The swinging pendulum of cancer immunotherapy

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The swinging pendulum of cancer immunotherapy personalization

Cancer immunotherapy has long offered the promise of producing cancer treatments that are more effective and less toxic than traditional chemotherapy and radiotherapy. That potential has only begun to be realized in the last 5 years with the first US FDA-approved cancer vaccine (sipuleucel-T), checkpoint inhibitors and adoptive cell therapy. While these therapies have been remarkably more effective than previous cancer immunotherapeutics, they are often limited by their inherently personalized nature. Indeed, each patient’s immune system and cancer are unique, limiting the scalability and generalizability of new approaches. However, emerging solutions may overcome these limitations, producing ‘off-the-shelf’ cancer immunotherapies that transform patient outcomes.

Tara S Abraham1 & Adam E Snook*,1 Department of Pharmacology & Experimental Therapeutics, Thomas Jefferson University, 1020 Locust Street Philadelphia, PA 19107, USA *Author for correspondence: Tel.: +1 215 503 7445 adam.snook@ jefferson.edu 1

First draft submitted: 29 December 2016; Accepted for publication: 27 February 2017; Published online: 5 May 2017 Keywords: adoptive cell therapy • antitumor immunity • bispecific killer cell engagers • BiKEs • bispecific T-cell engagers • BiTEs • cancer immunotherapy • CAR-T cells • checkpoint inhibitors • trispecific killer cell engagers • TriKEs • vaccines

Cancer immunotherapy Historically, it was believed that tumors express nonself or foreign antigens [1] . Indeed, approximately 20% of cancers have an infectious etiology and derive from viruses (oncoviruses), such as hepatitis B and C and human papilloma virus that cause hepatocellular carcinoma and cervical cancer, respectively [2] . Traditional cancer vaccines aimed to prevent infection with these cancercausing viruses have been very effective in preventing malignancies [3] . Additionally, cytokines, such as interleukins that boost immune cell growth and division, interferons that aid immune cells in neutralizing cancer and inhibit cancer cell growth and GM-CSF that boosts immune cell production can be administered to patients by injection either alone or as an adjuvant in combination with additional treatment. These early immunotherapeutic approaches were generalizable or nonspecific, designed to boost immune

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responses without directly targeting specific cancer cells, similar to the established trifecta of surgery, chemotherapy and radiotherapy employed in oncology. It is now established that many cancers rely on the expression of mutated or nonmutated self-antigens to drive the tumorigenesis process and these self-antigens may also be expressed by normal, healthy tissues [4] . Recently, cancer immunotherapy has focused on two areas: breaking immune tolerance and anergy that limit endogenous antitumor responses (active immunotherapy) or adoptive transfer of cancer-targeted immune effector cells that attack cancer cells (passive immunotherapy). The discovery of tumor-specific antigens was a pivotal advancement in the field, as tumor-specific antigens can induce protective antitumor T-cell responses [5] . Tumor antigens can be categorized according to their expression patterns: tumor-associated antigens (TAAs) are

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Review Abraham & Snook present on some normal healthy tissues and are overexpressed in tumors; neoantigens arise from somatic mutations or oncoviruses in cancer; oncofetal antigens are normally expressed only during fetal development; and cancer germline antigens are normally expressed only in germline cells in immunoprivileged sites [4] . Importantly, these tumor antigens serve as potential candidates for the development of targeted cancer immunotherapies [1] . Cancer immunotherapy personalization The development and US FDA approval of sipuleucel-T (Provenge® from Dendreon, Inc., WA, USA), the first autologous immune-based cellular therapy for the treatment of prostate cancer [6] , was a major breakthrough in the field of cancer immunotherapeutics, shifting the landscape to a highly personalized approach to cancer treatment. The sipuleucel-T vaccine involves the isolation of autologous dendritic cells from a patient with metastatic castration-resistant prostate cancer, which are then cultured ex vivo in the presence of tumor-specific antigens and growth factors, and subsequently infused back into the patient. Early Phase I/II clinical trials demonstrated increased T-cell responses to the vaccine antigen, modest clinical responses and limited treatment-related toxicity. Unfortunately, the modest survival benefit to patients receiving sipuleucel-T, the high cost (US$93,000 per course of treatment) [7] and labor-intensive process limited the utility of this approach and ultimately bankrupted Dendreon, Inc. Checkpoint inhibitors

