Generation of autoreactive CTL by tumour vaccines containing foreign T helper epitopes

Designing a therapeutic hepatitis B vaccine to circumvent immune tolerance

An effective prophylactic hepatitis B virus (HBV) vaccine has long been available but is ineffective for chronic infection. The primary cause of chronic hepatitis B (CHB) and greatest impediment for a therapeutic vaccine is the direct and indirect effects of immune tolerance to HBV antigens. The resulting defective CD4⁺/CD8⁺ T cell response, poor cytokine production, insufficient neutralizing antibody (nAb) and poor response to HBsAg vaccination characterize CHB infection. The objective of this study was to develop virus-like-particles (VLPs) that elicit nAb to prevent viral spread and prime CD4⁺/CD8⁺ T cells to eradicate intracellular HBV. Eight neutralizing B cell epitopes from the envelope PreS1 region were consolidated onto a species-variant of the HBV core protein, the woodchuck hepatitis core antigen (WHcAg). PreS1-specific B cell epitopes were chosen because of preferential expression on HBV virions. Because WHcAg and HBcAg are not crossreactive at the B cell level and only partially cross-reactive at the CD4⁺/CD8⁺ T cell level, CD4⁺ T cells specific for WHcAg-unique T cell sites can provide cognate T-B cell help for anti-PreS1 Ab production that is not curtailed by immune tolerance. Immunization of immune tolerant HBV transgenic (Tg) mice with PreS1-WHc VLPs elicited levels of high titer anti-PreS1 nAbs equivalent to wildtype mice. Passive transfer of PreS1 nAbs into human-liver chimeric mice prevented acute infection and cleared serum HBV from mice previously infected with HBV in a model of CHB. At the T cell level, PreS1-WHc VLPs and hybrid WHcAg/HBcAg DNA immunogens elicited HBcAg-specific CD4⁺ Th and CD8⁺ CTL responses.

A Theoretical Basis for the Efficacy of Cancer Immunotherapy and Immunogenic Tumor Dormancy: The Adaptation Model of Immunity

In the past decades, a variety of strategies have been explored to cure cancer by means of immunotherapy, which is less toxic compared with chemotherapy or radiation therapy, and could establish memory for long-lasting protection against tumor recurrence. These endeavors have been successful in offering therapeutic antibodies, vaccines, or cellular immunotherapies, which resulted in prolonging survival of some cancer patients; however, complete cures have not been consistently achieved. The conception, design, and implementation of these promising immunotherapeutic strategies have been influenced by two schools of thought in immunology, which include the "self-nonself" (SNS) model and the "danger" model. Further progress in cancer immunotherapy to achieve consistent cancer cures requires an evolution in our understanding of how the immune system works. The purpose of this review is to revisit premises and limitations of the SNS and danger models based on the outcomes of cancer immunotherapies by suggesting that both models are two sides of the same coin describing how the immune response is induced against cancer. However, neither explains how the immune response succeeds or fails in eliminating the tumor. To this end, the adaptation model has been proposed to explain efficacy of the immune response for achieving cancer cure.

Application of central immunologic concepts to cancer: helping T cells and B cells become intolerant of tumors

CD4-mediated T-cell help in the activation of CD8(+) T cells and B cells, through linked-recognition of antigenic determinants, is a long-standing concept foundational to our understanding of immunity (presence of help) versus tolerance (lack of help). Surprisingly, this function of CD4(+) T cells has not been extensively examined as a means to overcome immune tolerance of the self-antigens made by tumor cells. Hesitation to employ this powerful mechanism may be due to the potential to cause unwanted autoimmune pathology. In this issue of the European Journal of Immunology, Snook et al. [Eur. J. Immunol. 2014. 44: 1956-1966] identify a state of split tolerance, showing that CD4(+) T cells specific for a number of tumor-associated self-antigens are robustly tolerant, while their CD8(+) T-cell and B-cell counterparts are far less tolerant. Furthermore, the authors demonstrate that provision of linked foreign helper epitopes, such as influenza hemagglutinin, substantially enhances both CD8(+) T-cell and B-cell responses to tumor self-antigens without causing any overt autoimmune pathology. These findings provide a strong rationale to employ foreign helper epitopes in cancer vaccines and highlight the need to fully explore therapeutic strategies that are based on well-established immunologic concepts.

