Vaccination strategy determines the emergence and dominance of CD8+ T-cell epitopes in a FVB/N rat HER-2/neu mouse model of breast cancer

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.

Listeria monocytogenes as a Vector for Cancer Immunotherapy: Current Understanding and Progress

Listeria monocytogenes, a Gram-positive facultative anaerobic bacterium, is becoming a popular vector for cancer immunotherapy. Indeed, multiple vaccines have been developed utilizing modified Listeria as a tool for generating immune responses against a variety of cancers. Moreover, over a dozen clinical trials testing Listeria cancer vaccines are currently underway, which will help to understand the utility of Listeria vaccines in cancer immunotherapy. This review aims to summarize current views on how Listeria-based vaccines induce potent antitumor immunity and the current state of Listeria-based cancer vaccines in clinical trials.

Immunogenic peptide discovery in cancer genomes

As immunotherapies to treat malignancy continue to diversify along with the tumor types amenable to treatment, it will become very important to predict which treatment is most likely to benefit a given patient. Tumor neoantigens, novel peptides resulting from somatic tumor mutations and recognized by the immune system as foreign, are likely to contribute significantly to the efficacy of immunotherapy. Multiple in silico methods have been developed to predict whether peptides, including tumor neoantigens, will be presented by the major histocompatibility complex (MHC) Class I or Class II, and interact with the T cell receptor (TCR). The methods for neoantigen prediction will be reviewed here, along with the most important examples of their use in the field of oncology.

Schistosoma mansoni soluble egg antigens enhance T cell responses to a newly identified HIV-1 Gag H-2b epitope

Schistosome infection induces significant T helper type 2 (Th2) and anti-inflammatory immune responses and has been shown to negatively impact vaccine efficacy. Our goal was to determine if the administration of schistosome soluble egg antigens (SEA) would negatively influence the induction of cytotoxic T lymphocyte (CTL) and Th1-type T cell responses to an HIV candidate vaccine in the Th1-biased C57BL/6 mouse strain. Initial experiments failed, as we were unable to detect any response to the defined class I epitope for HIV-1 IIIB Gag. Therefore, we initiated an epitope mapping study to identify C57BL/6 (H-2(b)) T cell epitopes in HIV-1 IIIB Gag in order to perform the experiments. This analysis defined two previously unreported minimal class I H-2(b) and class II I-A(b) epitopes for HIV-1 IIIB Gag. The newly defined HIV-1 IIIB Gag epitopes were used to evaluate the influence of SEA on the generation of CTL and Th1-type HIV-1 IIIB Gag responses. Surprisingly, in contrast to our hypothesis, we observed that the coadministration of SEA with a Listeria monocytogenes vector expressing HIV-1 IIIB Gag (Lm-Gag) led to a significantly increased frequency of gamma interferon (IFN-γ)-producing CD8(+) and CD4(+) T cells in C57BL/6 mice compared to mice immunized with Lm-Gag only. These observations suggest that SEA contains, in addition to Th2-type and immune-suppressive molecules, substances that can act with the Lm-Gag vaccine to increase CTL and Th1-type vaccine-specific immune responses.

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.

A novel mycobacterial Hsp70-containing fusion protein targeting mesothelin augments antitumor immunity and prolongs survival in murine models of ovarian cancer and mesothelioma

Background

Although dendritic cell (DC) vaccines are considered to be promising treatments for advanced cancer, their production and administration is costly and labor-intensive. We developed a novel immunotherapeutic agent that links a single-chain antibody variable fragment (scFv) targeting mesothelin (MSLN), which is overexpressed on ovarian cancer and mesothelioma cells, to Mycobacterium tuberculosis (MTB) heat shock protein 70 (Hsp70), which is a potent immune activator that stimulates monocytes and DCs, enhances DC aggregation and maturation and improves cross-priming of T cells mediated by DCs.

