The new vaccines: building viruses that elicit antitumor immunity

Influence of tumors on protective anti-tumor immunity and the effects of irradiation

Innate and adaptive immunity plays important roles in the development and progression of cancer and it is becoming apparent that tumors can influence the induction of potentially protective responses in a number of ways. The prevalence of immunoregulatory T cell populations in the circulation and tumors of patients with cancer is increased and the presence of these cells appears to present a major barrier to the induction of tumor immunity. One aspect of tumor-mediated immunoregulation which has received comparatively little attention is that which is directed toward natural killer (NK) cells, although evidence that the phenotype and function of NK cell populations are modified in patients with cancer is accumulating. Although the precise mechanisms underlying these localized and systemic immunoregulatory effects remain unclear, tumor-derived factors appear, in part at least, to be involved. The effects could be manifested by an altered function and/or via an influence on the migratory properties of individual cell subsets. A better insight into endogenous immunoregulatory mechanisms and the capacity of tumors to modify the phenotype and function of innate and adaptive immune cells might assist the development of new immunotherapeutic approaches and improve the management of patients with cancer. This article reviews current knowledge relating to the influence of tumors on protective anti-tumor immunity and considers the potential influence that radiation-induced effects might have on the prevalence, phenotype, and function of innate and adaptive immune cells in patients with cancer.

Molecular cancer vaccines: Tumor therapy using antigen-specific immunizations

Vaccination against tumors promises selective destruction of malignant cells by the host's immune system. Molecular cancer vaccines rely on recently identified tumor antigens as immunogens. Tumor antigens can be applied in many forms, as genes in recombinant vectors, as proteins or peptides representing T cell epitopes.Analysis of various aspects indicates some advantage for peptide-based vaccines over the other modalities. Further refinements and extensively monitored clinical trials are necessary to advance molecular cancer vaccines from concepts into powerful therapy.

Immunochemical termination of self-tolerance

The ability to selectively induce a strong immune response against self-proteins, or increase the immunogenicity of specific epitopes in foreign antigens, would have a significant impact on the production of vaccines for cancer, protein-misfolding diseases, and infectious diseases. Here, we show that site-specific incorporation of an immunogenic unnatural amino acid into a protein of interest produces high-titer antibodies that cross-react with WT protein. Specifically, mutation of a single tyrosine residue (Tyr(86)) of murine tumor necrosis factor-alpha (mTNF-alpha) to p-nitrophenylalanine (pNO(2)Phe) induced a high-titer antibody response in mice, whereas no significant antibody response was observed for a Tyr(86) --> Phe mutant. The antibodies generated against the pNO(2)Phe are highly cross-reactive with native mTNF-alpha and protect mice against lipopolysaccharide (LPS)-induced death. This approach may provide a general method for inducing an antibody response to specific epitopes of self- and foreign antigens that lead to a neutralizing immune response.

Necrotic tumor cell death in vivo impairs tumor-specific immune responses

The manner in which cells die is believed to have a major impact on the nature of immune responses to their released Ags. In this study, we present the first direct analysis of tumor-specific immune responses to in vivo occurring tumor cell death through apoptosis or necrosis. Mice bearing thymidine kinase-transfected tumors were treated either with ganciclovir to induce tumor cell apoptosis in vivo or a vascular targeting agent, ZD6126, to induce tumor cell necrosis in vivo. In contrast to tumor apoptosis, induction of necrosis reduced the frequency and impaired the function of tumor-specific CD8(+) T cells. Adoptive transfer of lymphocytes from mice with apoptotic tumors into tumor-challenged mice resulted in a significant tumor protection, which was absent when splenocytes were transferred from mice with necrotic tumors. Anti-CD40 treatment reversed impaired Ag-specific CD8(+) T cell responses in these mice. These observations have not only fundamental importance for the development of immunotherapy protocols but also help to understand the underlying mechanism of in vivo immune responses to tumor cell death.

Colorectal cancer vaccines: principles, results, and perspectives

In the search for novel therapeutic approaches to treat patients with colorectal carcinoma, anticancer vaccination holds promise. A large body of preclinical and clinical evidence has demonstrated that the immune system can be polarized against malignant cells by means of several active specific immunotherapy strategies. Although no vaccination regimen can be currently recommended outside clinical trials, tumor response and immunologic findings observed in animal models and humans prompt researchers to explore further the antitumor potential of such biotherapy in an effort to reproduce in a larger set of patients the cascade of molecular events that characterizes the successful tumor immune rejection currently observed in a minority of vaccinated subjects. In this work, we summarize the principles and the main results of cancer vaccine strategies so far implemented for the treatment of patients with colorectal carcinoma. We also discuss the most recent preclinical tumor immunology insights that might change the way to design the next generation of cancer vaccines, hopefully improving the effectiveness of such a biotherapeutic approach.

Escape from immunotherapy: possible mechanisms that influence tumor regression/progression

Tumor escape is one major obstacle that has to be addressed prior to designing and delivering successful immunotherapy. There is compelling evidence to support the notion that immunogenic tumors, in murine models and cancer patients, can be rejected by the immune system under optimum conditions for activating adaptive and nonadaptive antitumor immune responses. Despite this capability, a large number of tumors continue to grow and evade recognition and/or destruction by the immune system. The limited success in current immunotherapeutic strategies may be due to a variety of reasons: failure of effector cells to compete with the growing tumor burden, production of humoral factors by tumors that locally block cytotoxicity, antigen/MHC loss, T-cell dysfunction, production of suppressor T cells-to name but a few causes for therapeutic ineffectiveness for the particular malignancy being treated. To optimize immunotherapy strategies, correction of immune-activating signals, eradication of inhibitory factors, and the evasion from newly developed immunoresistant tumor phenotypes need to be simultaneously considered.

