Preclinical validation of interleukin 6 as a therapeutic target in multiple myeloma
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- Rosean, T.R., Tompkins, V.S., Tricot, G. et al. Immunol Res (2014) 59: 188. doi:10.1007/s12026-014-8528-x
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Studies on the biologic and molecular genetic underpinnings of multiple myeloma (MM) have identified the pleiotropic, pro-inflammatory cytokine, interleukin-6 (IL-6), as a factor crucial to the growth, proliferation and survival of myeloma cells. IL-6 is also a potent stimulator of osteoclastogenesis and a sculptor of the tumor microenvironment in the bone marrow of patients with myeloma. This knowledge has engendered considerable interest in targeting IL-6 for therapeutic purposes, using a variety of antibody- and small-molecule-based therapies. However, despite the early recognition of the importance of IL-6 for myeloma and the steady progress in our knowledge of IL-6 in normal and malignant development of plasma cells, additional efforts will be required to translate the promise of IL-6 as a target for new myeloma therapies into significant clinical benefits for patients with myeloma. This review summarizes published research on the role of IL-6 in myeloma development and describes ongoing efforts by the University of Iowa Myeloma Multidisciplinary Oncology Group to develop new approaches to the design and testing of IL-6-targeted therapies and preventions of MM.
KeywordsIL-6 signaling in neoplastic plasma cellsMyeloma stem cells and minimal residual diseaseSmall-drug- and monoclonal antibody-based inhibitorsGenetically engineered mouse models of human myeloma
Interleukin-6 (IL-6) plays a prominent role in multiple myeloma (MM)
Complexity of IL-6 signaling in myeloma cells
Two principal sources of IL-6 in myeloma
IL-6 released by BMSCs may contribute to the immunosuppressive milieu in the TME of MM
IL-6 may shape, in part, the complex and poorly understood bidirectional interaction of myeloma cells with the surrounding stroma, which transforms the BM microenvironment into a tumor-promoting, bone-resorbing and immunosuppressive milieu (Fig. 2d). Immunosuppression in the TME of MM may be of great importance for the design and testing of novel myeloma therapies, because it suggests that drug-based approaches for killing myeloma cells should be combined with immune-based interventions to reverse the local immune suppression and unleash an effective anti-myeloma immune response. Besides IL-6-reactive suppressor T cells, such as CD4+ regulatory T (Treg) cells, distinct populations of myeloid cells, designated myeloid-derived suppressor cells, may block anti-myeloma immune responses . Myeloid lineage cells, such as macrophages, mast cells, dendritic cells and eosinophils, have long been recognized as significant sources of IL-6 in the BM microenvironment, and the myeloma-promoting role of eosinophils has been demonstrated recently . Eosinophils were also shown to be critical for maintaining survival niches of normal long-lived plasma cells in the BM , by virtue of secreting IL-6, IL-4, APRIL and other cytokines . IL-6 plays an important role in T-cell differentiation and activation; e.g., it functions as a major regulator of the balance between Treg cells and pro-inflammatory, IL-17-producing CD4+ T cells called Th17 cells. In conjunction with transforming growth factor-β, IL-6 stimulates the generation of Th17 cells but inhibits the induction of Treg cells. This results in an IL-6-dependent, pro-inflammatory environment, in which tumor-suppressive Treg responses are down regulated and may facilitate malignant growth, as Th17 cells have been shown to promote colon cancer in humans  and mice . On the other hand, Th17 cells have also been implicated as regulators of immune surveillance , suggesting that this subset of helper T cells may also have tumor-suppressive functions. What role Th17 cells play in myelomagenesis is not known. The role of CD8 T cells is in myeloma is similarly unclear, with some evidence pointing to tumor inhibition (cytotoxicity to myeloma cells) and other findings pointing to tumor promotion (see legend to Fig. 2d for details). Additional research on the inner mechanics of the immune system in myeloma is certainly warranted. These results and other findings not discussed have generated a great deal of interest in blocking IL-6 production specifically in BMSCs as a means to overcome the immune-suppressive microenvironment in MM.
