Immunotherapy in hematological malignancies: recent advances and open questions

Hematopoiesis is a highly dynamic regeneration process giving rise to all types of mammalian blood cells during embryonic development and adulthood [1]. It requires both self-renewal of hematopoietic stem cells (HSCs), that reside within the bone marrow niche, and a hierarchical cascade of progenitor differentiation [2]. HSCs are defined by their capability to differentiate into immature progenitor cells, i.e. myeloid cells (myeloid lineage) and lymphoid cells (lymphoid lineage) [2]. The myeloid cells produce platelets, erythrocytes, granulocytes, monocytes, macrophages and dendritic cells, whereas the lymphoid cells give rise to B and T lymphocytes, plasma cells and natural killer (NK) cells [3], as shown in Figure 1.

Hematopoiesis is governed by a combination of extrinsic and intrinsic factors, such as niche-associated factors, transcription factors, signal transduction pathways and chromatin modifiers. Any disruption of these factors may lead to serious hematological disorders, including hematologic malignancies, which can occur at any stage of blood cell differentiation; hence, affecting the production of blood cells. Consequently, HSCs would lose their functions of infection resistance and hemostasis [4]. Hematopoietic transformation can be resulted from the disruption of the transcription factors, including RUNX1/AML1, GATA3 and ETS that determine normal hematopoiesis [4,5]. Several studies have explored the relationship between the silencing of chromatin-modifying enzymes or misregulation of transcription factors and the development of hematological malignancies through epigenetic alterations [6]. Therefore, inhibiting the chromatin modifiers or reversing the transcription misregulation may rectify these epigenetic alterations, and thereby, procure treatments for these malignancies [7].

The microenvironment of hematologic malignancies consists of bone marrow (BM), blood vessels and peripheral lymphoid organs that provide nutrition and survival factors. Soluble factors such as VEGF, BFGF, IL-3, IL-6, nitric oxide (NO), APRIL and many others, could stimulate tumor cell survival and growth [8].

Bone marrow niche includes HSCs and nonhematopoietic cells, in other words, endothelial cells, fibroblasts, osteoblasts, macrophages, mast cells and mesenchymal stem cells (MSCs) [9]. Genetic alterations in the BM niche cells, in other words, HSCs or hematopoietic progenitor cells (HPCs) can modify the microenvironment and the proliferation of malignancies [10–12]. Any alterations in the signaling pathways can be a trigger to developing hematopoietic malignancies (Figure 2). In addition, the reduction of hematopoietic cells or HSCs can also enhance the progression of hematopoietic malignancies [13], by inhibiting the differentiation of HSCs into HPCs [14].

It is not only the niche that supports the survival of cancer cells at the expense of HSCs, hematological neoplasms can also hijack normal bone marrow to secrete high levels of stem cell factor (SCF), in order to attract normal CD34⁺ migrating into the cancer niche [15]. Abnormalities in the level of cytokines, such as IL-6, G-CSF, and TNF-α; CCL3, and CCL4; and the imbalance between suppressor and potentiator genes in the bone marrow microenvironment, could play important roles in the development of hematopoietic malignancies.

These microenvironment alterations contribute to the formation of a malignancy-favorable environment for supporting cancer survival throughout niche-associated factors. The cellular components of the bone marrow and lymph node niche, and the means by which they modulate normal and malignant hematopoiesis have been widely investigated in the past decade. Targeting malignant microenvironments, such as the niche remodeling; or the suppression of normal hematopoiesis, could be valuable approaches to treat hematopoietic cancers, overcome chemotherapeutic resistance and develop novel antineoplastic agents.

In this review, we highlight the recent immunomodulatory therapeutic approaches that are currently employed for the treatment of hematopoietic malignancies.

Hematopoietic cell transplantation

In the past two decades, hematopoietic cell transplantation was used for the treatment of several types of cancers including hematological cancers [16,17]. There are three known approaches that are used as cancer treatment regimens, in other words, the nonmyeloablative transplantation, also known as nonmyeloablative conditioning (NMA), myeloablative conditioning (MA) and reduced-intensity conditioning (RIC). MA requires a stem cell support, owing to the irreversible reduction in mature blood cells (cytopenia). On the other hand, patients who undergo NMA do not require stem cell support, as it is less likely to develop cytopenia. RIC does not fit the criteria of the previous two conditioning, however, stem cell support is crucial with this approach, with a possibility of reversible cytopenia [18,19]. Therefore, only hematopoietic cell transplantation approach NMA will be discussed as a hematological malignancy treatment regimen.

