The combined activation of positive costimulatory signals with modulation of a negative costimulatory signal for the enhancement of vaccine-mediated T-cell responses

Recombinant Viruses for Cancer Therapy

Recombinant viruses are novel therapeutic agents that can be utilized for treatment of various diseases, including cancers. Recombinant viruses can be engineered to express foreign transgenes and have a broad tropism allowing gene expression in a wide range of host cells. They can be selected or designed for specific therapeutic goals; for example, recombinant viruses could be used to stimulate host immune response against tumor-specific antigens and therefore overcome the ability of the tumor to evade the host's immune surveillance. Alternatively, recombinant viruses could express immunomodulatory genes which stimulate an anti-cancer immune response. Oncolytic viruses can replicate specifically in tumor cells and induce toxic effects leading to cell lysis and apoptosis. However, each of these approaches face certain difficulties that must be resolved to achieve maximum therapeutic efficacy. In this review we discuss actively developing approaches for cancer therapy based on recombinant viruses, problems that need to be overcome, and possible prospects for further development of recombinant virus based therapy.

New Combination Strategies Using Programmed Cell Death 1/Programmed Cell Death Ligand 1 Checkpoint Inhibitors as a Backbone

The discovery of immune checkpoints and subsequent clinical development of checkpoint inhibitors have revolutionized the field of oncology. The durability of the antitumor immune responses has raised the hope for long-term patient survival and potential cure; however, currently, only a minority of patients respond. Combination strategies to help increase antigen release and T-cell priming, promote T-cell activation and homing, and improve the tumor immune microenvironment, all guided by predictive biomarkers, can help overcome the tumor immune-evasive mechanisms and maximize efficacy to ultimately benefit the majority of patients. Great challenges remain because of the complex underlying biology, unpredictable toxicity, and accurate assessment of response. Carefully designed clinical trials guided by translational studies of paired biopsies will be key to develop reliable predictive biomarkers to choose which patients would most likely benefit from each strategy.

Immune checkpoint inhibitor combinations in solid tumors: opportunities and challenges

The emergence of immune 'checkpoint inhibitors' such as cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed death receptor 1 (PD-1) has revolutionized treatment of solid tumors including melanoma, lung cancer, among many others. The goal of checkpoint inhibitor combination therapy is to improve clinical response and minimize toxicities. Rational design of checkpoint combinations considers immune-mediated mechanisms of antitumor activity: immunogenic cell death, antigen release and presentation, activation of T-cell responses, lymphocytic infiltration into tumors and depletion of immunosuppression. Potential synergistic combinations include checkpoint blockade with conventional (radiation, chemotherapy and targeted therapies) and newer immunotherapies (cancer vaccines, oncolytic viruses, among others). Reliable biomarkers are necessary to define patients who will achieve best clinical benefit with minimal toxicity in combination therapy.

Is There Still Room for Cancer Vaccines at the Era of Checkpoint Inhibitors

Checkpoint inhibitor (CPI) blockade is considered to be a revolution in cancer therapy, although most patients (70%–80%) remain resistant to this therapy. It has been hypothesized that only tumors with high mutation rates generate a natural antitumor T cell response, which could be revigorated by this therapy. In patients with no pre-existing antitumor T cells, a vaccine-induced T cell response is a rational option to counteract clinical resistance. This hypothesis has been validated in preclinical models using various cancer vaccines combined with inhibitory pathway blockade (PD-1-PDL1-2, CTLA-4-CD80-CD86). Enhanced T cell infiltration of various tumors has been demonstrated following this combination therapy. The timing of this combination appears to be critical to the success of this therapy and multiple combinations of immunomodulating antibodies (CPI antagonists or costimulatory pathway agonists) have reinforced the synergy with cancer vaccines. Only limited results are available in humans and this combined approach has yet to be validated. Comprehensive monitoring of the regulation of CPI and costimulatory molecules after administration of immunomodulatory antibodies (anti-PD1/PD-L1, anti-CTLA-4, anti-OX40, etc.) and cancer vaccines should help to guide the selection of the best combination and timing of this therapy.

Reinstating endogenous antitumor immunity: The concept of therapeutic management of cancer

Strong evidence points to the role of cancer immunoediting and tumor immune infiltrates in regulating cancer progression. By understanding the immune tumor microenvironment, we can now target key pathways that suppress endogenous antitumor responses, thereby re-instating such immune responses and identifying novel targets for immune therapies. Therapies targeting oncogenic pathways and checkpoint blockades turn on a new paradigm shift in immune-therapy for cancer with remarkable clinical efficacy seen in various malignancies. However, a lot of cancer patients will fail to respond and therefore, it becomes crucial to identify biomarkers to predict who of the patients will most likely benefit from these therapies.

