Transgene-enforced co-stimulation of CD4+ T cells leads to enhanced and sustained anti-tumor effector functioning

Background The role of co-stimulation in CD4+ T cell activation by professional APC is well established, while less is known of the role co-stimulation plays when CD4+ T cells interact directly with tumor cells. Methods Through genetic engineering of human CD4+ T cells, we tested the hypothesis that integration of co-stimulatory signaling domains within a tumor-targeting chimeric Ag receptor (CAR), the IL-13Ralpha2-specific IL-13-zetakine (IL13zeta), would enhance CD4+ T cell mediated responses against tumors that fail to express ligands for co-stimulatory receptors. Results Compared with CD3zeta-mediated activation alone, CD4+ effector T cells expressing the IL13-CD28-41BBzeta CAR exhibited augmented/sustained MAPK and AKT activity, up-regulated Th1 cytokine production, and enhanced cytolytic potency against tumor targets. Moreover, upon recursive stimulation with tumor, the IL13-CD28-41BBzeta+ cells retained/recycled their lytic function, whereas IL-13zeta+ CD4+ cells became anergic/exhausted. These in vitro observations correlated with enhanced in vivo control of established orthotopic CNS glioma xenografts in immunodeficient mice mediated by adoptively transferred ex vivo-expanded CD4+ T cells expressing the co-stimulatory CAR. Discussion Together these studies demonstrate the importance of integrating co-stimulation with CD3zeta signaling events to activate fully CD4+ anti-tumor effector cells for sustained function in the tumor microenvironment.     

Signal transduction in T helper cells: CD4 coreceptors exert complex regulatory effects on T cell activation and function

The immune system provides a highly sophisticated surveillance mechanism to detect diverse antigens and to protect the host organism from invading pathogens and altered cells (e.g., virus-infected and tumor cells). Adaptive immune responses depend on the recognition of antigen by specific antigen receptors that are expressed on the surface of T and B lymphocytes. Helper T cells provide regulatory functions and direct the adaptive immune system to respond appropriately to a particular antigen (i.e., cytotoxic T cell responses against viral infections and tumor cells, humoral responses against extracellular bacteria and parasitic worms). Helper T cells express CD4 coreceptors, which recognize conserved domains on proteins expressed by the class II major histocompatibility complex, the same proteins that present antigen to the T cell receptor. Recent progress in T cell biology has identified multiple regulatory functions of CD4 during thymocyte development and antigen stimulation of mature T helper cells. Signaling pathways induced by engagement of CD4 independently of T cell receptor signaling mediate these regulatory functions. In this review, we discuss the regulation of T cell signaling and emphasize the functional consequences of proper and improper CD4 coreceptor signaling.     

Cutting edge: sustained expansion of CD8+ T cells requires CD154 expression by Th cells in acute graft versus host disease

Brief treatment with alphaCD154 Ab has been shown to prevent acute graft versus host disease (aGvHD). We extend these data to show that in the absence of CD154 function, donor T cells are unable to expand or generate high level anti-host CTL activity. Using transgenic (Tg) alloreactive CD8+ T cells adoptively transferred into allogeneic recipients, we show that short-term expansion of the CD8+ Tg T cells occurred in the absence of Th cells, and this short-term expansion could be facilitated with an agonistic alphaCD40. While CD40 agonism could enhance short-term expansion, sustained expansion of CD8+ Tg T cells required bona fide CD154-expressing CD4+ alloreactive Th cells. While CD154 was necessary for CD8+ Tg T cell sustained expansion, IL-2 was also implicated as essential. These observations suggest alphaCD154 therapy in GvHD is effective because the treatment causes an abortive CD8 alloresponse leading to the exhaustion or deletion of alloreactive CD8+ clones preventing the development of disease.     

Interleukin-15 rescues tolerant CD8+ T cells for use in adoptive immunotherapy of established tumors

CD8+ T cells can mediate eradication of established tumors, and strategies to amplify tumor-reactive T-cell numbers by immunization or ex vivo expansion followed by adoptive transfer are currently being explored in individuals with cancer. Generating effective CD8+ T cell-mediated responses to tumors is often impeded by T-cell tolerance to relevant tumor antigens, as most of these antigens are also expressed in normal tissues. We examined whether such tolerant T cells could be rescued and functionally restored for use in therapy of established tumors. We used a transgenic T-cell receptor (TCR) mouse model in which peripheral CD8+ T cells specific for a candidate tumor antigen also expressed in liver are tolerant, failing to proliferate or secrete interleukin (IL)-2 in response to antigen. Molecular and cellular analysis showed that these tolerant T cells expressed the IL-15 receptor alpha chain, and could be induced to proliferate in vitro in response to exogenous IL-15. Such proliferation abrogated tolerance and the rescued cells became effective in treating leukemia. Therefore, high-affinity CD8+ T cells are not necessarily deleted by encounter with self-antigen in the periphery, and can potentially be rescued and expanded for use in tumor immunotherapy.     

Th2 cells and cytokine networks in allergic inflammation of the lung

The cytokines released from Th2 and Th2-like cells are likely to be central to the pathophysiolgy of asthma and allergy, contributing to aberrant IgE production, eosinophilia and, perhaps, mucosal susceptibility to viral infection. IL-4 has emerged as a central target, not only for B cell IgE production, but also in the commitment of both CD4+ and CD8+ T cells to cells with Th2 effector function capable of secreting IL-5 resultlng in eosinophilic inflammation. In view of the central role of this cytokine and the evidence that glucocorticoids are unable to modify many IL-4 dependent effects, Th2 inhibitors may prove to be novel therapies for the treatment of bronchial asthma.     

