Keywords

4.1 Introduction

Multiple myeloma (MM) is a common hematologic malignancy with approximately 20,000 new cases diagnosed each year. Disease progression is characterized by the clonal expansion of malignant plasma cells associated with the clinical sequelae of anemia, lytic bone lesions, renal dysfunction, and compromised immunity. The advent of biologic-based therapies such as lenalidomide and bortezomib has resulted in improved patient outcomes in the last 5–10 years. However, curative outcomes remain elusive. There has been an increased appreciation of the critical role host immunity plays in the evolution of disease and the potential therapeutic efficacy of immune-based therapies. These treatment approaches hold the potential promise of selective targeting of the malignant clone, disruption of stromal-plasma cell interactions, and generation of sustained antitumor immunity and durable response. However, the development of clinically efficacious immunotherapy is dependent on achieving greater understanding of the complex interactions between the immunologic milieu and disease.

4.2 Immune Therapy for Myeloma: Overcoming Tumor-Associated Immune Suppression

Patients with myeloma exhibit prominent deficiencies in humoral immunity due to the dominance of the malignant plasma cell clone and the suppression of the normal B-cell repertoire [1]. A hallmark of the disease is the increased risk for viral and bacterial infections due to encapsulated organisms that are dependent on opsonization for systemic clearance [2]. Interactions between myeloma cells and stromal elements create an immunosuppressive microenvironment through the release of cytokines and soluble factors [3]. Myeloid suppressor cells and plasmacytoid dendritic cells (DCs) further contribute to the immunosuppressive milieu. Inhibition of antigen-presenting cell function may also contribute to tumor-associated tolerance and the loss of protective immunity. A variety of soluble factors such as vascular endothelial growth factor (VEGF) and indoleamine block the maturation and activation of antigen-presenting cells resulting in an increase of DCs with an inhibitory phenotype at the tumor site [4, 5].

The evolution of disease from monoclonal gammopathy of undetermined significance (MGUS) to MM is characterized by progressive deficiencies in T-cell immunity. There is a loss of complexity of the T-cell repertoire, including the absence of clones targeting defined myeloma-associated antigens such as SOX2 [6]. There is concomitant loss of effector cell function, expansion of regulatory T cells, and T-cell polarization toward an inhibitory phenotype in the tumor bed. Other T-cell subpopulations, such as Vγ9Vδ2 T cells, also demonstrate impaired activation. Loss of myeloma-specific immunity disrupts the homeostatic equilibrium, thereby allowing for the unrestrained growth of the malignant plasma cell clone.

The activation of immune effector cells and the targeting of tumor cells are modulated by the checkpoint inhibitor pathways mediated by CTL-4 and PD-1/PD-L1. In the non-disease setting, these negative costimulatory molecules maintain the normal equilibrium of host immunity by supporting the prevention of autoreactivity through the establishment of peripheral tolerance. In contrast, tumor cells upregulate these pathways as a means of preventing T-cell activation and blocking the killing of malignant cells by effector cells. PD-L1 expression has been demonstrated in human myeloma cell lines as well as primary cells. PD-1 is expressed by circulating and bone marrow-derived T cells. Of note, the percentage of T cells expressing PD-1 is increased in patients with bulk disease and after immune stimulation, potentially muting the induction of tumor-specific immunity [7–9].

Natural killer (NK) cells constitute a key cellular subset of the innate immune system with the potential to target malignant cells. NK cell reactivity is mediated through the expression of an array of inhibitory and activating receptors. Once activated, NK cells lyse target cells through secretion of cytotoxic granules such as perforin or granzyme B or via death receptors including Fas, and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-related pathways [10–12]. Studies have shown that NK cell function is preserved in the setting of MGUS and newly diagnosed MM; yet, with progression of the disease to advanced stages, there is a decline in the NK cell activity against MM [13–15]. Myeloma cells secrete TGF-β, IL-6, IL-10, and PGE2 which suppress NK cell function. The monoclonal immunoglobulin produced by MM may also directly impair NK cells. MM is associated with increased levels of soluble IL-2 receptor and IL-15 and displays the IL-15R, leading to an autocrine feedback loop that impairs NK cell activation, proliferation, and function [16–18]. Furthermore, the balance between activating and inhibitory NK cell receptors and ligands is changed. There is upregulation of inhibitory ligands on MM cells such as MHC class I and PD-L1 [7, 19, 20], while concurrently there is reduction in activating ligand expression. Moreover, soluble forms of activating ligand are secreted, and activating receptors such as DNAM-1, NKG2D, NKp30, and CD16 have reduced levels [21–25].

