Tissue-resident macrophages promote multiple myeloma early dissemination and progression via IL-6 and TNFa

Multiple myeloma (MM) is a plasma cell malignancy characterized by the presence of multiple foci in the skeleton. These distinct tumor foci indicate cycles of tumor growth and dissemination that seed new clusters and drives disease progression. Utilizing an intra-tibial Vk*MYC murine myeloma, we found that CD169 + radiation-resistant, tissue-resident macrophages (MPs) were critical for myeloma early dissemination and disease progression. While depletion of these MPs had no effect on tumor proliferation, it reduced myeloma BM egress and spreading to other bones, which improved overall survival as a single therapy and in combination with BM transplantation. Myeloma dissemination correlated with an increased inflammatory signature in BM MPs, as well as production of IL-6 and TNFα by tumor-associated MPs (TAMs). Exogenous i.v. IL-6 and TNFa could trigger myeloma intravasation in the BM by increasing vascular leakiness in the BM, and by enhancing myeloma motility by reducing CD138 adhesion. Moreover, mice lacking IL-6 had defects in myeloma dissemination as in MP-depleted recipients. While in TNFa or TNFaR deficient mice had defects in MM dissemination, engraftment was also impaired. These effects on myeloma dissemination required production of cytokines in the radiation-resistant compartment, containing these radiation-resistant BM MPs. Taken together, we propose BM egress of myeloma cells is regulated by localized inflammation in foci, driven in part by CD169 + MPs. To extend our analysis of Tumor associated macrophages (TAM) programming, we analyzed their transcriptome by RNA sequencing (RNA-seq). We FACS-purified TAMs (F4/80high, CD11blow) from bone marrow of myeloma bearing mice that both were in contact (TAM-IC) or not in contact (TAM-NIC) with myeloma, on the basis of GFP expression as well as BM macrophages from naïve mice as controls (Naive) for comparison. In addition, Vk*MYC myeloma (GFPhigh, CD138high) and polyclonal (GFP- CD138high B220low) BM PCs from naïve mice were also FACS-sorted and analyzed to investigate myeloma-macrophage interactions. Sorted MPs were clustered into distinct groups based on principal component analysis. Next, we performed pair-wise comparisons of the numbers of differentially expressed genes (DEGs) shared and uniquely expressed by the three MP subsets and found that a majority of DEGs were specific to each subset, even when comparing the two different TAM MPs.