Methods

Binding of this fusion protein with MSLN on the surface of tumor cells was measured by flow cytometry and fluorescence microscopy. The therapeutic efficacy of this fusion protein was evaluated in syngeneic and orthotopic mouse models of papillary ovarian cancer and malignant mesothelioma. Mice received 4 intraperitoneal (i.p.) treatments with experimental or control proteins post i.p. injection of tumor cells. Ascites-free and overall survival time was measured. For the investigation of anti-tumor T-cell responses, a time-matched study was performed. Splenocytes were stimulated with peptides, and IFNγ- or Granzyme B- generating CD3⁺CD8⁺ T cells were detected by flow cytometry. To examine the role of CD8⁺ T cells in the antitumor effect, we performed in vivo CD8⁺ cell depletion. We further determined if the fusion protein increases DC maturation and improves antigen presentation as well as cross-presentation by DCs.

Results

We demonstrated in vitro that the scFvMTBHsp70 fusion protein bound to the tumor cells used in this study through the interaction of scFv with MSLN on the surface of these cells, and induced maturation of bone marrow-derived DCs. Use of this bifunctional fusion protein in both mouse models significantly enhanced survival and slowed tumor growth while augmenting tumor-specific CD8⁺ T-cell dependent immune responses. We also demonstrated in vitro and in vivo that the fusion protein enhanced antigen presentation and cross-presentation by targeting tumor antigens towards DCs.

Conclusions

This new cancer immunotherapy has the potential to be cost-effective and broadly applicable to tumors that overexpress mesothelin.

Management of breast cancer by vaccine: fact or fiction

Breast cancer (BC) is the most common malignancy (75–80%) among women. Options for management of BC are multivariate. Available modalities include surgery, radiotherapy, chemotherapy and hormone therapy. Despite availability of improved therapeutic adjuncts, mortality from BC is 40%. Vaccination strategies against BC are emerging as a viable alternative. This review highlights the available results of this emerging therapeutic strategy.

Listeria monocytogenes and its products as agents for cancer immunotherapy

This review covers the use of Listeria monocytogenes and its virulence factors as cancer immunotherapeutics. We describe their development as vectors to carry protein tumor antigen and eukaryotic DNA plasmids to antigen-presenting cells and efforts to harness their tumor-homing properties. We also describe their use as vectors of angiogenic molecules to induce an immune response that will destroy tumor vasculature. The background knowledge necessary to understand the biology behind the rationale to develop Listeria as a vaccine vector for tumor immunotherapy is included as well as a brief summary of the major therapies that have used this approach thus far.

T cells sensitized with breast tumor progenitor cell vaccine have therapeutic activity against spontaneous HER2/neu tumors

Cancer progenitor cells are critical for tumor initiation and recurrence so they are an important therapeutic target. We tested whether T cells could recognize tumor antigens expressed by breast cancer progenitor cells and acquire therapeutic activity against established metastases or delay onset of spontaneous tumors. Breast tumors were derived from HER2/neu transgenic mice and propagated in vitro under conditions that selected progenitor cells which were then used as an irradiated whole cell vaccine. A minor subset of recently sensitized T cells was isolated from vaccine-draining lymph nodes then activated in vitro to achieve numerical expansion. We show that the tumor progenitor cell vaccines reversed tolerance to a known HER2/neu epitope, otherwise inhibited by Treg cells. Additional shared tumor antigens were recognized because a Neuneg subclone also induced a Th1 type immune response against breast tumors. Adoptive transfer of in vitro activated lymph node T cells-mediated regression of established metastases from multiple independently derived breast tumor lines. Moreover, adoptive transfer of effector T cells into Neu-tolerant mice, months before the onset of spontaneous tumors, significantly postponed tumor development. Interestingly, T-cell-mediated lysis of metastases stimulated an IgG response to HER2/neu as well as other shared antigens. In summary, tumor progenitor cells contain shared antigens which can lead to a cross-protective T-cell response. Moreover, antigens acquired during immune-mediated tumor destruction are presented in a manner conducive to reversal of tolerance and Ig class switching. These complementary effector mechanisms might augment therapy by eliminating refractory breast cancer stem cells.