4.3 Antibody-Mediated Strategies

The efficacy of antibody therapy is potentially dependent on several mechanisms involving accessory cell populations. Antibody-mediated binding of tumor cells may trigger Antibody-dependent cell-mediated cytotoxicity (ADCC) via immune effector populations that express Fcγ receptors, such as NK cells, neutrophils, mononuclear phagocytes, and DCs. After activation of Fc receptors, cytotoxicity is mediated through at least two different mechanisms: one involving the release of perforin and granzyme from effector cells, and the other involving death ligands Fas ligand and TRAIL [26]. Alternatively, cell lysis may be accomplished by the antibody-mediated activation of the classical complement cascade at the tumor site (CDC).

Release of tumor antigens via cell lysis may further amplify the antitumor immune response via cross-presentation of tumor-derived peptides via MHC class I molecules, resulting in activation of CD8+ cytotoxic T lymphocyte [27]. DCs are capable of presenting peptides from engulfed apoptotic cells on MHC class I molecule to elicit antigen-specific CD8+ T-cell responses [28]. ADCC mediated by monoclonal antibody (mAb) might trigger cross-presentation by DCs and promote adaptive immune responses, as DCs can engulf the resultant apoptotic tumor cells and subsequently present tumor antigens (Ags) on MHC class I and II molecules. In addition, cross-presentation can be mediated by phagocytosis of dying Ab-coated tumor cells through FcγRs [29]. As such, efficacy of humoral therapy is impacted by the underlying immune competence of the patient and may be associated with a secondary cellular immune response.

Antibody-based therapy has been pursued in an effort to selectively target myeloma cells while minimizing toxicity to normal tissues. Antibody therapies have focused on cell surface markers expressed by plasma cells such as CD38, CD138, and the tumor adhesion molecule CS1.

4.3.1 CS1

CS1, a cell surface glycoprotein (CD2 subset 1, CRACC, SLAMF7, CD319, or 19A24) member of the immunoglobulin gene superfamily, is highly expressed in CD138-purified primary tumor cells from the majority of MM patients (over 97 %), with low levels of circulating CS1 detectable in MM patient sera but not in healthy donors [30]. CS1 is believed to participate in promoting and supporting MM cell adhesion to bone marrow stromal cells [30].

Elotuzumab (HuLuc63) is a humanized anti-CS1 mAb which binds with high affinity to MM cells and significantly inhibits their adhesion to bone marrow stromal cells. This inhibition could result in inhibition of the stimulatory effects of bone marrow stromal cells on myeloma cell growth and survival [30]. Elotuzumab was also found to induce death of myeloma cells through ADCC [30, 31]. In vivo xenograft studies have shown that elotuzumab induces inhibition of MM tumor growth in mouse models [30, 31]. Elotuzumab triggered autologous ADCC against primary MM cells, and pretreatment with conventional or novel anti-MM drugs, especially lenalidomide, markedly enhanced elotuzumab-induced MM cell lysis [30].

Elotuzumab demonstrated minimal single-agent activity in phase I studies of patients with relapsed/refractory disease [32]. However, preclinical data suggested that elotuzumab demonstrates synergy with other biologic agents such as bortezomib and lenalidomide [30, 33]. In a phase I study of elotuzumab and bortezomib, the overall response rate (ORR) was 48 %, and responses were achieved in 67 % of bortezomib-refractory patients. The median time to tumor progression (TTP) was 9.46 months [34]. In a phase Ib combination study with lenalidomide and dexamethasone, the ORR was 82 % for all treated patients (n = 28), 95 % for lenalidomide-naive patients (n = 22), and 83 % among patients who had been refractory to their most recent treatment (n = 12) [35]. In a phase II study of the same combination, the ORR was 84 % for all patients (n = 71), with median progression-free survival (PFS) of 26.9 months after a median follow-up of 18.1 months [36]. Adverse events attributed to elotuzumab were tolerable and included infusional reactions [32, 35, 36]. Elotuzumab is also being investigated as a therapeutic strategy to delay progression from smoldering myeloma to clinically active disease.