To prevent the autoimmune destruction of healthy tissues, the immune system has a critical responsibility to distinguish self from foreign, so that immune cells can attack invading foreign pathogens and leave healthy self-tissues unharmed [8] . One way the immune system achieves this is by using checkpoints, or molecules expressed on immune cells that must be activated or inactivated to initiate or resolve an immune response [9] . Immune checkpoints are often inhibitory pathways that are important in maintaining self-tolerance and modulating the duration, amplitude and quality of an immune response to limit tissue damage [9,10] . Some cancers develop immune escape mechanisms that co-opt these pathways. Recently, monoclonal antibodies have been developed to block these ligand–receptor interactions to prevent the inhibition of tumor-specific T-cell responses as a treatment for several types of cancer [11] . Importantly, these checkpoint inhibitors do not target tumor cells directly, but target lymphocyte receptors and/or their ligands to promote endogenous antitumor immunity.

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The CTLA-4 receptor expressed by T cells counteracts the activity of the T-cell co-stimulatory receptor CD28, serving as an ‘off-switch’ to inhibit T-cell activity. Indeed, CTLA-4 and CD28 share the ligands CD80 (B7.1) and CD86 (B7.2) [10] . During the initial stages of T-cell activation, CD28 binds CD80/86 on antigen-presenting cells (APCs), providing a critical ‘signal 2’ that complements ‘signal 1’ mediated by binding of MHC–antigen complexes by the T-cell receptor (TCR). TCR activation results in increased expression of CTLA-4 on the cell surface resulting in competition with CD28. Because CTLA-4 has a higher affinity for its ligands than CD28 [12] , its expression decreases T-cell activation by outcompeting CD28 or sequestering the CD80/86 ligands. CTLA-4 can act to decrease T-helper cell activity as well as to increase Treg immunosuppressive activity [13,14] . The importance of CTLA-4 in immune regulation is underscored by the lethal systemic immune hyperactivation observed in Ctla4 knockout mice [15] . After significant preclinical success demonstrating that antibody-mediated blockade of the CTLA-4 pathway induced antitumor immunity in mouse models [16] , a clinical trial was initiated where one of two monoclonal antibodies against CTLA-4 (ipilimumab and tremelimumab) were administered to patients with advanced metastatic melanoma or ovarian cancer who were unresponsive to conventional therapies [17] . While a clinical response was observed in 10% of melanoma patients, 25–30% of patients exhibited immune-related toxicities, with colitis being the most prevalent [18] . Another Phase II clinical trial evaluated ipilimumab at different doses and showed that managing toxicities by administration of steroids and TNF blockers served to mitigate morbidity and mortality [19] . Furthermore, in a randomized, three-arm clinical trial of patients with advanced melanoma, patients received either a peptide vaccine containing the melanoma-specific antigen gp100 alone, the gp100 vaccine in combination with ipilimumab or ipilimumab alone [20] . A 3.5-month survival benefit was observed in patients receiving ipilimumab with or without the vaccine, when compared with patients receiving the vaccine alone [20] . Moreover, a long-term survival benefit of over 2 years was demonstrated in 18% of ipilimumab-treated patients, compared with only 5% of patients who received the gp100 vaccine alone [20] . Importantly, ipilimumab was the first immunotherapy to demonstrate a survival benefit in patients with metastatic melanoma, resulting in its FDA approval for the treatment of advanced melanoma in 2011, and subsequent trials for other malignancies. Because ipilimumab produces a clinical and survival benefit in a subset of patients, efforts to define biomarkers to predict clinical responses to anti-CTLA-4 therapy are


