Immunomodulatory molecule PD-L1 is expressed on malignant plasma cells and myeloma-propagating pre-plasma cells in the bone marrow of multiple myeloma patients

Biomarkers for checkpoint inhibition in hematologic malignancies

In the past few years we have seen remarkable paradigm shifts in the treatment of many solid tumors due to the introduction of inhibitors targeting immune checkpoints such as PD-1/PD-L1 and CTLA-4. Recent results indicate that checkpoint inhibition also represents a very promising approach for certain types of hematologic malignancies. Unfortunately, treatment with checkpoint inhibitors is also associated with substantial toxicities and high costs and only a subset of patients appears to derive clinical benefit from these treatments. This demonstrates the urgent need for biomarkers for the identification of patient populations that are likely to respond to this type of therapy and/or have fewer side effects. Here, we have reviewed available information on the prognostic and predictive value of biomarkers for anti-CTLA-4 and anti-PD-1/PD-L1 as the most commonly used checkpoint inhibitors. There are currently no reliable biomarkers capable of predicting responses to anti-CTLA-4 agents, such as ipilimumab, in hematologic malignancies. Certain polymorphisms in the CTLA-4 gene, however, seem to have an impact on the patients' outcome, especially in the case of chronic lymphocytic leukemia (CLL). There is now sufficient data supporting PD-L1 expression levels in the tumor tissue as an independent prognostic factor in B cell lymphomas such as diffuse large B-cell lymphoma (DLBCL). Overexpression of PD-L1 in the tumor tissue and elevated serum levels of soluble PD-L1 may also represent adverse prognostic factors in certain subtypes of T cell lymphomas. Finally, expression levels of PD-L1 also seem to predict responses to anti-PD-1/PD-L1 approaches in patients with Hodgkin lymphoma. Future studies will have to further delineate the prognostic/predictive role of PD-L1 expression as a biomarker in hematologic malignancies and may be able to identify confounding variables, which will hopefully to some extent be generalizable to other types of anti-tumor immunotherapies.

Established and Novel Prognostic Biomarkers in Multiple Myeloma

Multiple myeloma (MM) is an incurable plasma cell malignancy characterized by notable interpatient heterogeneity. There have been important advances in therapy and overall survival, but some patients with high-risk features still have poor survival rates. Therefore, accurate identification of this subset of patients has been integral to improvement of patient outcome. During the last few years, cytogenetics, gene expression profiling, MRI and PET/CT, as well as serum free light chain assays have been used as accurate biomarkers to better characterize the diverse course and outcome of the disease. With the recent advances of massive parallel sequencing techniques, the development of new models that better stratify high-risk groups are beginning to be developed. The use of multiparameter flow cytometry and next-generation sequencing have paved the way for assessment of minimal residual disease and better prognostication of post-therapeutic outcomes. Circulating tumor cells and circulating tumor DNA are promising potential biomarkers that demonstrate the spatial and temporal heterogeneity of MM. Finally, more prognostic markers are being developed that are specific to immunotherapeutic agents. In this review, we discuss these traditional and novel biomarkers that have been developed for MM and also those that can predict disease progression from precursor stages. Together, these biomarkers will help improve our understanding of the intrapatient and interpatient variabilities and help develop precision medicine for patients with high-risk MM.

Immunotherapy Strategies Against Multiple Myeloma

Multiple myeloma is a monoclonal B-cell malignancy characterized by an accumulation of malignant plasma cells in the bone marrow, the presence of a monoclonal protein in the serum and/or urine, decreased normal immunoglobulin levels, and lytic bone disease. Patients with multiple myeloma benefit from combination therapy including novel therapeutic agents followed by autologous stem cell transplantation prolonged maintenance therapy. However, multiple myeloma remains incurable; most patients with multiple myeloma will eventually become resistant to chemotherapy, and progression or relapse of the disease is inevitable. Immunotherapy represents a novel therapeutic approach with few adverse effects and good targeting capability that might be a powerful pool to allow long-term control of minimal residual disease. This article reviews the literature evaluating 4 major immunotherapeutic approaches for multiple myeloma including cellular immunotherapy, humoral immunotherapy, radio immunotherapy, and immunomodulation.