4.3.2 CD38

Malignant plasma cells strongly express CD38, making this a target of interest in the development of therapeutic antibodies in MM. Daratumumab is a human mAb that has been shown to effectively kill myeloma cells in vitro and in murine models [37]. In preclinical studies, it has been demonstrated that lenalidomide potently enhances the efficacy of this antibody [38]. In a phase I dose-escalation study in relapsed or refractory MM (RRMM) patients, 11/29 (38 %) had some reduction in paraprotein, including 5 (17 %) with more than 50 % reduction. Also a marked reduction in bone marrow (BM) plasma cells was seen in the higher doses [39]. Daratumumab is currently being evaluated in clinical trials in patients with relapsed/refractory MM in combination with dexamethasone and lenalidomide or bortezomib (trials NCT01615029 and NCT01620879, respectively).

Two other anti-CD38 Abs, SAR650984 and MOR03087, are also currently being evaluated in patients with RRMM (NCT01749969, NCT01084252, NCT01421186).

4.3.3 Interleukin-6 (IL-6)

Investigators have examined the efficacy of antibody therapy targeting the interaction of myeloma cells with critical aspects of the BM microenvironment including IL-6, insulin-like growth factor-1 (IGF-1), VEGF, and B-cell-activating factor (BAFF) [40, 41]. IL-6 is a pleiotropic cytokine that has been shown to play a crucial role in growth and survival of MM cells within the BM milieu. IL-6 is predominantly produced and secreted by BM stromal cells (BMSCs), mediating MM cell growth, preventing apoptotic cell death, promoting myeloma cell survival, and conferring drug resistance. IL-6 activates Ras/MEK/ERK cascade, JAK2/signal transducer and activator of transcription (STAT)-3 cascade, and PI3K/Akt cascade [42]. Siltuximab (CNTO 328) is a chimeric mAb targeting IL-6. Treatment of IL-6-dependent MM cell lines with siltuximab resulted in inhibited proliferation in a dose-dependent manner, both in the presence and absence of BMSCs [43]. In phase I studies minimal clinical activity was observed following single-agent therapy with siltuximab in patients with relapsed or relapsed and refractory disease [44, 45]. Preclinical studies demonstrated synergy between siltuximab and bortezomib [43], but in a randomized phase II study, combination therapy with bortezomib and siltuximab did not demonstrate enhanced response or survival as compared to bortezomib alone [46].

4.3.4 PD-1/PD-L1

The PD-1/PD-L1 pathway is upregulated in MM and provides a critical inhibitory signal that disrupts immune activation and promotes immune tolerance toward the myeloma cell. CT-011, a humanized anti-PD-1 mAb, enhances human NK cell function against autologous primary MM cells, through increased NK cell trafficking; enhanced immune complex formation between patient-derived NK cells and PD-L1-bearing, primary autologous MM tumor cells, and increased NK cytotoxicity [20]. Lenalidomide downregulates PD-L1 expression on myeloma cells and therefore synergized with CT-011 in the activation of NK cells and subsequent cytotoxicity against myeloma cells [47]. PD-1 blockade also enhances myeloma-specific T-cell immunity in vitro and in vivo [7, 8]. Preclinical studies demonstrated enhanced T-cell responses to autologous DC/myeloma fusion vaccines as manifested by polarization toward a Th1 phenotype, suppression of regulatory T cells, and increased cytotoxic T lymphocyte (CTL) response [9]. CT-011 prevented the vaccine-mediated increase in T-cell expression of PD-1 [9].

