Cancer Vaccines: Translation from mice to human clinical trials

Abstract

Therapeutic cancer vaccines have been a long-sought approach to harness the exquisite specificity of the immune system to treat cancer, but until recently have not had much success as single agents in clinical trials. However, new understanding of the immunoregulatory mechanisms exploited by cancers has allowed the development of approaches to potentiate the effect of vaccines by removing the brakes while the vaccines step on the accelerator. Thus, vaccines that had induced a strong T cell response but no clinical therapeutic effect may now reach their full potential. Here, we review a number of promising approaches to cancer vaccines developed initially in mouse models and their translation into clinical trials, along with combinations of vaccines with other therapies that might allow cancer vaccines to finally achieve clinical efficacy against many types of cancer.

Introduction

Cancer vaccines have been long sought as a therapy that could marshal the exquisite specificity of the immune system to fight cancer without the side effects of other less precisely targeted therapies. Despite successes in preventing cancers in mouse models, the difficulty in treating large established tumors in mice, together with failures in translation to humans, led to the expectation that cancer vaccines may not be feasible [1,2]. The successful pivotal trial and licensing of Sipuleucel-T (Provenge) in 2010 reinvigorated the field, despite the limited increase in survival time [3]. More academic labs and companies have invested in cancer vaccines since that milestone event. However, concurrently, other forms of immunotherapy have emerged that have taken hold of the field and attracted most of the new effort. These include checkpoint inhibitors such as anti-CTLA-4, anti-PD-1, and anti-PD-L1 that block immunoregulatory mechanisms inhibiting the immune response to cancer and allow the immune system to reject the cancer in some cases [4–7], and adoptive T cell therapies, involving T cells from patients selected for their anti-tumor activity or genetically engineered to express a T or B cell receptor targeting a cancer antigen, that are expanded ex vivo and infused [8–11]. Both of these approaches have now been licensed by the Food and Drug Administration (FDA) and are showing impressive results in certain malignancies. At the same time, newer cancer vaccines are emerging that also show great promise, and it is becoming clear from animal models that cancer vaccines can synergize with both checkpoint inhibitors [12,13] and adoptive cell therapy [2,8,14–16]. Moreover, the barrier to success of many cancer vaccines now appears to be the presence of so many immunosuppressive mechanisms in cancer that inhibit the immune response in the tumor microenvironment even when the vaccine can induce a response that is measurable systemically. Besides checkpoint molecules, there are immunosuppressive cytokines like TGF-beta [17–19] and IL-10 [20], and immunosuppressive cells like T-regulatory cells [21–23], myeloid-derived suppressor cells[24–27], regulatory NKT cells [28], tumor-associated M2 macrophages[27,29,30], B-regulatory cells [31], tolerogenic dendritic cells, and others. Therefore, we believe that the future of cancer immunotherapy is to combine cancer vaccines with these other therapies, especially treatments to overcome immunosuppression.

Here, we will review a sampling of cancer vaccines, including two from our own lab and several from others, that were originally developed in mouse models and then successfully translated into human clinical trials showing promising results. While this short review cannot cover a comprehensive survey of the field, these examples will illustrate how preclinical mouse tumor models are effective to develop cancer vaccines that can work in humans, and how different strategies for cancer vaccines can benefit cancer patients and potentially synergize with methods to overcome immunosuppression. Thus, cancer vaccines should soon become the third pillar of cancer immunotherapy and a key component of combination immunotherapy.

Novel promising cancer vaccine strategies from the Vaccine Branch, National Cancer Institute, NIH

Both the cellular and the humoral arms of the immune system can be used to attack cancer. However, antibodies are limited to detecting tumor antigens on the surface of intact tumor cells, whereas T cells can detect any protein made in the cell, because fragments of all proteins made are transported to the surface by Major Histocompatibility Complex (MHC) molecules such as human leukocyte antigens (HLA antigens), which serve an internal surveillance mechanism [32]. Thus, most cancer vaccines have aimed to induce T cells, because these can recognize any protein-based tumor antigen. Here, we will illustrate examples of both.