Induction of homologous rather than heterologous antigen-specific CD4 T cell responses is critical for functional CD8 T cell responses in mice transgenic for a foreign antigen

The development of a successful cancer vaccine requires the ability to break immunological tolerance to self-Ags expressed on tumor cells. The transgenic rat insulin promoter (RIP) OVA(LOW) mouse model has been reported to be hyporesponsive for both OVA-specific CD4 and CD8 T cell responses. The experiments described in the current study show that this hyporesponsiveness can be overcome by inclusion of GM-CSF and the TLR7 agonist imiquimod as adjuvants in a DNA immunization regimen with OVA-encoding plasmids. High frequencies of OVA-specific CD8 and CD4 T cells, including a response to a CD4 T cell epitope seen only in the RIP OVA(LOW) mice, were generated by this regimen. These responses were associated with the development of autoimmunity and increased protection to tumor challenge in the RIP OVA(LOW) mice. Heterologous CD4 T cell help has been shown to improve functional CD8 T cell responses, and we confirmed that inclusion of the CD4 T cell epitope pan HLA-DR-binding epitope improved CD8 T cell responses compared with self-Ag alone. Addition of GM-CSF and imiquimod, however, resulted in dominance of the pan HLA-DR-binding epitope-specific response over the OVA-specific CD4 T cell responses, decreased OVA-specific CD8 T cell numbers and function in tolerant RIP OVA(LOW) mice, and failure to induce diabetes. The results of this study suggest that the use of heterologous help needs to be evaluated carefully in the context of specific immunization regimes and that a preferable approach may be adjuvantization of DNA vaccines.

Alpha beta T cell receptor transfer to gamma delta T cells generates functional effector cells without mixed TCR dimers in vivo

The successful application of T cell-based immunotherapeutic applications depends on the availability of large numbers of T cells with the desired Ag specificity and phenotypic characteristics. Engineering of TCR-transferred T lymphocytes is an attractive strategy to obtain sufficient T cells with an Ag specificity of choice. However, the introduction of additional TCR chains into T cells leads to the generation of T cells with unknown specificity, due to the formation of mixed dimers between the endogenous and introduced TCR chains. The formation of such potentially autoaggressive T cells may be prevented by using gammadelta T cells as recipient cells, but the in vivo activity of such TCR-engineered gammadelta T cells has not been established. In the present study, we have investigated the in vivo functionality of TCR-transduced gammadelta T cells, in particular their Ag specific proliferative capacity, Ag specific reactivity, in vivo persistence, and their capacity to mount recall responses. The results demonstrate that alphabeta TCR engineering of gammadelta T cells forms a feasible strategy to generate Ag-specific effector T cells that do not express mixed TCR dimers. In view of increasing concerns on the potential autoimmune consequences of mixed TCR dimer formation, the testing of alphabeta TCR engineered gammadelta T cells in clinical trials seems warranted.

Enhancement of DNA tumor vaccine efficacy by gene gun-mediated codelivery of threshold amounts of plasmid-encoded helper antigen

Nucleic acid-based vaccines are effective in infectious disease models but have yielded disappointing results in tumor models when tumor-associated self-antigens are used. Incorporation of helper epitopes from foreign antigens into tumor vaccines might enhance the immunogenicity of DNA vaccines without increasing toxicity. However, generation of fusion constructs encoding both tumor and helper antigens may be difficult, and resulting proteins have unpredictable physical and immunologic properties. Furthermore, simultaneous production of equal amounts of highly immunogenic helper and weakly immunogenic tumor antigens in situ could favor development of responses against the helper antigen rather than the antigen of interest. We assessed the ability of 2 helper antigens (beta-galactosidase or fragment C of tetanus toxin) encoded by one plasmid to augment responses to a self-antigen (lymphoma-associated T-cell receptor) encoded by a separate plasmid after codelivery into skin by gene gun. This approach allowed adjustment of the relative ratios of helper and tumor antigen plasmids to optimize helper effects. Incorporation of threshold (minimally immunogenic) amounts of helper antigen plasmid into a DNA vaccine regimen dramatically increased T cell-dependent protective immunity initiated by plasmid-encoded tumor-associated T-cell receptor antigen. This simple strategy can easily be incorporated into future vaccine trials in experimental animals and possibly in humans.

Perspectives on the development of a therapeutic HER-2 cancer vaccine

With good reason, the majority of cancer vaccines tested, or being tested, have targeted the induction of anti-tumour CTL responses. However, the clinical success of monoclonal antibodies such as Rituximab/CD20, Trastuzumab/HER-2, Cetuximab/EGFR and Bevacisumab/VEGF suggests that their respective targets may also be relevant for cancer vaccines aiming at the induction of an effective humoral anti-tumour response to mimic, or potentially improve upon, the effects of monoclonal therapies. We report here an overview of the development of a protein vaccine targeting HER-2/neu, with an emphasis on the immunologic results obtained from the testing of the vaccine in animal models of disease and in toxicology programs, to its evaluation in three clinical trials in breast cancer patients.