In a phase I clinical trial, CT-011 was administrated to patients with advanced hematological malignancies including MM. CT-011 was safe and well tolerated, with clinical benefit seen in 33 % of patients (n = 15) including one complete remission (CR). Treatment with CT-011 was accompanied with an elevated percentage of peripheral blood CD4+ T cells [48]. In an ongoing phase II clinical trial, the safety of CT-011 alone, and in combination with a DC/myeloma fusion vaccine, is being evaluated following autologous stem cell transplantation (ASCT) (NCT01067287). Preliminary results have demonstrated that CT-011 has been well tolerated and that posttransplant administration of CT-011 was associated with the expansion of myeloma-specific T cells which persisted at 6 months following completion of therapy [49].

For additional Abs that are being evaluated in MM, see Table 4.1.

Table 4.1 mAb in clinical evaluation for multiple myeloma
Full size table

4.4 Cellular Immunotherapy for Multiple Myeloma

4.4.1 Allogeneic Transplantation

The unique potential efficacy of cellular immunotherapy for myeloma is highlighted by the observation that allogeneic transplantation induces durable remissions in a subset of patients due to the graft-versus-myeloma effect [50–53]. A summary of early data of myeloablative transplantation from the European Bone Marrow Transplant Registry demonstrated that 28 % of patients remained in remission 7 years posttransplant, suggesting that durable responses were potentially achievable [54]. However, the median survival was only 10 months due to extremely high treatment-related mortality, raising a difficult choice for physicians and patients as to the applicability of this strategy. In a more recent report of 158 patients undergoing autologous or allogeneic transplantation based on donor availability, the event-free survival (EFS) following allogeneic transplantation was 33 and 31 % at 5 and 10 years, respectively, consistent with the presence of a subgroup that appear to have sustained disease response [55]. The role of the graft-versus-myeloma effect in preventing disease recurrence was further supported by a retrospective report from the European registry, in which patients with limited or extensive chronic graft-versus-host (cGVHD) disease demonstrated markedly improved 3-year survival (84 and 58 %, respectively) as compared to those without cGVHD (29 %) [56].

Donor lymphocyte infusion (DLI) as a treatment for posttransplant relapse has been shown to induce disease response, achievement of molecular remission, reconstitution of TCR Vβ repertoire, and long-term disease control in a subset of patients [57–60]. However, DLI therapy is complicated by GVHD due to the lack of myeloma specificity of the alloreactive lymphocytes [61]. Efforts to limit toxicity through the use of reduced intensity conditioning regimens have resulted in a decrease in treatment-related mortality but a concomitant increase in the risk of relapse. As such, immune-based targeting of myeloma cells by alloreactive lymphocytes may carry the unique potential for curative outcomes; nonetheless, the lack of specificity and toxicity significantly limits its use.

Investigators have examined strategies to induce autologous cellular immune responses that selectively target myeloma-associated antigens while minimizing toxicity to normal tissue.

One such strategy is the use of cancer vaccines to foster the expansion of tumor-specific lymphocytes. Myeloma-associated antigens that have been explored as targets for immunotherapy include the idiotype protein, MUC1, WT1, PRAME, CYP1B1, and HSP96 [62–69]. Vaccine strategies have included the introduction of myeloma-specific antigens in the context of immune adjuvants and the loading of individual or whole-cell-derived antigens onto antigen-presenting cells such as DCs.

4.4.2 DC-Based Vaccines as a Platform for Antigen Presentation

DCs represent a diverse network of antigen-presenting cells that play a prominent role in mediating immune responsiveness [70]. Circulating DC populations have been identified as myeloid and plasmacytoid in origin with the capacity to elicit Th1 and Th2 responses, respectively. Plasmacytoid DCs have been shown to contribute to the stromal environment in myeloma and may contribute to tumor-mediated tolerance [4]. Myeloma antigens administered in the context of immune adjuvants may recruit and activate native DC populations that subsequently internalize and present tumor antigens [71–73]. However, functional deficiencies have been demonstrated in DCs derived from myeloma patients which may impact their ability to elicit immunologic responses [5]. Alternatively, myeloid DCs with strong expression of costimulatory molecules and stimulatory cytokines may be generated ex vivo through cytokine stimulation of precursor populations [74]. DCs generated ex vivo and loaded with myeloma-associated antigens may act as a platform for cancer vaccines [75]. Strategies to introduce tumor antigens include pulsing with peptides, proteins, or lysates [76], electroporation with tumor-derived RNA or DNA [8, 77, 78], loading of tumor-derived apoptotic bodies [79], transduction with viral vectors expressing tumor antigens potentially enhanced by costimulatory molecules [80–83], and the use of whole-cell fusion between DCs and myeloma cells [84–86].

