It is now known that the microenvironment plays a prominent role in both the biology and etiology of Multiple Myeloma (MM), with infiltrating cells providing tumor-promoting signals in the bone marrow (BM) and aiding drug resistance. In particular, M2-like macrophages accumulate in the BM and have been shown to promote tumor growth. Macrophages can be ‘educated’ to become either M1 - which are associated with inflammation and fighting pathogens, or M2 - which are associated with anti-inflammatory responses and occur in response to signals driven by the CSF1/CSF1R (colony-stimulating factor 1 and CSF1 receptor) axis.
In a recent study, published in Leukemia in June 2017 by Qiang Wang, Yong Lu and colleagues from the Department of Cancer Immunology, The First Hospital of Jilin University, China, and the Department of Cancer Biology, Lerner Research Institute, USA, the role of macrophages and myeloma-associated macrophages (MAMs) were studied using a blocking antibody targeting CSF1R.
In vitro experiments using murine cells:
- Treatment of cells with a monoclonal antibody against CSF1R (CS7) inhibited differentiation pathways, survival, and proliferation of macrophages and MAMs
- Lower (non-apoptotic) doses of CS7 polarized MAMs into M1-like macrophages, as assessed by gene array analysis of the transcriptional profile, ingenuity pathway analysis (IPA), and measurement of cell-surface CD206 (a marker for M2’s)
In vivo experiments in murine MM Models:
- Injection of CS7 into mice bearing MM tumors, significantly inhibited tumor growth as assessed by bioluminescent imaging
- MM cell number was also depleted in hind leg and spleen, and CS7-treated mice had notably smaller spleens
- Injection of diphtheria toxin (DT) into MMDTR mice leads to depletion of monocytes and macrophages; DT injection led to a significantly decreased tumor burden in these mice (with an MM disease model), further confirming a role for macrophages in supporting MM
- The effects of CS7 treatment on cell subtypes in blood, spleen, BM and peritoneal cavity were investigated; depletion of macrophages and MAMs was confirmed, indicating that CSF1R inhibition prevents the development of these cells in vivo
- CS7-treated mice also showed decreased numbers of CD206+ splenic macrophages compared to mice treated with the control antibody
- CS7 did not alter the percentage of CD206+ BM macrophages and MAMs, but these cells expressed significantly lower levels of IL10, IL12a, and TNFαa than control treated mice, suggesting a skew towards the M1-macrophage phenotype and creating an ‘anti-MM microenvironment’
- The effect of CS7 on MDSCs was minimal
- There was no effect of CS7 in mice devoid of T cells (immunodeficient Rag-/-) with an established MM model, indicating that lymphocytes are required for CSF1R-driven effects
- The percentage of CD4+ and CD8+ cells expressing granzyme-B was higher in the BM and lymph nodes of CS7 treated mice (immunocompetent KAL mice with an MM model)
- Depletion of CD4+ cells abrogated CS7-induced inhibition of tumor growth and BM infiltration
Further in vitro experiments:
- CS7 treatment enhanced the antigen-presenting capacity of MAMs and promoted a tumor-specific CD4+ T cell response
- IPA confirmed activation of signaling pathways indicative of immune activation
- Treatment of mice with an established MM model, with either bortezomib or CS7 alone, led to a decreased tumor burden, but the effect was enhanced when both agents were administered together; the same effect was observed when combining CS7 and melphalan
- A humanized blocking antibody against CSF1R (CS4) was similarly effective in blocking differentiation, survival, and proliferation of human macrophages
This study is the first to show a definitive role for macrophages in supporting MM development, and in particular, M2 macrophages were pinpointed as the responsible subset. CSF1R signaling polarizes macrophages towards the M2 phenotype and therefore blocking this pathway reverts polarization towards the M1 phenotype, leading to decreased MM tumor growth and burden. This work suggests therapeutic potential for CSF1R blocking antibodies, and indeed additional benefits when used in combination with other therapies.