The nonmyeloablative transplantation relies on the use of immune cells of a donor to destroy the cancer of a recipient individual. Therefore, NMA is the most commonly used hematopoietic approach, in which it can reduce the toxicity of other cancer therapies, such as chemo- and radiotherapies [20]. NMA is being exploited as a timing regimen that emphasizes the immunosuppression, decreases the risk of rejection by residual recipient immunity, and produces mixed chimerism [21]. To determine the chimerism in the recipient blood samples, quantification of the percentage of hematopoietic cell recovery from donors or recipients origins can be performed. Mixed chimerism describes a state when both the recipient and donor hematopoietic stem cells coexist [22].

Among other studies, a multicenter 10-year retrospective study involved 70 allogeneic hematopoietic stem cell transplantation (HSCT), including 33 patients with mantle cell lymphoma, allowing better progression-free survival (PFS) rates when treated with NMA [23]. A different study with a prospective Phase II trial has used NMA with myeloma patients and the findings suggested that NMA regimen brought clinical advantages to the patients [24]. Another study was also evaluating NMA on lymphoma patients reporting that NMA could enhance the overall survival rate [25].

Nevertheless, there are number of cases that have demonstrated a graft-versus-host disease (GvHD) and progression or relapse of the treated cancer after the use of NMA [26]. In addition, despite of the promising results of the clinical studies using NMA in hematological malignances, more than 50% of those studies have no published results, which would require more considerations in the near future [27]. Consequently, this treatment approach is directed to the infirm patients, who might be inclined to other diseases. Hence, NMA can be used to decrease the risk of cancer therapy-related morbidity and to cure vicious cancers, such as hematopoietic malignancies [28].

T-cell immunotherapy

The use of T cells as a potential approach for immunotherapy has gained major recognition in the past two decades [29]. T cells have the potential to recognize diverse antigens from pathogens, tumors and the environment, while maintaining immunological memory and self-tolerance [30]. T-cell immunotherapy, or adoptive T-cell (ATC) therapy, is the process of infusing autologous T cells into cancer patients for treatment purposes [31]. An important question in assessing ATC therapeutic strategies is whether T-cell therapy has the capacity to recognize tumor-associated antigens (TAAs) [32]. Although immune cells are recruited to the tumor microenvironment, they display diminished anti-tumor functions due to immunosuppression from tumor-derived signals [33]. The aim of ATC therapy is to overcome the tumor-induced immunosuppression by ex vivo activation and expansion of cancer-specific T cells, and then reinfusion for subsequent killing upon recognizing tumor antigens [34]. In this section, we will discuss the efforts taken to develop tumor-specific T cells for ATC therapy in the context of hematological malignancies.

Marrow infiltrating lymphocytes

Marrow-infiltrating lymphocytes (MILs) are likely to provide similar benefits to tumor-infiltrating lymphocytes (TILs) as they possess antigenic specificity for hematological malignancies [35]. It has been observed that antigen-specific memory T cells are enriched in the bone marrow and are involved in controlling the tumor dormancy, providing therapeutic activity [36]. As opposed to TILs, MILs exists in all patients, and therefore, can be easily harvested and rapidly expanded [37]. Ex vivo activation and expansion of MILs have demonstrated significant antitumor specificities for mature plasma cells and their clonogenic precursors in multiple myeloma compared with their peripheral blood counterparts [38]. The first clinical study using MILs for ATC therapy showed a direct correlation between their tumor specificity and clinical response in multiple myeloma, as well as, the persistence of tumor-specific T cells in the bone marrow [37].

Chimeric antigen receptor T cells

Challenges associated with the ex vivo expansion of tumor-specific T cells have led to the development of engineered T cells, however, the response of these cells was limited to tumor antigens presented by major histocompatibility complex (MHC) [31]. The genetic engineering of T cells to have antitumor specificities could allow for rapid production of tumor-specific T cells for therapeutic applications [29]. Chimeric antigen receptors (CARs) are synthetic receptors that enable T cells to recognize and eliminate cells, expressing their cognate ligands [39]. Patients autologous T cells are isolated and genetically modified to express a CAR that is specific to TAA, then expanded and infused into the same patient [31]. T cells expressing CARs allow for combining their cytotoxic functions with a high degree of antibody specificity by recognizing specific TAAs in a MHC-independent manner [40]. This ability is particularly advantageous, as the loss of MHC is a common immune evasion mechanism of malignant cells to escape T-cell recognition [41].

