Listeria-based vaccination against the pericyte antigen RGS5 elicits anti-vascular effects and colon cancer protection.

Colorectal cancer (CRC) remains a leading cause of cancer-related mortality despite efforts to improve standard interventions. As CRC patients can benefit from immunotherapeutic strategies that incite effector T cell action, cancer vaccines represent a safe and promising therapeutic approach to elicit protective and durable immune responses against components of the tumor microenvironment (TME). In this study, we investigate the pre-clinical potential of a Listeria monocytogenes (Lm)-based vaccine targeting the CRC-associated vasculature. CRC survival and progression are reliant on functioning blood vessels to effectively mediate various metabolic processes and oxygenate underlying tissues. We, therefore, advance the strategy of initiating immunity in syngeneic mouse models against the endogenous pericyte antigen RGS5, which is a critical mediator of pathological vascularization. Overall, Lm-based vaccination safely induced potent anti-tumor effects that consisted of recruiting functional Type-1-associated T cells into the TME and reducing tumor blood vessel content. This study underscores the promising clinical potential of targeting RGS5 against vascularized tumors like CRC.

Targeting Ubiquitin-like Protein, ISG15, as a Novel Tumor Associated Antigen in Colorectal Cancer.

Colorectal cancer (CRC) is the third leading cause of cancer-related deaths in both men and women in the United States. While immune checkpoint inhibitor (ICI) therapy is demonstrating remarkable clinical responses, the resistance and immune-related toxicities associated with ICIs demonstrate the need to develop additional immunotherapy options for CRC patients. Cancer vaccines represent a safe and promising treatment approach for CRC. As previously developed tumor-associated antigen (TAA)-based cancer vaccines for CRC are not demonstrating promising results, we propose that interferon-stimulated gene 15 (ISG15) is a novel TAA and therapeutic target for CRC. Our work demonstrates the anti-tumor efficacy of a Listeria-based vaccine targeting ISG15, designated Lm-LLO-ISG15, in an immunocompetent CRC murine model. The Lm-LLO-ISG15-mediated anti-tumor response is associated with an increased influx of functional T cells, higher production of multiple intracellular cytokines response, a lower number of regulatory T cells, and a greater ratio of effector to regulatory T cells (Teff/Treg) in the tumor microenvironment.

Interferon stimulated gene 15 (ISG15) in cancer: An update.

Among the plethora of defense mechanisms which a host elicits after pathogen invasion, type 1 interferons play a central role in regulating the immune system's response. They induce several interferon-stimulated genes (ISGs) which play a diverse role once activated. Over the past few decades, there have been several studies exploring the role of ISGs in cancer and ISG15 is among the most studied for its pro and anti-tumorigenic role. In this review, we aim to provide an update on the recent observations and findings related to ISG15 in cancer. We provide a brief overview about the initial observations and important historical findings which helped scientists understand structure and function of ISG15. We aim to provide an overview of ISG15 in cancer with an emphasis on studies which delve into the molecular mechanism of ISG15 in modulating the tumor microenvironment. Further, the dysregulation of ISG15 in cancer and the molecular mechanisms associated with its pro and anti-tumor roles are discussed in respective cancer types. Finally, we discuss multiple therapeutic applications of ISG15 in current cancer therapy.

A Listeria-based vaccine targeting ISG15 exerts anti-tumor efficacy in renal cell carcinoma.

Renal cell carcinoma (RCC) is the deadliest form of urological cancer and is projected to be the fourth most common neoplasm in the USA in males by 2040. In addition to the current poor prognosis with 5-year survival rates hardly reaching 15%, the prevalence of resistance to currently available systemic therapies has also established an urgent need to develop new treatment regimen(s) for advanced RCC. Interferon-stimulated gene 15 (ISG15) is the first identified ubiquitin-like modifier and has been intensively studied for its central role in innate immunity against intracellular pathogens. However, in this study, we identified ISG15 as a novel tumor-associated antigen and prognostic marker in RCC. Further, we therapeutically targeted elevated ISG15 expression by means of a Listeria monocytogenes (Lm)-based vaccine, designated Lm-LLO-ISG15, in both subcutaneous and orthotopic RCC mouse models. Treatment with Lm-LLO-ISG15 resulted in an influx of tumor-infiltrating effector T cells and significant anti-tumor efficacy in both subcutaneous and orthotopic RCC tumor models. Treatment with Lm-LLO-ISG15 also generated a robust interferon-gamma response and attracted a larger pool of polyfunctional T cells into the tumor microenvironment. Importantly, the therapeutic efficacy of Lm-LLO-ISG15 in RCC is comparable to that of anti-PD-1 and sunitinib, the current frontline therapies for RCC patients. Collectively, our work illustrates that targeting ISG15 in RCC with a CTL-based immunotherapy such as Lm-LLO-ISG15 is a promising and potentially translatable therapeutic strategy to enhance survival in RCC patients.

