Work

HLA-DR modulation and PD-1/PD-L2 checkpoint signaling identify a mechanistic potency axis for mesenchymal stromal cell immunosuppression

Year: 2026

Type: article

Abstract: INTRODUCTION: Mesenchymal stromal cells exhibit potent immunomodulatory properties and are under active investigation for the treatment of immune-mediated disorders. However, their clinical translation is hindered by the lack of standardized potency assays. METHODS: Here, we established a reproducible mixed lymphocyte reaction platform by systematically optimizing peripheral blood mononuclear cell donor composition, culture conditions, and co-culture ratios to define a robust activation window.Using this optimized mixed lymphocyte reaction-based potency assay, we compared bone marrow and adipose-derived mesenchymal stromal cells across independent donor batches. RESULTS: Both sources of mesenchymal stromal cells effectively suppressed T-cell proliferation, with the adipose-derived source demonstrating greater inhibitory activity under the experimental conditions, while a conserved lower threshold of suppression was observed across both sources. Mesenchymal stromal cells reduced early (CD25+) and late (CD25+ HLA-DR+) T-cell activation, with downregulation of CD25 and HLA-DR emerging as sensitive correlates of functional potency. Notably, bone marrow-derived mesenchymal stromal cells exerted stronger suppression of late-stage HLA-DR+ activation and preferentially inhibited CD8+ T-cell expansion, demonstrating source-dependent immunomodulatory functions. Mechanistically, this suppression was associated with modulation of the PD-1 pathway, characterized by decreased soluble PD-1, increased soluble PD-L1, and induction of soluble PD-L2 by mesenchymal stromal cells. PD-L2 levels inversely correlated with T-cell proliferation in the mixed lymphocyte reaction, supporting a PD-1/PD-L2 regulatory axis linked to mesenchymal stromal cell potency. CONCLUSION: These findings support a standardized and mechanistically informed mixed lymphocyte reaction-based potency assay framework integrating functional suppression, HLA-DR modulation, and PD-1/PD-L2 pathway signaling as complementary indicators of mesenchymal stromal cell activity.

Source: Stem Cells Translational Medicine

DOI: 10.1093/stcltm/szag090