CXCL12/CXCR4 blockade induces multimodal antitumor effects that prolong survival in an immunocompetent mouse model of ovarian cancer

The chemokine CXCL12 and its receptor CXCR4 are expressed widely in human cancers, including ovarian cancer, in which they are associated with disease progression at the levels of tumor cell proliferation, invasion, and angiogenesis. Here, we used an immunocompetent mouse model of intraperitoneal papillary epithelial ovarian cancer to show that modulation of the CXCL12/CXCR4 axis in ovarian cancer has multimodal effects on tumor pathogenesis associated with induction of antitumor immunity. siRNA-mediated knockdown of CXCL12 in BR5-1 cells that constitutively express CXCL12 and CXCR4 reduced cell proliferation in vitro, and tumor growth in vivo. Similarly, treatment of BR5-1-derived tumors with AMD3100, a selective CXCR4 antagonist, resulted in increased tumor apoptosis and necrosis, reduction in intraperitoneal dissemination, and selective reduction of intratumoral FoxP3(+) regulatory T cells (Treg). Compared with controls, CXCR4 blockade greatly increased T-cell-mediated antitumor immune responses, conferring a significant survival advantage to AMD3100-treated mice. In addition, the selective effect of CXCR4 antagonism on intratumoral Tregs was associated with both higher CXCR4 expression and increased chemotactic responses to CXCL12, a finding that was also confirmed in a melanoma model. Together, our findings reinforce the concept of a critical role for the CXCL12/CXCR4 axis in ovarian cancer pathogenesis, and they offer a definitive preclinical validation of CXCR4 as a therapeutic target in this disease.

Nondominant CD8 T cells are active players in the vaccine-induced antitumor immune response

We previously reported that CD8(+) T cells are directed predominantly toward the immunodominant Her-2/neu (neu) epitope RNEU(420-429) in nontolerized FVB/N but not tolerized HER-2/neu (neu-N) mice. In this study, we screened overlapping peptides of the entire neu protein and identified six new epitopes recognized by vaccine-induced neu-N-derived T cells. Evaluation of individual nondominant responses by tetramer staining and IFN-γ secretion demonstrate that this repertoire is peripherally tolerized. To address the role that the complete CD8(+) T cell repertoire plays in vaccine-induced antitumor immunity, we created a whole-cell vaccine-expressing neu cDNA that has been mutated at the RNEU(420-429) anchor residue, thereby abrogating activation of immunodominant epitope responses. Studies comparing the mutated and nonmutated vaccines indicate that nondominant CD8(+) T cells can induce antitumor immunity when combined with regulatory T cell-depleting agents in both neu-N and FVB/N mice. Collectively, these studies demonstrate that the neu-directed T cell repertoire is not intrinsically incapable of eradicating tumors. Rather, they are suppressed by mechanisms of peripheral tolerance. Thus, these studies provide new insights into the function of the complete T cell repertoire directed toward a clinically relevant tumor Ag in tumor-bearing hosts.

Dendritic cells transfected with Her2 antigen-encoding RNA replicons cross-prime CD8 T cells and protect mice against tumor challenge

Antigen-specific T cells can be induced by direct priming and cross-priming. To investigate cross-priming as a vaccination approach dendritic cells were transfected with cytopathogenic viral RNA-replicons that expressed domains of the tumor-associated Her2-antigen and injected into MHC-discordant mice that did not allow direct priming. Upon tumor challenge 75% of the vaccinated, but none of the mock-vaccinated mice remained tumor-free. The anti-tumor effect required T cells and correlated with the vigor of the cross-primed CD8 T cell response. Her2-specific antibodies were not detected. This study highlights the potential of T cell cross-priming in cancer immunotherapy.

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.