Monoclonal Antibody: A New Treatment Strategy against Multiple Myeloma

2015 was a groundbreaking year for the multiple myeloma community partly due to the breakthrough approval of the first two monoclonal antibodies in the treatment for patients with relapsed and refractory disease. Despite early disappointments, monoclonal antibodies targeting CD38 (daratumumab) and signaling lymphocytic activation molecule F7 (SLAMF7) (elotuzumab) have become available for patients with multiple myeloma in the same year. Specifically, phase 3 clinical trials of combination therapies incorporating daratumumab or elotuzumab indicate both efficacy and a very favorable toxicity profile. These therapeutic monoclonal antibodies for multiple myeloma can kill target cells via antibody-dependent cell-mediated cytotoxicity, complement-dependent cytotoxicity, and antibody-dependent phagocytosis, as well as by direct blockade of signaling cascades. In addition, their immunomodulatory effects may simultaneously inhibit the immunosuppressive bone marrow microenvironment and restore the key function of immune effector cells. In this review, we focus on monoclonal antibodies that have shown clinical efficacy or promising preclinical anti-multiple myeloma activities that warrant further clinical development. We summarize mechanisms that account for the in vitro and in vivo anti-myeloma effects of these monoclonal antibodies, as well as relevant preclinical and clinical results. Monoclonal antibody-based immunotherapies have already and will continue to transform the treatment landscape in multiple myeloma.

CAR T cell therapy for multiple myeloma: where are we now and where are we headed?

While recent progress has been made in the management of multiple myeloma, it remains a highly fatal malignancy especially among patients with relapsed-refractory disease. Immunotherapy with adoptive T cells targeting myeloma-associated antigens are at various stages of development and have brought about a new hope for cure. This is a review on the emerging field of adoptively transferred engineered T cell based approaches, with an in-depth focus on chimeric antigen receptors (CAR) targeting multiple myeloma. The recent results from CAR T cells targeting B cell maturation antigen are encouraging but eventual resistance to the CAR T cell therapies remain problematic. With newer approaches in therapies for multiple myeloma, the role of transplantation is evolved to form a platform for T cell therapies.

Activation of NK cells and disruption of PD-L1/PD-1 axis: two different ways for lenalidomide to block myeloma progression

Natural Killer (NK) cells play a critical role against tumor cells in hematological malignancies. Their activating receptors are essential in tumor cell killing. In Multiple Myeloma (MM) patients, NK cell differentiation, activation and cytotoxic potential are strongly impaired leading to MM escape from immune surveillance in tissues and bone marrow. Mechanisms used by MM to affect NK cell functions are mediated by the release of soluble factors, the expression of activating and inhibitory NK cell ligands, and the expression of immune check-point inhibitors. Lenalidomide represents an efficient clinical approach in MM treatment to improve patients' survival. Lenalidomide does not only promotes tumor apoptosis, but also stimulates T and NK cells, thereby facilitating NK-mediated tumor recognition and killing. This occurs since Lenalidomide acts on several critical points: stimulates T cell proliferation and cytokine secretion; decreases the expression of the immune check-point inhibitor Programmed Death-1 (PD-1) on both T and NK cells in MM patients; decreases the expression of both PD-1 and PD-L1 on MM cells; promotes MM cell death and abrogates MM/stromal microenvironment cross-talk, a process known to promote the MM cell survival and proliferation. This leads to the inhibition of the negative signal induced by PD-1/PD-L1 axis on NK cells, restoring NK cell cytotoxic functions. Given the importance of an effective immune response to counteract the MM progression and the promising approaches using anti-PD-1/PD-L1 strategies, we will discuss in this review how Lenalidomide could represent an adequate approach to re-establish the recognition against MM by exhausted NK cell.

Novel Immunotherapies for Multiple Myeloma

PURPOSE OF REVIEW

The treatment landscape of multiple myeloma is rapidly changing; however, despite improvement in patients' survival, it still remains a largely incurable disease. One hallmark of myeloma is substantial immune dysfunction leading to an increased infection rate and the inability of immune surveillance to detect neoplastic cells. Here, we critically analyze clinical approaches to harness the immune system to overcome this defect with a focus on antibody based and adoptive cellular therapies.

RECENT FINDINGS

Clinical trials exploring these immunotherapies to treat myeloma are now well underway and show promising results. In relapsed myeloma, monoclonal antibodies directed against plasma cell antigens and immune checkpoints have already shown substantial efficacy. In parallel, trials of adoptive cellular therapy have exciting promise in myeloma, having induced dramatic responses in a handful of early study participants. Taken together, immunotherapeutic approaches hold enormous potential in the field of multiple myeloma and in the near future can be combined with or even replace the current standard of care.