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ongoing [21–24] . One potential biomarker is mutational load, reflecting the induction of T-cell responses targeting neoepitopes produced by mutations [21] and the emerging dichotomous efficacy of checkpoint blockade for cancers typically characterized by high mutational load, such as melanoma, lung and microsatellite-instable colorectal cancer [25] . However, because mutational load alone was not sufficient to predict patient outcomes [21] , additional studies are required to develop a companion diagnostic. Given the severity and risk of toxicity from CTLA-4 blockade, such a diagnostic would greatly improve the management of patients potentially eligible for this therapy. Like CTLA-4 blockade with ipilimumab, monoclonal antibodies have been developed targeting the PD-1/PD-L1 checkpoint pathway to treat cancer. PD-1 engagement with its ligand PD-L1 inhibits T-cell activation. High levels of PD-1 expression can be induced on T cells by chronic antigen exposure, seen in chronic viral infections and cancer, leading to exhaustion or anergy in antigen-specific T cells [26] . PD-L1 can be expressed by some normal cells and is often overexpressed in cancer [27] . It is also upregulated in the tumor microenvironment in response to inflammation [28] , inhibiting cytokine production and cytotoxic activity of PD-1+ tumor-infiltrating lymphocytes (TILs) [27,29] . Like CTLA-4, PD-1 is also highly expressed on Treg cells, which are abundant in tumors. PD-1 engagement on Tregs can enhance their proliferation [30] , potentially enabling Tregs to further suppress effector TILs. Moreover, PD-1 is more broadly expressed than CTLA-4 and has been detected on other activated non-T lymphocytes such as B cells and natural killer (NK) cells [31,32] , potentially reducing their antibody production or cytotoxic activity, respectively. Thus, it is possible that anti-PD-1 treatment can suppress Treg proliferation, enhance NK-cell activity in tumors and boost antibody production by B cells. Together, increased PD-1 expression by TILs and increased ligand expression by tumor cells provided an important rationale for the blockade of this pathway to enhance antitumor immune responses. Indeed, preclinical studies of PD-1 or PD-L1 blockade resulted in increased antitumor immunity, while only mild phenotypes were observed in Pdcd−/− and Cd247−/− mice lacking this pathway, suggesting that PD-1 pathway blockade would produce only limited immunerelated toxicity [27] . Clinically, blockade of the PD-1 receptor has successfully treated melanoma [33] , head and neck cancer [34] and Hodgkin lymphoma [35] . In a clinical trial involving patients with various cancers, PD-L1 blockade induced durable tumor regression and prolonged stabilization of disease, including nonsmall-cell lung cancer (NSCLC), a tumor that has not


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been previously responsive to immunotherapy [36] . In 2016, atezolizumab (Tecentriq® from Genentech, Inc., CA, USA) became the first and only anti-PD-L1 antibody to be approved by the FDA for the treatment of metastatic NSCLC and for locally advanced or metastatic urothelial carcinoma [37] . However, PD-1 and PD-L1 are characterized by significant heterogeneity in expression patterns across cells and cancers [29] . The conflicting reports on an association between expression patterns and response to therapy [29,38–39] pose a challenge for anti-PD-1/PD-L1 therapies and question the utility of these molecules as potential predictive biomarkers [40] for therapeutic efficacy. Neoantigen immunotherapy

Mouse model studies performed in the 1970s with carcinogen-induced sarcomas suggested that naturally occurring tumor immunity is directed against unique tumor antigens, rather than universal antigens shared by tumors of the same type [41] . That concept has been supported by checkpoint inhibitor studies, which also suggest that endogenous immunity induced by these therapies targets neoepitopes [21] . Now, advancements in next-generation sequencing technology [25] , as well as mass spectrometry-based immunopeptidome identification technology [42] , make identification of neoepitopes in patient tumors significantly more feasible than ever before. Moreover, these approaches could be combined to identify neoepitopes that may serve as patientspecific vaccine targets [43–45] . While, these approaches have not yet been translated into clinical trials demonstrating clinical efficacy of personalized neoepitope vaccination, evidence from animal models and human studies strongly suggests that this could be a highly effective new cancer immunotherapy paradigm [46] . Adoptive cell transfer

