This study establishes a reproducible technique for the culture of human peritoneal mesothelial cells. Direct explants, as well as enzymatically degraded specimens, of human omentum have been used as the source of cells. Cells were grown on collagen and gelatin coated matrices and were maintained in supplemented Ham's F-12 medium containing 10% (vol/vol) Fetal calf serum. Morphologically and ultrastructurally, the cells formed a homogeneous population. They were polygonal when confluent and devoid of contaminating fibroblasts, endothelial cells and macrophages. Cultured mesothelial cells co-expressed cytokeratin and vimentin and synthesized laminin, fibronectin, mesosecrin, non-specific esterase and collagen Types I and III but not Type IV. Ultrastructural features included numerous surface microvilli, cytoplasmic vesicles and an abundant endoplasmic reticulum. The stimulation of mesothelial cells by the calcium ionophore A23187 demonstrated that the two major products of arachidonic acid metabolism were prostacyclin and prostaglandin E2. The peritoneal mesothelial cell may be pivotal in the initiation of the inflammatory response during peritonitis and its establishment in culture will provide the basis for an in vitro model of peritoneal inflammation.

Interleukin 1 (IL1) is a primary regulator of inflammatory and immune responses. Via its type I receptor it activates specific protein kinases, including the NF kappa B inducing kinase (NIK) and three distinct mitogen-activated protein (MAP) kinase cascades. These modulate a number of transcription factors including NF kappa B, AP1 and CREB each of which regulate a plethora of immediate early genes central to the inflammatory response. Phase I clinical trials of the soluble type I receptor and IRAP indicate that these have potential anti-inflammatory effects.

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.

Recent data have implicated nuclear factor B (NF-B