Over the past decade, CAR T-cell therapy has been one of the most outstanding therapeutic advances for certain hematopoietic malignancies, such as leukemia and B cell lymphoma [42,43]. Initial clinical trials have used CAR T cells directed against CD19, expressed by a wide range of B cells (pro to memory B cells) [44]. CD19 is known to be expressed in various B cell malignancies, such as acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), and B cell lymphoma, and its absence in other hematopoietic cancers makes it an ideal CAR target [45]. In 2010, Kochenderfer et al. reported on dramatic regression of advanced follicular lymphoma using anti-CD19 CAR T cells [46]. For 39 weeks post CAR T-cell infusion, B cells were absent, while other blood counts were promptly recovered [46]. Another study included 15 patients suffering from diffuse large B cell lymphoma, indolent lymphoma or CLL. It was reported that eight out of 15 patients have achieved a complete remission (CR), while four patients have achieved a partial remission (PR), after receiving a single infusion of anti-CD19 CAR T cells following conditioning chemotherapy [47]. Brentjens et al. have reported rapid tumor eradication and achievement of CR in adult patients with refractory B cell ALL (B-ALL) [48]. It has also been reported that anti-CD19 CAR T-cell therapy is therapeutically effective in high-risk B-ALL patients, including those with Philadelphia chromosome-positive disease that occur in relapsed B-ALL patients who undergone allogeneic hematopoietic stem cell transplant [49]. In a trial aimed at treating relapsed/refractory (R/R) CLL, the overall response was 57%, with approximately 29% CR, 4+ years of CAR T-cell persistence and no relapse [50].

CAR T cells have demonstrated great success in treating certain hematological malignancies [31]. However, this therapeutic approach face certain limitations on the path of development [51]. Upon CAR T-cell activation, excessive cytokine production can potentially lead to cytokine toxicity, or cytokine release syndrome, which could be a potential life-threatening condition [52]. Efforts are being made to define diagnostic criteria for severe cytokine release syndrome, where therapeutic intervention is required to minimize the associated symptom [49]. Another treatment-related toxicity includes on-target off-tumor toxicity. It was reported that anti-CD19 CAR T cells may lead to B-cell aplasia and hypogammaglobinemia, due to off-tumor targeting of CD19 on normal B-cell progenitors [45]. Both effects are well tolerated and manageable by administering immunoglobulins intravenously [39]. Other obstacles include the downregulation of target antigens or tumor antigen heterogeneity that can lead to tumor relapse, a commonly seen phenomenon in CD19⁺ B-cell malignancies [53]. Several efforts to overcome these drawbacks were recently investigated, such as utilizing more than one CAR T cells targeting different antigens [54] or using bispecific CAR T cells [55]. In the upcoming years, advances in gene editing and the development of innovative techniques to manipulate the immune system will guide this new era of cell-based therapeutics, enhancing the rate of cures from hematological malignancies [43].

Natural killer cell immunotherapy

NK cells are cytokine-secreting cytolytic subset of innate lymphoid cells that can induce death in virally-infected or tumor cells [56]. They constitute of approximately 5–15% of circulating lymphocytes, and therefore, represent one of the three major lymphocyte lineages including T and B lymphocytes [57]. As opposed to adaptive lymphocytes, NK cells are involved in innate immune response [58]. One of the key structural features of NK cells is the presence of cytoplasmic granules that contain proteases and perforins [59]. NK cells can deliver immediate response upon recognizing stress and ‘danger’ signals, from foreign molecules [60]. Additionally, NK cells play an important role in modulating immune responses via rapid production of various cytokines and chemokines, eradication of antigen-presenting cells and activation of T cells [57].

NK cells can attack tumor cells by downregulating HLA class I molecules via ‘missing self-recognition’ or overexpression of ligands for NK activating receptors as ‘induced self-recognition’ [61]. NK cells are recruited to the tumor microenvironment under the direction of pro-inflammatory chemokines secreted from innate and adaptive immune cells [62]. Normally, NK activation is inhibited by the interaction of their inhibitory receptors with MHC class I molecules. However, many cancer cells downregulate their MHC class I expression to avoid CD8⁺ T cells [63]. Therefore, NK cells possess therapeutic potential for cancer, where other immune cells fail to recognize and eliminate cancer cells due to MHC class I downregulation [64].