Adoptive cell transfer (ACT) involves the infusion of tumor-specific cytotoxic and/or helper T lymphocytes into patients to recognize, target and destroy tumor cells. The discovery of TAAs was pivotal in the success of this approach. Traditional vaccines developed using proteins or peptides against known TAAs or whole cancer cells induced T-cell responses, but did not eliminate tumors in patients with a large tumor burden. In contrast, ACT allows for the transfer of an enormous number of tumor-specific T cells that may be capable of eliminating significant tumor burdens in patients [47] . Prior to transfer, T cells undergo robust ex vivo expansion and acquire effector phenotypes. Coupled with their ability to traffic to tumor sites, these observations suggest that ACT can target and eliminate metastatic disease [48] . Importantly, transferred T cells can also develop immunologic memory,

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Review Abraham & Snook producing a long-term therapeutic effect that may persist for years after T-cell transfer [49] . While some tumor-specific T cells can be expanded nonspecifically in bulk culture, tumor specificity can be enhanced through repeated antigen-specific stimulation ex vivo [50] or through genetic engineering of isolated T cells [51] . Although the genetic manipulation of T cells to express a relevant TCR provides the opportunity for an unlimited source of tumor-specific T cells, this requires the identification and isolation of a tumor-reactive TCR from patients’ T cells with naturally occurring antitumor activity [52] , a process impeded by self-tolerance and the rarity of such T cells. Adoptive cell therapy utilizing genetically engineered TCRs targeting TAAs has expanded across a range of cancers such as synovial cell sarcoma [53] and colorectal cancer [54] , despite the weak responses [55] and autoimmune toxicities [56] reported in some of these studies. The development of chimeric antigen receptors (CARs) provides a more universal approach to targeting TAAs than using TCRs restricted to a particular HLA molecule, which are highly polymorphic in the population. CARs are hybrid receptors formed by the fusion of an extracellular tumor-specific antibody fragment, a CD3-derived immunoreceptor tyrosine-based activation motif (ITAM) signaling chain and often a co-stimulatory signaling domain [57] , allowing the CAR to mediate T-cell-like cytotoxicity with antibody-like specificity (i.e., without the requirement for MHC presentation by HLA molecules). Indeed, CARs targeting an expanding array of TAAs have been, and continue to be, engineered and successfully used to treat diseases including renal cell carcinoma [58] and neuroblastoma [59] , targeting carbonic anhydrase and CD171, respectively. CAR-T cells have been the most successful in treating hematologic malignancies by targeting CD19 [60–62] . Indeed, in a clinical trial, CD19-targeting CAR-T cells induced complete and lasting remission in the first three reported patients [49,61] . CAR-T cells expanded >1000-fold in vivo when compared with the initial amount transferred, leading to long-term persistence in peripheral blood and bone marrow (>6 months) with minimal acute treatment-related toxicities [49,61] . Overall, CD19-targeting CAR-T cells have been remarkably successful in the treatment of leukemia (>70% overall response rate across various trials and centers) [63] , suggesting that this approach could improve patient outcomes in other cancers expressing compelling targets such as GUCY2C in gastric, esophageal, pancreatic and colorectal cancers [64] . However, this approach remains highly personalized: autologous T cells are infused to the same patient, limiting the scalability and increasing the cost of this approach.

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Immunoscore®

The recent advancement and success of cancer immunotherapy is now shifting cancer treatment paradigms to reflect the hypothesis that cancer progression, prognosis and treatment are profoundly influenced by the immune system [65] . With mounting evidence to support this hypothesis, the landscape of current prognostic scoring systems may evolve to include immune context by evaluating systemic and local immunologic biomarkers [66,67] . Traditional clinical and pathological risk predictions in cancer patients reflect the assumption that tumor progression is a cell-autonomous process and utilizes histopathological evaluation of tissue samples from surgically resected primary tumors [68] . The assessment criteria include tumor size, tissue integrity, cell morphology, histology grade, aberrant expression of protein and genetic markers, evidence of malignant transformation, senescence and proliferation, invasive margin, depth of invasion and extent of vascularization [68] . Additionally, tumor-draining and regional lymph nodes, as well as distant organs, are analyzed for evidence of metastasis [68] . This information is compared with patients with similar disease progression characteristics and corresponding clinical outcome to estimate patient prognosis in terms of disease-free, disease-specific and overall survival. However, clinical outcome can vary among patients with the same histological tumor stage. Moreover, the highly personalized nature of immunotherapies reveals a demand for a new level of personalization in the immunological context to determine prognostic information. Indeed, Immunoscore® (from the French National Institute of Health and Medical Research, Paris, France) is a prognostic tool that aims to integrate immune information including number and phenotypic markers of TILs as well as characteristics of the tumor microenvironment with traditional histological scoring and classification methods [65,66,68] . The development of such a platform is important to more accurately predict patient outcomes by considering the critical importance of the antitumor immune response in cancer, and may serve to better inform therapeutic choice and efficacy on an individual basis. The future of ‘off-the-shelf’ therapies Induced pluripotent stem cell therapy