Myeloid-derived suppressor cells in mammary tumor progression in FVB Neu transgenic mice

Female mice transgenic for the rat proto-oncogene c-erb-B2, under control of the mouse mammary tumor virus (MMTV) promoter (neuN), spontaneously develop metastatic mammary carcinomas. The development of these mammary tumors is associated with increased number of GR-1(+)CD11b(+) myeloid derived suppressor cells (MDSCs) in the peripheral blood (PB), spleen and tumor. We report a complex relationship between tumor growth, MDSCs and immune regulatory molecules in non-mutated neu transgenic mice on a FVB background (FVB-neuN). The first and second tumors in FVB-neuN mice develop at a median of 265 (147-579) and 329 (161-523) days, respectively, resulting in a median survival time (MST) of 432 (201 to >500) days. During tumor growth, significantly increased number of MDSCs is observed in the PB and spleen, as well as, in infiltrating the mammary tumors. Our results demonstrate a direct correlation between tumor size and the number of MDSCs infiltrating the tumor and an inverse relationship between the frequency of CD4(+) T-cells and MDSCs in the spleen. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assessment of enzyme and cytokine transcript levels in the spleen, tumor, tumor-infiltrating non-parenchymal cells (NPCs) and mammary glands revealed a significant increase in transcript levels from grossly normal mammary glands and tumor-infiltrating NPCs during tumor progression. Tumor NPCs, as compared to spleen cells from wild-type (w/t) mice, expressed significantly higher levels of arginase-1 (ARG-1), nitric oxide synthase (NOS-2), vascular endothelial growth factor (VEGF-A) and significantly lower levels of interferon (IFN)-gamma, interleukin (IL)-2 and fms-like tyrosine kinase-3 ligand (Flt3L) transcript levels. Transcript levels in the spleens of tumor-bearing (TB) mice also differed from normal mice, although to a lesser extent than transcript levels from tumor-infiltrating NPCs. Furthermore, both spleen cells and NPCs from TB mice, but not control mice, suppressed alloantigen responses by syngeneic control spleen cells. Correlative studies revealed that the number of MDSCs in the spleen was directly associated with granulocyte colony stimulating factor (G-CSF) transcript levels in the spleen; while the number of MDSCs in the tumors was directly correlated with splenic granulocyte macrophage stimulating factor (GM-CSF) transcript levels, tumor volume and tumor cell number. Together our results support a role for MDSCs in tumor initiation and progressive, T-cell depression and loss of function provide evidence which support multiple mechanisms of MDSC expansion in a site-dependent manner.

Antiangiogenesis immunotherapy induces epitope spreading to Her-2/neu resulting in breast tumor immunoediting

Targeting tumors using cancer vaccine therapeutics has several advantages including the induction of long-term immunity, prime boost strategies for additional treatments and reduced side effects compared to conventional chemotherapeutics. However, one problem in targeting tumor antigens directly is that this can lead to antigen loss or immunoediting. We hypothesized that directing the immune response to a normal cell type required for tumor growth and survival could provide a more stable immunotherapeutic target. We thus examined the ability of an antiangiogenesis, Listeria monocytogenes (Lm)-based vector to deliver extracellular and intracellular fragments of the mouse vascular endothelial growth factor receptor-2/Flk-1 molecule, Lm-LLO-Flk-E1, and Lm-LLO-Flk-11 respectively, in an autochthonous model for Her-2/neu(+) breast cancer. We found that these vaccines could cause epitope spreading to the endogenous tumor protein Her-2/neu and significantly delay tumor onset. However, tumors that grew out overtime accumulated mutations in the Her-2/neu molecule near or within cytotoxic T lymphocytes epitopes. We show here for the first time how an antiangiogenesis immunotherapy can be used to delay the onset of a spontaneous tumor through epitope spreading and determine a possible mechanism of how immunoediting of an endogenous tumor protein can allow for tumor escape and outgrowth in an autochthonous mouse model for Her-2/neu(+) breast cancer.