Tumoral CD105 promotes immunosuppression, metastasis, and angiogenesis in renal cell carcinoma.

CD105 (endoglin) is a transmembrane protein that functions as a TGF-beta coreceptor and is highly expressed on endothelial cells. Unsurprisingly, preclinical and clinical evidence strongly suggests that CD105 is an important contributor to tumor angiogenesis and tumor progression. Emerging evidence suggests that CD105 is also expressed by tumor cells themselves in certain cancers such as renal cell carcinoma (RCC). In human RCC tumor cells, CD105 expression is associated with stem cell-like properties and contributes to the malignant phenotype in vitro and in xenograft models. However, as a regulator of TGF-beta signaling, there is a striking lack of evidence for the role of tumor-expressed CD105 in the anti-tumor immune response and the tumor microenvironment. In this study, we report that tumor cell-expressed CD105 potentiates both the in vitro and in vivo tumorigenic potential of RCC in a syngeneic murine RCC tumor model. Importantly, we find that tumor cell-expressed CD105 sculpts the tumor microenvironment by enhancing the recruitment of immunosuppressive cell types and inhibiting the polyfunctionality of tumor-infiltrating CD4+ and CD8+ T cells. Finally, while CD105 expression by endothelial cells is a well-established contributor to tumor angiogenesis, we also find that tumor cell-expressed CD105 significantly contributes to tumor angiogenesis in RCC.

Listeria-based immunotherapy directed against CD105 exerts anti-angiogenic and anti-tumor efficacy in renal cell carcinoma.

Targeting tumor-associated angiogenesis is currently at the forefront of renal cell carcinoma (RCC) therapy, with sunitinib and bevacizumab leading to increased survival in patients with metastatic RCC (mRCC). However, resistance often occurs shortly after initiation of therapy, suggesting that targeting the tumor-associated vascular endothelium may not be sufficient to eradicate RCC. This study reports the therapeutic efficacy of a Listeria (Lm)-based vaccine encoding an antigenic fragment of CD105 (Lm-LLO-CD105A) that targets both RCC tumor cells and the tumor-associated vasculature. Lm-LLO-CD105A treatment reduced primary tumor growth in both subcutaneous and orthotopic models of murine RCC. The vaccine conferred anti-tumor immunity and remodeled the tumor microenvironment (TME), resulting in increased infiltration of polyfunctional CD8+ and CD4+ T cells and reduced infiltration of immunosuppressive cell types within the TME. We further provide evidence that the therapeutic efficacy of Lm-LLO-CD105A is mediated by CD8+ T cells and is dependent on the robust antigenic expression of CD105 by RCC tumor cells. The result from this study demonstrates the safety and promising therapeutic efficacy of targeting RCC-associated CD105 expression with Lm-based immunotherapy.

Clinical Experience and Recent Advances in the Development of Listeria-Based Tumor Immunotherapies.

The promise of tumor immunotherapy to significantly improve survival in patients who are refractory to long-standing therapies, such as chemotherapy and radiation, is now being realized. While immune checkpoint inhibitors that target PD-1 and CTLA-4 are leading the charge in clinical efficacy, there are a number of other promising tumor immunotherapies in advanced development such as Listeria-based vaccines. Due to its unique life cycle and ability to induce robust CTL responses, attenuated strains of Listeria monocytogenes (Lm) have been utilized as vaccine vectors targeting both infectious disease and cancer. In fact, preclinical studies in a multitude of cancer types have found Listeria-based vaccines to be highly effective at activating anti-tumor immunity and eradicating tumors. Several clinical trials have now recently reported their results, demonstrating promising efficacy against some cancers, and unique challenges. Development of the Lm-based immunotherapies continues with discovery of improved methods of attenuation, novel uses, and more effective combinatorial regimens. In this review, we provide a brief background of Listeria monocytogenes as a vaccine vector, discuss recent clinical experience with Listeria-based immunotherapies, and detail the advancements in development of improved Listeria-based vaccine platforms and in their utilization.