Current ACT therapies are labor intensive and costly, requiring the isolation, ex vivo expansion and reinfusion of autologous T cells from individual patients. Unlimited access to antigen-specific T cells would significantly broaden the scope and delivery of T-cell therapies. In this context, stem cells have unlimited selfrenewal capacity, the ability to differentiate into most tissue cell types, and have been used as a new source for




the generation of immune cells [69] . Induced pluripotent stem cells (iPSCs) have been used to generate dendritic cells [70] , T cells [71] and other cytokine-producing cells [72] with antitumor activity in vitro [73] . Though advances in iPSC techniques are promising as potential cancer immunotherapies, the low yield of immune cells derived from iPSCs [74] presents a limitation for its application. Furthermore, the unknown antigen specificity and HLA restriction complicate the functional characterization of these cells. The issue of unpredictable TCR rearrangement and consequent antigen specificity can be circumvented by genetically engineering a TCR of known specificity into iPSCs [75] , but this requires the isolation and cloning of antigen-specific T cells, a process that is limited to the tumor-specific TCRs detected in patient samples of the appropriate HLA type. The genetic engineering of iPSCs with CARs, however, couples the unlimited availability of iPSCs with the generation of phenotypically characterized, functional and expandable tumor-specific T cells [71] . Indeed, iPSCs transduced with a CD19 CAR construct were able to inhibit tumor growth in a xenograft model [71] . Although iPSC-derived immune cells are promising for cancer immunotherapeutic applications, further studies and characterization of these cells are necessary before clinical relevance can be evaluated in human patients. CAR-NK cell therapy

CAR-T cells can be engineered to recognize TAAs but they pose a safety risk associated with recognition of target antigen in normal healthy tissues [49] . Because CAR-T cells can persist in patient circulation for long periods of time, they hold the potential to cause persistent on-target/off-tumor toxicities that could have adverse health implications [49] . Additionally, adoptively transferred T cells induce production of proinflammatory cytokines upon activation, which can lead to a harmful and potentially lethal cytokine storm [76] . In that context, much more short-lived NK cells are also potent cytotoxic effector cells and may provide advantages over T cells when used in CAR cancer immunotherapy [77] . Their brief lifespan enables NK cells to be rapidly cleared from the circulation, reducing potential toxicities. Even transferred allogenic NK cells induce a clinical response before they are rejected a few days after transfer. Importantly, NK cells can mediate cytotoxicity through several mechanisms involving different pathways and receptors, whereas T-cell cytotoxicity is via CAR-restricted mechanisms. Still, supply is limited as NK cells comprise only 10% of circulating blood lymphocytes [77] . Importantly, NK-92 is a cytotoxic NK-cell line established from patients with clonal NK-cell lymphoma that can serve as an


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open cellular platform for CAR-NK immunotherapy [78,79] . These cells can be genetically manipulated to recognize TAAs at a higher efficiency than NK cells from an allogenic donor, have been well characterized in preclinical models [79–81] and have been the only cell line product to show a clinical benefit with minimal toxicity in advanced cancer patients [82,83] . This continuously growing, highly active NK-derived cell line can provide a donor-independent ‘off-the-shelf’ option for CAR therapies, with significantly more availability and at significantly less cost in preparation and administration than CAR-T cells (US$20,000 vs US$250,000 per infusion cycle, respectively) [78] . Bispecific T-cell engagers, bispecific killer cell engagers & trispecific killer cell engagers

