This study aims to evaluate the therapeutic efficacy of human umbilical cord mesenchymal stem cells (hUC-MSCs) in alleviating pulmonary inflammation and fibrosis in a collagen-induced arthritis (CIA) mouse model, and to explore the underlying immunomodulatory mechanisms.

A CIA model was established in male DBA/1 mice through intradermal immunization with bovine type II collagen. On day 28 post-immunization, mice were randomly assigned to the CIA+PBS group or the CIA+MSC treatment group (n=6 per group), alongside a non-immunized control group (n=4). The treatment group received a single tail vein injection of 1×10⁶ hUC-MSCs. Arthritis Index (AI) and systemic conditions were monitored throughout the study. On day 60, lung tissues were harvested for systematic analysis. Histopathological changes and collagen deposition were assessed via H&E and Masson’s trichrome staining (Szapiel scoring). Cytokine profiles (IFN-γ, TNF-α, IL-6, IL-17A, and TGF-β) were quantified by ELISA. Fibrotic markers (FN, α-SMA) were characterized using immunohistochemistry and qRT-PCR. Additionally, immunofluorescence (IF) was employed for CD3/α-SMA co-localization, while lineage-specific transcription factors (T-bet, GATA3, RORγt, and FOXP3) were analyzed to determine T-lymphocyte differentiation patterns.

hUC-MSC administration significantly ameliorated articular swelling and reduced AI scores compared to the CIA+PBS group (P < 0.05). Systemic conditions, including activity levels and weight gain, were notably improved (Fig. 1). Histopathological restoration showed that hUC-MSCs effectively restored alveolar architecture, significantly reduced septal thickening, and attenuated the extent of collagen deposition in the lung parenchyma (P < 0.05). A significant downregulation of pro-inflammatory cytokines (IFN-γ, TNF-α, IL-6, and IL-17A) and the pro-fibrotic factor TGF-β was observed in the lung tissues of hUC-MSC-treated mice (Fig. 2). Consistent with these findings, the expression of fibrotic markers (FN and α-SMA) was markedly decreased at both protein and mRNA levels (Fig. 3). IF staining revealed a spatial interactome between CD3⁺T-cells and α-SMA⁺ myofibroblasts in fibrotic regions. While CIA mice exhibited a profound immune dysregulation (elevated T-bet, RORγt, and FOXP3), hUC-MSC treatment specifically and significantly suppressed the expression of the Th17-specific transcription factor RORγt (P < 0.05). Correlation analysis indicated that pulmonary fibrosis markers were positively associated with the levels of RORγt and T-bet (Fig. 4).

hUC-MSCs demonstrate significant therapeutic potential in mitigating both systemic arthritis and pulmonary damage in the CIA model. These protective effects are likely mediated through the suppression of Th17 differentiation and the subsequent inhibition of myofibroblast activation. Our findings provide a robust experimental foundation for the clinical translation of MSC-based therapies in the management of rheumatoid arthritis-associated interstitial lung disease.