Dendritic cell vaccine therapy for colorectal cancer.

Colorectal cancer (CRC) remains a leading cause of cancer-related deaths in the United States despite an array of available treatment options. Current standard-of-care interventions for this malignancy include surgical resection, chemotherapy, and targeted therapies depending on the disease stage. Specifically, infusion of anti-vascular endothelial growth factor agents in combination with chemotherapy was an important development in improving the survival of patients with advanced colorectal cancer, while also helping give rise to other forms of anti-angiogenic therapies. Yet, one approach by which tumor angiogenesis may be further disrupted is through the administration of a dendritic cell (DC) vaccine targeting tumor-derived blood vessels, leading to cytotoxic immune responses that decrease tumor growth and synergize with other systemic therapies. Early generations of such vaccines exhibited protection against various forms of cancer in pre-clinical models, but clinical results have historically been disappointing. Sipuleucel-T (Provenge®) was the first, and to-date, only dendritic cell-based therapy to receive FDA approval after significantly increasing overall survival in prostate cancer patients. The unparalleled success of Sipuleucel-T has helped revitalize the clinical development of dendritic cell vaccines, which will be examined in this review. We also highlight the promise of these vaccines to instill anti-angiogenic immunity for individuals with advanced colorectal cancer.

Harnessing Liposome Interactions With the Immune System for the Next Breakthrough in Cancer Drug Delivery.

Liposomal nanoparticles are a heterogeneous group of engineered drug carriers that have tremendous therapeutic potential in the treatment of cancer. They increase tumor drug delivery, significantly attenuate drug toxicity, and protect the drug from degradation. However, two decades after approval of the first nanoparticle-mediated anticancer drug, pegylated liposomal doxorubicin (Doxil), there has yet to be a major shift in cancer treatment paradigms. Only two anticancer nanoparticles are used in the first-line treatment of cancer patients, with all others relegated to the refractory or salvage setting. Herein, we discuss new insights into the mechanisms underlying in vivo interactions between liposomes and the tumor immunologic milieu, and the knowledge gaps that need to be addressed in order to realize the full clinical potential of cancer nanomedicines. We also discuss immunopharmacology insights from a parallel field, Cancer Immunotherapy, which have the potential to generate breakthroughs in Cancer Nanomedicine.

Liposome-induced immunosuppression and tumor growth is mediated by macrophages and mitigated by liposome-encapsulated alendronate.

Liposomal nanoparticles are the most commonly used drug nano-delivery platforms. However, recent reports show that certain pegylated liposomal nanoparticles (PLNs) and polymeric nanoparticles have the potential to enhance tumor growth and inhibit antitumor immunity in murine cancer models. We sought herein to identify the mechanisms and determine whether PLN-associated immunosuppression and tumor growth can be reversed using alendronate, an immune modulatory drug. By conducting in vivo and ex vivo experiments with the immunocompetent TC-1 murine tumor model, we found that macrophages were the primary cells that internalized PLN in the tumor microenvironment and that PLN-induced tumor growth was dependent on macrophages. Treatment with PLN increased immunosuppression as evidenced by increased expression of arginase-1 in CD11b+Gr1+ cells, diminished M1 functionality in macrophages, and globally suppressed T-cell cytokine production. Encapsulating alendronate in PLN reversed these effects on myeloid cells and shifted the profile of multi-cytokine producing T-cells towards an IFNγ+ perforin+ response, suggesting increased cytotoxic functionality. Importantly, we also found that PLN-encapsulated alendronate (PLN-alen), but not free alendronate, abrogated PLN-induced tumor growth and increased progression-free survival. In summary, we have identified a novel mechanism of PLN-induced tumor growth through macrophage polarization and immunosuppression that can be targeted and inactivated to improve the anticancer efficacy of PLN-delivered drugs. Importantly, we also determined that PLN-alen not only reversed protumoral effects of the PLN carrier, but also had moderate antitumor activity. Our findings strongly support the inclusion of immune-responsive tumor models and in-depth immune functional studies in the preclinical drug development paradigm for cancer nanomedicines, and the further development of chemo-immunotherapy strategies to co-deliver alendronate and chemotherapy for the treatment of cancer.