Poxvirus-based active immunotherapy synergizes with CTLA-4 blockade to increase survival in a murine tumor model by improving the magnitude and quality of cytotoxic T cells

The dramatic clinical benefit of immune checkpoint blockade for a fraction of cancer patients suggests the potential for further clinical benefit in a broader cancer patient population by combining immune checkpoint inhibitors with active immunotherapies. The anti-tumor efficacy of MVA-BN-HER2 poxvirus-based active immunotherapy alone or in combination with CTLA-4 checkpoint blockade was investigated in a therapeutic CT26-HER-2 lung metastasis mouse model. MVA-BN-HER2 immunotherapy significantly improved the median overall survival compared to untreated controls or CTLA-4 blockade alone (p < 0.001). Robust synergistic efficacy was achieved with the combination therapy (p < 0.01). Improved survival following MVA-BN-HER2 administration was accompanied by increased tumor infiltration by HER-2-specific cytotoxic T lymphocytes (CTL). These tumor-specific CTL had characteristics similar to antiviral CTL, including strong expression of activation markers and co-expression of IFNγ and TNFα. Combination with CTLA-4 blockade significantly increased the magnitude of HER-2-specific T cell responses, with a higher proportion co-expressing TNFα and/or IL-2 with IFNγ. Furthermore, in mice treated with MVA-BN-HER2 (alone or in combination with CTLA-4 blockade), the inducible T cell co-stimulator (ICOS) protein was expressed predominantly on CD4 and CD8 effector T cells but not on regulatory T cells (T(reg)). In contrast, mice left untreated or treated solely with CTLA-4 blockade harbored elevated ICOS(+) Treg, a phenotype associated with highly suppressive activity. In conclusion, poxvirus-based active immunotherapy induced robust tumor infiltration by highly efficient effector T cells. Combination with CTLA-4 immune checkpoint blockade amplified this response resulting in synergistically improved efficacy. These hypothesis-generating data may help elucidate evidence of enhanced clinical benefit from combining CTLA-4 blockade with poxvirus-based active immunotherapy.

Immune checkpoint modulation: rational design of combination strategies

Immune recognition and elimination of malignant cells require a series of steps orchestrated by the innate and the adaptive arms of the immune system. The majority of tumors have evolved mechanisms that allow for successful evasion of these immune responses. Recognition of these evasive processes led to the development of immunotherapeutic antibodies targeting the co-stimulatory and co-inhibitory receptors on T cells, with the goal of enhancement of T cell activation or reversal of tumor-induced T cell inhibition. Several of these agents, such as antibodies targeting cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed death receptor 1 (PD-1) have already demonstrated significant promise in clinical trials. Clinical benefit of these antibodies as single agents, however, has been limited to a subset of patients and has not been observed in all tumor types. These limitations call for the development of rational combination strategies aiming to extend therapeutic benefit to a broader range of patients. These include: 1) modalities that enhance antigen presentation, such as radiation, cryotherapy, chemotherapy, targeted agents, vaccines, toll-like receptor (TLR) agonists, type I interferon, and oncolytic viruses; 2) additional agents aiming to reverse T cell dysfunction, such as other immune checkpoint inhibitors; and 3) agents targeting other immune inhibitory mechanisms, such as inhibitors of indoleamine dioxygenase (IDO), regulatory T cells, and myeloid-derived suppressor cells (MDSCs). It is becoming increasingly evident that the efficacy of specific combinations will likely not be universal and that the choice of a treatment modality may need to be tailored to fit the needs of each individual patient.

Potentiation of immunomodulatory antibody therapy with oncolytic viruses for treatment of cancer