Due to certain limitations in CAR T-cell therapy, including the high costs associated with manufacturing the drug, and its patient-specificity, CAR NK cells represent an appealing solution [61]. Allogenic CAR NK cells can be used without causing GvHD, and thus represent a potential system for generating ‘off-the-shelf’ or ‘universal’ therapeutic cells [65]. Several sources of CAR NK cells occur and have been used in preclinical and clinical settings, including cell lines, such as NK-92, UCB and hematopoietic pluripotent stem cells [65–67]. Numerous clinical trials involving CAR NK cells are targeting CD19, CD22, B-cell maturation antigen (BCMA), CD33 or CD7 for hematological malignancies [68]. In early 2020, Liu et al. published a large-scale CAR NK cell trial (NCT03056339), where NK cells were engineered with an anti-CD19 CAR for treatment of R/R CD19⁺ cancers [69]. In the Phase I and II trials, eight out of the 11 participants had objective response, including seven that achieved CR. Remarkably, those eight patients showed response to the treatment within the first month postinfusion.

However, similar to CAR T cells, there are inevitable limitations, from loss of target antigens, to tumor heterogeneity and suppressive tumor microenvironment [68]. Overall, NK cell therapy is evolving as a promising area of clinical research, with manageable safety and preliminary signs of efficacy in patients with certain hematological malignancies [64]. The results of clinical trials that involve alloreactive NK cells in patients with hematological malignancies are encouraging [70]. Further clinical studies with larger patient cohorts are required to validate these promising early results [64].

Monoclonal antibodies

Another mechanism of NK cell-mediated elimination of cancer cells is via antibody-dependent cellular cytotoxicity (ADCC). This approach involves the administration of monoclonal antibodies (mAb) directed against TAAs that leads to the recognition and killing of cancer cells [64]. An example of therapeutic mAb in the context of hematological malignancies includes rituximab, an anti-CD20 used for the treatment of lymphoma. Rituximab binds to CD20 antigens that expressed on the surface of immune system B cells, subsequently eradicating the cancerous cells [71]. Several other new US FDA approved drugs using mAb targeting CD20 have been employed to treat hematological malignancies, in other words, ofatumumab and obinutuzumab [72,73]. Ofatumumab consist of a humanized monoclonal antibody that interact with a different CD20 epitope than rituximab, therefore, inducing a better ADCC [72]. The use of obinutuzumab has also shown a greater ADCC and antitumor effect [73].

In addition to CD20, other FDA approved drugs were used to target other antigens expressed by malignant cells. A study by de Weers et al. has shown that the administration of daratumumab, a mAb targeting CD38 antigen, prompts the killing of multiple myeloma and other hematological tumors [74]. Moreover, brentuximab vedotin, an antibody drug conjugate (ADC) against CD30 that is expressed in Hodgkin Reed–Sternberg cells and lymphoma tumor cells, has also shown a significant antitumor effect [75].

Bispecific T-cell engagers

Another fast-growing research for cancer targeted therapy is the development of bispecific T-cell engagers (BiTEs). BiTEs have a well-reported structure in which two single chain variable fragments bind to two different antigens, one for the immune cell receptor and the other for the cancer antigen, together by a peptide linker, in order to localize the immune response against cancerous cells [76]. The first FDA-approved BiTE for the treatment of ALL, is blinatumomab, which was approved in 2014 and has the ability to bind to anti-CD3 chain of T cells and anti-CD19 of B cells together [77]. The first trials that involved blinatumomab have gained unsuccessful outcomes, such as neurological complications and cytokine release syndrome [78]. Further studies have demonstrated that by reducing the prolonged doses at a level that fulfil both high cancer cell killing and tolerable level of toxicity can lead to effective and safe use of this drug against hematological malignancies [79,80].

Recently, extended efforts have been made to engineer BiTEs targeting different multiple myeloma expressed antigens [81–84]. A study by Hipp et al. demonstrated a novel BiTE (BI 836909), whereby BCMAs, in other words, highly expressed antigens on multiple myeloma cells and malignant plasma cells but not on naive B cells, are connected to anti-CD3ε fragments of T cells [81]. Results showed that the upregulated expression of CD25 and CD69 indicated T cell activity in BCMA positive cells, whereas, the BCMA negative cells had no expression of such cytokines. In regard to cell lysis activity of BI 836909, BCMA-positive multiple myeloma cell lysis has increased substantially compared with the BCMA-negative cells. These results have shown the highly potent ability of BI 836909 to induce multiple myeloma cell lysis upon activation of T cells, while sparing the healthy B cells of which BCMA was not expressed [81].