TCR-like antibody drug conjugates mediate killing of tumor cells with low peptide/HLA targets.

The currently marketed antibody-drug conjugates (ADC) destabilize microtubule assembly in cancer cells and initiate apoptosis in patients. However, few tumor antigens (TA) are expressed at high densities on cancer lesions, potentially minimizing the therapeutic index of current ADC regimens. The peptide/human leukocyte antigen (HLA) complex can be specifically targeted by therapeutic antibodies (designated T cell receptor [TCR]-like antibodies) and adequately distinguish malignant cells, but has not been the focus of ADC development. We analyzed the killing potential of TCR-like ADCs when cross-linked to the DNA alkylating compound duocarmycin. Our data comprise proof-of-principle results that TCR-like ADCs mediate potent tumor cytotoxicity, particularly under common scenarios of low TA/HLA density, and support their continued development alongside agents that disrupt DNA replication. Additionally, TCR-like antibody ligand binding appears to play an important role in ADC functionality and should be addressed during therapy development to avoid binding patterns that negate ADC killing efficacy.

Ubiquitin-like Molecule ISG15 Acts as an Immune Adjuvant to Enhance Antigen-specific CD8 T-cell Tumor Immunity.

ISG15 is an ubiquitin-like protein induced by type I interferon associated with antiviral activity. ISG15 is also secreted and known to function as an immunomodulatory molecule. However, ISG15's role in influencing the adaptive CD8 T-cell responses has not been studied. Here, we demonstrate the efficacy of ISG15 as a vaccine adjuvant, inducing human papilloma virus (HPV) E7-specific IFNγ responses as well as the percentage of polyfunctional, cytolytic, and effector CD8 T-cell responses. Vaccination with ISG15 conferred remarkable control and/or regression of established HPV-associated tumor-bearing mice. T-cell depletion coupled with adoptive transfer experiments revealed that ISG15 protective efficacy was CD8 T-cell mediated. Importantly, we demonstrate that ISG15 vaccine-induced responses could be generated independent of ISGylation, suggesting that responses were mostly influenced by free ISG15. Our results provide more insight into the immunomodulatory properties of ISG15 and its potential to serve as an effective immune adjuvant in a therapeutic tumor or infectious disease setting.

Liposome promotion of tumor growth is associated with angiogenesis and inhibition of antitumor immune responses.

Liposomes have tremendous potential as drug carriers in the treatment of cancer. However, despite enhanced tumor drug delivery and decreased toxicity, patient survival rates have not improved significantly compared to corresponding free drug treatments. Importantly, we found that a liposomal nanoparticle currently used as a drug carrier in cancer patients enhanced tumor growth in an immune competent murine model of cancer. This was associated with increased tumor angiogenesis and suppression of antitumor immune responses as indicated by decreased cytokine production by tumor macrophages and cytotoxic T cells, diminished tumor infiltration of tumor-specific T cells, and decreased number of dendritic cells in tumor draining lymph nodes. These results suggest that carrier-induced immunosuppression and angiogenesis have the potential to reduce the antitumor effects of drugs loaded within. These findings may have significant implications for the current use and future development of anticancer nanoparticles and further investigations are urgently needed. FROM THE CLINICAL EDITOR: This study discusses important implications of nanoliposome-based drug delivery systems in cancer therapy, and demonstrates that nanoliposomes may have immunosuppressive and angiogenetic properties, directly counterbalancing their anti-cancer activity, which may also have important clinical implications related to more widespread applications of such systems.

Attenuated Listeria monocytogenes: a powerful and versatile vector for the future of tumor immunotherapy.