Induction of antigen-specific CTL responses using antigens conjugated to short peptide vectors

Linear peptides (SynB vectors) with specific sequence motifs have been identified that are capable of enhancing the transport of a wide range of molecules into cells. These peptide vectors have been used to deliver exogenous peptides and protein Ags across the cell membrane and into the cytoplasm of cells. Specifically, in vitro analysis indicated that these SynB peptides enhanced the uptake of two 9-mer peptide Ags, NP(147-155) and Mtb(250-258) (T cell epitopes of influenza nucleoprotein and Mycobacterium tuberculosis, respectively) and the M. tuberculosis Ag Mtb8.4 protein, into K562 cells when covalently linked to the respective Ags. Furthermore, selected SynB vectors, when conjugated to these same Ags and used as immunogens, resulted in considerably enhanced Ag-specific CTL responses. Several SynB vectors were tested and resulted in varying levels of cellular uptake. The efficiency of uptake correlated with the ability of the SynB construct to deliver each epitope in vivo and induce specific CTL responses in mice. These data suggest that peptide vectors, such as SynB that transport target Ags across the cell membrane in a highly efficient manner, have significant potential for vaccine delivery.

Rational approaches to human cancer immunotherapy

Over most of the 20th century, immunotherapy for cancer was based on empiricism. Interesting phenomena were observed in the areas of cancer, infectious diseases, or transplantation. Inferences were made and extrapolated into new approaches for the treatment of cancer. If tumors regressed, the treatment approaches could be refined further. However, until the appropriate tools and reagents were available, investigators were unable to understand the biology underlying these observations. In the early 1990s, the first human tumor T cell antigens were defined and dendritic cells were discovered to play a pivotal role in antigen presentation. The current era of cancer immunotherapy is one of translational research based on known biology and rationally designed interventions and has led to a rapid expansion of the field. The beginning of the 21st century brings the possibility of a new era of effective cancer immunotherapy, combining rational, immunological treatments with conventional therapies to improve the outcome for patients with cancer.

Prime-boost vaccines encoding an intracellular idiotype/GM-CSF fusion protein induce protective cell-mediated immunity in murine pre-B cell leukemia

Two vaccines against an intracellularly expressed B cell idiotype were assessed for their ability to induce protective immunity in mice against challenge with a pre-B cell leukemia. One vaccine was based on a plasmid expression vector and the other was a recombinant vaccinia virus; both vaccines expressed a polypeptide derived from the complementarity-determining regions (CDR(2)-CDR(3)) of the leukemic clone-specific immunoglobulin heavy chain (IgH), as a fusion product with mouse granulocyte-macrophage colony-stimulating factor (mGM-CSF). Mice inoculated with either vaccine showed significantly higher survival rates than controls after challenge with leukemia cells. However, protection from tumor challenge was optimal when the DNA vaccine was used for priming, followed by a booster immunization with the vaccinia virus recombinant. This vaccination protocol induced resistance not only to the first tumor challenge given shortly afterwards, but also to a second challenge given months later. Both CD4(+) and CD8(+) T cells contributed to protection in vaccinated mice. These data suggest that such a vaccine regimen might reduce the incidence of recurrence in patients with minimal residual disease after conventional therapy.

Adenovirus-transduced dendritic cells stimulate cellular immunity to melanoma via a CD4(+) T cell-dependent mechanism

We previously showed that genetic immunization of C57BL/6 mice with recombinant adenovirus encoding human TRP2 (Ad-hTRP2) was able to circumvent tolerance and induce cellular and humoral immune responses to murine TRP2 associated with protection against metastatic growth of B16 melanoma. In the present study we compared delivery of Ad-hTRP2 with cultured dendritic cells (DC) and direct injections of Ad-hTRP2. We show that application of Ad-hTRP2 with cultured DC enhanced protective immunity to B16 melanoma cells. Most importantly, delivery of recombinant adenovirus with DC alters the character of the immune response resulting in preferential stimulation of strong cellular immunity in the absence of significant humoral immunity to the encoded antigen. Adoptive transfer of lymphocytes from mice immunized with Ad-hTRP2-transduced DC confirmed that cellular components of the immune response were responsible for rejection of B16 melanoma. The protective efficacy of Ad-hTRP2-transduced DC clearly depended on the presence of CD4(+) T helper cells. Furthermore, AD-hTRP2-transduced DC, but not direct injection of Ad-hTRP2, were effective in the presence of neutralizing anti-adenoviral antibodies. These preclinical studies demonstrate the superiority of melanoma vaccines consisting of cultured DC transduced with recombinant adenoviruses encoding melanoma antigens.

Cancer vaccines

Cancer vaccines have been extensively tested in animal models, and in humans. Initial studies focused on first generation vaccines based on whole cell preparations or tumor lysates derived from autologous or allogeneic tumors. Clinical studies conducted with such candidate vaccines contributed to establish the feasibility of immunizing cancer patients against their own tumors. Significant clinical benefits were observed, both in terms of long term survival and recurrence rate, in some of these trials. More recently, however, cancer vaccines targeting well-characterized tumor-associated antigens, i.e. molecules selectively or preferentially expressed by cancer cells but not by normal cells, have been designed and tested in humans. Results obtained as of today with these second-generation vaccines suggest that they are safe and that they can elicit humoral and cellular responses against tumor-specific antigens, without inducing unacceptable clinical signs of autoimmunity. Advances in tumor biology and tumor immunity have helped to better understand the mechanisms displayed by a number of tumors to escape host immunity. This bulk of new knowledge will be used to design future cancer vaccines, which will likely target multiple TAAs, presented by different antigen presentation platforms, in association with synthetic adjuvants and/or immunostimulatory cytokines. Lastly, specific tools allowing to assess in a qualitative and quantitative manner immune responses are critically needed in order to establish correlates between clinical and immune responses in patients receiving experimental vaccines.