Monoclonal Antibodies in Multiple Myeloma: A New Wave of the Future

In 2015, 2 monoclonal antibodies were approved for the treatment of relapsed or refractory multiple myeloma (RRMM), elotuzumab and daratumumab. Elotuzumab is a monoclonal IgG-κ antibody directed against SLAMF7 (signaling lymphocytic activation molecule F7), a cell surface receptor involved in natural killer cell activation. Daratumumab is a monoclonal IgG-κ antibody that binds to CD38, a transmembrane protein found on the surface of myeloma cells and responsible for cellular adhesion and ectoenzymatic activity. Both elotuzumab and daratumumab act through recruitment of the immune system to enhance cellular cytotoxicity directed against myeloma cells. Elotuzumab requires lenalidomide and dexamethasone combined to enhance progression-free survival in patients with RRMM, and daratumumab has both single-agent and combination activity with either lenalidomide or the proteasome inhibitor bortezomib in RRMM. The adverse effect profile of both agents mainly consists of allergic-type infusion reactions. Other considerations for monoclonal antibody use in the treatment of MM include the potential for interference in serum protein electrophoresis testing and cross-reactivity of daratumumab with CD38 present on red blood cells. In the present report, we discussed the clinical development of daratumumab and elotuzumab and newer immunologic approaches to the treatment of MM.

The multi-faceted potential of CD38 antibody targeting in multiple myeloma

CD38, an adenine dinucleotide phosphate (ADP) ribose cyclase and a cyclic ADP ribose hydrolase, is widely expressed on the surface of multiple myeloma (MM) cells. It is known to play a pivotal role in the downstream pathways that mediate MM cell growth, signal transduction, and adhesion. The clinical use of CD38 monoclonal antibodies (MoAbs), such as daratumumab, either as monotherapy or in combination with other anti-MM agents, has produced impressive results in patients who have failed standard MM therapy. CD38 MoAbs exhibit several cytotoxic mechanisms on MM cells. In addition to the classical effector mechanisms associated with antibody therapy, CD38 MoAbs induce MM apoptosis and clonal T-cell expansion. Here, we summarize the results of some pivotal clinical studies using a human CD38 MoAb, daratumumab, in patients with MM, discuss the anti-MM effector mechanisms induced by CD38 MoAbs, and review the potential tumor antigens that may be suitable targets for immunotherapy of MM. Finally, we present a paradigm of immunotherapy for MM patients using CD38 MoAbs followed by GM-CSF and an immune checkpoint inhibitor in patients who have undergone high dose chemotherapy and autologous stem cell transplant. CD38 MoAbs have emerged as a novel and ultimately very promising immunotherapeutic agent for MM because of its ability to induce MM cytotoxicity through both arms of the adaptive immune responses.

Checkpoint inhibitors in hematological malignancies

Inhibitory molecules such as PD-1, CTLA-4, LAG-3, or TIM-3 play a role to keep a balance in immune function. However, many cancers exploit such molecules to escape immune surveillance. Accumulating data support that their functions are dysregulated in lymphoid neoplasms, including plasma cell myeloma, myelodysplastic syndrome, and acute myeloid leukemia. In lymphoid neoplasms, aberrations in 9p24.1 (PD-L1, PD-L2, and JAK2 locus), latent Epstein-Barr virus infection, PD-L1 3'-untranslated region disruption, and constitutive JAK-STAT pathway are known mechanisms to induce PD-L1 expression in lymphoma cells. Clinical trials demonstrated that PD-1 blockade is an attractive way to restore host's immune function in hematological malignancies, particularly classical Hodgkin lymphoma. Numerous clinical trials exploring PD-1 blockade as a single therapy or in combination with other immune checkpoint inhibitors in patients with hematologic cancers are under way. Although impressive clinical response is observed with immune checkpoint inhibitors in patients with certain cancers, not all patients respond to immune checkpoint inhibitors. Therefore, to identify best candidates who would have excellent response to checkpoint inhibitors is of utmost importance. Several possible biomarkers are available, but consensus has not been made and pursuit to discover the best biomarker is ongoing.