In order for peptide fragments of protein tumor antigens to bind to MHC molecules, they must fit in the peptide binding groove, and higher affinity binding usually corresponds to greater immunogenicity [33]. Tumor antigens did not evolve to be good MHC binders, so we developed a strategy called “epitope enhancement” in which we modified the sequence to improve affinity without affecting the surface seen by the T cell receptor (TCR), so that we could still elicit T cells recognizing the unmodified antigen [34–36]. Others have seen similar improvements by such epitope enhancement of cancer antigens [37,38] or modifications improving T cell recognition [39]. We applied epitope enhancement to peptides we mapped to bind to HLA-A*0201, the most common human class I MHC molecule, present in nearly half the population, from TARP, a prostate and breast cancer antigen described by Ira Pastan’s lab [40]. We showed improved immunogenicity in HLA-A2-transgenic mice and ability to recognize the natural sequence, and confirmed this with human T cells, which could kill human cancer cells [41]. We translated this to a phase I clinical trial in HLA-*0201 stage D0 prostate cancer patients, in which a rising prostate-specific antigen (PSA) indicated microscopic recurrence after prostate removal, despite no radiographic evidence of tumor. In this setting, the rate of rise of PSA is a validated predictor of clinical course and response to therapy [42–47]. Patients got immunized 5 times with peptides either in Montanide-ISA51 adjuvant or coated onto autologous dendritic cells (DCs), and since the two arms gave comparable results, they were pooled for statistics. We found that 74% of vaccinated patients had a decreased PSA slope at 1 year compared to their own baseline slope (p = 0.0004), and the median tumor growth rate constant fell in half (p = 0.003), with no significant adverse events [48] 78% also made a new T cell response to the peptides. Based on these results, a randomized placebo-controlled phase II trial, with more peptides to expand beyond HLA-A*0201, has been opened.

Human epidermal growth factor receptor 2 (HER2) is a driver oncogene product expressed on the surface of about 25% of breast cancers and to a lesser extent on many other solid tumors, where it is accessible to antibodies. Monoclonal antibodies to HER2, and other HER2-directed agents, have already shown clinical benefit [49–51]. However, no vaccine is available to induce such antibodies. We made an adenovirus expressing the extracellular & transmembrane (ECTM) domains of rodent HER2 and used it to immunize either HER2 transgenic mice that develop autochthonous breast cancers or wild type mice with large established syngeneic TUBO breast tumors [52–54]. The vaccine prevented growth of the autochthonous tumors and cured nearly 100% of mice with large established TUBO mammary tumors (up to 2 cm), and established lung metastases[54], but surprisingly was independent of effector T cells, requiring only antibody production. The HER2 antibodies were Fc-receptor (FcR) independent, unlike trastuzumab [55], but rather blocked HER2 phosphorylation [54]. We translated this to a human phase I trial with an adenovirus expressing human HER2 ECTM, used to transduce autologous DCs. First tested in non-breast cancer patients naïve to HER2 therapies (with gastroesophageal, colorectal, ovarian, lung and bladder cancers), it was safe and showed clinical benefit in 5/11 (45%) of evaluable patients at the second and third dose levels [56]. This trial has now been opened to breast cancer patients who have failed other HER2-directed therapies. In addition, there are peptide-based and other vaccines under study to induce T cell responses to HER2 [57,58].

Representative other examples of promising novel cancer vaccines Prostvac

Cancer/testis antigens, whose expression is limited to cancer and non-vital organs, can be a good target for vaccines. A hurdle of targeting such antigens is that we need to break self-tolerance. Prostate-specific antigen (PSA) is such a self-antigen expressed in prostate cancer. Schlom and his coworkers developed PROSTVAC to deal with this problem by incorporating two simultaneous approaches. One is the use of viral vectors expressing PSA, which induces strong inflammation to induce massive T cell responses. The viral vaccines in therapeutic use have been shown to be able to break tolerance in multiple mouse tumor models [59–61]. Furthermore, they have demonstrated that the T cell-mediated immune responses induced by a viral vector can be maximized by taking a prime-boost strategy in which immune responses were primed by a recombinant vaccinia virus followed by boosts with a recombinant fowlpox virus [62,63]. Second is to strengthen co-stimulation for T cells by transducing genes of three co-stimulator molecules, LFA-3, ICAM-1 and B7, whose ligands are CD2, LFA-1, and CD28, respectively (together called TRICOM vectors). The effect of the combination is more dramatic when there is a weak TCR signal or low frequency of T cells against target cells [64]. These conditions represent the environment to which T cells are likely to be exposed in tumors, as tumors frequently decrease the expression of class I MHC. The TRICOM vectors have also been shown to improve not only the quantity/magnitude of T cell responses but also the quality of the responses by inducing T cells with higher functional avidity [65,66] .