Identification of the immune suppressive mechanisms active within the tumor microenvironment led to development of immunotherapeutic strategies aiming to reverse the immunosuppression and to enhance the function of tumor-infiltrating lymphocytes. Of those, cancer therapy with antibodies targeting the immune costimulatory and coinhibitory receptors has demonstrated significant promise in the recent years, with multiple antibodies entering clinical testing. The responses to these agents, however, have not been universal and have not been observed in all cancer types, calling for identification of appropriate predictive biomarkers and development of combinatorial strategies. Pre-existing immune infiltration in tumors has been demonstrated to have a strong association with response to immunotherapies, with the type I interferon (IFN) pathway emerging as a key player in tumor innate immune recognition and activation of adaptive immunity. These findings provide a rationale for evaluation of strategies targeting the type I IFN pathway as a means to enhance tumor immune recognition and infiltration, which could potentially make them susceptible to therapeutics targeting the cosignaling receptors. To this end in particular, oncolytic viruses (OVs) have been demonstrated to enhance tumor recognition by the immune system through multiple mechanisms, which include upregulation of major histocompatibility complex and costimulatory molecules on cancer cells, immunogenic cell death and antigen release, and activation of the type I IFN pathway. Evidence is now emerging that combination therapies using OVs and agents targeting immune cosignaling receptors such as 4-1BB, PD-1, and CTLA-4 may work in concert to enhance antitumor immunity and therapeutic efficacy. Our evolving understanding of the interplay between OVs and the immune system demonstrates that the virus-induced antitumor immune responses can be harnessed to drive the efficacy of the agents targeting cosignaling receptors and provides a strong rationale for integration of such therapies in clinic.

A combination trial of vaccine plus ipilimumab in metastatic castration-resistant prostate cancer patients: immune correlates

We recently reported the clinical results of a Phase I trial combining ipilimumab with a vaccine containing transgenes for prostate-specific antigen (PSA) and for a triad of costimulatory molecules (PROSTVAC) in patients with metastatic castration-resistant prostate cancer. Thirty patients were treated with escalating ipilimumab and a fixed dose of vaccine. Of 24 chemotherapy-naïve patients, 58 % had a PSA decline. Combination therapy did not exacerbate the immune-related adverse events associated with ipilimumab. Here, we present updated survival data and an evaluation of 36 immune cell subsets pre- and post-therapy. Peripheral blood mononuclear cells were collected before therapy, at 13 days and at 70 days post-initiation of therapy, and phenotyped by flow cytometry for the subsets of T cells, regulatory T cells, natural killer cells, and myeloid-derived suppressor cells. Associations between overall survival (OS) and immune cell subsets prior to treatment, and the change in a given immune cell subset 70 days post-initiation of therapy, were evaluated. The median OS was 2.63 years (1.77-3.45). There were trends toward associations for longer OS and certain immune cell subsets before immunotherapy: lower PD-1(+)Tim-3(NEG)CD4EM (P = 0.005, adjusted P = 0.010), higher PD-1(NEG)Tim-3(+)CD8 (P = 0.002, adjusted P = 0.004), and a higher number of CTLA-4(NEG) Tregs (P = 0.005, adjusted P = 0.010). We also found that an increase in Tim-3(+) natural killer cells post- versus pre-vaccination associated with longer OS (P = 0.0074, adjusted P = 0.015). These results should be considered as hypothesis generating and should be further evaluated in larger immunotherapy trials.

Therapeutic cancer vaccines: past, present, and future

Therapeutic vaccines represent a viable option for active immunotherapy of cancers that aim to treat late stage disease by using a patient's own immune system. The promising results from clinical trials recently led to the approval of the first therapeutic cancer vaccine by the U.S. Food and Drug Administration. This major breakthrough not only provides a new treatment modality for cancer management but also paves the way for rationally designing and optimizing future vaccines with improved anticancer efficacy. Numerous vaccine strategies are currently being evaluated both preclinically and clinically. This review discusses therapeutic cancer vaccines from diverse platforms or targets as well as the preclinical and clinical studies employing these therapeutic vaccines. We also consider tumor-induced immune suppression that hinders the potency of therapeutic vaccines, and potential strategies to counteract these mechanisms for generating more robust and durable antitumor immune responses.

Cancer vaccines targeting carcinoembryonic antigen: state-of-the-art and future promise

Concurrent with the US FDA's approval of the first therapeutic cancer vaccine, and supported by mounting clinical evidence indicating that targeting carcinoembryonic antigen (CEA) can safely overcome pre-existing tolerance, a multitude of novel CEA cancer vaccines are now in various stages of development. Since cancer-driven immune suppression often limits the efficacy of vaccines, numerous strategies are being examined in both preclinical and clinical settings to overcome immunosuppressive elements, including the combined use of vaccines with certain chemotherapies, immune checkpoint inhibitors, small-molecule targeted therapies and radiation. This review discusses the current state and future direction of therapeutic cancer vaccines targeting CEA, based on advances achieved over the last 5 years.