Challenges to the development of antigen-specific breast cancer vaccines

Continued progress in the development of antigen-specific breast cancer vaccines depends on the identification of appropriate target antigens, the establishment of effective immunization strategies, and the ability to circumvent immune escape mechanisms. Methods such as T cell epitope cloning and serological expression cloning (SEREX) have led to the identification of a number target antigens expressed in breast cancer. Improved immunization strategies, such as using dendritic cells to present tumor-associated antigens to T lymphocytes, have been shown to induce antigen-specific T cell responses in vivo and, in some cases, objective clinical responses. An outcome of successful tumor immunity is the evolution of antigen-loss tumor variants. The development of a polyvalent breast cancer vaccine, directed against a panel of tumor-associated antigens, may counteract this form of immune escape.

Creating therapeutic cancer vaccines: notes from the battlefield

With the identification of tumor antigens and a knowledge of how to vaccinate against them, the field of tumor immunology faces new challenges. In this article, the authors argue that successful immunotherapies of the future will activate anti-tumor T cells without inducing their anergy or apoptotic death.

Immunogenicity of enhanced green fluorescent protein (EGFP) in BALB/c mice: identification of an H2-Kd-restricted CTL epitope

Enhanced green fluorescent protein (EGFP) is a novel marker gene product, which is readily detectable using techniques of fluorescence microscopy, flow cytometry, or macroscopic imaging. In the present studies, we have examined the immunogenicity of EGFP in murine models. A stable transfectant of the transplantable CMS4 sarcoma of BALB/c origin expressing EGFP, CMS4-EGFP-Zeo, was generated. Splenocytes harvested from mice immunized with a recombinant adenovirus expressing EGFP (Ad-EGFP) were restimulated in vitro with CMS4-EGFP-Zeo. Effector lymphocytes displayed strong cytotoxicity against CMS4-EGFP-Zeo, but not against mock-transfected CMS4-Zeo tumor cells. A number of candidate H2-Kd-binding peptides derived from the EGFP protein were chosen according to an epitope prediction program and synthesized. These peptides were tested for their ability to bind to H2-Kd molecules and stimulate IFNgamma-production by splenocytes harvested from Ad-EGFP-immunized mice. Using this methodology, the peptide, HYLSTQSAL (corresponding to EGFP200-208) which strongly binds to H2-Kd molecules, was identified as a naturally occurring epitope of EGFP. These results should facilitate the use of EGFP as a model tumor antigen in BALB/c mice.

Recombinant yellow fever viruses are effective therapeutic vaccines for treatment of murine experimental solid tumors and pulmonary metastases

We have genetically engineered an attenuated yellow fever (YF) virus to carry and express foreign antigenic sequences and evaluated the potential of this type of recombinant virus to serve as a safe and effective tumor vaccine. Live-attenuated YF vaccine is one of the most effective viral vaccines available today. Important advantages include its ability to induce long-lasting immunity, its safety, its affordability, and its documented efficacy. In this study, recombinant live-attenuated (strain 17D) YF viruses were constructed to express a cytotoxic T-lymphocyte epitope derived from chicken ovalbumin (SIINFEKL). These recombinant viruses replicated comparably to the 17D vaccine strain in cell culture and stably expressed the ovalbumin antigen, and infected cells presented the antigen in the context of major histocompatibility complex class I. Inoculation of mice with recombinant YF virus elicited SIINFEKL-specific CD8(+) lymphocytes and induced protective immunity against challenge with lethal doses of malignant melanoma cells expressing ovalbumin. Furthermore, active immunotherapy with recombinant YF viruses induced regression of established solid tumors and pulmonary metastases. Thus, recombinant YF viruses are attractive viral vaccine vector candidates for the development of therapeutic anticancer vaccines.

Human papillomavirus genotype 16 vaccines for cervical cancer prophylaxis and treatment

More than 11% of the global cancer incidence in females is due to human papillomavirus (HPV) infections, with HPV genotype 16 the most prevalent viral type to infect the cervix. Vaccine strategies currently target HPV 16 genes E6 and E7, constitutively expressed in cervical cancer cells, and L1 and L2, HPV surface antigens. Recent developments in HPV vaccine research are reviewed. Most studies focus on vaccine models showing improved immunogenicity or dual induction of both humeral and cellular systems. Preclinical studies show that (1) L1 /E7 chimeric viral-like proteins induce both neutralizing L1 antibodies and E7-specific T cells; (2) rerouting a cytosolic tumor antigen into the endosomal/lysosomal compartment can improve the therapeutic potency of DNA vaccines; and (3) accelerated E7 protein degradation leads to enhanced antigen presentation in the context of major histocompatability complex class I. Clinical studies show that (1) HPV 16 E7 peptide vaccination can be safely delivered to patients with terminal disease; and (2) HPV-16 capsid proteins harbor at least one HLA-A*201 restricted cytotoxic T lymphocyte (CTL) epitope.