For over a century, inactivated or attenuated bacteria have been employed in the clinic as immunotherapies to treat cancer, starting with the Coley's vaccines in the 19th century and leading to the currently approved bacillus Calmette-Guérin vaccine for bladder cancer. While effective, the inflammation induced by these therapies is transient and not designed to induce long-lasting tumor-specific cytolytic T lymphocyte (CTL) responses that have proven so adept at eradicating tumors. Therefore, in order to maintain the benefits of bacteria-induced acute inflammation but gain long-lasting anti-tumor immunity, many groups have constructed recombinant bacteria expressing tumor-associated antigens (TAAs) for the purpose of activating tumor-specific CTLs. One bacterium has proven particularly adept at inducing powerful anti-tumor immunity, Listeria monocytogenes (Lm). Lm is a gram-positive bacterium that selectively infects antigen-presenting cells wherein it is able to efficiently deliver tumor antigens to both the MHC Class I and II antigen presentation pathways for activation of tumor-targeting CTL-mediated immunity. Lm is a versatile bacterial vector as evidenced by its ability to induce therapeutic immunity against a wide-array of TAAs and specifically infect and kill tumor cells directly. It is for these reasons, among others, that Lm-based immunotherapies have delivered impressive therapeutic efficacy in preclinical models of cancer for two decades and are now showing promise clinically. In this review, we will provide an overview of the history leading up to the development of current Lm-based immunotherapies, the advantages and mechanisms of Lm as a therapeutic vaccine vector, the preclinical experience with Lm-based immunotherapies targeting a number of malignancies, and the recent findings from clinical trials along with concluding remarks on the future of Lm-based tumor immunotherapies.

The ubiquitin-like protein, ISG15, is a novel tumor-associated antigen for cancer immunotherapy.

The recent announcement of the first FDA-approved therapeutic vaccine for prostate cancer, Sipuleucel-T, is a watershed moment for the field of tumor immunotherapy. However, while Sipuleucel-T provides a powerful tool to clinicians for the most prevalent form of cancer in men, there remains an unmet need for a similar therapeutic strategy against breast cancer, the most prevalent cancer in women. While current breast cancer vaccines in development target several antigens, the most prevalent is the tumor-associated antigen, HER2. Initial results with HER2 vaccines appear promising in terms of efficacy; however, the lack of HER2 overexpression by a majority of breast tumors and the safety concerns associated with current HER2-targeted immunotherapy suggest that additional therapeutic strategies would be beneficial. Recently, several studies have identified ISG15 as a molecule highly expressed in numerous malignancies. ISG15 is a small ubiquitin-like protein regulated by type-I interferon and classically associated with viral defense. Elevated ISG15 expression in breast cancer is especially well documented and is independent of HER2, progesterone receptor, and estrogen receptor status. Additionally, high ISG15 expression in breast cancer correlates with an unfavorable prognosis and poor responses to traditional treatment strategies such as chemotherapy and radiation. To overcome these challenges, we employ a novel strategy to specifically target tumor-associated ISG15 expression with immunotherapy. We demonstrate that vaccination against ISG15 results in significant CD8-mediated reductions in both primary and metastatic mammary tumor burden. These results validate ISG15 as a tumor-associated antigen for cancer immunotherapy.

ISG15 disrupts cytoskeletal architecture and promotes motility in human breast cancer cells.

The interferon-stimulated gene 15 (ISG15) pathway is highly elevated in breast cancer; however, very little is known about how the ISG15 pathway contributes to breast tumorigenesis. In the current study, using the gene disruption approach, we demonstrate that both ISG15 and UbcH8 (ISG15-specific conjugating enzyme) disrupt F-actin architecture and formation of focal adhesions in ZR-75-1 breast cancer cells. In addition, ISG15 and UbcH8 promote breast cancer cell migration. We also demonstrate that ISG15 inhibits ubiquitin/26S proteasome-mediated turnover of proteins implicated in tumor cell motility, invasion and metastasis. Together, our results suggest that the aberrant activation of the ISG15 pathway confers a motile phenotype to breast cancer cells by disrupting cell architecture and stabilizing proteins involved in cell motility, invasion and metastasis. Because the cellular architecture is conserved and the ISG15 pathway is constitutively activated in tumor cells of different lineages, it is reasonable to assume that our observations in breast cancer must hold true for many other tumors.

Targeting tumor vasculature with novel Listeria-based vaccines directed against CD105.