PROSTVAC was investigated in 2 Phase II studies in metastatic castration-resistant prostate cancer (mCRPC). The first trial was a placebo-controlled, multicenter trial in 125 mCRPC patients with 2:1 randomization that showed overall survival benefit (25.1 months vs. 16.6 months; P = 0.0061) with PROSTVAC compared to the empty poxviral vector. No advantage in time to progression (TTP) was detected as shown similarly in the case of Sipuleucel-T, whereas both did show benefit in overall survival (OSS) [3]. The second trial investigated the influence of GM-CSF with PROSTVAC in a total of only 32 patients that showed no difference in the T cell response or survival between the group receiving GM -CSF and no GM-CSF [67]. PROSTVAC received Special Protocol Assessment (SPA) from the FDA in collaboration with NCI that lead to the PROSPECT study, a global, randomized, double-blind, placebo-controlled phase III trial in patients with mCRPC (NCT01322490). In September 2017, an independent Data Monitoring Committee (DMC) determined that continuation of the trial was futile based on a preplanned interim analysis after fully enrolling approximately 1300 patients with OSS longer than 30 months. It is unclear what led to the divergence in outcome between the phase III trial and the phase II trial. Newly available treatment options could have affected the overall course after the vaccination, in addition to the limited effect of the vaccine as a single agent on the tumor microenvironment during the prolonged treatment course, abrogating the benefit that had been seen in the phase II study. Prostvac might be another example to reinforce previous evidence that a cancer vaccine alone is not sufficient to gain the control of cancer, emphasizing the need for combinations with other agents, such as checkpoint inhibitors, to overcome negative immunoregulatory factors. Additional studies with combination strategies with nivolumab (NCT02933255) in CRPC and with docetaxel in metastatic castration sensitive prostate cancer (NCT02649855) are enrolling the patients. The use of PROSTVAC in the adjuvant setting for high-risk patients and in combination with ipilimumab in the neoadjuvant setting (NCT02506114) as well as in biochemically recurrent or localized prostate cancer is also under investigation.

Human Papillomavirus (HPV) therapeutic vaccines

Among human cancer viruses, HPV has been studied most and most successfully with visible outcome while hepatitis B virus vaccines have certainly reduced incidence of hepatocellular carcinoma, and there are ongoing efforts for hepatitis C virus, Epstein-Barr Virus (EBV), Human T-Lymphotropic Virus (HTLV)-1 and Merkel cell polyoma virus. Even though preventive vaccines covering the high-risk HPV subtypes such as HPV16 and HPV18 can affect the scourge of HPV associated cancer, the vaccination rate has not been satisfactory so far and there are many individuals who already have persistent HPV infection and are at risk of developing cancer, revealing the unmet need of developing therapeutic HPV vaccines.

Early experience with peptide vaccines raised the concern that they may work only in mice but never in human patients. This reputation was set by a long list of clinical trials with peptide vaccines that never achieved success [1]. However, recent work on a long peptide vaccine against HPV, which causes cervical cancer, clearly demonstrated that peptide vaccines work in humans if we design them properly. What made the HPV vaccine different from others? In many previous studies, short peptides with minimal length (9 to 11 amino acid residues long) for binding with a class I MHC, called minimal cytotoxic T lymphocyte (CTL) epitopes, were used as a vaccine. Using minimal CTL epitopes has potential problems. For CD8⁺ T cells to be fully activated by recognizing a peptide/MHC complex, they need “help” from CD4⁺ T cells. The help includes 1) “licensing” professional antigen presenting cells (APCs) through CD40-CD40L interaction, which activates and matures them [68–70], and 2) supplying a growth factor, IL-2. It has been shown in both viral infection and tumor models that in the absence of CD4⁺ T cell help long-lasting protection cannot be induced [71,72]. Another potential problem with a short minimal CTL epitope is that it replaces peptides presented by class I MHC on the cell surface, loading directly without processing. As class I MHC molecules are expressed on almost all types of cells, this means that administered peptides will not be presented only by professional APCs but primarily by non-professional APCs that do not provide proper second signals to CD8⁺ T cells, because they do not have co-stimulatory molecules. Thus, additional CD4⁺ T cell epitopes peptide must be included in a peptide vaccine, and it may be preferable to use the epitope from vaccine-targeting antigen.

