HLA-DR MODULATION AND PD-1/PD-L2 CHECKPOINT SIGNALLING DEFINE A MECHANISTIC POTENCY AXIS FOR MESENCHYMAL STROMAL CELL IMMUNOSUPPRESSION

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. 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 system, we compared bone marrow and adipose derived Mesenchymal stromal cells across independent donor batches.

Both sources effectively suppressed T cell proliferation, with the adipocyte derived source consistently showing greater inhibitory activity, 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 these markers emerging as a sensitive correlate of functional potency. Notably, bone marrow derived mesenchymal stromal cells exerted stronger suppression on late-stage activation and preferentially suppressed CD8 ⁺ T cell expansion.

Mechanistically, this immunosuppression was associated with modulation of the PD-1 pathway, characterized by decreased soluble PD-1, increased PD-L1, and induction of mesenchymal stromal cells derived PD-L2. PD-L2 levels inversely correlated with T cell proliferation, identifying a PD-1/PD-L2 regulatory axis linked to the cell’s potency.

These findings define a standardized and mechanistically informed potency assay framework for assessing mesenchymal stromal cell immunomodulatory function.