Tuning the Innate Immune Multiple Myeloma Microenvironment By Modulating IRF4

The tumor microenvironment (TME) has emerged as a key contributor to cancer progression in numerous solid tumors as well as blood cancers that evolve in the bone marrow (BM). In the context of multiple myeloma (MM), survival of malignant plasma cells is supported by interactions with the BM microenvironment, which is comprised of various components including cells, extracellular matrix, and soluble factors. Therefore, the development of strategies that specifically target the MM BM microenvironment is crucial for improving current disease prognosis. Within the BM niche, M2-like myeloma-associated macrophages (MAMs) play a significant role in cancer cell survival and progression and have been implicated in producing an immune-suppressive TME by generating inflammatory mediators, growth factors, cytokines, chemokines, etc. Macrophages exhibit plasticity with regard to their polarization state, which enable them to dynamically respond and adapt to different environmental stimuli in order to fulfill important immune functions. Thus, the reprogramming of M2-like, pro-tumoral MAMs toward a M1-like, anti-tumoral phenotype represents a promising therapeutic strategy. In MM, inflammatory and anti-viral pathways promote disease progression. Interferon regulatory factor 4 (IRF4) is a transcription factor that has previously been shown to promote MM progression through enhanced survival of malignant plasma cells. Under physiological conditions, IRF4 also regulates macrophage polarization, where its expression drives alternative activation of macrophages primarily to M2-like (anti-inflammatory) phenotypes. However, the extent to which IRF4 activation or direct inhibition governs the polarization status and pro-tumoral activity of macrophages in the MM microenvironment has not been fully elucidated. In this study, we sought to further explore the role of IRF4 in macrophage polarization and investigated the potential of modulating IRF4 expression to reprogram myeloma-associated macrophages (MAMs) as a novel therapeutic strategy. First, we developed myeloma-tailored macrophage polarization models using primary human peripheral blood-derived monocytes and the THP1 monocyte cell line. Our M1 and M2 polarization models utilize a customized cocktail of myeloid growth factors and cytokines, supplemented with interleukin (IL)-6 to mimic the IL-6-enriched MM microenvironment. Characterization of M1 (CD80, TNFα, and CXCL10) and M2 (MRC1, FN1, and CCL22) markers confirmed the pres