In addition to conventional antibodies specific for TAAs that elicit antibody-dependent cell-mediated cytotoxicity, bispecific T-cell engagers (BiTEs) represent a new class of immunotherapeutic molecules that are engineered to retarget T cells to tumor cells. BiTEs consist of two single-chain variable fragments (scFvs) connected in tandem by a flexible linker [84] . One scFv binds to a T-cell-activating molecule (e.g., CD3 within the TCR complex), while the other has specificity for a TAA, allowing the BiTE to physically link the T cell to a tumor cell and stimulate T-cell activation, cytotoxic activity and cytokine production [84] . BiTEs induce the formation of immunological synapses [85] to direct T cells toward tumor cells. They are highly selective for their targets [86] , requiring simultaneous engagement of the T cell and tumor cell to elicit cytotoxicity and cytokine production [86] , limiting the risk of off-target toxicities. Additionally, BiTEs do not require co-stimulatory molecule (CD80/CD86) engagement for T-cell activation [86–88] , suggesting an unconventional mechanism of T-cell activation. Alternatively, memory T cells require less stimulation for activation than naive T cells, suggesting that memory T cells might be the predominant effectors in BiTE-mediated antitumor efficacy [89] . Indeed, in a study comparing BiTE-associated cytotoxic activity of CD8 + CD45A+ (naive) and CD8 + CD45O + (memory) CD8 + T cells, cytotoxicity preferentially occurred in previously primed T cells [87] . In fact, blinatumomab (Blincyto® from Amgen Inc., CA, USA), a BiTE engineered to bind CD19 on B cells and certain leukemia cells and the CD3–TCR complex, is approved for the treatment of adults with Philadelphia chromosome (Ph)-negative relapsed/refractory B-cell precursor acute lymphoblastic leukemia by the FDA after a singlearm multinational Phase II study showed that 42% of patients achieved complete remission with a median duration of response (time from response to relapse or death) of approximately 6 months [90] .


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Review Abraham & Snook Like BiTEs, bispecific killer cell engagers (BiKEs) are designed to engage NK cells and tumor cells. BiKEs engage NK activating receptors (e.g., CD16) to trigger cytotoxicity and have been designed to recognize several TAAs including CD19 [91] and HLA class II [92] for B-cell malignancies, CD30 for Hodgkin lymphoma [93] , HER2 for breast cancer [94] and CD33 for acute myeloid leukemia [95] . Many of these produced cytotoxicity against cancer cells in vitro and a clinical trial infusing varying regimens of anti-CD16/CD30 to 16 Hodgkin lymphoma patients produced treatment-related toxicities limited to mild fever observed in only six patients [93] . One of the 16 patients in this trial achieved complete remission, three experienced partial remissions lasting 5–9 months and four patients had stable disease for 3 to >6 months [93] . IL-2 pretreatment before a second BiKE infusion course resulted in a significant increase in circulating NK cells in all five treated patients, which coincided with the conversion of two cases of stable disease into one complete remission and one partial remission [93] . Another NK receptor that has been targeted by BiKEs is NKG2D, an activating receptor that can trigger cytotoxicity upon binding the polymorphic MHC class I chain-related molecules (MIC), MICA and MICB, expressed by stressed, transformed and infected cells [96] . BiKEs have been engineered to bind to NKG2D on NK cells and TAAs on cancer cells such as CD138 overexpressed in multiple myeloma [97] and CEA expressed in colon carcinoma [98] . Moreover, since NKG2D is also expressed by T cells, BiKEs targeting this receptor have the benefit of co-stimulating both NK cells and T cells to mediate tumor killing [99] . Furthermore, a class of trispecific killer cell engagers (TriKEs) has emerged, building upon the existing bispecific molecules [100] by adding a third scFv binding arm to bind NK-cell proliferation receptors (e.g., IL-15) and drive NK-cell expansion [101] . In a study comparing a TriKE (targeting CD16 and IL-15 on NK cells and the AML antigen CD133) with its BiKE predecessor (targeting CD16 on NK cells and AML antigen CD133), TriKEs induced superior NK-cell-mediated killing of CD33expressing myeloma cell lines and primary AML blasts by increasing degranulation, cytokine production, proliferation and survival in vitro [101] . Importantly, the superior in vivo efficacy of the TriKE was demonstrated in a xenograft model in which human NK cells were adoptively transferred to mice to eradicate engrafted human CD33 + myeloma cells, showing a reduction in tumor burden 3 weeks after infusion and enhanced NK-cell expansion (∼350-fold) compared with the BiKE [101] .

