Targeting mtDAMPed macrophages for MM therapy

In this issue of Blood, Jibril et al¹ demonstrate that multiple myeloma (MM) cells release cell-free, circulating mitochondrial DNA (mtDNA), a form of mitochondrial damage associated molecular patterns (mtDAMPs), which activate macrophages via the guanosine monophosphate-adenosine monophosphate synthase (cGAS)/guanosine monophosphate-adenosine monophosphate (cGAMP)/stimulating interferon gene (STING)-signaling pathway, thereby promoting the retention of tumor cells within the bone marrow (BM) microenvironment and disease progression in murine models.

Despite unprecedented therapeutic advances during the last 2 decades, MM remains as an incurable disease. Therefore, there is still an urgent need for more efficacious, well-tolerated drugs.² It is well established that the BM tumor microenvironment (TME) plays a fundamental role in MM pathogenesis. The multifaceted role of macrophages in this disease has only recently been elucidated. Tumor-associated macrophages (TAMs), predominantly immunosuppressive CD206⁺ M2 macrophages, are a fundamental component of the MM TME. They support MM cell homing to and colonization of the BM. In addition, they play a critical role in MM cell proliferation, survival, chemo-protection, drug resistance, as well as in direct immune suppression. Unlike macrophages derived from healthy donors, macrophages derived from the BM of patients with MM lack the ability to present antigens, to engulf tumor cells, and to stimulate adaptive immune responses.³ In addition, they downregulate expression of crucial cytotoxic T-cell factors. Attacking vulnerabilities of TAMs in the MM TME therefore represents a promising therapeutic avenue to further improve patient outcome.

Mitochondria are the “engines” of the cell but also are fundamental for amino acid metabolism, protein synthesis, gluconeogenesis, fatty acid oxidation, generation of reactive oxygen species (ROS), calcium homeostasis, and the initiation of apoptosis. Tumor cells are characterized by altered bioenergetic processes, such as an increased glucose metabolism, altered calcium regulation, altered ROS production, and inhibition of apoptotic processes. These changes may result, at least in part, from free cytoplasmic mtDNA, which characteristically contains unmethylated CpG nucleotide motifs and belongs to the group of mtDAMPs of the innate immune system.⁴ 

Our understanding of the cyclic cGAS/cGAMP/STING signaling pathway, which is responsible for danger sensing by the innate immune system, has grown dramatically. Upon mtDNA binding, the DNA sensor cGAS catalyzes the production of cGAMP, which in turn binds to and activates the adapter protein STING. Activated STING then migrates from the endoplasmic reticulum to the Golgi apparatus and activates downstream interferon regulatory transcription factor (IRF)-3 and NF-κB signaling cascades, thus inducing the expression of type I interferon and other inflammatory factors (eg, interleukin-6, tumor necrosis factor). cGAS and STING agonists may represent a promising strategy for cancer immunotherapy. Indeed, preclinical data demonstrated that STING agonists significantly promote antitumor immunity in acute myeloid leukemia, breast cancer, and small cell lung cancer.⁵ In the context of macrophages, cGAS agonists have been proposed to trigger antitumor effects via repolarization of tumor-promoting M2-type TAMs into M1-type inflammatory macrophages, leading to enhancement of major histocompatibility complex class molecules or costimulatory molecules that drive recruitment, maturation, activation, and differentiation of CD4⁺ and CD8⁺ T cells to produce intense antitumor responses.⁶ Based on these findings, there are ongoing preclinical and early clinical studies evaluating the ability of STING agonists in tumors cells or tumor-infiltrating immune cells (including dendritic cells) to elicit immunostimulatory effects, alone or in combination with a conventional chemo- and immunotherapeutics or radiotherapy. However, STING activation may also contribute to cancer initiation and progression, for example, by activating cancer-associated inflammation; by hampering the immune response through infiltration of the TME with immunosuppressive cells such as T-regulatory cells (Tregs), myeloid-derived suppressor cells, or TAMs; or by upregulating the expression of immune checkpoints, programmed death-ligand 1 (PD-L1) on tumor cells and programmed cell death protein 1 (PD-1) on T cells. Moreover, STING-induced activation of indolamine-2,3-dioxygenase restricts T-cell proliferation, while promoting Treg differentiation and hampering the antigen-presenting ability of dendritic cells.

In the present study, Jibril et al show elevated serum levels of free cytoplasmic mtDNA in the serum of mice engrafted with a human MM cell line versus healthy controls. Moreover, the authors demonstrate that mtDNA levels are higher in the peripheral blood of patients with MM compared with healthy controls and that they are further elevated in the MM BM. These data confirm previous data, which have linked elevated mtDNA levels to the transition of monoclonal gammopathy of undetermined significance to smoldering myeloma and MM and to disease progression.⁷ Together, they indicate a potential role for mtDNA as a valuable, new prognostic biomarker. Utilizing a xenograft as well as an isogeneic murine MM model, the authors then elegantly prove that mtDNA originates from tumor cells and that MM cell-derived mtDNA induces upregulation of chemokines in macrophages and promotes the retention of malignant plasma cells in the BM via activation of the cGAS/cGAMP/STING pathway. Conversely, STING inhibition by H151 resulted in the egress of MM cells from the BM, reducing the tumor volume and prolonging survival in the KaLwRij-5TGM1 mouse model. Of note, no significant differences were observed in the size or polarization of M1- or M2-type phenotypes upon STING inhibition (see figure). These data appear to conflict with recent data suggesting a therapeutic role for STING agonists in combination with bortezomib and with bortezomib and immune checkpoint inhibitors.^8,9 Further efforts are therefore needed to expand our knowledge on the multifaceted functional roles of mtDNA and cGAS/cGAMP/STING signaling in MM biology. For example, in MM, the pathophysiologic impact of mtDNAs, mtDNA mutations, and gene/copy number alterations, in particular,⁴ is mostly unknown. Likewise, more studies are needed to decipher the function of the cGAS/cGAMP/STING-signaling pathway on each specific cell within the BM as well as on the surrounding milieu. Ultimately, only the thorough evaluation of mtDNA and cGAS/cGAMP/STING functions within the BM TME will allow us to rationally design therapeutic strategies to exploit this pathway for our patients. Specifically, when should a STING agonists be used and when should a cGAS/STING antagonists be tried?

In summary, the results presented in this article raise a multitude of questions and will stimulate a wide range of follow-up studies on the role of mtDNA, macrophages, and the cGAS/cGAMP/STING pathway in MM and other malignancies.

Conflict-of-interest disclosure: The author declares no competing financial interests.