Background: Tumor necrosis factor (TNF)-like protein 1A (TL1A) is a member of the TNF superfamily and contributes to the pathogenesis of Crohn's disease (CD) by stimulating T-helper (Th) 1 cells. In addition to Th1, recent studies have focused on the role of Th17 cells in the pathogenesis of CD. Here we tried to clarify the role of TL1A in Th1 and Th17 immunity in CD. Methods: TL1A expression was assessed by quantitative reverse-transcription polymerase chain reaction (RT-PCR) in lamina propria (LP) macrophages (LP-Mφs) from normal controls (NC) and patients with CD or ulcerative colitis (UC). Purified LP CD4+ T cells were stimulated with TL1A and/or IL-23 and interferon gamma (IFN-γ) and interleukin (IL)-17 levels were analyzed. We also examined the effect of TL1A on naïve CD4+ Tcell differentiation. Results: We found that LP-Mφs are a major producer of TL1A. TL1A expression was markedly enhanced in LP-Mφs from CD patients compared with NC or UC patients. IL-23, in addition to TL1A, was induced in LP-Mφs by commensal bacteria stimulation. TL1A and IL-23 synergistically promoted the production of IFN-γ and IL-17 by LP T cells, while TL1A alone did not induce cytokine production. Furthermore, TL1A promoted Th17 differentiation from naïve T cells by LP-Mφs; however, IL-23 did not show any synergistic effects on Th17 differentiation. Conclusions: TL1A expressed in LP-Mφs might play an important role in the pathogenesis of CD by inducing Th1 and Th17 immunity. IL-23 differentially regulated these functions of TL1A on memory and naïve T cells.
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