A study by Zwaveling et al using a mouse model with the TC1 cell line, which is an epithelial cell tumor line transformed by HPV16 E6 and E7, demonstrated that a HPV16-derived 35-amino-acid long peptide (HPV E7_43–77) containing both a CD8⁺ T cell epitope and a CD4⁺ T cell epitope induced a far more robust CD8⁺ T cell response capable of eliminating an established tumor while a minimal CTL epitope (HPV E7_49–57) did not [73]. One mechanism by which the long peptide can induce such CD8⁺ T cell responses is that a long peptide requires processing by a mechanism unique to professional APCs, such as cross-presentation, to be presented by class I MHC. In contrast, short minimum CTL epitopes can be loaded onto class I MHC by replacing a peptide already bound to class I MHC molecules on cell surface, and hence can be presented by any cells expressing class I MHC molecules without a requirement for endogenous processing. Thus, non-professional APCs cannot present the antigen. It was also shown that the long peptide increased the duration of antigen presentation in vivo, which contributes to the induction of long-lasting immune response [74].

Based on the unexpected strong efficacy of the long peptide in mouse tumor models, Melief and coworkers launched a development pipeline for ISA101, a mixture of HPV16 E6 and E7 synthetic long peptide (HPV16-SLP) with Montanide ISA-51. A Phase II study in vaginal intraepithelial neoplasia (VIN) Grade III showed viral specific immunogenicity in all vaccinated (n=22) patients and clinical response in 79%, with a complete response in 47% at 12 months of follow-up that was maintained at 24 months of follow-up. No toxicity CTCAE grade III or higher was observed [75–77]. Several additional studies as a single agent or in combination using ISA101 are pending final reports. A study of ISA101 in combination with standard of care paclitaxel and carboplatin chemotherapy in advanced cervical cancer was presented during ASCO-SITC 2017 by Melief and Welters [78], reporting an objective response rate (ORR) of 67% vs 25% and disease control rate (ORR+SD) of 94% vs 79% in patients without prior systemic chemotherapy or with prior systemic chemotherapy and survival benefit correlating with HPV-specific immune response.

Another Phase IIb study was successfully conducted by Cornelia Trimble and coworkers using VGX-3100, a plasmid mixture targeting HPV 16 and 18 E6 and E7 proteins delivered by electroporation (Cellectra) for CIN 2/3 patients (N=167), showing 18.2% (95% CI 1.3–34.4; p=0.034) higher histopathologic regression (48.2 vs 30.0%) in modified intention-to-treat analysis with higher concomitant viral clearance in vaccine treated patients (40.2% vs 14.3%, 95% CI 8.0–39.2; p=0.003)[79]. This study laid the groundwork for additional studies in the full spectrum of precancerous and cancerous lesions caused by HPV (NCT03180684, NCT02163057, NCT02172911), including a phase III study for CIN 2/3 (NCT03185013).

