TL1A is a novel TNF-like factor that acts as a costimulator of IFN-γ secretion through binding to the death domain-containing receptor, DR3. The aim of this study was to test the hypothesis that TL1A may play an important role in inflammatory bowel disease (IBD) by functioning as a Th1-polarizing cytokine. The expression, cellular localization, and functional activity of TL1A and DR3 were studied in intestinal tissue specimens as well as isolated lamina propria mononuclear cells from IBD patients and controls. TL1A mRNA and protein expression was up-regulated in IBD, particularly in involved areas of Crohn’s disease (CD; p < 0.03 vs control). TL1A production was localized to the intestinal lamina propria in macrophages and CD4+ and CD8+ lymphocytes from CD patients as well as in plasma cells from ulcerative colitis patients. The amount of TL1A protein and the number of TL1A-positive cells correlated with the severity of inflammation, most significantly in CD. Increased numbers of immunoreactive DR3-positive T lymphocytes were detected in the intestinal lamina propria from IBD patients. Addition of recombinant human TL1A to cultures of PHA-stimulated lamina propria mononuclear from CD patients significantly augmented IFN-γ production by 4-fold, whereas a minimal effect was observed in control patients. Our study provides evidence for the first time that the novel cytokine TL1A may play an important role in a Th1-mediated disease such as CD.

TL1A is a TNF-like cytokine that binds to the death-domain receptor (DR)3 and provides costimulatory signals to activated lymphocytes. Through this interaction, TL1A induces secretion of IFN-␥ and may, therefore, participate in the development of T helper-1-type effector responses. In this study, we investigated whether interactions between TL1A and DR3 are involved in the pathogenesis of chronic murine ileitis. We demonstrate that alternative splicing of DR3 mRNA takes place during the activation of lymphocytes, which results in up-regulation of the complete͞transmembrane (tm) form of DR3. Using two immunogenetically distinct animal models of Crohn's disease, we demonstrate that induction of intestinal inflammation is associated with significant up-regulation of TL1A and tm DR3 in the inflamed mucosa. In addition, within isolated lamina propria mononuclear cells from mice with inflammation, TL1A is primarily expressed on CD11c high dendritic cells. We also report that TL1A acts preferentially on memory CD4 ؉ ͞CD45RB lo murine lymphocytes by significantly inducing their proliferation, whereas it does not affect the proliferation of the naïve CD4 ؉ ͞ CD45RB hi T helper cell subpopulation. Finally, we demonstrate that TL1A synergizes with both the cytokine-dependent IL-12͞IL-18 pathway and with low-dose stimulation of the T cell receptor to significantly induce the secretion of IFN-␥ via an IL-18-independent pathway. Our results raise the possibility that interaction(s) between TL1A expressed on antigen-presenting cells and tm DR3 on lymphocytes may be of particular importance for the pathogenesis of chronic inflammatory conditions that depend on IFN-␥ secretion, including inflammatory bowel disease. Blockade of the TL1A͞DR3 pathway may, therefore, offer therapeutic opportunities in Crohn's disease.Crohn's disease ͉ cytokines ͉ mucosal inflammation T he differentiation of naïve CD4 ϩ lymphocytes into IFN-␥-secreting Th1 ''effector'' cells is a multistep process that involves several cell types, costimulatory molecules, transcription factors, and secreted cytokines (1). Antigen-presenting cell (APC)-derived IL-12 is essential for the induction of IFN-␥, an effect that is greatly enhanced by IL-18 (2). IL-12 up-regulates T-bet, a transcription factor that is critical for the stabilization of a T helper (Th)1-polarized phenotype (3). Recently, additional cytokines that play prominent roles during Th1 responses have been described, such as IL-27 and IL-23 (4). Engagement of the T cell receptor (TCR) provides further signals for the induction of IFN-␥, both in parallel to and independently of cytokinemediated pathways (1).Members of the TNF and TNF-receptor superfamilies of proteins (TNFSFPs and TNFRSFPs, respectively) are abundantly expressed in the immune system, and are critically involved in the differentiation, proliferation, and apoptosis of immune cells (5). Several members of these families induce secretion of IFN-␥ upon ligand͞receptor binding, thereby enhancing Th1-type responses (6-8). TL1A (TNFSPF15) is a r...

Tumor necrosis factor ␣ (TNF-␣) is an important mediator of programmed cell death, and TNF-␣ blockade significantly improves disease severity in several inflammatory conditions, including Crohn's disease (CD), one of the idiopathic inflammatory bowel diseases. However, the precise mechanism(s) of action of anti-TNF-␣ therapy in CD remains poorly understood. SAMP1͞YitFc mice develop a spontaneous ileitis with similarities to human CD in regard to histological features as well as response to conventional treatments. In this report, we tested the novel hypothesis that the beneficial effects of anti-TNF-␣ therapy in CD are mediated by a mechanism that involves down-regulation of intestinal epithelial cell (IEC) apoptosis. Similar to the efficacy of monoclonal anti-TNF-␣ antibodies in human CD, a single injection of a chimeric anti-murine TNF-␣ antibody into SAMP1͞YitFc mice resulted in a marked suppression of intestinal inflammation and epithelial cell damage compared with mice injected with an isotype control antibody. These effects were associated with a significant reduction in apoptosis of freshly isolated IEC as assessed by propidium iodide staining and DNA laddering. In contrast, an increase in lamina propria mononuclear cell apoptosis was observed in anti-TNF-␣-treated mice compared with control. These results were confirmed in vivo by using the terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling-assay. In addition, neutralization of TNF-␣ reduced membrane bound FAS͞CD95 expression in IEC from SAMP1͞YitFc mice compared with control antibody. These data demonstrate a novel mechanism of action of anti-TNF-␣ therapy that involves homeostatic regulation of mucosal cell apoptosis, which results in the net decrease of chronic inflammation typically found in CD.

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The endogenous purine nucleoside adenosine is an important antiinflammatory mediator that contributes to the control of CD4+ T cell responses. While adenosine clearly has direct effects on CD4+ T cells, it remains to be determined whether actions on APC such as dendritic cells (DC) are also important. In this report we characterize DC maturation and function in BMDC stimulated with LPS in the presence or absence of the nonselective adenosine receptor agonist NECA (5′-N-ethylcarboxamidoadenosine). We found that NECA inhibited TNF-α and IL-12 in a concentration-dependent manner, whereas IL-10 production was increased. NECA-treated BMDC also expressed reduced levels of MHC class II and CD86 and were less effective at stimulating CD4+ T cell proliferation and IL-2 production compared with BMDC exposed to vehicle control. Based on real-time RT-PCR, the A2A adenosine receptor (A2AAR) and A2BAR were the predominant adenosine receptors expressed in BMDC. Using adenosine receptor subtype selective antagonists and BMDC derived from A2AAR−/− and A2BAR−/−mice, it was shown that NECA modulates TNF-α, IL-12, IL-10, and CD86 responses predominantly via A2BAR. These data indicate that engagement of A2BAR modifies murine BMDC maturation and suggest that adenosine regulates CD4+ T cell responses by selecting for DC with impaired immunogencity.