibrosis will create a conceptual and practical framework that could achieve the specific blockade of fibrogenic pathways, allow for the estimation of risk of fibrotic complications, permit the detection of early fibrotic changes and, eventually, enable the development of treatments customized to the type and stage of each patients IBD.

Localization of intestinal interleukin 1 activity and protein and gene expression to lamina propria cells

BACKGROUND Interleukin 1 (IL-1) is a key mediator of bowel inflammation, but there is limited knowledge about the amount and site of production of this cytokine in the gastrointestinal tract under physiological or pathological conditions. METHODS Epithelial and lamina propria mononuclear cells were isolated from control, and Crohns disease- and ulcerative colitis-involved mucosa to investigate the capacity of these cells to generate IL-1 bioactivity, IL-1 alpha and IL-1 beta immunoreactivity, and gene expression. RESULTS Control lamina propria mononuclear cells produced substantial amounts of IL-1 alpha and IL-1 beta, which increased dramatically when inflammatory bowel disease cells were used. Epithelial cells from control, Crohns disease, and ulcerative colitis intestine displayed no IL-1 bioactivity or immunoreactivity. Lamina propria mononuclear cells contained moderate to large quantities of IL-1 alpha and IL-1 beta messenger RNA (mRNA), respectively, whereas epithelial cells had none. The absence of IL-1 transcripts in epithelial cells was selective, because mRNA for HLA-DR antigens was present in control and inflammatory bowel disease cells. CONCLUSIONS In normal and inflamed human intestine there is a distinct compartmentalization of IL-1, as mononuclear but not epithelial cells generate this cytokine. The high levels of IL-1 in inflammatory bowel disease may explain several of its local and systemic manifestations, and blockade by specific antagonists could have important therapeutic effects.