Objectives <p>The aim of this study is to explore whether T follicular helper cells (Tfh) cells participate in the progression of systemic sclerosis related interstitial lung disease (SSc-ILD), and to observe whether mesenchymal stem cells (MSCs) therapy are able to regulate Tfh cells and the potential mechanism of treating SSc-ILD.</p> Methods <p>The expression of CXCR5⁺PD-1⁺CD4⁺T cells in SSc-ILD patients was verified by flow cytometry, immunohistochemistry and immunofluorescence. In vitro differentiated Tfh cells were cocultured with human lung fibroblasts. Peripheral blood mononuclear cells (PBMCs) were directly cocultured with MSCs. Naive CD4⁺ T cells isolated from PBMCs were cocultured with MSCs under Tfh 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. Bleomycin-induced SSc-ILD mice model received Bcl-6 inhibitor or MSCs treatment respectively. Then, the histopathology of lungs and pulmonary Tfh cells were examined in these mice.</p> Results <p>Increased frequency of Tfh cells and intracellular IL-21 were found in SSc-ILD patients and positively correlated with mRSS. CXCR5⁺PD-1⁺ Tfh cells could drive the differentiation of myofibroblasts in vitro. Tertiary lymphoid structures (TLS), where Tfh cells localized, was found in lung biopsies from pulmonary fibrosis patients and BLM induced mice model. The upregulated frequency of Tfh cells and germinal center (GC) B cells were also found in the lung and spleen of BLM mice model. After depletion of Tfh cells, the proportion of GC B cells was reduced and the manifestation of lung fibrosis was attenuated. In addition, MSCs treatment contributed to remissive pulmonary fibrosis and reduced number of pulmonary TLS, followed by decreased frequency of Tfh cells both in lung of mice model and in peripheral circulation of SSc patients. In vitro, MSCs could downregulate the proportion of Tfh cells in SSc patients through suppressing their differentiation and proliferation.</p> Conclusion <p>Tfh cells, which could promote fibrosis both in vivo and in vitro, play pivotal role in the pathogenesis of SSc-ILD. MSCs therapy can downregulate the percentage of Tfh cells and inhibit their proliferation and differentiation, which may be the mechanism for their effective treatment for SSc-ILD.</p>