4.4.3 Myeloma Vaccines: Single-Antigen Approaches

The idiotype protein represents a truly tumor-specific antigen created by the unique immunoglobulin gene arrangement intrinsic to the malignant clone [87]. Vaccination with the idiotype protein in conjunction with granulocyte-macrophage colony-stimulating factor (GM-CSF) or IL-12 was associated with antigen-specific T-cell responses. Prolonged disease-free progression was observed in patients exhibiting an immunologic response [88]. Responses have also been observed following vaccination with antigen-presenting cells pulsed with M protein or with DCs loaded with idiotype and exposed to CD40L to induce maturation [89–93]. Vaccination with idiotype-pulsed antigen-presenting cells posttransplant was associated with improved progression-free survival as compared to a historical control cohort.

A peptide-based vaccine for WT1 administered with immune adjuvant has been shown to elicit immunologic response in patients with hematological malignancies and a decrease in measures of disease [94, 95]. In a recent study, WT1-specific immunity following allogeneic transplantation for myeloma was associated with long-term disease control. Peptide-based vaccine for MUC1 is currently being explored in patients with myeloma (NCT01232712). Expression of several cancer-testis antigens has been demonstrated and has been shown to be targeted by donor-derived humoral responses following allogeneic transplantation, confirming their potential immunogenicity. The cancer-testis antigen, NY-ESO, demonstrates increased expression by plasma cells in the setting of advanced disease, creating an appealing target for immune-based therapy [96]. Repetitive stimulation with DCs pulsed with an NY-ESO-derived peptide elicits a strong CTL response in vitro, demonstrating an activated phenotype capable of lysing primary myeloma cells [97]. Recent studies have identified a series of antigens recognized by T cells in patients following syngeneic transplantation.

Several other peptides which are highly expressed on myeloma cells and are important in the pathogenesis of the disease have been identified as potential immunogenic targets. Heteroclitic XBP1 (X-box-binding protein 1) (unspliced 184–192 and spliced 367–375), CD138 (syndecan-1)260–268, and CS1239-247 were shown each alone and in a cocktail combination of the four to generate specific CTLs enriched for effector and activated T cells, Ag-specific cytotoxicity against MM cell lines, as well as increased degranulation, proliferation, and INF-γ secretion [98–101].

4.4.4 Myeloma Vaccines: Whole-Cell Approaches

The use of whole-cell-derived antigens for vaccination may elicit a broad polyclonal response that is better able to target the heterogeneity of the myeloma cell population [86]. Consistent with this hypothesis, a murine model demonstrated the emergence of idiotype-negative variants following idiotype-based vaccinations, while whole myeloma cell-based vaccines did not induce resistance [102]. DCs pulsed with tumor lysates have been shown to induce myeloma-associated immunity, although the clinical efficacy was uncertain [76].

The authors have developed a vaccine model in which patient-derived myeloma cells are fused with autologous DCs, creating a hybridoma which expresses a broad array of myeloma antigens in the context of enhanced costimulation [86]. In a murine model, DC/MM fusions were shown to be protective against lethal challenge with syngeneic myeloma cells, and therapeutic efficacy was further enhanced by coadministration of IL-12 [103]. In preclinical human studies, fusion of DCs and MM cells elicited the expansion of activated T cells that potently lysed autologous myeloma cells in vitro.

