Mesenchymal Stem Cells (MSC) from Patients with Multiple Myeloma Promote Myeloid Cells to Become Granulocytic-Myeloid-Derived Suppressor Cells (G-MDSC) with Immunosuppressive, Bone Resorption and Pro-Angiogenic Activity

Purpose

A well-recognized feature of MM is the intimate relationship between plasma cells and bone marrow microenvironment, which is mainly composed of MSC, endothelial cells, immune cells and extracellular matrix. G-MDSC accumulate in the tumor microenvironment during tumor development promoting tumor growth and immunosuppression.

Aim

Analyzing MSC from MGUS, Smoldering myeloma (SMM) and MM patients in promoting G-MDSC generation.

Methods

Human peripheral blood mononucleated cells (PBMC) isolated from healthy subjects (HS) were cultured alone and with HS- (n=10), MGUS- (n=10), SMM- (n=4) or MM-MSC (n=14)(1:100). After 6 days, G-MDSC were isolated using anti-CD66b magnetic microbeads and the phenotype(CD11b+CD33+CD14-HLADR-) was confirmed by cytofluorimetric analysis.

Results

Only G-MDSC educated by SMM- and MM-MSC co-cultures (MSCed-G-MDSC) were able to suppress T cell proliferation when cultured with normal lymphocytes (p<0.001) compared to G-MDSC control (isolated from PBMC cultured in medium alone). SMM- and MM-MSCed-G-MDSC significantly up-regulated Arg1, NOS2, TNFα and CEBPA, a transcription factor promoting suppressive phenotype. Since also the angiogenic factor BV8 was significantly up-regulated, we next investigated the pro-angiogenic effect in vitro co-culturing MSCed-G-MDSC with Human Brain Microvascular Endothelial Cells (HBMEC) (1:2). After 5 h, we observed that MM-MSCed-G-MDSC were able to increase both tube length and number of branch points compared to G-MDSC control (p<0.05). Moreover, MM-MSCed-G-MDSC were able to digest bone matrix in vitro (p<0.01).

Adding Bortezomib (5 nM), Lenalidomide (10 μM) or Pomalidomide (1 nM) during co-culture with MM-MSC, isolated G-MDSC showed a significant reduction of pro-angiogenic and bone resorption activity (p<0.05) but did not lose immunosuppressive ability.

Conclusion

MSC play a key role promoting tumor microenvironment transformation in SMM and MM patients.Indeed, only SMM- and MM-MSC and not MGUS-MSC are able to activate myeloid cells in G-MDSC with immunosuppressive, pro-angiogenic, and bone resorptionactivity.