Immune checkpoint inhibitors

Host immunosurveillance plays a crucial role in eradicating pathogens, as well as, reducing any autoimmune responses. Such processes are significantly mediated by the effector T cells, which requires a two-level activation or inhibition, to change their fate: costimulatory and coinhibitory signals. These signals can ensure a proper effector T-cell functioning, known as immune checkpoints [85]. For instance, activation of effector T cell requires binding to a peptide. It becomes fully activated once it binds to a ligand from the B7-1 family. Interestingly, PD-1 and CTLA4 receptors and their ligands constitute different groups in the CD28/B7-1 family [86].

Host immunosurveillance can also identify cancer cells and eradicate them by inducing their apoptosis [87]. However, this process is often not impeccable. Different hematopoietic malignancies have been reported to upregulate the expression of the inhibitory molecules, PD-1 and CTLA4, thereby, escaping immunosurveillance [87]. Remarkably, targeting those immune checkpoint inhibitors have revolutionized cancer treatment. Earlier clinical trials of drugs targeting immune checkpoint inhibitors have been reviewed extensively [85–87]. Herein, we shed the light on recent clinical studies employing PD-1 inhibitors for the treatment of various hematopoietic malignancies.

PD-1 inhibitors were used in many clinical trials as a potential treatment approach of different hematological malignancies, including multiple myeloma. Increased PD-1 signaling was found to be associated with multiple myeloma, by preventing immunosurveillance [88]. The PD-1 inhibitor, pembrolizumab, was recently used in a Phase IB clinical trial with R/R multiple myeloma (RRMM) [89]. Pembrolizumab is a humanized antibody that inhibits PD-1 receptor signaling by blocking its association with its ligands. Pembrolizumab is used as a monotherapy for RRMM and was shown to be generally safe with no significant severe adverse effects. However, 56% of patients (n = 17) exhibited stable disease with a maiden period of 4 months. Interestingly, all 17 patients were previously treated with the immunomodulator, lenalidomide and had at least one refractory disease. Thus, despite lacking the desired objective response rate, pembrolizumab might be a promising anti-RRMM therapy, especially when used in combination with other anticancer drugs.

Classical Hodgkin lymphoma (cHL) is a hematopoietic malignancy characterized by amplified PD-L1 expression due to a genetic amplification of chromosome 9p24.1 [90]. The first line of treatment after the failure of the conventional therapy of R/R cHL is salvage chemotherapy in combination with autologous stem cell transplant (ASCT). The second line of treatment would be the use of brentuximab vedotin, which has a considerably high overall response rate (75%), but low durable response. Therefore, the effect of high PD-L1 and PD-L2 expression should be targeted by PD-1 antibodies.

Recently, the effect of PD-1 blockade in R/R cHL patients was assessed in a Phase II clinical study [90]. The efficiency and safety of pembrolizumab were tested after ASCT in 30 patients suffering from R/R cHL. Interestingly, the study reported a PFS in 82% of patients accounting for 60–80% of PFS rates. Moreover, pembrolizumab has shown a considerably safe profile, with 40% of patients showing one grade 2 or higher immune-related adverse event, and 30% with one grade 3 adverse events.

Nivolumab, an anti-PD-1 drug, was also used to assess its efficiency and safety on RR cHL patients [91]. Nivolumab combined with brentuximab vedotin before ASCT therapy has resulted in a considerably high objective response rate of 82% and an overall response rate of 61% (n = 61) [92]. Moreover, most reported adverse events are of grades 1 and 2, thus, indicating the safety of this treatment approach for R/R cHL.

Primary mediastinal large B-cell lymphoma (PMBCL) is a subtype of large B-cell lymphoma characterized by an amplified PD-1 axis [90,91]. A study conducted by Armand et al. analyzed the results of Phase 1B and Phase II clinical trials to assess pembrolizumab's efficiency and safety profile on R/R PMBCL patients [93]. Interestingly, pembrolizumab was shown to be safe and resulted in a high response rate in R/R PMBCL patients. The reported objective response rate of the Phase 1B study was 48%, while it was 45% for the Phase II study. Moreover, the two trials demonstrated similar safety profiles of grade 3 or 4 drug-related adverse events in 24 and 23% of patients, respectively. Remarkably, the data from the two clinical studies have contributed and accelerated the FDA's approval of pembrolizumab as a drug for R/R PMBCL. All of which indicated the importance of targeting disease immune-biomarkers as a monotherapy or in combination with other treatment approaches.

