Proteomic Analysis of Reprograming of Mesenchymal Stem/Stromal Cells (MSC) Following Interferon Gamma Identifies Pathways That Are Upregulated in Suppression

Assessing potency of mesenchymal stem/stromal cells (MSC) used for immunologic applications such as the treatment of GVHD or other inflammatory disorders has been a challenge. Expression of PD-L1 or production of indoleamine-pyrrole 2,3-dioxygenase (IDO-1) has been proposed as potential potency markers. To screen for other pathways involved with suppression we undertook proteomic analysis of IFN-γ stimulated MSC. MSC isolated and expanded from normal healthy donors to 70-80% confluence were treated overnight with human rIFN-γ (30ng/ml). MSC were harvested using TrypLE Select and then immunologic and proteomic studies were performed. IFN-γ exposure increased a) MSC expression of IDO-1 and PD-L1, b) MSC suppression of 3^rd party T lymphocyte proliferation, c) MSC inhibition of development of IFN-γ producing T lymphocytes, and d) MSC promotion of Treg expansion. Cellular proteomic changes that occur with IFN-γ exposures were studied in paired samples of control and IFN-γ treated MSC. Samples were prepared using a modified Filter Aided Sample Prep (FASP) and digests were separated using 2D liquid chromatography and analysed by tandem mass spectrometry. Data was processed with the Global Proteome Machine and only proteins with at least two confident peptides were reported. A total of 7621 proteins were identified of which 5575 were seen in all samples and 232 proteins were significantly upregulated in the IFN-γ treated cells relative to their controls. The proteomic analysis identified constitutive proteins seen in MSC. The upregulated proteins were significantly enriched (p<10^-17) for GO processes such as "response to IFN-γ" and "cytokine mediated signaling pathway". Known inhibitory mediators (such as IDO-1, PD-L1, PGE2, galectin-9) were upregulated. Interestingly adhesion molecules (ICAM-1, VCAM-1, and CCL9) were increased. Other proteins with increased expression include Bone Marrow Stromal 2 (BST2). Conclusion: Proteomic analysis of response of MSC to IFN-γ has identified a signature of proteins upregulated with the activation of immune suppressive functions of MSC. Once confirmed these findings will support the development of a potency test for immunosuppressive potential of given MSC preparations - something that is sorely needed in the clinical manufacturing of MSC products.

Acknowledgments: Q.D. is holding a postdoctoral fellowship from MS Society of Canada. This research was supported by The Bihlers' Professorship in Stem Cell Research to D.W.