A novel human Her-2/neu chimeric molecule expressed by Listeria monocytogenes can elicit potent HLA-A2 restricted CD8-positive T cell responses and impact the growth and spread of Her-2/neu-positive breast tumors

PURPOSE

The aim of this study was to efficiently design a novel vaccine for human Her-2/neu-positive (hHer-2/neu) breast cancer using the live, attenuated bacterial vector Listeria monocytogenes.

EXPERIMENTAL DESIGN

Three recombinant L. monocytogenes-based vaccines were generated that could express and secrete extracellular and intracellular fragments of the hHer-2/neu protein. In addition, we generated a fourth construct fusing selected portions of each individual fragment that contained most of the human leukocyte antigen (HLA) epitopes as a combination vaccine (L. monocytogenes-hHer-2/neu chimera).

RESULTS

Each individual vaccine was able to either fully regress or slow tumor growth in a mouse model for Her-2/neu-positive tumors. All three vaccines could elicit immune responses directed toward human leukocyte antigen-A2 epitopes of hHer-2/neu. The L. monocytogenes-hHer-2/neu chimera was able to mimic responses generated by the three separate vaccines and prevent spontaneous outgrowth of tumors in an autochthonous model for Her-2/neu-positive breast cancer, induce tumor regression in transplantable models, and prevent seeding of experimental lung metastases in a murine model for metastatic breast cancer.

CONCLUSION

This novel L. monocytogenes-hHer-2/neu chimera vaccine proves to be just as effective as the individual vaccines but combines the strength of all three in a single vaccination. These encouraging results support future clinical trials using this chimera vaccine and may be applicable to other cancer types expressing the Her-2/neu molecule such as colorectal and pancreatic cancer.

An anti-vascular endothelial growth factor receptor 2/fetal liver kinase-1 Listeria monocytogenes anti-angiogenesis cancer vaccine for the treatment of primary and metastatic Her-2/neu+ breast tumors in a mouse model

Thirty years after angiogenesis was shown to play an enabling role in cancer, modern medicine is still trying to develop novel compounds and therapeutics to target the tumor vasculature. However, most therapeutics require multiple rounds of administration and can have toxic side effects. In this study, we use anti-angiogenesis immunotherapy to target cells actively involved in forming new blood vessels that support the growth and spread of breast cancer. Targeting a central cell type involved in angiogenesis, endothelial cells, we immunized against host vascular endothelial growth factor receptor 2 to fight the growth of Her-2/neu(+) breast tumors. Using the bacterial vector, Listeria monocytogenes (Lm), we fused polypeptides from the mouse vascular endothelial growth factor receptor 2 molecule (fetal liver kinase-1) to the microbial adjuvant, listeriolysin-O, and used Lm to deliver the Ags and elicit potent antitumor CTL responses. Lm-listeriolysin-O-fetal liver kinase-1 was able to eradicate some established breast tumors, reduce microvascular density in the remaining tumors, protect against tumor rechallenge and experimental metastases, and induce epitope spreading to various regions of the tumor-associated Ag Her-2/neu. Tumor eradication was found to be dependent on epitope spreading to HER-2/neu and was not solely due to the reduction of tumor vasculature. However, vaccine efficacy did not affect normal wound healing nor have toxic side effects on pregnancy. We show that an anti-angiogenesis vaccine can overcome tolerance to the host vasculature driving epitope spreading to an endogenous tumor protein and drive active tumor regression.

Cancer immunotherapy using Listeria monocytogenes and listerial virulence factors

Our laboratory is interested in how immunogenicity may be modulated in vivo in order to better design more effective immunotherapeutics against cancer. Our main approach is to use a facultative intracellular bacterium, Listeria monocytogenes, which has the unusual ability to live and grow in the cytoplasm of the cell and is thus an excellent vector for targeting passenger antigens to the major histocompatibility complex (MHC) class I pathway of antigen processing with the generation of authentic CTL epitopes. We have used this approach to target tumor antigens expressed on breast, melanoma and cervical cancer. We are also exploring the role of Listerial virulence factors in potentiating adaptive immune responses by activating innate immunity. Specifically, we are using these proteins as adjuvants for B cell lymphomas.