Tumor-specific immunotherapy based on dominant models of natural tolerance

The assumption that cancer immunotherapy may be based on the existence of autoreactive lymphocytes recognizing self-antigens on cancer cells, obviously opens a new opportunity. Nevertheless this analysis, relying on a recessive model of natural tolerance, limits the approach to try to activate peripheral lymphocytes, by increasing co-stimulatory signals or using modified self-antigens for immunization. Here we hypothesize that, based on emerging dominant tolerance notions in autoimmunity, it would be possible to induce a specific autoimmunity against tumor cells and arrest their growth following the removal of regulatory T cells. These immunoregulatory cells suppress available immunocompetent autoreactive cells capable of destroying tumor cells. Therefore, in order to reach a complete tumor-specific autoimmunity it is necessary to combine the T cell immunosuppression which abrogates the regulatory cells, with the cancer vaccines, which induces extensive proliferation of lymphoid cells directed towards specificities on tumor cells.

Recent developments in the virus therapy of cancer

Cancer is one of the leading causes of death in the United States. Although there has been significant progress in the areas of cancer etiology, diagnostic techniques, and cancer prevention, adequate therapeutic approaches for many cancers have lagged behind. One promising line of investigation is the virus therapy of cancer. This approach entails the use of viruses, such as retroviruses, adenovirus, and vaccinia virus, to modify tumor cells so that they become more susceptible to being killed by the host immune response, chemotherapeutic agents, or programmed cell death. This review discusses recent advances in the virus therapy of cancer from both basic science and clinical perspectives. Given the potential of viruses to kill tumor cells directly or transduce desired gene products to allow a vigorous host antitumor immune response, the virus therapy of cancer holds great promise in the treatment of cancer.

HLA class I expression on human cancer cells. Implications for effective immunotherapy

Early studies demonstrated the role of cytotoxic T cells as an immune defence mechanism against tumour cells. The demonstration of tumour antigen peptides and their presentation to T cells on major histocompatibility complex class I molecules highlighted the importance of these molecules in effective anti-tumour responses. It is well established that many tumours escape T cell recognition by loss or down regulation of class I molecule expression on the cell surface of tumour cells. Tumours which have lost class I expression are immunoselected and as a result have a propensity for growth and metastatic spread. With the development of cancer vaccine strategies for clinical use, there will be a future role for histocompatibility laboratories in determining class I expression on tumour cells in individual patients. These studies of expression will require not just the demonstration of total class I expression but the demonstration of locus and allele specific class I molecules involved in the relevant tumour peptide presentation. These studies will be pivotal in tailoring individual patient therapies. The identification of appropriate monoclonal antibody reagents for class I expression and techniques used on different kinds of tissue sections will be a component of the forthcoming 13th International Histocompatibility Workshop.

Cutting edge: CD4+ T cell control of CD8+ T cell reactivity to a model tumor antigen

Neoantigens resulting from the inherent genomic instability of tumor cells generally do not trigger immune recognition. Similarly, transfection of tumors with model Ags often fails to elicit CD8+ T cell responses or alter a tumor's growth rate or lethality. We report here that the adoptive transfer of activated Th1-type CD4+ T cells specific for a model tumor Ag results in the de novo generation of CD8+ T cells with specificity to that Ag and concomitant tumor destruction. The anti-tumor effects of the CD4+ T cells required the presence of both MHC class I and class II on host cells, as evidenced by experiments in knockout mice, suggesting that CD4+ T cells enhanced the ability of host APC to activate endogenous CD8+ T cells. These results indicate that the apparent inability of tumor cells expressing highly immunogenic epitopes to activate tumor-specific CD8+ T cells can be altered by activated CD4+ T cells.

The promise of nucleic acid vaccines

Establishing the effective use of 'naked' nucleic acids as vaccines would undoubtedly be one of the most important advances in the history of vaccinology. While nucleic acids show much promise for use as vaccine vectors in experimental animals, not a single naked nucleic acid vector has been approved for use in humans. Indeed, data from human clinical trials is scant: nucleic acid vaccines have not been clearly demonstrated to have any convincing efficacy in the prevention or treatment of infectious disease or cancer. Here we illustrate possible mechanisms underlying effective nucleic acid vaccination. We focus on progress that has been made in the improvement of their function. Additionally, we identify promising new strategies and try to forecast future developments that could lead to the real success of nucleic acid vaccines in the prevention and treatment of human disease.

Induction of P815 tumor immunity by recombinant Semliki Forest virus expressing the P1A gene

The methylcholantrene-induced P815 mastocytoma tumor is derived from DBA/2 mice and expresses a weak tumor rejection antigen, P815A. The P1A gene, which encodes for the P815A antigen, is silent in most normal tissues with the exception of testis and placenta. These characteristics make P815 an interesting mouse model for the human MAGE-type tumor antigens. Recombinant Semliki Forest virus particles (rSFV) were constructed that expressed variants of the P815 antigen. Such particles, when used for vaccination, express the antigen only transiently since the viral vector is incapable of productive replication. Nevertheless, mice vaccinated with rSFV generated strong CTL responses and were protected against P815 tumor challenge.

DNA-based vaccination against tumors expressing the P1A antigen

Based on experience acquired in the last few years, we describe some technical steps and provide suggestions on how to induce an immune response against tumors expressing the weakly immunogenic antigen P1A by means of a DNA-based vaccination approach. P1A is the product of a normal mouse gene, which shares many characteristics with already identified human tumor-associated antigens, and therefore represents a useful experimental model to evaluate the efficacy of new vaccination strategies potentially applicable to the field of human tumors. Information gained with this model has been applied with success in other experimental settings, and thus we think that the procedure described herein may constitute a valid platform that can be implemented and further refined.

