Hello

Look at links here 

Good luck

http://www.ncbi.nlm.nih.gov/pubmed/20941614

http://austinpublishinggroup.com/stem-cells/fulltext/jscr-v1-id1016.php

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3759509/

http://stemcellassays.com/2012/06/potency-assays-mesenchymal-stromal-cell-based-products/

Dear Adomas,

A mixed lymphocyte reaction (MLR) assay using PBMCs along with either the MSCs or their conditioned medium could help you develop a potency assay. You could look into an immunosupressive cytokine or molecule regulated by MSCs and correlate that to the MLR assay. Ultimately just the quantification of that particular molecule or cytokine could be your surrogate marker for immunosupressive potency.

Hope this gives you fair idea!

Good luck!

beside what other recommended above i would recommend to start with basic BM-MSCs parameters (cell morphology, proliferation, CFU, CD markers, and differentiation capacity). 

Thank You for the suggestions