Immune checkpoint blockade: the role of PD-1-PD-L axis in lymphoid malignancies

The co-inhibitory receptor programmed cell death (PD)-1, expressed by immune effector cells, is credited with a protective role for normal tissue during immune responses, by limiting the extent of effector activation. Its presently known ligands, programmed death ligands (PD-Ls) 1 and 2, are expressed by a variety of cells including cancer cells, suggesting a role for these molecules as an immune evasion mechanism. Blocking of the PD-1-PD-L signaling axis has recently been shown to be effective and was clinically approved in relapsed/refractory tumors such as malignant melanoma and lung cancer, but also classical Hodgkin’s lymphoma. A plethora of trials exploring PD-1 blockade in cancer are ongoing. Here, we review the role of PD-1 signaling in lymphoid malignancies, and the latest results of trials investigating PD-1 or PD-L1 blocking agents in this group of diseases. Early phase studies proved very promising, leading to the clinical approval of a PD-1 blocking agent in Hodgkin’s lymphoma, and Phase III clinical studies are either planned or ongoing in most lymphoid malignancies.

Anti-PD-1/PD-L1 therapy for infectious diseases: learning from the cancer paradigm

OBJECTIVES

Immune checkpoint pathways regulate optimal host immune responses against transformed cells, induce immunological memory, and limit tissue pathology. Conversely, aberrant immune checkpoint activity signifies a poor prognosis in cancer and infectious diseases. Host-directed therapy (HDT) via immune checkpoint blockade has revolutionized cancer treatment with therapeutic implications for chronic infections, thus laying the foundation for this review.

METHODS

Online literature searches were performed via PubMed, PubMed Central, and Google using the keywords "immune checkpoint inhibition"; "host-directed therapy"; "T cell exhaustion"; "cancer immunotherapy"; "anti-PD-1 therapy"; "anti-PD-L1 therapy"; "chronic infections"; "antigen-specific cells"; "tuberculosis"; "malaria"; "viral infections"; "human immunodeficiency virus"; "hepatitis B virus"; "hepatitis C virus"; "cytomegalovirus" and "Epstein-Barr virus". Search results were filtered based on relevance to the topics covered in this review.

RESULTS

The use of monoclonal antibodies directed against the antigen-experienced T-cell marker programmed cell death 1 (PD-1) and its ligand PD-L1 in the context of chronic infectious diseases is reviewed. The potential pitfalls and precautions, based on clinical experience from treating patients with cancer with PD-1/PD-L1 pathway inhibitors, are also described.

CONCLUSIONS

Anti-PD-1/PD-L1 therapy holds promise as adjunctive therapy for chronic infectious diseases such as tuberculosis and HIV, and must therefore be tested in randomized clinical trials.

Targeting the PD-1/PD-L1 axis in multiple myeloma: a dream or a reality?

The programmed cell death protein 1 (PD-1)/programmed cell death ligand 1 (PD-L1) pathway is a negative regulator of immune activation that is upregulated in multiple myeloma and is a critical component of the immunosuppressive tumor microenvironment. Expression is increased in advanced disease and in the presence of bone marrow stromal cells. PD-1/PD-L1 blockade is associated with tumor regression in several malignancies, but single-agent activity is limited in myeloma patients. Combination therapy involving strategies to expand myeloma-specific T cells and T-cell activation via PD-1/PD-L1 blockade are currently being explored.

Genetically Modified T-Cell-Based Adoptive Immunotherapy in Hematological Malignancies

A significant proportion of hematological malignancies remain limited in treatment options. Immune system modulation serves as a promising therapeutic approach to eliminate malignant cells. Cytotoxic T lymphocytes (CTLs) play a central role in antitumor immunity; unfortunately, nonspecific approaches for targeted recognition of tumor cells by CTLs to mediate tumor immune evasion in hematological malignancies imply multiple mechanisms, which may or may not be clinically relevant. Recently, genetically modified T-cell-based adoptive immunotherapy approaches, including chimeric antigen receptor (CAR) T-cell therapy and engineered T-cell receptor (TCR) T-cell therapy, promise to overcome immune evasion by redirecting the specificity of CTLs to tumor cells. In clinic trials, CAR-T-cell- and TCR-T-cell-based adoptive immunotherapy have produced encouraging clinical outcomes, thereby demonstrating their therapeutic potential in mitigating tumor development. The purpose of the present review is to (1) provide a detailed overview of the multiple mechanisms for immune evasion related with T-cell-based therapies; (2) provide a current summary of the applications of CAR-T-cell- as well as neoantigen-specific TCR-T-cell-based adoptive immunotherapy and routes taken to overcome immune evasion; and (3) evaluate alternative approaches targeting immune evasion via optimization of CAR-T and TCR-T-cell immunotherapies.