Role of IL-6 in myeloma stemness, minimal residual disease (MRD) and targeted therapies
A large body of clinical evidence indicates that a small but therapy-resistant subpopulation of myeloma cells comprises the principal roadblock to curing myeloma . Depending on the investigator team, these cells are variably referred to as myeloma-initiating, myeloma-propagating, transit-amplifying, dormant or drug-resistant myeloma cells or, a term the UI Myeloma MOG prefers, multiple myeloma stem cells (MMSCs) [36–40]. The tight association of these cells with minimal residual disease (MRD) , acquired drug resistance and relapsed myeloma underscores the urgency to develop new therapies for targeting the MMSC  in order to achieve the long-term goal of finding a cure for myeloma (Fig. 3a); . We acknowledge that the MMSC model has not been fully embraced by the myeloma community, owing in part to a lack of a coherent set of criteria for defining these cells unambiguously [44–48]. Nonetheless, the model has catalyzed myeloma research, enhanced our appreciation of the complexity of myeloma biology and promoted the development of small-drug inhibitors and immunotherapies in myeloma (Fig. 3b). The latter include monoclonal antibodies, chimeric antigen receptor (CAR)-T cells and dendritic cell (DC) vaccines. CAR-T cells harbor an engineered T-cell receptor that consists of three parts: the antibody portion, which denotes specificity; a suicide signal for programed cell death; and a co-stimulatory region that contains both CD28, a potent proliferation signal, and 41BB, a critical survival signal . CAR-T cells reactive to CD19 have been successfully used for treatment of B-cell lymphoma . Potential targets for CAR-T cells in myeloma include an isoform of CD44, CD44v6 , B-cell maturation antigen (BCMA)  and syndecan-1 (CD138) . DC vaccination strategies rely on professional antigen-presenting cells  that are generally suppressive in cancer  yet critical in the development of functional anti-tumor immune responses . Two methods have been explored to devise DC-based myeloma vaccines. The first involves stimulating DCs ex vivo with tumor cell lysates, which allows the DCs to cross-present the tumor antigens to CD8+ T cells. A phase 2 study using DCs that had been loaded with Id (immunoglobulin idiotype peptides) revealed the vaccine to be safe, albeit not particularly effective . Another study that employed DCs loaded with RNA that encodes certain myeloma antigens (MAGE3, survivin, BCMA) induced the intended immune response in myeloma patients, with 3 of 12 patients experiencing a complete remission and the majority of the remaining patients demonstrating a partial response . The second method relies on the fusion of DCs with myeloma cells, followed by the adoptive transfer of the DC/MM hybrids to patients. This approach has also shown promise in clinical trials, which demonstrated both stabilization of disease due to expansion of activated CD4+ T cells and CTLs in vivo  and compatibility with ASCT-based frontline therapies of myeloma . It is possible that the therapeutic approaches depicted in Fig. 3b will synergize with emerging IL-6/JAK/STAT3-targeted drugs to kill MMSCs in BM survival niches. However, more needs to be learned about the specific role of IL-6 in MMSCs before combination therapies of this sort can be seriously considered. The importance of IL-6 for breast cancer stem cells [61, 62], including mammosphere-based in vitro models of breast cancer stemness , is firmly established. IL-6/JAK/STAT3 signaling also plays an important role in cancer stem cell-like cells in lung cancer , colon cancer [65, 66], glioblastoma  and liver cancer . These studies provide a blueprint for future work aimed at elucidating the role of IL-6 in MMSCs.