Synergistic effect of CTLA-4 blockade and cancer chemotherapy in the induction of anti-tumor immunity

Several chemotherapeutics exert immunomodulatory effects. One of these is the nucleoside analogue gemcitabine, which is widely used in patients with lung cancer, ovarian cancer, breast cancer, mesothelioma and several other types of cancer, but with limited efficacy. We hypothesized that the immunopotentiating effects of this drug are partly restrained by the inhibitory T cell molecule CTLA-4 and thus could be augmented by combining it with a blocking antibody against CTLA-4, which on its own has recently shown beneficial clinical effects in the treatment of patients with metastatic melanoma. Here we show, using two non-immunogenic murine tumor models, that treatment with gemcitabine chemotherapy in combination with CTLA-4 blockade results in the induction of a potent anti-tumor immune response. Depletion experiments demonstrated that both CD4(+) and CD8(+) T cells are required for optimal therapeutic effect. Mice treated with the combination exhibited tumor regression and long-term protective immunity. In addition, we show that the efficacy of the combination is moderated by the timing of administration of the two agents. Our results show that immune checkpoint blockade and cytotoxic chemotherapy can have a synergistic effect in the treatment of cancer. These results provide a basis to pursue combination therapies with anti-CTLA-4 and immunopotentiating chemotherapy and have important implications for future studies in cancer patients. Since both drugs are approved for use in patients our data can be immediately translated into clinical trials.

Synergistic effect of CTLA-4 blockade and cancer chemotherapy in the induction of anti-tumor immunity

Several chemotherapeutics exert immunomodulatory effects. One of these is the nucleoside analogue gemcitabine, which is widely used in patients with lung cancer, ovarian cancer, breast cancer, mesothelioma and several other types of cancer, but with limited efficacy. We hypothesized that the immunopotentiating effects of this drug are partly restrained by the inhibitory T cell molecule CTLA-4 and thus could be augmented by combining it with a blocking antibody against CTLA-4, which on its own has recently shown beneficial clinical effects in the treatment of patients with metastatic melanoma. Here we show, using two non-immunogenic murine tumor models, that treatment with gemcitabine chemotherapy in combination with CTLA-4 blockade results in the induction of a potent anti-tumor immune response. Depletion experiments demonstrated that both CD4⁺ and CD8⁺ T cells are required for optimal therapeutic effect. Mice treated with the combination exhibited tumor regression and long-term protective immunity. In addition, we show that the efficacy of the combination is moderated by the timing of administration of the two agents. Our results show that immune checkpoint blockade and cytotoxic chemotherapy can have a synergistic effect in the treatment of cancer. These results provide a basis to pursue combination therapies with anti-CTLA-4 and immunopotentiating chemotherapy and have important implications for future studies in cancer patients. Since both drugs are approved for use in patients our data can be immediately translated into clinical trials.

Clinical evaluation of TRICOM vector therapeutic cancer vaccines

We have developed an "off-the-shelf" vector-based vaccine platform containing transgenes for carcinoma-associated antigens and multiple costimulatory molecules (designated TRICOM). Two TRICOM platforms have been evaluated both preclinically and in clinical trials. PROSTVAC consists of rV, rF-PSA-TRICOM and is being used in prostate cancer therapy trials. PANVAC consists of rV, rF-CEA-MUC1-TRICOM; the expression of the two pan-carcinoma transgenes CEA and MUC-1 renders PANVAC vaccination applicable for therapeutic applications for a range of human carcinomas. Many new paradigms have emerged as a consequence of completed and ongoing TRICOM vaccine trials, including (1) clinical evidence of patient benefit may be delayed, because multiple vaccinations may be necessary to induce a sufficient anti-tumor immune response; (2) survival, and not strict adherence to RECIST criteria or time-to-progression, may be the most appropriate trial endpoint when TRICOM vaccines are used as monotherapy; (3) certain patient populations are more likely to benefit from vaccine therapy as compared to other therapeutics; and (4) TRICOM vaccines combined with standard-of-care therapeutics, either concomitantly or sequentially, are feasible because of the limited toxicity of vaccines.

Toward maximizing immunotherapy in metastatic castration-resistant prostate cancer - rationale for combinatorial approaches using chemotherapy

Prostate cancer is particularly suited for active immunotherapy because of the expression of a distinctive number of antigens which are overexpressed on prostate cancer cells and cell lines. There is evidence in this disease that tumors promote immune tolerance starting early in the disease course. As such, chemotherapy, by suppressing tumors and activating immune system homeostatic mechanisms, may help overcome this tumor-induced immune tolerance. Sipuleucel-T which has recently been approved in the US, is an autologous cellular product immunotherapy that induces immune activity likely through activation of dendritic cells. This was associated with a survival benefit in the absence of significant toxicity. However, a post hoc analysis of phase III trial participants found a substantial survival benefit to receiving docetaxel some months after sipuleucel-T. However, another phase III immunotherapy trial combining a prostate cancer therapeutic vaccine GVAX plus docetaxel versus standard docetaxel therapy in advanced prostate cancer, observed a lower overall survival with the vaccine regimen. These trials highlight major unresolved questions concerning the optimum choice, dosing, and timing of chemotherapy relative to active immunotherapy and the overall merits of considering this approach. The ideal treatment approach remains unclear; advances in biomarker validation and trial design may likely improve our ability to assess biologic benefit irrespective of the development of true antitumor immunity.