A phase I clinical trial was completed in which successive cohorts of patients with advanced myeloma underwent vaccination with escalating doses of autologous DC/MM fusions [104]. Patients had undergone a median of four prior treatment regimens. Myeloma cells were derived from bone marrow aspirates, and DCs were generated from adherent mononuclear cells cultured with GM-CSF and IL-4 and matured with TNF-α. Patients underwent serial vaccination in conjunction with GM-CSF. Vaccine-associated toxicity consisted of transient grade 1–2 vaccine site reactions most commonly, while clinically significant autoimmunity was not observed. Biopsy of the vaccine bed demonstrated a dense infiltrate of CD8+ T cells consistent with T-cell expansion occurring at the site of vaccination. Vaccination resulted in the expansion of myeloma-specific T cells in the majority of patients as manifested by the percent of CD4+ and/or CD8+ T cells expressing IFN-γ following ex vivo exposure to autologous tumor lysate. On SEREX analysis, humoral responses against novel proteins were noted after vaccination. These findings were consistent with the induction of myeloma-specific immunity in patients with advanced disease. Of note, 66 % of patients demonstrated a period of disease stability ranging from several months to greater than 2 years after vaccination.

The authors have completed a phase II clinical trial in which patients underwent vaccination with DC/MM fusions in conjunction with autologous stem cell transplantation. It was postulated that vaccine response would be augmented following transplant-mediated cytoreduction and in the context of lymphopoietic reconstitution with the associated depletion of regulatory T cells. It was demonstrated that the posttransplant period was associated with the expansion of myeloma-reactive T cells which were further boosted by vaccination with DC/MM fusions. Vaccination was associated with the conversion of partial to complete responses greater than 100 days posttransplant in a subset of patients. A clinical trial is now underway examining the efficacy of PD-1 blockade in conjunction with the DC/MM fusion vaccine following autologous transplantation, and a national cooperative group study for the assessment of DC/MM fusion vaccine with lenalidomide versus lenalidomide maintenance alone is being planned.

4.4.5 NK Cell Therapy

Augmentation of NK cell-mediated immunity has been explored as therapy for MM. Preclinical models have demonstrated that thalidomide and lenalidomide increase the production of IL-2 by T cells, which stimulates NK cell activation and function against MM [105]. Lenalidomide has also been shown to increase CD16 and LFA-1 expression on NK cells, which facilitates an ADCC response against MM [106]. Lenalidomide also modulates the balance of NK cell-activating and inhibitory ligand expression on MM cells. It decreases expression of PD-L1 and enhances expression of ULBP-1(NKG2D ligand) on MM cells, which both result in improved NK cell immune response, as well as recognition and lysis of MM tumor targets [20, 107]. Bortezomib decreases MM expression of MHC class I and enhances the sensitivity of myeloma to NK cell-mediated lysis [108].

The importance of NK cell-mediated immunity in modulating disease outcome was highlighted by the observation that levels of autologous NK cells reinfused with autologous transplantation correlates with absolute lymphocyte recovery after ASCT for MM and non-hodgkin lymphoma [109]. Lymphocyte subset analyses revealed that an absolute NK cell count of 80/μL or more on day +15 post-SCT correlated significantly with improved progression-free survival [110]. In patients with MM undergoing allogeneic SCT, killer-cell immunoglobulin-like receptor (KIR)-ligand mismatch predicting for NK activation was protective against relapse [111]. In addition, the infusion of T-cell-depleted, haploidentical, KIR-mismatched NK cells, followed by delayed autograft stem cell rescue, has been shown to induce a near-complete/complete response rate of nearly 50 % [112]. Improved disease-free and overall survival was observed in myeloma patients who received grafts from donors with KIR haplotype B, which is associated with more activating receptor genes than KIR haplotype A [113]. Lenalidomide therapy for patients with progressive MM following allogeneic SCT has been associated with an overall response rate of 66 %, and immunomonitoring data show that lenalidomide augments NK cell expression of the activating receptor NKp44 [114]. Moreover, in a recent phase I/II study of lenalidomide given early after allogeneic SCT for MM, lenalidomide treatment resulted in an increase of activating receptors NKp30 and NKp44 on NK cells, as well as an increase in NK cell-mediated cytotoxicity directed against myeloma associated with an increase in the rate of complete remission [115].