Cell-based vaccines for the stimulation of immunity to metastatic cancers

We are developing vaccines for inducing immunity to metastatic cancers. Although primary tumors are frequently cured by surgery, chemotherapy, or radiation therapy, metastatic lesions often do not respond to these treatments or proliferate after conventional therapy is terminated. Vaccine therapy for established metastases as well as prophylactic vaccine treatment to prevent outgrowth of latent metastatic tumor cells would therefore be beneficial. Our goal is to activate CD4+ and CD8+ T lymphocytes; however, we have focused on activating tumor-specific CD4+ T-helper lymphocytes because of their pivotal role as regulatory cells and in the generation of long-term immunological memory. The vaccines are based on the premise that tumor cells express potentially immunogenic antigens that could be targeted for T-cell activation, and that if appropriately genetically modified, tumor cells could be antigen presenting cells for these antigens. To facilitate direct antigen presentation to CD4+ T cells, tumor cells have been transfected with syngeneic major histocompatibility complex class II, co-stimulatory molecule, and/or superantigen genes. In vivo studies in three mouse tumor models demonstrate that vaccination protects against future challenge with wild-type tumor, cures some solid primary tumors, reduces established metastatic disease, and extends mean survival time. Antigen presentation studies demonstrate that in vivo vaccine efficacy is directly related to in vitro antigen presentation activity. The relevance of antigen presentation activity of the vaccines is further confirmed by in vivo studies demonstrating that during the immunization process, the vaccines directly present tumor-encoded antigens to CD4+ T lymphocytes. Adaptation of these vaccines for the treatment of human metastatic cancers is discussed.

Production of recombinant subunit vaccines: protein immunogens, live delivery systems and nucleic acid vaccines

The first scientific attempts to control an infectious disease can be attributed to Edward Jenner, who, in 1796 inoculated an 8-year-old boy with cowpox (vaccinia), giving the boy protection against subsequent challenge with virulent smallpox. Thanks to the successful development of vaccines, many major diseases, such as diphtheria, poliomyelitis and measles, are nowadays kept under control, and in the case of smallpox, the dream of eradication has been fulfilled. Yet, there is a growing need for improvements of existing vaccines in terms of increased efficacy and improved safety, besides the development of completely new vaccines. Better technological possibilities, combined with increased knowledge in related fields, such as immunology and molecular biology, allow for new vaccination strategies. Besides the classical whole-cell vaccines, consisting of killed or attenuated pathogens, new vaccines based on the subunit principle, have been developed, e.g. the Hepatitis B surface protein vaccine and the Haemophilus influenzae type b vaccine. Recombinant techniques are now dominating in the strive for an ideal vaccine, being safe and cheap, heat-stable and easy to administer, preferably single-dose, and capable of inducing broad immune response with life-long memory both in adults and in infants. This review will describe different recombinant approaches used in the development of novel subunit vaccines, including design and production of protein immunogens, the development of live delivery systems and the state-of-the-art for nucleic acids vaccines.

Cancer therapy using a self-replicating RNA vaccine

'Naked' nucleic acid vaccines are potentially useful candidates for the treatment of patients with cancer, but their clinical efficacy has yet to be demonstrated. We sought to enhance the immunogenicity of a nucleic acid vaccine by making it 'self-replicating'. We accomplished this by using a gene encoding an RNA replicase polyprotein derived from the Semliki forest virus, in combination with a model antigen. A single intramuscular injection of a self-replicating RNA immunogen elicited antigen-specific antibody and CD8+ T-cell responses at doses as low as 0.1 microg. Pre-immunization with a self-replicating RNA vector protected mice from tumor challenge, and therapeutic immunization prolonged the survival of mice with established tumors. The self-replicating RNA vectors did not mediate the production of substantially more model antigen than a conventional DNA vaccine did in vitro. However, the enhanced efficacy in vivo correlated with a caspase-dependent apoptotic death in transfected cells. This death facilitated the uptake of apoptotic cells by dendritic cells, providing a potential mechanism for enhanced immunogenicity. Naked, non-infectious, self-replicating RNA may be an excellent candidate for the development of new cancer vaccines.

Co-delivery of T helper 1-biasing cytokine genes enhances the efficacy of gene gun immunization of mice: studies with the model tumor antigen beta-galactosidase and the BALB/c Meth A p53 tumor-specific antigen

DNA-based immunization is currently being investigated as a new method for the induction of cellular and humoral immunity directed against viral disease and cancer. In the present study we characterized and compared the immune responses induced in mice following particle-bombardment of the skin ('gene gun' immunization) with those elicited by intracutaneous injection of a recombinant adenoviral vector. Using the well characterized beta-galactosidase (beta gal) model Ag system we find that both in vivo gene transfer systems elicit potent and long-lasting anti-beta gal-specific CD8+ and CD4+ T cell responses. However, gene gun immunization predominantly promotes the production of anti-beta gal antibodies of the gamma 1 isotype, indicative of a Th2-biased immune response, while intradermal injection of recombinant adenovirus primarily leads to the production of anti-beta gal gamma 2a antibodies, indicative of a Th1-biased immune response. Since viral infections are generally associated with the production of large amounts of IFN-alpha and IL-12, we investigated whether administration of expression plasmids encoding these Th1-associated cytokines along with antigen-encoding cDNA can influence the nature of the immune response resulting from gene gun immunization. We observed that co-delivery of IFN-alpha or IL-12 resulted in increased production of anti-beta gal gamma 2a antibodies. This suggests a shift towards a Th1 phenotype of the resulting immune response, thus mimicking a viral infection. Importantly, gene gun immunization of mice with a naturally occurring tumor antigen, the tumor-specific p53 mutant antigen expressed by the chemically induced BALB/c Meth A sarcoma, required co-delivery of IL-12 for the induction of effective antitumor immunity. These results have important implications for the design of clinically relevant gene gun immunization strategies for tumor immunotherapy.