MUC1

Finn and coworkers have established the usefulness of the link between tumor-associated antigens (TAA) or tumor specific antigen and disease-associated antigens (DAA) in the research on MUC1, justifying how such an antigen can be a target for a prophylactic vaccine. A DAA is defined as a self-antigen abnormally expressed during acute or chronic inflammation or infection. For example, hypoglycosylated MUC1 is a DAA for mumps that can induce anti-MUC1 antibody but a TAA for pancreatic cancer, ovarian cancer and several other cancer types. Aberrant, hypoglycosylated MUC1 has been identified also in adenocarcinoma and hematologic malignancies. Typically, immune recognition requires an MHC-restricted process to present antigens and only rare situations of MHC-unrestricted TCR recognition have been described. Magarian-Blander et al. published that a repeated MUC1 tandem epitope was able to induce the activation of cytotoxic T cells without presentation by MHC molecules even though antibody to the TCR and CD3 complex blocked the process[80]. Also, mice reconstituted with such TCR-transduced bone marrow cells were able to control MUC1-positive human tumors [81]. A history of childhood mumps might be protective against ovarian cancer according to the case-control studies and the protection seemed to be related to the presence of the anti-MUC1 antibody that rejected tumor cells in mice [82]. If the immunity against MUC1 provided immune protection against the development of ovarian cancer, can the same strategy be used for the cancer prevention by enhancing immune surveillance in high-risk individuals? Accordingly, the Finn group has been focusing on the development of a prophylactic cancer vaccine [83,84].

A MUC1 vaccine, a synthetic 100-amino acid peptide containing 5 MUC1 20-residue repeats, induced antibody responses and long-lasting immune memory in 43.6% of 39 individuals with advanced colonic adenoma[85]. High circulating myeloid-derived suppressor cells correlated with poor antibody response. Based on this work, a randomized, placebo-controlled Phase II study (NCT02134925) is underway in 120 patients with advanced adenomas with the end point of immunogenicity and 25% reduction in polyp recurrence after 3 yrs.

Vaccines for hematologic malignancies

Hematologic malignancies have been the target of many pioneering cancer treatments, exemplified by the first cell therapy (hematopoietic stem cell transplantation), first anti-cancer monoclonal antibody (rituximab), first tyrosine kinase inhibitor (imatinib) and first CAR-T cell therapy (ALL). However, with the success of monoclonal antibodies and kinase inhibitors, the need for vaccine studies has not seemed pressing.

Idiotype vaccines in lymphoma, pioneered by Levy, fascinated many researchers with the idea of providing active immunotherapies evoking polyclonal response while preserving normal B cells as a new concept in cancer immunotherapy from early 1990’s [86]. B cell tumors express unique surface immunoglobulin and idiotype (Id) refers to the unique amino acid sequences within the complementarity determining regions (CDR) of the variable regions of the heavy and light chain of such an Ig. Thus, these are among the early tumor-specific personalized antigens to be studied in humans as targets of cancer vaccines. A study (n=20) lead by Larry Kwak [87] demonstrated not only humoral but also a tumor cell-specific T cell response after vaccination in follicular lymphoma patients in remission after chemotherapy. However, such an id vaccine did not show advantage either in combination with rituximab (n=349) [88] or as a single agent first-line treatment in advanced disease (n=675) [89] in the subsequent two multi-center randomized Phase III studies. Even though Schuster reported improved PFS in advanced stage follicular lymphoma [90] when the vaccine was given when the patients were in remission after chemotherapy, this did not advance for further development. Instead, B cell malignancies became a new focus of adoptive cell therapy.

WT1, the top-ranked cancer antigen in the Prioritization of Cancer antigens proposed by NCI in 2009 considering therapeutic function, immunogenicity, and specificity, is overexpressed both in hematologic malignancies and solid tumors [91]. Oka reported the immunogenicity of the 9-mer WT1 peptide Db126 inducing a CTL response and rejection of WT1-expressing tumor cells in mice immunized with the Db126 peptide [92]. However, no vaccine studies of WT1 have reached Phase III and only a few small studies done in NCI and NHLBI showed disappointing clinical efficacy so far in hematologic malignancies. Currently, studies targeting malignant mesothelioma and ovarian cancer are accruing patients.

In myeloid malignancies, the PR1 vaccine had a long-waited report. PR1 is a HLA-A2 restricted 9-mer peptide derived from proteinase-3 and neutrophil elastase that are normal components of neutrophil azurophilic granules overexpressed in myeloid blast cells. A Phase I/II study treating recurrent or high-risk AML/MDS and CML patients (n=66) with repeated doses of PR1 showed an immune response in 53% and clinical response in 24% with minimal toxicity [93].

