Myeloma Derived Mitochondrial Damage Associated Molecular Patterns Promote Pro-Tumoral Expansion By Inducing a Pro-Inflammatory Signature in the Bone Marrow Microenvironment

Multiple Myeloma (MM) is a malignancy of antibody producing B cells (plasma cells), leading to the accumulation of tumour cells within the bone marrow (BM) microenvironment. Mitochondrial DNA (mtDNA) is a damage associated molecular pattern (DAMP), the mitochondrial genome contains islands of unmethylated CpG nucleotide motifs that have been shown to activate and promote memory B cell proliferation and antibody secretion.

Recent studies have indicated that mtDNA is elevated in the circulation of trauma and cancer patients and have highlighted the need to determine the functional purpose of elevated mtDNA within the BM microenvironment. Here we investigate the hypothesis that multiple myeloma cells secrete mtDNA and other mtDAMPs to induce a pro-inflammatory bone marrow microenvironment, supportive of the progression and survival of MM. As well as investigating how MM affects the haematopoietic stem cell niche through establishing a syngeneic myeloma mouse model.

We show that immunocompromised NSG mice transplanted with human MM cell lines released higher levels of MM derived mtDNA detected in the plasma by real-time PCR. Furthermore, in vitro experiments showed that pro-inflammatory cytokines IL-1B, IL-6, and IL-8 were elevated in both primary and cell line myeloma samples upon treatment with CpG oligonucleotides. Murine bone marrow derived macrophages (BMDMs) treated with mtDNA or mtDAMPs showed increased expression of pro-inflammatory cytokines. To understand the significance of the mtDAMP proinflammatory induction, C57BL/6 mice were successfully engrafted with murine 5TGM1 myeloma cell line to establish a syngeneic mouse model. Analysis of the haematopoietic stem and progenitor cell populations showed that 5TGM1 induced the expansion of these cell populations when compared to control WT animals. To understand the significance of mtDAMPs in regulating this HSPC expansion we used blocking antibodies to TLR9 (receptor for mtDNA) and FPR1 (receptor for formylated mitochondrial proteins) in 5TGM1 engrafted animals. Results show that myeloma tumour burden was reduced when FPR1 and TLR9 are blocked compared to control animals.

Here we highlight the involvement of mtDAMPs in MM to create a favourable proinflammatory microenvironment which provides the conditions for MM survival and proliferation. This data highlights the importance of mtDAMPs in MM disease progression and suggests targeting these activating receptor pathways may provide a novel therapeutic intervention.