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Vaccines

Traditional patient-specific therapeutic cancer vaccines have been generated from immune cells or tumor products obtained from cancer patients. Although the generation of patient-specific therapeutic cancer vaccines from patient-isolated immune cells or tumor products is an arduous, resourceintensive and costly process, it has yielded therapies with clinical benefit [6,102–103] . The recent resurgence in cancer vaccine development has shifted the focus of this field to ‘off-the-shelf ’ vaccines that require the identification of TAAs shared between patients and universally expressed in malignancies. Several of these ‘off-the-shelf ’ vaccines have shown efficacy as cancer therapeutics including a MAGE-A3 vaccine to treat NSCLC [104] , a poxviral-based PSA-targeted vaccine to treat metastatic castration-resistant prostate cancer [105] and the BLP25 liposome vaccine targeting mucin 1 for the treatment of NSCLC [106] . The recent discoveries that T-cell leukemia/lymphoma 1 oncoprotein is overexpressed by a range of B-cell lymphomas, but only minimally by normal B cells [107] , and that GUCY2C is universally expressed in metastatic colorectal cancer but not extramucosal tissues [108–110] establish these as two of the several emerging targets for next-generation vaccines. Furthermore, a new category of in situ vaccines represents vaccines with a similar ‘off-the-shelf ’ goal in mind, but goes a step further to ‘depersonalize’ vaccines, as they are generated in vivo without the prerequisite identification/isolation of a specific TAA [111] . In fact, in situ vaccines aim to take advantage of the complete antigenic repertoire of the tumor, rather than a single specific antigen [112] . TAAs are released upon tumor cell death and can be subsequently processed and presented by APCs. In situ vaccines utilize local or intratumoral immunomodulation to facilitate this process and induce immunogenic tumor cell death, increase the number of APCs at the tumor site [113,114] and promote their activation [115] , and enhance cross-presentation of TAAs to tumor-reactive T cells [116] , which can in turn mount a systemic antitumor immune response and protective immunologic memory, as well as inhibit immunosuppression [117,118] . Conclusion The field of cancer therapeutics has gradually evolved with the discovery of the extensive involvement of the immune system in the efficacy of conventional cancer therapies and patient outcomes. The discovery of TAAs represents a significant breakthrough in the field, allowing the direct targeting of tumor cells based on the molecules they express, conferring a higher level of specificity than conventional cancer therapies




Expansion

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Reinfusion

Large-scale therapies

Culture

T-cell source

Genetic engineering/ antigen-specific expansion/ non-specific TIL expansion

Patient

Traditional cancer immunotherapy – T cells obtained from patient – Highly personalized – Limited scalability – Costly

‘Off-the-shelf’ therapies

Future of cancer immunotherapy – Does not require patient cells – ‘Off-the-shelf’ availability – Homogeneous product – Large scalability and engineering capacity

Figure 1. The future of cancer immunotherapy personalization. Currently, many forms of cancer immunotherapy require highly personalized preparation of materials, such as genetically modified T cells for cellular therapies. However, these require collection of cells or tissues from the patient, are expensive and have limited scalability. In contrast, the future of cancer immunotherapy may see the large-scale manufacturing of ‘off-the-shelf’ drugs with unlimited and large-scale production, homogeneity and reproducibility.