DNA vaccines: technology and application as anti-parasite and anti-microbial agents

DNA vaccines have been termed The Third Generation of Vaccines. The recent successful immunization of experimental animals against a range of infectious agents and several tumour models of disease with plasmid DNA testifies to the powerful nature of this revolutionary approach in vaccinology. Among numerous advantages, a major attraction of DNA vaccines over conventional vaccines is that they are able to induce protective cytotoxic T-cell responses as well as helper T-cell and humoral immunity. Here we review the current state of nucleic acid vaccines and cover a wide range of topics including delivery mechanisms, uptake and expression of plasmid DNA, and the types of immune responses generated. Further, we discuss safety issues, and document the use of nucleic acid vaccines against viral, bacterial and parasitic diseases, and cancer. The early potential promise of DNA vaccination has been fully substantiated with recent, exciting developments including the movement from testing DNA vaccines in laboratory models to non-human primates and initial human clinical trials. These advances and the emerging voluminous literature on DNA vaccines highlight the rapid progress that has been made in the DNA immunization field. It will be of considerable interest to see whether the progress and optimism currently prevailing can be maintained, and whether the approach can indeed fulfil the medical and commerical promise anticipated.

Induction of anti-tumor immunity elicited by tumor cells expressing a murine LFA-3 analog via a recombinant vaccinia virus

T cell activation requires binding of the T cell receptor to the major histocompatibility molecule-peptide complex in the presence of adhesion and/or costimulatory molecules such as B7-1 (CD80), B7-2 (CD86), ICAM-1 (CD54), and LFA-3 [corrected]. The major ligand of CD2 is CD48, the murine analog of human leukocyte function-associated antigen 3 (LFA-3). To determine the effect of LFA-3 expression on the immunogenicity of tumor cells, we constructed a recombinant vaccinia virus containing the murine LFA-3 gene (designated rV-LFA-3). rV-LFA-3 was shown to be functional in vitro in terms of expression of LFA-3, T cell proliferation, adhesion, and cytotoxicity. Subcutaneous inoculation of rV-LFA-3-infected murine colon adenocarcinoma tumor cells (MC38) into immunocompetent syngeneic C57BL/6 mice resulted in complete lack of tumor growth. Inoculation of MC38 cells infected with equal doses of control wild-type vaccinia virus resulted in tumor growth in all animals. In addition, partial immunological protection was demonstrated against subsequent challenge with uninfected parental tumor cells up to 56 days after vaccination with rV-LFA-3-infected cells. Anti-tumor memory was also demonstrated by using gamma-irradiated MC38 cells and cells from another carcinoma model (CT26). These studies demonstrate that expression of LFA-3 via a poxvirus vector can be used to induce anti-tumor immunity.

Isolation of High Avidity Melanoma-Reactive CTL from Heterogeneous Populations Using Peptide-MHC Tetramers

Immunogenic peptides of human tumor Ag have been used to generate antigen-specific CTL. However, the vast majority of these peptide-specific CTL clones are of low avidity and are peptide, but not tumor, reactive. Peptide-MHC tetramers have been shown to bind specific TCRs with sufficient affinity to be useful reagents for flow cytometry. In this paper we demonstrate that peptide-MHC tetramers can also be used to selectively identify high avidity tumor-reactive CTL and enrich, from a heterogeneous population, the subpopulation of peptide-reactive T cells that can lyse tumor targets. The melanoma proteins, MART-1 and gp100, were used to induce potentially tumor-reactive T cells, and the intensity of T cell staining by TCR binding of specific peptide-MHC tetramers was assessed. A range of fluorescence intensity was detected, and the magnitude of tetramer binding was correlated with T cell avidity. The population of peptide-reactive T cells was phenotypically similar with regard to expression of TCR and adhesion molecules, suggesting that this differential avidity for tumor cells reflected differential affinity of the TCR for its peptide-MHC ligand. Sorting, cloning, and expansion of tetramerhigh CTL from a heterogeneous population of peptide-stimulated PBMCs enabled rapid selection of high avidity tumor-reactive CTL clones, which retained their functional and tetramerhigh phenotype on re-expansion. These results demonstrate that the avidity of a T cell for its tumor target is due to the specific affinity of the TCR for its peptide-MHC ligand, that this interaction can be described using peptide-MHC tetramers and used to isolate high avidity tumor-reactive CTL.

Developing recombinant and synthetic vaccines for the treatment of melanoma

To develop new vaccines for the treatment of patients with cancer, target antigens presented on tumor cell surfaces have been cloned. Many of these antigens are non-mutated differentiation antigens and are expressed by virtually all melanomas, making them attractive components for a widely efficacious melanoma vaccine. These antigens are also expressed by melanocytes, however, and are likely to be subject to immune tolerance. A central challenge for tumor immunologists has thus been the breaking of tolerance to cancer antigens. We review recent clinical trials using experimental cancer vaccines, including recent evidence that therapeutic vaccines can induce objective responses in patients with metastatic malignant melanoma. We focus on the foundations of these approaches in new experimental animal models designed to test novel vaccines and report on what these new models predict for the future development of therapeutic vaccines for cancer.