Emerging IL-6 inhibitory strategies to treat and prevent myeloma
The considerations presented above suggest that targeting IL-6 production in the TME affords a viable strategy for the treatment and prevention of MM. Indeed, the lingering pessimism that followed the disappointing results of early clinical trials using an IL-6-targeted monoclonal antibody in myeloma patients  has recently been lifted as a consequence of exciting new results with agents that either nonspecifically inhibit IL-6 as part of their highly pleiotropic action (e.g., lenalidomide and related immunomodulatory drugs) or specifically target IL-6 in both myeloma and BMSCs . Specific inhibition of IL-6 can be achieved with monoclonal antibodies (mAbs) to IL-6 (siltuximab) or the IL-6R (tocilizumab), with recombinant proteins that function as IL-6R antagonists, or with small molecules that inhibit the cellular IL-6 signal transduction pathway or the cross talk of that pathway with other signaling networks in myeloma (Fig. 3c). Among the recombinant proteins mentioned above is Sant-7, FE999301, and a newly designed immunotoxin, designated IL6(T23)-PE38KDEL, that consists of IL-6 and Pseudomonas exotoxin and exhibits efficacy in a mouse models of MM . Small-molecule inhibitors of IL-6 signaling include drugs that target JAK tyrosine kinases, which are now in clinical trial for lymphoma , but also show promise for myeloma as evidenced by down-regulation of survival and angiogenesis pathways in myeloma [73–75]. Additional therapeutic opportunities (not included in Fig. 3c) are provided by compounds that either inhibit biochemical pathways that can activate IL-6 signaling indirectly (e.g., NFκB inhibitors , such as 6-amino-4-quinazoline ; Hsp90 inhibitors, such as SNX-2112 ; PI3 K/mTor inhibitors, such as NVP-BEZ235 ) or activate biochemical pathways that can inhibit IL-6 signaling indirectly (e.g., PPARγ inhibitors ). The most effective use of new IL-6-targeted therapies will depend in large measure on enhanced understanding of the biological role of IL-6 in myeloma, and on identifying the patients that may benefit the most. We believe the mouse models described in the following will make a nice contribution to the preclinical validation of IL-6-targeted therapies for myeloma.
IL-6-dependent plasma-cell tumors (PCTs) in laboratory mice
Peritoneal plasmacytoma in inbred BALB/c mice
Accelerated transgenic mouse models of IL-6-driven PCT
Interleukin-6-dependent mouse models of human myeloma may further our understanding of the role of IL-6 in the natural history of myeloma and permit us to test emerging IL-6/JAK/STAT3-targeted therapies in the genetically and environmentally controlled setting of a preclinical mouse study. Immune-competent mouse models of myeloma, such as iMycIL6, may also provide a good opportunity to elucidate the mechanism by which the pro-inflammatory, immune-suppressive bone marrow microenvironment promotes myeloma development and acquisition of drug resistance in myeloma. In keeping with the spirit of the recently proposed blueprint for a cure of myeloma , IL-6-targeted therapies may afford further improvements for patients with MM. A potentially promising approach is therapies that combine the targeting of IL-6 in the myeloma microenvironment with immune-based strategies for eradicating MM stem cells and minimal residual disease. However, despite enormous progress in cancer immunotherapy in the past few years, additional research is warranted to translate this promise to myeloma. Outstanding problems include the optimization of DC vaccination methods (e.g., development of more effective delivery vehicles) and the incorporation of CAR-T cells in therapeutic regimens that mainly rely at this juncture on monoclonal antibodies. The success and failure of the much heralded new era of individualized myeloma therapy will depend, in part, on the effective collaboration of basic, translational and clinical scientists in innovative frameworks such as the UI Myeloma MOG.
This research was performed by TRR in partial fulfillment of the requirements for the degree Doctor of Philosophy in the Graduate Immunology Program of the University of Iowa. He wishes to thank Dr. Kristin Ness for expert technical assistance and mouse husbandry. This work was supported in part by NIH Predoctoral Training Grant 5T32 AI007485 (to TRR), by R01CA152105 from the NCI (to FZ); by a Translational Research Program award from the Leukemia and Lymphoma Society (to FZ); by institutional start-up funds from the Department of Internal Medicine, Carver School of Medicine, University of Iowa (to FZ and GT); by P30CA086862 from the NCI; by a Senior Research Award from the Multiple Myeloma Research Foundation (to SJ); by a career development award from NCI P50CA097274 (to SJ); and by R01CA151354 from the NCI (to SJ).
Conflict of interest
The authors declare no competing financial interest.