Immunomodulatory drugs

Immunomodulatory drugs (IMiDs) are thalidomide analogues that combine higher immunomodulation and anticancer properties with less toxicity profiles [94]. Thalidomide was initially used for the treatment of multiple myeloma and has shown a significant anticancer activity via several mechanisms, including inhibition of angiogenesis and TNF-α synthesis [94]. Other IMiDs, such as lenalidomide and pomalidomide, are able to increase the production of IL-10, an anti-inflammatory cytokine, and to inhibit the production of pro-inflammatory cytokines such as TNF-α, IL-1β and IL-6. In fact, these analogues have been reported to be up to 50,000-fold more potent than thalidomide at inhibiting TNF-α production by peripheral blood mononuclear cells in vitro [95]. IMiDs can potently enhance the functions of T cells and NK cells by inducing IL-2 and interferon-γ production [96]. Particularly, lenalidomide has shown to increase the T-cell proliferation and cytokine production by enhancing AP-1 transcriptional activity [97], and reducing CTLA4 inhibition via directly inducing tyrosine phosphorylation of CD28 on T cells [98]. Other studies have also suggested that IMiDs can inhibit the activity of Treg cells [99], and can exert a CASP8 effect on multiple myeloma cells [100].

Through the two past decades, many clinical trials have taken place to study the effect of IMiDs on several hematopoietic malignancies including multiple myeloma, acute myeloid leukemia (AML) and CLL (Table 1). A recent randomized, open-label Phase III study (Myeloma XI) included patients who are diagnosed newly with multiple myeloma, and have compared the PFS and overall survival of those who received induction therapy [101]. These patients were either assigned to maintenance therapy with lenalidomide or were kept under observation. The results demonstrated that the median PFS was significantly higher, while the overall survival was slightly higher for patients who received lenalidomide therapy. Both PFS and overall survival outcomes draw preferences to the use of lenalidomide as a maintenance therapy. However, 45% of patients receiving lenalidomide had high adverse effects, while only 17% of patients on observation had serious adverse effects [101]. Overall, the results of this study demonstrated the improvement of treatment outcomes using lenalidomide as a maintenance therapy.

In a Phase III trial, in which stem cell transplant-eligible patients who are newly diagnosed with multiple myeloma have received either lenalidomide with a low dose of dexamethasone until progression (LD), lenalidomide with a low dose of dexamethasone for 72 weeks (LD72), or melphalan with prednisone and thalidomide for 72 weeks (MPT). The findings of the 4-year PFS rate were considerably higher for LD group of patients compared with LD72 and MPT groups, which were comparable [102]. Moreover, LD and LD72 groups had higher 4-year overall survivals when compared with the MPT group [102].

Lenalidomide was also tested intensively for the treatment of other hematologic malignancies. One study has reported the use of both lenalidomide and pomalidomide, both second-generation IMiDs, which have direct and immune-driven activities on AML. These activities were presented by the suppression of leukemia progress in vivo, AML cell death in vitro, and the increase of NK cell activity in slowing leukemia's development [103]. Furthermore, lenalidomide combined with other drugs was used to treat different lymphoma types. A Phase III study where rituximab was used with lenalidomide compared with a placebo, was shown an improved efficacy of rituximab in terms of PFS rate for patients with indolent non-Hodgkin's lymphoma [104].

Another study featured the use of obinutuzumab along with lenalidomide in the treatment of R/R follicular B cell lymphoma. Out of 86 evaluable patients, 79% have reached a CR, which is the primary end point of the study, while 38% achieved an overall response by the end of the induction therapy [105]. Several studies have also reported the enhanced efficacy of using lenalidomide with different drugs in the treatment of CLL, in regards to altering the cytokines secretion [106], or reaching to higher complete responses at different doses within patients tolerable levels [107].