A look backward and forward in the regulatory and treatment history of multiple myeloma: Approval of novel-novel agents, new drug development, and longer patient survival

The past decade has seen significant advances in our understanding and treatment of multiple myeloma (MM) and its precursor diseases. These advances include gains in knowledge of the underlying pathobiology including molecular and cellular prognostic factors for disease progression. In parallel we have witnessed the availability of novel therapeutics. Together these advances have translated into improvements in long-term clinical benefit and survival in MM. Indeed, it has been shown that patients diagnosed in the last decade have experienced almost doubling of median survival time. We aim to review and give further insight into drug development and novel drug approvals that have revolutionized the treatment of MM.

Mesenchymal Stromal Cells Can Regulate the Immune Response in the Tumor Microenvironment

The tumor microenvironment is a good target for therapy in solid tumors and hematological malignancies. Indeed, solid tumor cells' growth and expansion can influence neighboring cells' behavior, leading to a modulation of mesenchymal stromal cell (MSC) activities and remodeling of extracellular matrix components. This leads to an altered microenvironment, where reparative mechanisms, in the presence of sub-acute inflammation, are not able to reconstitute healthy tissue. Carcinoma cells can undergo epithelial mesenchymal transition (EMT), a key step to generate metastasis; these mesenchymal-like cells display the functional behavior of MSC. Furthermore, MSC can support the survival and growth of leukemic cells within bone marrow participating in the leukemic cell niche. Notably, MSC can inhibit the anti-tumor immune response through either carcinoma-associated fibroblasts or bone marrow stromal cells. Experimental data have indicated their relevance in regulating cytolytic effector lymphocytes of the innate and adaptive arms of the immune system. Herein, we will discuss some of the evidence in hematological malignancies and solid tumors. In particular, we will focus our attention on the means by which it is conceivable to inhibit MSC-mediated immune suppression and trigger anti-tumor innate immunity.

T cells in multiple myeloma display features of exhaustion and senescence at the tumor site

BACKGROUND

Multiple myeloma is an incurable plasma cell malignancy that is mostly restricted to the bone marrow. Cancer-induced dysfunction of cytotoxic T cells at the tumor site may be responsible for immune evasion and therapeutical failure of immunotherapies. Therefore, enhanced knowledge about the actual status of T cells in myeloma bone marrow is urgently needed. Here, we assessed the expression of inhibitory molecules PD-1, CTLA-4, 2B4, CD160, senescence marker CD57, and CD28 on T cells of naive and treated myeloma patients in the bone marrow and peripheral blood and collected data on T cell subset distribution in both compartments. In addition, T cell function concerning proliferation and expression of T-bet, IL-2, IFNγ, and CD107a was investigated after in vitro stimulation by CD3/CD28. Finally, data was compared to healthy, age-matched donor T cells from both compartments.

METHODS

Multicolor flow cytometry was utilized for the analyses of surface molecules, intracellular staining of cytokines was also performed by flow cytometry, and proliferation was assessed by ³H-thymidine incorporation. Statistical analyses were performed utilizing unpaired T test and Mann-Whitney U test.

RESULTS

We observed enhanced T cell exhaustion and senescence especially at the tumor site. CD8+ T cells expressed several molecules associated with T cell exhaustion (PD-1, CTLA-4, 2B4, CD160) and T cell senescence (CD57, lack of CD28). This phenotype was associated with lower proliferative capacity and impaired function. Despite a high expression of the transcription factor T-bet, CD8+ T cells from the tumor site failed to produce IFNγ after CD3/CD28 in vitro restimulation and displayed a reduced ability to degranulate in response to T cell stimuli. Notably, the percentage of senescent CD57+CD28- CD8+ T cells was significantly lower in treated myeloma patients when compared to untreated patients.