Transfectant influenza A viruses are effective recombinant immunogens in the treatment of experimental cancer

Using reverse genetics methods, we constructed three different transfectant influenza A viruses encoding an Ld-restricted, nine amino-acid-long fragment, corresponding to amino-acid residues 876-884, of beta-galactosidase (beta-gal). Sequences encoding this epitope were nested within the hemagglutinin (HA) or neuraminidase (NA) open reading frames. Alternatively, an independent beta-gal mini-gene, preceded by an endoplasmic reticulum insertion signal sequence, was placed in a bicistronic arrangement in the NA RNA segment of the virus. All three transfectants mediated the presentation of the epitope to a beta-gal-specific CTL clone. Furthermore, each of the three transfectant viruses expressing the beta-gal fragment elicited specific cytolytic responses in vivo. Most importantly, these H1N1 transfectants mediated the regression of established murine pulmonary metastases. Tumor regression in mice was also achieved in the presence of preexisting immunity against an H3N2 influenza A virus serotype. Nononcogenic and nonintegrating, transfectant influenza A viruses are attractive candidates for development as antitumor vaccines.

gp100/pmel 17 is a murine tumor rejection antigen: induction of "self"-reactive, tumoricidal T cells using high-affinity, altered peptide ligand

Many tumor-associated antigens are nonmutated, poorly immunogenic tissue differentiation antigens. Their weak immunogenicity may be due to "self"-tolerance. To induce autoreactive T cells, we studied immune responses to gp100/pmel 17, an antigen naturally expressed by both normal melanocytes and melanoma cells. Although a recombinant vaccinia virus (rVV) encoding the mouse homologue of gp100 was nonimmunogenic, immunization of normal C57BL/6 mice with the rVV encoding the human gp100 elicited a specific CD8(+) T cell response. These lymphocytes were cross-reactive with mgp100 in vitro and treated established B16 melanoma upon adoptive transfer. To understand the mechanism of the greater immunogenicity of the human version of gp100, we characterized a 9-amino acid (AA) epitope, restricted by H-2Db, that was recognized by the T cells. The ability to induce specific T cells with human but not mouse gp100 resulted from differences within the major histocompatibility complex (MHC) class I-restricted epitope and not from differences elsewhere in the molecule, as was evidenced by experiments in which mice were immunized with rVV containing minigenes encoding these epitopes. Although the human (hgp10025-33) and mouse (mgp10025-33) epitopes were homologous, differences in the three NH2-terminal AAs resulted in a 2-log increase in the ability of the human peptide to stabilize "empty" Db on RMA-S cells and a 3-log increase in its ability to trigger interferon gamma release by T cells. Thus, the fortuitous existence of a peptide homologue with significantly greater avidity for MHC class I resulted in the generation of self-reactive T cells. High-affinity, altered peptide ligands might be useful in the rational design of recombinant and synthetic vaccines that target tissue differentiation antigens expressed by tumors.

Development and application of PROVAX adjuvant formulation for subunit cancer vaccines

A major challenge facing the development of subunit vaccines comprised of well-defined recombinant antigens is their weak immunogenicity and inability to induce effective cytotoxic T cell (CTL) responses. Adjuvants aimed at increasing the immunogenicity of recombinant antigens remain a focus in vaccine development. The potency of an adjuvant is linked to specific stimulation of T cell responses, involving TH1 and TH2 subsets of CD4(+) T helper cells and CD8(+) CTL and B cell-mediated antibody responses. As a result of the existence of two distinct intra-cellular pathways for antigen processing, immunization with exogenous antigens often shows a greater propensity for T helper and antibody responses, but not CD8(+) CTL responses. However, existing experimental evidence suggests that CD8(+) CTLs, which are critical in the elimination of viral-infected and neoplastic cells, can be elicited with soluble antigens when delivered in appropriate formulations or adjuvants. This review focuses on the properties of PROVAX adjuvant in inducing antigen-specific CTL responses, antibody responses and tumor regression in experimental models and its potential application for the development of recombinant cancer vaccines.

Experimental vaccine strategies for cancer immunotherapy

Recently, cancer immunotherapy has emerged as a therapeutic option for the management of cancer patients. This is based on the fact that our immune system, once activated, is capable of developing specific immunity against neoplastic but not normal cells. Increasing evidence suggests that cell-mediated immunity, particularly T-cell-mediated immunity, is important for the control of tumor cells. Several experimental vaccine strategies have been developed to enhance cell-mediated immunity against tumors. Some of these tumor vaccines have generated promising results in murine tumor systems. In addition, several phase I/II clinical trials using these vaccine strategies have shown extremely encouraging results in patients. In this review, we will discuss many of these promising cancer vaccine strategies. We will pay particular attention to the strategies employing dendritic cells, the central player for tumor vaccine development.

Eradication of established murine tumors using a novel cell-free vaccine: dendritic cell-derived exosomes

Dendritic cells (DCs) are professional antigen presenting cells with the unique capacity to induce primary and secondary immune responses in vivo. Here, we show that DCs secrete antigen presenting vesicles, called exosomes, which express functional Major Histocompatibility Complex class I and class II, and T-cell costimulatory molecules. Tumor peptide-pulsed DC-derived exosomes prime specific cytotoxic T lymphocytes in vivo and eradicate or suppress growth of established murine tumors in a T cell-dependent manner. Exosome-based cell-free vaccines represent an alternative to DC adoptive therapy for suppressing tumor growth.

