Inflammatory bowel disease (IBD) is a chronic disorder whose etiology is linked to triggering events, including viral infections, that lead to immunoregulatory dysfunction in genetically susceptible people. Characteristic pathological changes include increased mononuclear leukocyte influx into the intestinal mucosa as well as mucosal smooth muscle cell (M-SMC) hyperplasia. Virus infection or viral mimic [polyinosinic acid:polycytidylic acid (polyI:C)] treatment of human colon M-SMCs in vitro increases cell surface hyaluronan (HA), and nonactivated mononuclear leukocytes bind to virus-induced HA structures by interactions that involve the HA-binding receptor CD44. In this study, confocal microscopy reveals increased HA on poly I:C-treated M-SMC surfaces within 3 hours, arrayed in coat-like structures. By 17 hours, novel, lengthy cable structures are evident, and these are primarily responsible for mediating leukocyte adhesion. Immunohistochemical staining demonstrates components of the inter-alpha-trypsin inhibitor (IalphaI) complex in both coat-like and cable structures. M-SMCs co-treated with polyI:C and a polyclonal antibody to IalphaI display HA in coats but with diminished cables, and they bind significantly fewer leukocytes than M-SMCs treated with polyI:C alone. Western blot data suggest that heavy chains of IalphaI are specifically associated with cable structures. Staining of tissue sections from patients with IBD demonstrates the presence of HA in inflamed colon tissue, and shows that HA-associated IalphaI staining increases in the mucosa of inflamed IBD specimens compared to noninflamed sections from the same patient, establishing a probable link between the observations in vitro and the progression of the inflammatory process in IBD.

Hyaluronan (HA), a major component of the extracellular matrix (ECM), plays a key role in regulating inflammation. Inflammation is associated with accumulation and turnover of HA polymers by multiple cell types. Increasingly through the years, HA has become recognized as an active participant in inflammatory, angiogenic, fibrotic, and cancer promoting processes. HA and its binding proteins regulate the expression of inflammatory genes, the recruitment of inflammatory cells, the release of inflammatory cytokines, and can attenuate the course of inflammation, providing protection against tissue damage. A growing body of evidence suggests the cell responses are HA molecular weight dependent. HA fragments generated by multiple mechanisms throughout the course of inflammatory pathologies, elicit cellular responses distinct from intact HA. This review focuses on the role of HA in the promotion and resolution of inflammation.

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There is mounting evidence that perturbations in endoplasmic reticulum (ER) function play a key role in the pathogenesis of a broad range of diseases. We have examined the ability of ER stress to modulate leukocyte binding to colonic and aortic smooth muscle cells. In vitro, control smooth muscle cells bind few leukocytes, but treatment with compounds that induce ER stress, including tunicamycin, A23187, and thapsigargin, promotes leukocyte binding. Likewise, dextran sulfate, another agent capable of inducing ER stress and promoting inflammation in vivo, strongly induces leukocyte adhesion. The bound leukocytes are released by hyaluronidase treatment, indicating a critical role for hyaluronan-containing structures in mediating leukocyte binding. Affinity histochemistry demonstrated that hyaluronan accumulates and is present in cable-like structures in the treated, but not the untreated, cultures and that these structures serve as attachment sites for leukocytes. Hyaluronan-rich regions of both murine and human inflamed colon contain numerous cells that stain intensely for ER-resident chaperones containing the KDEL sequence, demonstrating a relationship between ER stress and hyaluronan deposition in vivo. These results indicate that ER stress may contribute to chronic inflammation by forming a hyaluronan-rich extracellular matrix that is conducive to leukocyte binding.

Platelets, in addition to exerting hemostatic activity, contribute to immunity and inflammation. The recent report that platelets express CD40 led us to hypothesize that CD40 ligand (CD40L)-positive T cells could bind to platelets, cause their activation, and trigger granular RANTES release, creating a T cell recruitment feedback loop. Platelets were cocultured with resting or activated autologous T cells and their activation was assessed by P-selectin expression. RANTES binding to endothelial cells was assessed by confocal microscopy, and its biological activity was demonstrated by a T cell adhesion assay. CD40L-positive T cells induced platelet activation through a contact-mediated, CD40-dependent pathway resulting in RANTES release, which bound to endothelial cells and mediated T cell recruitment. Soluble CD40L induced the same events via p38, but not extracellular signal-regulated kinase, phosphorylation. These results show the existence of a novel platelet-dependent pathway of immune response amplification which brings these nonimmune cells close to the level of pathogenic relevance traditionally attributed to classical immune cells.

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In addition to mesenchymal cells, endothelial cells may contribute to fibrosis through the process of endothelial-to-mesenchymal transition (EndoMT). We investigated whether human intestinal microvascular endothelial cells (HIMEC) undergo EndoMT and contribute to fibrosis in human and experimental inflammatory bowel disease (IBD). HIMEC were exposed to TGF-β1, IL-1β, and TNF-α or supernatants of lamina propria mononuclear cells (LPMC) and evaluated for morphological, phenotypic, and functional changes compatible with EndoMT. Genomic analysis was used to identify transcription factors involved in the transformation process. Evidence of in situ and in vivo EndoMT was sought in inflamed human and murine intestine. The combination of TGF-β1, IL-1β and TNF-α, or activated LPMC supernatants induced morphological and phenotypic changes consistent with EndoMT with a dominant effect by IL-1. These changes persisted after removal of the inducing agents and were accompanied by functional loss of acetylated LDL-uptake and migratory capacity, and acquisition of de novo collagen synthesis capacity. Sp1 appeared to be the main transcriptional regulator of EndoMT. EndoMT was detected in microvessels of inflammatory bowel disease (IBD) mucosa and experimental colonic fibrosis of Tie2-green fluorescent protein (GFP) reporter-expressing mice. In conclusion, chronic inflammation induces transdifferentiation of intestinal mucosal microvascular cells into mesenchymal cells, suggesting that the intestinal microvasculature contributes to IBD-associated fibrosis through the novel process of EndoMT.