IPH2101 is a fully human mAb which cross-reacts with KIR2DL1, KIR2DL2, and KIR2DL3 receptors and prevents their inhibitory signaling, thereby enhancing in vitro and in vivo NK cell killing of autologous tumor cells [107, 116]. In a phase I trial of IPH2101 in patients with RRMM, the drug was safe and tolerable, but objective responses were not observed [117]. Data suggest that combination of IPH2101 and lenalidomide may exert synergistic effects, as IPH2101 suppresses negative regulatory signals and lenalidomide augments NK cell function and upregulates activating ligands [107].

Other classes of drugs with anti-MM effects may also confer efficacy, at least in part, through recovery or enhancement of the NK cell versus MM effect. For example, histone deacetylase inhibitors increase the tumor surface expression of ligands for the activating NK receptors NKG2D and DNAM-1, thereby facilitating tumor cell recognition by NK cells and augmenting NK cell-mediated lysis of myeloma cells [118, 119].

Ex vivo expansion of NK cells from MM patients using good manufacturing practice (GMP)-compliant components has been demonstrated. NK cells expanded on average 1,600-fold. These expanded NK cells showed significant cytotoxicity against primary autologous MM cells and were able to retain their tolerance against normal cells [120]. Phase I studies utilizing this technology are underway. Another successful method of ex vivo NK cell expansion using coculture with K562 cells transfected with 41BBL and membrane-bound interleukin-15 has resulted in 804 and 351 fold expansion from healthy donors and myeloma patients, respectively. These cells killed both allogeneic and autologous primary myeloma cells as well as inhibited myeloma tumor growth in a murine model [121]. Phase II clinical trials have been initiated examining this approach in relapsed high-risk MM (NCT01313897) and asymptomatic MM (NCT01884688).

4.4.6 Engineered T Cells

A promising area of cancer immunotherapy involves the ex vivo expansion of activated T cells that target tumor cells. One strategy has been the development of chimeric antigen receptor cells (CARs) in which antibody targeting a cell surface protein on the malignant cell is transduced into the T-cell receptor apparatus, such that selective binding of the tumor is associated with activation of receptor and cell-mediated lysis. An important advance was the cotransduction of a costimulatory molecule such as 41BB to facilitate T-cell expansion and survival. Promising results have been obtained using CARs targeting CD19 in patients with advanced chronic lymphocytic leukemia and acute lymphocytic leukemia with persistence of the engineered cells in the circulation associated with long-term protection [122–124]. Investigators have begun exploring myeloma-specific targets such as CD38, B-cell maturation antigen (BCMA), and CS1. Of note, the choice of antibody epitope appears to have an important effect on T-cell efficacy. The Ag should be expressed by MM cells on their surface, but not by essential cells or organs. It should be expressed by all tumor cells or be essential for their maintenance.

In a recent study, BCMA was found to have restricted expression on plasma cells. Anti-BCMA-CAR-transduced T cells killed MM cell lines and were able to eradicate tumors in a mouse model. The anti-BCMA-CAR-transduced T cells produced IFN-γ when stimulated with primary MM cells and killed primary MM cells [125].

In another recently published study, NY-ESO-1 was found to be expressed in ~10 % of MM patients. A high-affinity CAR recognizing the immuno-dominant NY-ESO-1157–165 peptide in the context of the HLA-A*02:01 molecule was constructed. These cells (called redirected T cells) had subpopulations of effector and memory cells. They were able to lyse target cells and express IFN-γ. The memory cells showed signs of differentiation upon Ag restimulation and secreted IL-2. Moreover, these redirected T cells were protective against tumor growth in a mouse model [126].

4.5 Concluding Remarks

Potent anti-myeloma immunity has been demonstrated in the allogeneic transplant setting. However, the lack of specificity of alloreactive T cells represents a major limitation of this approach. In the autologous setting, a number of antigens have been identified on malignant plasma cells which may be targeted by both humoral and cell-mediated immunotherapeutic strategies, and encouraging results have been demonstrated both preclinically and in clinical trials. Future directions will focus on: (1) integrating immunotherapeutic approaches in the setting of low disease burden and (2) combining both cellular and humoral immunotherapy with immunomodulatory drugs to enhance autologous anti-MM immunity and improve patient outcome.