Objective. The aim of this study was to investigate the effect of umbilical cord mesenchymal stem cells (UC-MSCs) on circulating T follicular helper (cTfh) cells in primary SS (pSS) patients. Methods. The percentage of CXCR5 + PD-1 + CD4 + T cells in peripheral blood mononuclear cells (PBMCs) was analysed by flow cytometry. PBMCs were co-cultured with UC-MSCs by cell-to-cell contact or in a trans-well system. Naive CD4 + T cells were isolated from PBMCs and then co-cultured with UC-MSCs under

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... h cell-polarizing conditions. The percentage of CXCR5 + PD-1 + CD4 + T cells, carboxyfluorescein succinimidyl ester (CFSE) fluorescence intensity and annexin V were determined by flow cytometric analysis. Real-time PCR and Luminex cytokine assay were performed to detect mRNA expression and supernatant protein levels. The activity of indoleamine 2,3-dioxygenase (IDO) was measured by HPLC. Results. Increased frequency of cTfh cells was found in pSS patients and was positively correlated with serum anti-La/SSB levels and the European League Against Rheumatism SS Disease Activity Index score. In vitro, UC-MSCs suppressed the differentiation and proliferation of cTfh cells. Real-time PCR analysis showed significantly higher IDO mRNA expression on UC-MSCs when co-cultured with naive CD4 + T cells under Tfh cell-polarizing conditions in pSS patients. However, IDO mRNA expression on UC-MSCs was only a little higher when UC-MSCs were co-cultured with naive CD4 + T cells in a trans-well system. In addition, HPLC showed increased IDO enzymic activity in the supernatant of UC-MSCs co-cultured with naive CD4 + T cells under Tfh cell-polarizing conditions in pSS. The addition of the IDO inhibitor 1-MT partly reversed the suppressive effect of UC-MSCs on the differentiation of cTfh cells. Conclusion. These results suggest an inhibitory effect of UC-MSCs on the differentiation of cTfh cells via the secretion of IDO, and soluble factors secreted by activated CD4 + T cells might contribute to IDO secretion by UC-MSCs.