Therapeutic cancer vaccines: current status and moving forward

Concurrent with U.S. Food and Drug Administration (FDA) approval of the first therapeutic cancer vaccine, a wide spectrum of other cancer vaccine platforms that target a diverse range of tumor-associated antigens is currently being evaluated in randomized phase II and phase III trials. The profound influence of the tumor microenvironment and other immunosuppressive entities, however, can limit the effectiveness of these vaccines. Numerous strategies are currently being evaluated both preclinically and clinically to counteract these immunosuppressive entities, including the combined use of vaccines with immune checkpoint inhibitors, certain chemotherapeutics, small-molecule targeted therapies, and radiation. The potential influence of the appropriate patient population and clinical trial endpoint in vaccine therapy studies is discussed, as well as the potential importance of biomarkers in future directions of this field.

Immune modulation by chemotherapy or immunotherapy to enhance cancer vaccines

Chemotherapy has been a mainstay in cancer treatment for many years. Despite some success, the cure rate with chemotherapy remains unsatisfactory in some types of cancers, and severe side effects from these treatments are a concern. Recently, understanding of the dynamic interplay between the tumor and immune system has led to the development of novel immunotherapies, including cancer vaccines. Cancer vaccines have many advantageous features, but their use has been hampered by poor immunogenicity. Many developments have increased their potency in pre-clinical models, but cancer vaccines continue to have a poor clinical track record. In part, this could be due to an inability to effectively overcome tumor-induced immune suppression. It had been generally assumed that immune-stimulatory cancer vaccines could not be used in combination with immunosuppressive chemotherapies, but recent evidence has challenged this dogma. Chemotherapies could be used to condition the immune system and tumor to create an environment where cancer vaccines have a better chance of success. Other types of immunotherapies could also be used to modulate the immune system. This review will discuss how immune modulation by chemotherapy or immunotherapy could be used to bolster the effects of cancer vaccines and discuss the advantages and disadvantages of these treatments.

Enhancement of dendritic cells as vaccines for cancer

Dendritic cells are the most potent antigen-presenting cells known; owing to their ability to stimulate antigen-specific cytolytic and memory T-cell responses, their use as cancer vaccines is rapidly increasing. While clinical trials provide evidence that dendritic cells vaccines are safe and elicit immunological responses in most patients, few complete tumor remissions have been reported and further technological advances are required. An effective dendritic cell vaccine must possess and maintain several characteristics: it must migrate to lymph nodes, have a mature, Th1-polarizing phenotype expressed stably after infusion and present antigen for sufficient time to produce a T-cell response capable of eliminating a tumor. While dendritic cells are readily matured ex vivo, their phenotype and fate after infusion are rarely evaluable; therefore, strategies to ensure that dendritic cells access lymphoid tissues and retain an immunostimulatory phenotype are required. In order to best exploit dendritic cells as vaccines, they may require genetic modification and combination with other strategies including adoptive T-cell transfer, inhibition of regulatory T cells or modulation of inflammatory pathways.

The consequence of immune suppressive cells in the use of therapeutic cancer vaccines and their importance in immune monitoring

Evaluating the number, phenotypic characteristics, and function of immunosuppressive cells in the tumor microenvironment and peripheral blood could elucidate the antitumor immune response and provide information to evaluate the efficacy of cancer vaccines. Further studies are needed to evaluate the correlation between changes in immunosuppressive cells and clinical outcomes of patients in cancer vaccine clinical trials. This paper focuses on the role of T-regulatory cells, myeloid-derived suppressor cells, and tumor-associated macrophages in cancer and cancer immunotherapy and their role in immune monitoring.

Vaccine Immunopotentiators of the Future

Vaccine adjuvants or immunopotentiators comprise a diverse group of molecules or formulations. Despite a wealth of different candidates, there is a need for better vaccine adjuvants in both veterinary and human medicine. For human vaccines, the immunopotentiator choice has been limited to aluminum salts, until recently. By contrast, a whole range of adjuvants is employed for inactivated veterinary vaccines, due to less stringent safety and regulatory requirements and proven superior vaccine performance. This review highlights recent developments and future trends in immunopotentiators.