Potency Assays for Mesenchymal Stromal Cells

A cell preparation can satisfy every identity criterion and still do nothing. Identity assays confirm what a cell is. Potency assays attempt the harder question — what it does — and for any cell-based product intended for therapeutic use, that is the question regulators and physicians both care about.

Why differentiation assays underperform as potency measures

Tri-lineage differentiation — osteogenic, adipogenic, chondrogenic — is part of the ISCT identity definition and is genuinely useful for that purpose. As a potency measure it is weak, for a straightforward reason: it does not correspond to the mechanism by which mesenchymal stromal cells are thought to act in most applications.

If the therapeutic effect is paracrine and immunomodulatory, then a preparation's capacity to become bone in a dish predicts very little about it. The assay is also slow, semi-quantitative and poorly suited to lot release.

Immunopotency: closer to the mechanism

Assays in this class measure what the cells actually do to immune effectors. Suppression of activated T-cell proliferation in co-culture is the most established. Induction of indoleamine 2,3-dioxygenase following interferon-gamma stimulation is quantifiable and mechanistically linked. Macrophage polarisation assays assess the shift toward a reparative phenotype.

These are more informative and more mechanistically defensible — and correspondingly more complex, requiring responder cells that introduce their own variability.

Secretome and vesicle analytics

Where the product is the secretome, the analytics move to the vesicles themselves: particle concentration and size distribution by nanoparticle tracking analysis, tetraspanin profiling for CD9, CD63 and CD81, quantification of key soluble factors, and where relevant, microRNA cargo profiling.

These measures are quantitative, reproducible and directly descriptive of the material being supplied — which is why they belong on a certificate of analysis rather than in a marketing summary.

The assay matrix

The field's working consensus is that no single assay is sufficient. A defensible potency strategy combines a quantitative identity measure, at least one mechanistically relevant functional readout, and a stability-indicating parameter — then correlates that panel against clinical or in vivo outcome data over time.

For a physician evaluating a supplier, the useful question is not whether potency is claimed but which assays were run, on which lot, and what the results were. Potency asserted without a named method is not potency.