Immunotherapeutic vaccines

The concept of cancer therapeutic vaccination relies on the administration of TAAs, tumor specific antigens (TSAs) or whole cells to enhance active immunity of cancer patients through T-cells upregulation [108]. Despite the extensive clinical trials that took place in the 2000s, there is still no FDA approved therapeutic vaccines for the treatment of any type of hematological malignancies [109,110]. Nonetheless, more efforts have been conducted to develop a hematologic cancer vaccine by using different formulations. In general, therapeutic vaccines can be divided into cell-based or antigen specific vaccines, each of which holds a promising cancer targeting approach.

Tumor specific vaccines aim at utilizing antigens, specifically expressed on the surface of cancer cells, in an attempt to allow immune cells to recognize and target tumor cells, while sparing healthy ones that do not express them. Cancer derived proteins and multi-peptides are the main constituents of this therapeutic approach, both of which can be stemmed from mutated, such as neoantigens, or non-mutated proteins [111]. Since the 1990's, more attention has been drawn toward the use of heat shock proteins (HSPs), which are overexpressed in many types of cancer cells that are associated with their proliferation and metastasis, in cancer vaccination [112]. HSP105, a therapeutic target for several cancer types, including non-Hodgkin lymphoma [113], has recently undergone Phase I clinical trial involving 30 patients diagnosed with either esophageal or colorectal cancers [114]. The level of HSP105 was recorded, and the results showed a clear declination of HSP105 level of expression for those who were vaccinated [114]. Other emerging studies have focused on using WT1 for AML patients [115,116]. A Phase II trial study of 22 AML patients with complete remission has reported that 47% of those patients reached an overall score of 3 years, and there were detectable CD4⁺ and CD8⁺ immunological responses of 44 and 86%, respectively. Thereby, WT1 protein was considered a potential AML targeting approach, while still calling for larger clinical trials to address other possible risks and success rates [115].

Another highly investigated hematological cancer vaccine approach is the use of whole tumor cells to trigger the immune system rather than using specific components of the cells. This strategy of cancer immunization has the advantage in which the injection of whole cancer cells employs multiple cancer specific antigens as therapeutic targets all at once. However, the clear disadvantages of this approach are the difficulty of cell line harvesting and the high cost of the procedure [117]. A new study on AML mice model has described the use of Cryogel^® (Aspen Aerogels, MA, USA) a microporous biomaterial, as a cancer vaccine. Cryogel was used as an immune cell activator and a delivery system for the combination of GM-CSF, TLR9 and AML-associated antigens, specifically WT1, which are the main immunogenic components [118]. The results proved the efficacy of the Cryogel in inducing an immunogenic response, with a full protection against AML challenge (day 0) and rechallenge (day 100), whereas there was no mice survival beyond 29 days. In addition, subcutaneous bolus injection of the immunogenic components (GM-CSF and antigen) showed lower number of CD11c⁺ and CD86⁺ dendritic cells in the draining lymph nodes compared with the Cryogel vaccine. The survival rate of the untreated group was lower (between 25 and 30 days), followed by the chemotherapy alone group (between 30 and 40 days), and finally, the chemotherapy with Cryogel group that showed 100% mice survival rate, despite the presence of the antigens indicating that this effect depends on the GM-CSF [118].

Besides, the ability of dendritic cell-based vaccines to trigger both innate and adaptive immunities have favored their wide application on hematologic malignancies [119]. Several studies on the use of dendritic cell-based vaccines for AML patients have demonstrated high efficacy, in other words, triggering the immune system, and safety levels that might lead to their potential use as vaccines for hematopoietic malignancies [120,121].

Conclusion & future perspective

Blood cancer immunotherapeutic approaches have been a great success in recent years. This attributed to their distinguishable features, including their proximity to the immune cell microenvironment and the simplicity of their isolation and manipulation [122]. Several regimens of immunotherapy have been studied and approved by the FDA, while many are still in the late stages of clinical development, including adoptive cellular transfer, antibody-based therapies and immune checkpoint inhibitors. Many of these immunotherapies have been discussed in details as efficient therapeutic approaches for hematological malignancies.

However, in spite of the enormous achievements of immunotherapy in hematological malignancies over the past years, managing their associated severe adverse effects is an unmet need to be solved and required further investigations. Combined the use of multiple immune therapeutics could offer a more effective therapeutic approach for hematological malignancies in the near future. Moreover, the precise identification of new molecular targets and new molecular pathways, to reduce the damage of normal cells, is still needed. Altogether, these future and supplemental studies will lead to the development of novel therapeutic approaches that will complement the existing treatment strategies and improve the overall longevity and lifestyle of cancer patients.