Based on the structural characteristics, we amplified the gene of the “two cytokine receptor domains”, and named it as IL23R-CHR. Human and mouse IL23R share 66% amino acid sequence identity. They both contain a WQPWS sequence similar to the cytokine receptor signature WSXWS motif. Mouse IL23R is expressed in mouse T helper cells, bone marrow, dendritic cells and macrophages. On the other hand, IL23R-CHRis expressed on the cell surface to recognize and respond to IL-23 by the WQPWS sequence. Up to now, both the extracellular and intracellular domains of IL-23R have been reported, but the CHR domain has hardly been analyzed, soluble receptors consisting only of the extracellular part are potent inhibitors of ligand activity. They bind the ligands with the same specificity and affinity as the membrane bound receptors without eliciting an intracellular signals. Our data showed that the exogenous soluble human IL23R-CHR protein could bind with human/mouse IL-23 complex and inhibit the AP24534 943319-70-8 binding of mouse IL-23 to endogenic mouse IL-23 receptor complex on CD4+ T cell surface in vitro. In this study, we created a 3D structure of IL23R-CHR by homology modeling based on template using Swissmodel online modeling service. The secondary structure was predicted to be dominated by b sheet, and our circular dichroism spectra confirmed the prediction. Although IL23RCHR showed a good binding affinity to IL-23 complex in vitro, our results are not all the same with a previous report, which described that hIL23R-Ig could not bind to human IL-23 in vitro, and could not act as an effective antagonist of IL-23. However, in later publication, hIL23R-Ig was found to bind human IL-23 using competition ELISA and had a good affinity. Additionally, Yu et al demonstrated that a naturally occurring IL23R variant D9 was able to bind human IL-23 in vitro. In our binding assays, native PAGE showed that IL23R-CHR bind to IL-23 in a dose-dependent manner, and direct binding ELISA assay measured the binding affinity to be around 90 nM. Furthermore, we observed that Th17 cells differentiation level was significantly down-regulated by targeting IL-23 with soluble IL23R-CHR protein, which was consistent with the results in IL23R deficient mice. Moreover, we also demonstrated that IL23R-CHR mediated-Th17 suppression was through blockage of IL-23 signals on CD4+ T cells, resulting in lower RORct expression, and therefore lowering the IL-17/IL-22 expression. The cytokines assays by intracellular staining and ELISA suggested that IL23RCHR can inhibit naive CD4+ T cells polarizing into Th17 cells in a dose-responsive manner. IL-23 regulates Th17 development and adjusts IL-23/IL-17 inflammation axis by controlling the expression of many Th17 related genes. Thus, to understand the molecular mechanism involved in differentiation of Th17 cells, we used quantitative PCR to evaluate mRNA levels of RORct and other molecules implicated in Th17 differentiation. As a result, the repression of RORct, IL-17a and IL-22 mRNA was observed. Yu et al explained that IL-23 signals could be blocked to result in the inhibition of STAT3 phosphorylation, and the critical role for STAT3 expression in Th17 development has been described since IL-23 induces a positive feed back loop in terms of IL23R expression and for further IL-23 responsiveness, STAT3 activation is required for IL23-mediated induction of its own receptor. Consequently, IL23-induced activation of STAT3 also plays an important role in IL-17 production.