Vaccines against neo-antigen epitopes

With the recent discoveries that cancers with multiple somatic mutations are the most immunogenic and thus most responsive to checkpoint inhibitors [6] and that tumor-infiltrating lymphocytes often target such neo-antigens [94,95], there have been recent attempts at developing personalized cancer vaccines targeting such neo-antigen epitopes [96,97]. However, some of the earliest neo-antigens that fit this description are actually mutant Kras and mutant p53, a frequently mutated tumor oncogene and tumor suppressor gene, that were studied as vaccine candidates over a decade earlier [98–101]. In these studies, it was found that the most common Kras mutations at codon 12 and 13 fall in a sequence predicted to bind to HLA-A*0201, and indeed these peptides did bind to HLA-A*0201. In addition, many p53 mutations were in such segments that bound to HLA-A*0201 or other common human HLA molecules. Early clinical trials showed immune responses in cancer patients with mutant ras or p53 [102,103] and there was a significant correlation between cytokine responses to these peptides in vaccinated patients and their overall survival, with median survivals of 393 days with a cytotoxic T cell response to these peptides compared to 98 days without, or of 470 days with an interferon-gamma response to the peptides vs 88 days without, although cause and effect could not be proven [103]. Recently, binding of a mutant Kras peptide to HLA-Cw08 with recognition by a protective tumor infiltrating lymphocyte population was also reported [104]. Thus, from these mutant ras and p53 peptides, there is already precedent for use of neo-antigen-based vaccines.

Combinations of vaccines with other cancer therapies

From the history of clinical trials cancer vaccines, we have multiple proofs that vaccines can induce antigen-specific T cells but the response from vaccine alone is not sufficient to control the cancer. From recent breakthroughs in basic studies of immune regulation, such as checkpoint receptors and various types of regulatory T and B cells and myeloid cells, we now understand that the limiting hurdle to cancer vaccine efficacy is not necessarily the induction of an adequate magnitude T cell or antibody response, but the ability of that response to eradicate tumors in the face of these counterposing forces in the hostile tumor microenvironment. Thus, vaccine combinational strategies with cytokines, radiation, chemotherapy or hormonal therapy have been explored for possible synergy, and more recently, vaccine combinations with checkpoint inhibitors such as anti-CTLA-4 or anti-PD-1/PD-L1 are actively investigated to allow T cells induced by the vaccine to enter the tumor and to be effective there without inhibition by ligands such as PD-L1 [105,106]. As CTLA-4 is a receptor that turns off activated T cells and may prevent their entrance into the tumor parenchyma [107], and may also serve as a target for antibody-dependent killing of T-regulatory cells [108], and PD-1/PD-L1 interaction tunes down the T cell receptor and costimulatory signaling cascade by counteracting kinases in these signaling pathways [109], blocking such CTLA-4 or PD-1/PD-L1 interaction is expected to permit the immune responses generated by vaccines to more effectively control cancers. This is an innovative strategy that may reinvigorate the immune cells that were not successful enough in providing adequate immune surveillance.

A combination of PROSTVAC with ipilumumab (anti-CTLA-4) was studied in 30 patients with no increased immune-related adverse events and with a PSA decline in 58% of 24 chemotherapy-naïve metastatic castrate resistant prostate cancer (mCRPC) patients [110]. Vaccines combined with either ipilimumab or nivolumab (anti-PD-1) in recurrent or high-risk melanoma patients showed acceptable toxicity, potential survival benefit, and decreased recurrence[111] [112]. Glisson and coworkers [113] presented an ORR of 33% during ESMO 2017 in a study on ISA101 with Nivolumab in HPV16-positive incurable solid tumors (n=22), mostly oropharyngeal cancers (NCT02426892). Studies of vaccines in combination with checkpoint inhibitors are summarized in Table 1.

Table 1.