The FDA approval of bevacizumab (Avastin®, Genentech/Roche), a monoclonal antibody raised against human VEGF-A, as second-line therapy for colon and lung carcinoma validated the approach of targeting human tumors with angiogenesis inhibitors. While the VEGF/VEGFR pathway is a viable target for anti-angiogenesis tumor therapy, additional targets involved in tumor neovascularization have been identified. One promising target present specifically on tumor vasculature is endoglin (CD105), a member of the TGF-ß receptor complex expressed on vascular endothelium and believed to play a role in angiogenesis. Monoclonal antibody therapy and preventive vaccination against CD105 has met with some success in controlling tumor growth. This report describes the in vivo proof-of-concept studies for two novel therapeutic vaccines, Lm-LLO-CD105A and Lm-LLO-CD105B, directed against CD105 as a strategy to target neovascularization of established tumors. Listeria-based vaccines directed against CD105 lead to therapeutic responses against primary and metastatic tumors in the 4T1-Luc and NT-2 mouse models of breast cancer. In a mouse model for autochthonous Her-2/neu-driven breast cancer, Lm-LLO-CD105A vaccination prevented tumor incidence in 20% of mice by week 58 after birth while all control mice developed tumors by week 40. In comparison with previous Listeria-based vaccines targeting tumor vasculature, Lm-LLO-CD105A and Lm-LLO-CD105B demonstrated equivalent or superior efficacy against two transplantable mouse models of breast cancer. Support is provided for epitope spreading to endogenous tumor antigens and reduction in tumor vascularity after vaccination with Listeria-based CD105 vaccines. Reported here, these CD105 therapeutic vaccines are highly effective in stimulating anti-angiogenesis and anti-tumor immune responses leading to therapeutic efficacy against primary and metastatic breast cancer.

A novel role for ATM in regulating proteasome-mediated protein degradation through suppression of the ISG15 conjugation pathway.

Ataxia Telangiectasia (A-T) is an inherited immunodeficiency disorder wherein mutation of the ATM kinase is responsible for the A-T pathogenesis. Although the precise role of ATM in A-T pathogenesis is still unclear, its function in responding to DNA damage has been well established. Here we demonstrate that in addition to its role in DNA repair, ATM also regulates proteasome-mediated protein turnover through suppression of the ISG15 pathway. This conclusion is based on three major pieces of evidence: First, we demonstrate that proteasome-mediated protein degradation is impaired in A-T cells. Second, we show that the reduced protein turnover is causally linked to the elevated expression of the ubiquitin-like protein ISG15 in A-T cells. Third, we show that expression of the ISG15 is elevated in A-T cells derived from various A-T patients, as well as in brain tissues derived from the ATM knockout mice and A-T patients, suggesting that ATM negatively regulates the ISG15 pathway. Our current findings suggest for the first time that proteasome-mediated protein degradation is impaired in A-T cells due to elevated expression of the ISG15 conjugation pathway, which could contribute to progressive neurodegeneration in A-T patients.

Listeria-derived ActA is an effective adjuvant for primary and metastatic tumor immunotherapy.

Tumor immunotherapy is currently at the cusp of becoming an important aspect of comprehensive cancer treatment in the clinic. However, the need for improved adjuvants to augment immune responses against tumor antigens is always present. In this paper, we characterize the Listeria monocytogenes-derived actin-nucleating protein, ActA, as a novel adjuvant for use in tumor immunotherapy. ActA is a virulence factor that is expressed on the cell surface of L. monocytogenes and facilitates the production of actin tails that propel Listeria throughout the cytosol of an infected host cell. It is believed that this ActA-dependent cytosolic motility allows Listeria to evade adaptive host cell defenses and facilitates its invasion into a proximal uninfected host cell. However, there is evidence that ActA fused to a tumor antigen and delivered by L. monocytogenes can perform a beneficial function in tumor immunotherapy as an adjuvant. Our investigation of this adjuvant activity demonstrates that ActA, either fused to or administered as a mixture with a tumor antigen, can augment anti-tumor immune responses, break immune tolerance and facilitate tumor eradication, which suggests that ActA is not only an effective adjuvant in tumor immunotherapy but can also be applied in a number of therapeutic settings.

Listeria and Salmonella bacterial vectors of tumor-associated antigens for cancer immunotherapy.

This review covers the use of the facultative intracellular bacteria, Listeriamonocytogenes and Salmonella enterica serovar typhimurium as delivery systems for tumor-associated antigens in tumor immunotherapy. Because of their ability to infect and survive in antigen presenting cells, these bacteria have been harnessed to deliver tumor antigens to the immune system both as bacterially expressed proteins and encoded on eukaryotic plasmids. They do this in the context of strong innate immunity, which provides the required stimulus to the immune response to break tolerance against those tumor-associated antigens that bear homology to self. Here we describe differences in the properties of these bacteria as vaccine vectors, a summary of the major therapies they have been applied to and their advancement towards the clinic.