Introduction: The bone marrow (BM) microenvironment plays an important role in multiple myeloma (MM). The BM niche is composed of multiple cell types including macrophages. Macrophages polarize into pro-inflammatory macrophage-1 (M1) or alternative M2 states that promote tumor growth and metastasis. We evaluated the proportion of M2 macrophages in BM from MM pts either showing complete response (CR) or progressive disease (PD), the effects of MM cells on M1 and M2 differentiation, and the role of Trib1 in M2 differentiation in MM BM. Since the JAK-STAT signaling pathway plays key roles in macrophages, we also evaluated the effects of the JAK2 inhibitor ruxolitinib (RUX) on M2 polarization in MM.

Methods: Using immunofluorescence (IFC), we determined the proportion of M1 and M2 macrophages in BM biopsies and aspirates from MM pts with PD or CR. The BM biopsy samples were stained with antibodies directed against human iNOS and CD86 for M1 and arginase 1(ARG1) and CD36 for M2 cells. MM BM aspirates were also examined using flow cytometric analysis (FCA). Human monocytes isolated from healthy subjects or the THP1 monocyte cell line were co-cultured with MM cell lines (RPMI8226 and U266) or primary MM tumor cells. The effects of RUX at low concentrations (IC20) on M2 polarization were determined. The percentages of M1 and M2 macrophages were determined using FCA. Total RNA was extracted from monocytes. Quantitative PCR was measured with TaqMan technology. For the in vivo studies, human MM tumors (LAGκ-2) were surgically implanted into the left superficial gluteal muscle of SCID mice and tumor volume measured on a weekly basis.

Results: The proportion of M2 macrophages (CD36+/ARG1+) was markedly increased in BM biopsies or mononuclear cells from MM pts with PD compared with those in CR using IFC staining. FCA also showed the percentage of M2 macrophages in BM was significantly increased in MM pts with PD (n=25) compared to those in CR (n=10; P=0.005) whereas there was no difference in the percentage of M1 (CD86+/iNOS+) macrophages in BM derived from MM pts with PD compared to those in CR. Trib1 gene mRNA levels were higher among pts with PD compared to those in CR whereas the gene expression of Trib2 and Trib3 was not different. Next, we co-cultured MM cell lines (U266) or fresh MM BMMCs with purified healthy human monocytes for one week. The percentage of M2 cells markedly increased and the proportion of M1 cells decreased. Trib1 gene expression increased during co-culture whereas there was no change in expression of the other two Tribs. When direct cell-to-cell contact occurred between the MM tumor cells and the monocytes, the percentage of M2 macrophages markedly increased. We investigated the effects of the JAK2 inhibitor RUX on M2 differentiation induced with MM tumor cells. After exposure to a low concentration of RUX, the percentage of M2 cells decreased when the monocytes were co-cultured with MM tumor cells. Trib1 gene expression of the monocytes treated with RUX was also notably reduced compared with cells not treated with the JAK2 inhibitor. Using our human MM xenograft model LAGκ-2, RUX (1.5mg/kg) reduced tumor growth and decreased the proportion of M2 macrophages in the tumor tissue of MM tumor-bearing SCID mice.

Conclusion: M2 cells are present at high levels in BM derived from MM pts with PD compared to those in CR, MM cells induce monocytes to become M2 macrophages and increase Trib1 gene expression. This induces monocyte differentiation into M2 macrophages that support MM tumor cell growth.. Notably, the JAK2 inhibitor RUX inhibits both M2 macrophage polarization and Trib1 gene expression in MM, and reduces tumor growth in SCID mice bearing human MM. These results suggest that RUX may be effective for treating MM pts.


No relevant conflicts of interest to declare.

Author notes


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