Authors: Adip G. Bhargav; Rawan Al-Kharboosh; Cesar Garcia; Alfredo Quiñones-Hinojosa, MD (Rochester, MN)

Introduction: Targeting Brain Tumor Initiating Cells (BTICs) implicated in recurrence and treatment resistance of Glioblastoma (GBM) is a tremendous therapeutic challenge. Furthermore, tumor-supportive myeloid cells create a hostile tumor environment that contributes to refractory disease. Human Adipose-derived Mesenchymal Stem Cells (hAMSCs) are promising delivery vehicles of anti-GBM cargo that can target BTICs. However, the immunomodulatory effects and interaction of hAMSCs with the GBM stroma remain unclear. Here, we present data on the hAMSC—microglia interaction and implications for BTIC targeting. Methods: hAMSCs were harvested, isolated, and purified from patient lipoaspirates while primary, patient-derived GBM tissue was used to isolate BTICs. hAMSCs were allowed to condition media for 48 hours, after which, media containing secreted, soluble factors was filtered and used for experiments to assess: 1) polarization of microglia via flow cytometry and 2) migration to determine tropism of hAMSCs via transwell assay. Gene expression was analyzed to dissect hAMSC—microglia interaction in the context of GBM. Appropriate 1-way ANOVA analysis was used to examine experimental findings. Results: hAMSCs preferentially migrated to anti-inflammatory, tumor-supportive, polarized microglia. Furthermore, hAMSCs polarized microglia to a pro-inflammatory, tumor-suppressive phenotype as measured by CXCR4/CD206 expression. Infiltrating microglia are known to secrete IL-4 in the tumor stroma; however, there was no loss in stemness and no risk of oncogenicity with hAMSCs exposure to IL-4. In the presence of BTICs, hAMSCs reduced BTIC stem markers, proliferation, and sphere-forming capacity. Finally, hAMSC treatment of microglia increased endogenous microglial expression of BMP4, a potent anti-glioma protein. Conclusion: hAMSC—microglia interactions suggest a site-specific immunodulatory role for hAMSC therapy in GBM. hAMSCs preferentially target microglia and promote a tumor-suppressive microglial phenotype. Thus, hAMSCs can work both directly and indirectly by suppressing BTIC oncogenicity and promoting a pro-inflammatory immune phenotype, respectively. These results suggest hAMSCs play a dual role in suppressing GBM.