Autologous Human Monocyte-Derived Dendritic Cells Genetically Modified to Express Melanoma Antigens Elicit Primary Cytotoxic T Cell Responses In Vitro: Enhancement by Cotransfection of Genes Encoding the Th1-Biasing Cytokines IL-12 and IFN-α

DNA-based immunization strategies designed to elicit cellular antitumor immunity offer an attractive alternative to protein- or peptide-based approaches. In the present study we have evaluated the feasibility of DNA vaccination for the induction of CTL reactivity to five different melanoma Ags in vitro. Cultured, monocyte-derived dendritic cells (DC) were transiently transfected with plasmid DNA encoding human MART-1/Melan-A, pMel-17/gp100, tyrosinase, MAGE-1, or MAGE-3 by particle bombardment and used to stimulate autologous PBMC responder T cells. CTL reactivity to these previously identified melanoma Ags was reproducibly generated after two or three stimulations with genetically modified DC. Co-ordinate transfection of two melanoma Ag cDNAs into DC promoted CTL responders capable of recognizing epitopes from both gene products. Coinsertion of genes encoding the Th1-biasing cytokines IL-12 or IFN-α consistently enhanced the magnitude of the resulting Ag-specific CTL reactivity. Importantly, DC transfected with a single melanoma Ag cDNA were capable of stimulating Ag-specific CTL reactivity restricted by multiple host MHC alleles, some of which had not been previously identified. These results support the inherent strengths of gene-based vaccine approaches that do not require prior knowledge of responder MHC haplotypes or of relevant MHC-restricted peptide epitopes. Given previous observations of in situ tumor HLA allele-loss variants, DC gene vaccine strategies may elicit a greater diversity of host therapeutic immunity, thereby enhancing the clinical utility and success of such approaches.

CTL response and protection against P815 tumor challenge in mice immunized with DNA expressing the tumor-specific antigen P815A

A DNA immunization approach was used to induce an immune response against the tumor-specific antigen P815A in DBA/2 mice. The P1A gene, which encodes the P815A antigen, was modified by the addition of a short sequence coding for a tag epitope recognized by the monoclonal antibody AU1, and cloned into the eukaryotic expression vector pBKCMV, resulting in plasmid pBKCMV-P1A. L1210 cells stably transfected with pBKCMV-P1A expressed P1A mRNA and were lysed by the syngeneic P815A-specific cytotoxic clone CTL-P1:5, thus confirming that the tag-modified P1A protein underwent correct processing and presentation. A single intramuscular injection of 100 microg of pBKCMV-P1A induced the expression of P1A mRNA for at least 4 months. Eighty percent of DBA/2 mice injected three times with 100 microg of pBKCMV-P1A generated cytotoxic T lymphocytes (CTL) that lysed P815 tumor cells, whereas mock-inoculated animals failed to show any cytotoxicity. Moreover, experiments designed to evaluate the protection of pBKCMV-P1A-immunized mice against a lethal challenge with P815 tumor cells showed that 6 of 10 immunized mice rejected the tumor, and 2 mice showed prolonged survival compared to control animals.

Recombinant viruses as vaccines and immunological tools

Recombinant viruses have been investigated as candidate vaccines, and have also been used extensively as immunological tools. Recent advances in this area include the following: the construction and testing of a recombinant simian immunodeficiency virus encoding human interferon-gamma; the development of new vectors such as recombinant poliovirus; and the generation of polyepitope vaccines. Basic immunological research has benefited from the use of recombinant viruses to further understand the role of molecules such as CD40 ligand, nitric oxide and interleukin-4.

HLA and cancer: implications for cancer immunotherapy and vaccination

Both animal models and studies in humans indicate that the immune response has enormous potential for the treatment of cancer, and that HLA plays a central role in this. This role is likely to become increasingly important in years to come, as effective strategies for immunotherapy of cancer are developed. In the last few years, much of the focus of tumour immunology has been on the identification and characterization of tumour-associated antigens that represent HLA-restricted tumour-specific targets. In the next few years the emphasis is likely to change, such that the HLA type and antigen profile of an individual's tumour may define the most appropriate form of therapy for the patient. The ability of tumour cells to down-regulate or lose expression of some or all HLA molecules may prove to be a major barrier to the effectiveness of such treatments. The next few years should show whether the potential of the advances in tumour immunology made over the last decade can be realized, or whether the application of these advances as therapeutic strategies falls short of their promise.

Identification of a Kb-restricted CTL epitope of beta-galactosidase: potential use in development of immunization protocols for "self" antigens

The use of recombinant and synthetic vaccines in the treatment of cancer has recently been explored using model tumor associated antigens (TAA), many of which do not model the immunological state of affairs in which the TAA is expressed by normal tissues. One potentially useful model Ag is beta-galactosidase (beta-gal). Because the activity of this enzyme is so easily detectable, this gene has been inserted into a large number of recombinant viruses and tumors useful to the cancer vaccinologist. In addition, numerous transgenic mouse colonies that have tissue-specific expression of beta-gal have been developed, enabling the modeling of tolerance to "self" Ags. Since most of these mice have an H-2b background, we generated cytotoxic T lymphocytes (CTL) capable of recognizing beta-gal-expressing tumor cells of C57BL\6 origin and have determined that their restriction element is the K(b) molecule. Using an allele-specific epitope forecast to generate a panel of candidate peptides, we have determined that the K(b)-restricted sequence is DAPIYTNV and corresponds to amino acids 96-103 of the intact beta-gal molecule. A recombinant vaccinia virus (rVV-ES beta-gal96-103) was constructed that encoded the peptide epitope preceded by an endoplasmic reticulum insertion signal sequence. Tumor cells infected with this rVV were recognized by the original CTL that had been used to identify the epitope. Furthermore, splenocytes of mice immunized with a rVV encoding the full-length beta-gal molecule and restimulated with the DAPIYTNV peptide specifically recognized tumor cells expressing beta-gal. The identification of this immunogenic beta-gal sequence enables the modeling of immunization strategies in animal models of malignant disease in which the target antigen is a "self" protein.