CONCLUSIONS

T cells from the bone marrow of myeloma patients were more severely impaired than peripheral T cells. While our data suggest that terminally differentiated cells are preferentially deleted by therapy, immune-checkpoint molecules were still present on T cells supporting the potential of checkpoint inhibitors to reactivate T cells in myeloma patients in combination therapies. However, additional avenues to restore anti-myeloma T cell responses are urgently needed.

miR-24, miR-30b and miR-142-3p interfere with antigen processing and presentation by primary macrophages and dendritic cells

Antigen uptake, processing and presentation by antigen presenting cells (APCs) are tightly coupled processes which consequently lead to the activation of innate and adaptive immune responses. However, the regulatory role of microRNA (miRNAs) in these critical pathways is poorly understood. In this study, we show that overexpression of miR-24, miR-30b and miR-142-3p attenuates uptake and processing of soluble antigen ovalbumin (Ova) in primary human macrophages and dendritic cells. MiRNA mimic transfected APCs exhibit defects in antigen presentation (Ova and CMV antigen) to CD4+ T-cells leading to reduced cell proliferation. Using transgenic OT-II mice we demonstrated that this impairment in T-cell proliferation is specific to antigen provided i.e., Ova. Further, human T-cells co-cultured with miRNA transfected dendritic cells secrete low levels of T helper (Th)-1 polarization associated cytokines. Analysis of molecules regulating APC and T-cell receptor interaction shows miRNA-mediated induced expression of Programmed Death-Ligand 1 (PD-L1) which inhibits T-cell proliferation. Blocking PD-L1 with antibodies rescues miRNA-mediated inhibition of T cell priming by DCs. These results uncover regulatory functions of miR-24, miR-30b and miR-142-3p in pairing innate and adaptive components of immunity.

Programmed death-1 immune checkpoint blockade in the treatment of hematological malignancies

The use of tumor-specific monoclonal antibodies (MAbs) has revolutionize the field of cancer immunotherapy. Although treatment of malignant diseases with MAbs is promising, many patients fail to respond or relapse after an initial response. Both solid tumors and hematological malignancies develop mechanisms that enable them to evade the host immune system by usurping immune checkpoint pathways such as PD-1, PD-2, PDL-1, or PDL-2 (programmed cell death protein-1 or 2 and PD-Ligand 1 or 2), which are expressed on activated T cells and on T-regulatory, B cells, natural killers, monocytes, and dendritic cells. One of the most exciting anticancer development in recent years has been the immune checkpoint blockade therapy by using MAbs against immune checkpoint receptor and/or ligands. Anti-PD1 antibodies have been tested in clinical studies that included patients with hematological malignancies and showed remarkable efficacy in Hodgkin lymphoma (HL). In our review, we will focus on the effect of PD-1 activation on hematological malignancies and its role as a therapeutic target. Key messages The programmed death 1 (PD1) immune checkpoint is an important homeostatic mechanism of the immune system that helps in preventing autoimmunity and uncontrolled inflammation in cases of chronic infections. However, PD1 pathway is also operated by a wide variety of malignancies and represents one of the most important mechanisms by which tumor cells escape from the surveillance of the immune system. Blocking of immune checkpoints by the use of monoclonal antibodies opened a new era in the field of cancer immunotherapy. Results from clinical trials are promising, and currently, this approach has been proven effective and safe in patients with solid tumors and hematological malignancies.

The Role of Immunotherapy in Multiple Myeloma

Multiple myeloma is the second most common hematologic malignancy. The treatment of this disease has changed considerably over the last two decades with the introduction to the clinical practice of novel agents such as proteasome inhibitors and immunomodulatory drugs. Basic research efforts towards better understanding of normal and missing immune surveillence in myeloma have led to development of new strategies and therapies that require the engagement of the immune system. Many of these treatments are under clinical development and have already started providing encouraging results. We, for the second time in the last two decades, are about to witness another shift of the paradigm in the management of this ailment. This review will summarize the major approaches in myeloma immunotherapies.

Dissecting the multiple myeloma-bone microenvironment reveals new therapeutic opportunities

Multiple myeloma is a plasma cell skeletal malignancy. While therapeutic agents such as bortezomib and lenalidomide have significantly improved overall survival, the disease is currently incurable with the emergence of drug resistance limiting the efficacy of chemotherapeutic strategies. Failure to cure the disease is in part due to the underlying genetic heterogeneity of the cancer. Myeloma progression is critically dependent on the surrounding microenvironment. Defining the interactions between myeloma cells and the more genetically stable hematopoietic and mesenchymal components of the bone microenvironment is critical for the development of new therapeutic targets. In this review, we discuss recent advances in our understanding of how microenvironmental elements contribute to myeloma progression and, therapeutically, how those elements can or are currently being targeted in a bid to eradicate the disease.