Selected therapeutic cancer vaccine trials in combination with checkpoint inhibitors

Checkpoint Inhibitor	Vaccine/Target	Cancer Types	Phase/ClinicalTrials.gov Identifier
Anti-CTLA-4
Ipilimumab	GVAX	Pancreas	1, 2	NCT00836407, NCT01896869
	NeoVax (neoantigen peptides)	Kidney	1	NCT02950766
	Pxca-Vec	Multiple solid tumors	1	NCT02977156
	NY-ESO-1pulsed DC	Multiple solid tumors	1	NCT02070406
	Prostvac	Prostate	1, 2	NCT00113984, NCT02506114
	gp100	Melanoma	3	NCT00094653
			2	NCT00032045, NCT00077532
	gp100, MART-1	Melanoma	1	NCT00025181, NCT00028431
	IDO peptide	Melanoma	1	NCT02077114
	6MHP (MHC Class II peptides)	Melanoma	1, 2	NCT02385669
	MART-1	Melanoma	2	NCT00084656
Tremelimumab	PrCa VBIR	Prostate	1	NCT02616185
	MART-1	Melanoma	1	NCT00090896
Anti-PD-1
Nivolumab	GVAX, CRS207/mesothelin	Pancreas	2	NCT02243371, NCT03190265
	GVAX	Pancreas	1, 2	NCT02451982, NCT03161379
	ISA 101/HPV16 E6 and E7	HPV16 cancers	2	NCT02426892
	CIMAvax/EGF	NSCLC	1, 2	NCT02955290
	CD40L, GM	NSCLC (adenocarcinoma)	1, 2	NCT02466568
	NY-ESO-1, MART-1, gp100	Melanoma	1	NCT01176461
	PD-L1/IDO peptide	Melanoma	1, 2	NCT03047928
	NY-ESO-1 pulsed DC	Multiple solid tumors	1	NCT02775292
	WT1	Ovary	1	NCT02737787
	HHS-110 (Viagenpumatucel-L)	NSCLC (adeno)	1, 2	NCT02439450
	NEO-PV01	Bladder, NSCLC, melanoma	1	NCT02897765
	Prostvac	Prostate	1, 2	NCT02933255
	DCVax-L (tumor lysate)	GBM	2	NCT03014804
	TG4010/MUC1	NSCLC	2	NCT02823990
	Synthetic long peptide	Follicular lymphoma	1	NCT03121677
Pembrolizumab	pTVG-HP DNA	mCRPC	1, 2	NCT02499835
	6MHP (MHC Class II peptides)	Melanoma	1, 2	NCT02515227
	Synthetic long peptide	NSCLC	1	CT03166254
	GVAX	Pancreas	1	NCT03153410
			2	NCT02648282, NCT03006302
	GVAX	CRC	2	NCT02981524
	Peptide	Pancreas, CRC	1	NCT02600949
	DPX-Survivac	Ovary	2	NCT03029403
	Oncolytic MG1-MAGE-A3	NSCLC	1, 2	NCT02879760
	P53MVA	Ovary	2	NCT03113487
		Multiple solid tumors	1	NCT02432963
	mRNA-4157	Multiple solid tumors	1	NCT03313778
	BCG	Bladder	1	NCT02808143
	Intratumoral DC	NHL	1, 2	NCT03035331
	HSPPC-96	GBM	2	NCT03018288
	Vigil TM bi-shRNAfurin-GM-CSF	Melanoma	1	NCT02574533
	DC	Melanoma	1	NCT03092453
Anti-PD-L1
Atezolizumab	CDX-1401/NY-ESO-1	NSCLC	2	NCT02495636
		Ovary, peritoneum	1, 2	NCT03206047
	RO7198457	Multiple solid tumors	1	NCT03289962
Avelumab	TG4001 (MVA-HPV-IL2)	SCCHN	1, 2	NCT03260023
	PVX-410 (tumor antigen peptides)	MM	1	NCT02886065
Durvalumab	PVX-410 (tumor antigen peptides)	TNBC	1	NCT02826434
	Neoantigen DNA	TNBC	1	NCT03199040
	TPIV200/huFR-1/Folate Receptor	Ovary	2	NCT02764333
	PVX-410 (tumor antigen peptides)	Smoldering myeloma	1	NCT02886065
	INO3112/HPV DNA and IL-12	HPV16 and 18 SCCHN	1, 2	NCT03162224
	DC/AML fusion cell	AML	2	NCT03059485
	BCG	Bladder	2	NCT02901548
Anti-DLL1/PD-1
CT-011	DC fusion cells	Myeloma, AML	2	NCT01067287, NCT01096602
Two or more checkpoint inhibitors
Ipi, Nivo	GVAX	Pancreas	2	NCT03190265
Ipi, Nivo, Pem	HAM (dorgenmeltucel-L)	Melanoma	2	NCT02054520
Ipi, Nivo	NY-ESO-1, gp100	Melanoma	1	NCT01176474
Tre, Dur	Poly ICLC	Multiple solid tumors	1, 2	NCT02643303
	mRNA	NSCLC	1, 2	NCT03164772
Nivo, Ate	DSP-7888/WT-1	Multiple solid tumors	1	NCT03311334
	DC	CRC	1, 2	NCT03152565