allowed. While tumor cells have evolved mechanisms of immune evasion, immunotherapies have been developed to block pathways that inhibit antitumor effectors. However, the heterogeneity of tumors across cancer types and individuals poses a challenge for the broad application of immunotherapies to treat cancers. As such, the current immunotherapy paradigms remain highly personalized, requiring, for example, the isolation, ex vivo expansion and reinfusion of patient cells in ACT approaches. Moreover, many current immunotherapy platforms aim to induce tumorspecific responses by infusing autologous immune effector cells or tumor products into the patient. These techniques require extensive and costly resources and skilled manpower. Thus, many immunotherapy paradigms essentially call for the development of an expensive and ‘brand new drug’ on an individual basis, without the potential for broad clinical application (Figure 1) . Thus, recent therapeutic developments in the pipeline have shifted the focus to those with broader utility, scalability and accessibility for patients, a change needed to meet the demand for improved treatments and patient outcomes in cancer.


Future perspective Cancer immunotherapy is advancing at an unprecedented rate. The first cancer immunotherapy was approved in 1990 with Bacillus Calmette–Guérin for superficial bladder tumors. In 2008, the first therapeutic cancer vaccine was approved in Russia for renal cancer. Since then, greater than ten cancer immunotherapy indications have been approved, including a prostate cancer vaccine, oncolytic viral therapy in melanoma, the first BiTE therapy and several checkpoint inhibitor approvals in melanoma, bladder cancer, Hodgkin lymphoma, renal cancer and lung cancer. In the context of numerous ongoing clinical trials examining novel therapies or approved drugs for new indications, we expect to see many more FDA approvals. However, for cancer immunotherapy to impact the oncology field like chemotherapy and radiotherapy did previously, it must provide solutions that overcome the most prominent hurdles to its widespread application. Notably, these include transitioning from patient-specific approaches to centralized, large-scale and ‘depersonalized’ approaches. This will make cancer immunotherapy available to many more patients


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Review Abraham & Snook and significantly reduce costs. Additionally, combining immunotherapies with traditional chemotherapy, targeted therapies or radiotherapy may further broaden their application and efficacy in patients who do not respond to immunotherapy. Cancer immunotherapy was not chosen as the Breakthrough of the Year for 2013 by ‘Science’ because of the field’s accomplishments to that point [119] . Rather, the remarkable success of checkpoint inhibitors and ACT in early clinical trials began to reveal the promise of immunotherapy, which had remained elusive for decades. With continued investment in cancer immunotherapy development and the growing acceptance of these therapies by oncologists, the next decade should see many more FDA approvals,

broad adoption of cancer immunotherapy and a transformation of patient outcomes not seen since the golden age of chemotherapy in the mid-20th century. Financial & competing interests disclosure Funding was provided by PhRMA Foundation (to AE Snook) and Margaret Q Landenberger Research Foundation (to AE Snook). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.

Executive summary Cancer immunotherapy • Cancer immunotherapy seeks to exploit the remarkable specificity of the immune system to treat cancer. • Common cancer immunotherapy approaches may target identified tumor-associated antigens that include mutated proteins, viral proteins or normal self-proteins, which are selectively expressed or overexpressed in cancer.

Cancer immunotherapy personalization • The most effective cancer immunotherapeutics often require significant personalization. • The US FDA-approved prostate cancer vaccine sipuleucel-T and adoptive cell therapy (ACT) approaches require ex vivo manipulation of patient immune cells prior to infusion. • Checkpoint inhibitors targeting CTLA-4 and PD-1/PD-L1 are most effective in patients with tumors possessing a high mutational load or expressing PD-L1, respectively, necessitating the development and use of companion diagnostics. • Immunoscore® seeks to integrate immunological assessment into conventional histopathologic evaluation to predict patient outcomes and guide therapy.

The future of ‘off-the-shelf’ therapies • Reprogrammed induced pluripotent stem cells or natural killer cells may be viable alternatives to patient T-cell-based ACT, permitting large-scale manufacturing and ‘depersonalization’. • Bispecific T-cell engagers, bispecific killer cell engagers and trispecific killer cell engagers are proteins which redirect immune cells in vivo, overcoming the hurdles of ACT. • Vaccines comprised of viral vectors or similar ‘off-the-shelf’ platforms could further expand the number of ‘off-the-shelf’ cancer immunotherapies.

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