Introduction: Endothelial progenitor cells (EPCs) have been linked to angiogenesis in many cancers including multiple myeloma (MM). Most published reports on EPCs are based upon myeloid lineage cells, which can support angiogenesis, but cannot form blood vessels on their own. A recently described subpopulation of EPCs called endothelial colony forming cells (ECFCs) have a high clonogenic potential, an ability to differentiate into more mature endothelial cells, as well as in vitro and in vivo vasculogenesis. The role of ECFCs in the pathobiology of MM is unknown. Objective: Determine the in vitro interaction of ECFCs and MM cells by using a matrigel assay, an MTS proliferation assay, and cell cycle analysis. Methods: ECFCs derived from cord blood, or peripheral blood, of normal volunteers and MM patients were combined with human MM cells. MM cells and ECFCs were cultured alone or in co-culture and studied for in vitro tubulogenesis and were evaluated using a 2-dimensional matrigel assay, with images captured at 6 and 24 hours post culture, and analyzed using Image J. Vascular tubules regressed after 24 hours of culture. Therefore, to evaluate the interaction of ECFCs and MM cells over a longer duration, GFP-expressed ECFCs and cherry red expressing MM cells were cultured either alone or in co-culture for 3-5 days. T-tests were used to evaluate the differences between the 2 groups. Results: In matrigel cultures, MM cells preferentially associated with ECFCs as over 90% of MM cells migrated towards the newly formed vascular tubes. No statistical differences in the number of closed units, vascular area and branch length was observed between ECFCs cultured alone or with MM cells. There was also no detectable difference in the proliferation of ECFCs or MM cells. In longer term cultures (3-5 days), the growth of MM cells was significantly reduced in the MM-ECFC co-culture compared to MM cells cultured alone (1.7 Vs. 2.8 fold of baseline, p=0.008). ECFC growth was largely unaffected by MM cells. Summary: Cell cycle analysis showed an increased G1 phase and decreased S phase when MM cells were co-cultured with ECFCs compared to MM cells cultured alone. Conclusion: We demonstrated a tropism of MM cells for ECFCs. In contrast to the proliferation reported when HUVECs are co-cultured with MM cells, ECFCs keep MM cells in a quiescent state by causing G1 arrest. We hypothesize that ECFCs may induce a vascular niche for MM cells. The quiescent state could theoretically result in resistance to chemotherapeutics, which tend to target dividing cells. MM cells could subsequently acquire other molecular events allowing them to dislodge from the vascular niche and contribute to relapse. Additional work to understand the mechanisms underlying the interactions of ECFCs and MM cells, and whether this interaction could be targeted for therapy, are warranted.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5300. doi:1538-7445.AM2012-5300