Summary and Conclusions

In conclusion, we propose that the previous failures of cancer vaccines to eradicate or control tumors was most often not due to failure to induce an adequate immune response, but rather the inability of that response to achieve its full potential in the face of a myriad of negative regulatory mechanisms invoked by the tumor to counteract the immune system. Most tumors that become clinically evident have already escaped natural immunosurveillance by exploiting host immune regulatory mechanisms that evolved to prevent autoimmunity and inflammation. As the tumor cells are largely self, anti-tumor immunity must overcome these barriers to autoimmunity, and tumors have taken advantage of these mechanism to escape such immunosurveillance. In addition, other aspects of the tumor microenvironment may be hostile to infiltrating T cells, such as hypoxia or extracellular potassium ion concentrations [114]. Thus, such mechanisms are already in place the tumor microenvironment of clinically evident tumors, providing a barrier to the efficacy of tumor-specific T cells, whether the T cells are elicited by a vaccine or exogenously administered as adoptive T cell therapy. Overcoming these barriers with newly developed checkpoint inhibitors or inhibitors of regulatory cells may finally allow the immune response induced by a vaccine to achieve its full potential [107]. Most major efforts to improve the efficacy of cancer vaccine therapy are now focused on such combinations of vaccines with modalities to block checkpoints or remove or block activity of other negative regulatory cells, such as T-regulatory cells, myeloid-derived suppressor cells (MDSCs), regulatory type II NKT cells, M2 macrophages, or other regulatory mechanisms such as indoleamine oxidase (IDO) or cytokines like IL-10, IL-13 or TGF-beta.

We have also learned that while some tumors are naturally immunogenic, and have elicited immune responses that can function on their own as long as immune checkpoints or other regulators are blocked, other tumors (so-called “cold” tumors) [115,116] [6,117,118] fail to elicit immune responses on their own. This may be due to intrinsic lack of immunogenicity as in the case of tumors with low mutational burden [6] or active immunosuppression through activation of pathways such as beta-catenin signaling [118]. In these cases, vaccines can provide the immunogenicity that the tumors lack, inducing an immune response that can then function when combined with such inhibitors of immune regulation. Other interventions are now understood to also function at least in part by inducing an immune response, such as the abscopal effects of radiotherapy [119,120] or the immunogenic cell death induced by some types of chemotherapy [121–123], and these modalities may also synergize with vaccines and with checkpoint inhibitors or other strategies to block immunoregulatory mechanisms. New targets for cancer vaccines, such as neo-antigens created by tumor-unique mutations [6,97], may facilitate the development of more effective cancer vaccines. Also, the recent discovery that tumor immunity is strongly influenced by the gut microbiome may provide another approach to potentiating the efficacy of cancer vaccines [124–126]. With all of these approaches to improve the efficacy of cancer vaccines, cancer vaccines may finally be in a position to achieve their full potential and to take their rightful place as the third leg of the stool of cancer immunotherapy alongside adoptive cell therapy [8] and checkpoint inhibition that have recently been licensed as novel efficacious treatments for cancer.

Highlights.

• Animal model successes have been translated to clinical trials from phase I to III.

• Cancer vaccines have elicited antigen-specific T cells but limited efficacy alone.

• Overcoming negative regulation may allow vaccine clinical efficacy.

• Vaccine studies in combination with checkpoint inhibitors are ongoing.

• Mutation-generated neoantigens provide novel target antigens for cancer vaccines.