In the FVB/N HER-2/neu transgenic mouse both peripheral and central tolerance limit the immune response targeting HER-2/neu induced by Listeria monocytogenes-based vaccines

Listeria monocytogenes-based vaccines for HER-2/neu are capable of breaking tolerance in FVB/N rat HER-2/neu transgenic mice. The growth of implanted NT-2 tumors, derived from a spontaneously occurring tumor in the FVB/N HER-2/neu transgenic mouse, was significantly slower in these mice following vaccination with a series of L. monocytogenes-based vaccines for HER-2/neu. Mechanisms of T cell tolerance that exist in these transgenic mice include the absence of functional high avidity anti-HER-2/neu CD8(+) T cells and the presence of CD4(+)CD25(+) regulatory T cells. The in vivo depletion of these regulatory T cells resulted in the slowing in growth of tumors even without the treatment of mice with an anti-HER-2/neu vaccine. The average avidities of responsive CD8(+) T cells to six of the nine epitopes in HER-2/neu we examined, four of which were identified in this study, are shown here to be of a lower average avidity in the transgenic mice versus wild type FVB/N mice. In contrast, the average avidity of CD8(+) T cells to three epitopes that showed the lowest avidity in the wild-type mice did not differ between wild type and transgenic mice. This study demonstrates the ability of L. monocytogenes-based vaccines to impact upon tolerance to HER-2/neu in FVB/N HER-2/neu transgenic mice and further defines some of the aspects of tolerance in these mice.

Immunoediting sculpts tumor epitopes during immunotherapy

Immunoediting of tumor-associated antigens occurs in response to immune pressure. We show that the mutation of residues within epitopes of HER-2/neu leads to the outgrowth of autochthonous tumors after immunizing HER-2/neu transgenic mice with Listeria monocytogenes therapeutic vaccines expressing fragments of HER-2/neu. Three of these vaccines target the extracellular domain (LmLLO-EC1, LmLLO-EC2, and LmLLO-EC3), and two of these vaccines target the intracellular domain (Lm-LLO-IC1 and Lm-LLO-IC2). Mutations occurred in the regions of the HER-2/neu molecule targeted by the Listeria strain expressing that region, which suggests that the rate of generation of escape mutants was a significant factor in the efficacy of each vaccine. A longer delay in the onset of tumors after immunotherapy occurred with the vaccine that targeted the kinase domain. We verified that the mutations in this domain occurred within novel CD8(+) T-cell epitopes, and that the mutation of these residues abrogated CTL responses to these epitopes. The long delay in the onset of tumors after immunotherapy targeting the kinase domain may be because this region of HER-2/neu cannot undergo extensive mutations without impairing its ability to signal cell growth.

Listeria monocytogenes as a vector for tumor-associated antigens for cancer immunotherapy

As a facultative intracellular bacterium, Listeria monocytogenes has adapted to live within the cytosol of the host cell. It is actively taken up by antigen-presenting cells through phagocytosis, and as Listeria survive within these cells, it is an ideal vector for the delivery of antigens to be processed and presented through both the class I and II antigen-processing pathways. Once phagocytosed, Listeria produces virulence factors within the phagolysosome of the host cell, which allows it to break out of this organelle and live in the host cytosol. It is possible that these virulence factors can enhance the immunogenicity of tumor-associated antigens, which are poorly immunogenic. Recent progress in the development of this bacterium as a vaccine vector for tumor-associated antigens is discussed in the context of bacterial vectors in general. In several mouse models, Listeria-based vaccines have been demonstrated to be an effective method of influencing tumor growth and eliciting potent antitumor immune responses. Safety issues and the transition of Listeria into human clinical trials will also be discussed in this review.