A temporal role of type I interferon signaling in CD8+ T cell maturation during acute West Nile virus infection

A genetic absence of the common IFN-α/β signaling receptor (IFNAR) in mice is associated with enhanced viral replication and altered adaptive immune responses. However, analysis of IFNAR(-/-) mice is limited for studying the functions of type I IFN at discrete stages of viral infection. To define the temporal functions of type I IFN signaling in the context of infection by West Nile virus (WNV), we treated mice with MAR1-5A3, a neutralizing, non cell-depleting anti-IFNAR antibody. Inhibition of type I IFN signaling at or before day 2 after infection was associated with markedly enhanced viral burden, whereas treatment at day 4 had substantially less effect on WNV dissemination. While antibody treatment prior to infection resulted in massive expansion of virus-specific CD8(+) T cells, blockade of type I IFN signaling starting at day 4 induced dysfunctional CD8(+) T cells with depressed cytokine responses and expression of phenotypic markers suggesting exhaustion. Thus, only the later maturation phase of anti-WNV CD8(+) T cell development requires type I IFN signaling. WNV infection experiments in BATF3(-/-) mice, which lack CD8-α dendritic cells and have impaired priming due to inefficient antigen cross-presentation, revealed a similar effect of blocking IFN signaling on CD8(+) T cell maturation. Collectively, our results suggest that cell non-autonomous type I IFN signaling shapes maturation of antiviral CD8(+) T cell response at a stage distinct from the initial priming event.     

Timing and magnitude of type I interferon responses by distinct sensors impact CD8 T cell exhaustion and chronic viral infection

Type I interferon (IFN-I) promotes antiviral CD8(+)T cell responses, but the contribution of different IFN-I sources and signaling pathways are ill defined. While plasmacytoid dendritic cells (pDCs) produce IFN-I upon TLR stimulation, IFN-I is induced in most cells by helicases like MDA5. Using acute and chronic lymphocytic choriomeningitis virus (LCMV) infection models, we determined that pDCs transiently produce IFN-I that minimally impacts CD8(+)T cell responses and viral persistence. Rather, MDA5 is the key sensor that induces IFN-I required for CD8(+)T cell responses. In the absence of MDA5, CD8(+)T cell responses to acute infection rely on CD4(+)T cell help, and loss of both CD4(+)T cells and MDA5 results in CD8(+)T cell exhaustion and persistent infection. Chronic LCMV infection rapidly attenuates IFN-I responses, but early administration of exogenous IFN-I rescues CD8(+)T cells, promoting viral clearance. Thus, effective antiviral CD8(+)T cell responses depend on the timing and magnitude of IFN-I production.     

Dual function of the NK cell receptor 2B4 (CD244) in the regulation of HCV-specific CD8+ T cells

The outcome of viral infections is dependent on the function of CD8+ T cells which are tightly regulated by costimulatory molecules. The NK cell receptor 2B4 (CD244) is a transmembrane protein belonging to the Ig superfamily which can also be expressed by CD8+ T cells. The aim of this study was to analyze the role of 2B4 as an additional costimulatory receptor regulating CD8+ T cell function and in particular to investigate its implication for exhaustion of hepatitis C virus (HCV)-specific CD8+ T cells during persistent infection. We demonstrate that (i) 2B4 is expressed on virus-specific CD8+ T cells during acute and chronic hepatitis C, (ii) that 2B4 cross-linking can lead to both inhibition and activation of HCV-specific CD8+ T cell function, depending on expression levels of 2B4 and the intracellular adaptor molecule SAP and (iii) that 2B4 stimulation may counteract enhanced proliferation of HCV-specific CD8+ T cells induced by PD1 blockade. We suggest that 2B4 is another important molecule within the network of costimulatory/inhibitory receptors regulating CD8+ T cell function in acute and chronic hepatitis C and that 2B4 expression levels could also be a marker of CD8+ T cell dysfunction. Understanding in more detail how 2B4 exerts its differential effects could have implications for the development of novel immunotherapies of HCV infection aiming to achieve immune control.     

The extrinsic RNA-sensing pathway for adjuvant immunotherapy of cancer

Infection with RNA viruses presents a typical pattern of virus products, double-stranded RNA (dsRNA), and induces the maturation of antigen-presenting dendritic cell (mDC). There are several dsRNA sensors that are differentially distributed on the cell membrane and in the cytoplasm and are variably expressed depending on the cell type. Among these sensors, TLR3 links to the adaptor TICAM-1 (TRIF), which is characterized by its unique multipronged signaling cascades for cytokine/chemokine production, apoptosis and autophagy in both immune and tumor cells. In the context of mDC maturation, various cellular events are further induced in response to dsRNA; these include cross-priming followed by CD8+ CTL induction, NK activation and proliferation of CD4+ T cells including Th1, Th2, Treg and Th17 cells. In this review, we focus on the potential role of dsRNA in modulating the inflammatory milieu around mDCs and tumor-associated antigens to drive specific cellular effectors against the tumor.     

Th cells act via two synergistic pathways to promote antiviral CD8+ T cell responses

The mechanisms of how Th cells promote CD8(+) T cell responses during viral infections are largely unknown. In this study, we unraveled the mechanisms of T cell help for CD8(+) T cell responses during vaccinia virus infection. Our results demonstrate that Th cells promote vaccinia virus-specific CD8(+) T cell responses via two interconnected synergistic pathways: First, CD40L expressed by activated CD4(+) T cells instructs dendritic cells to produce bioactive IL-12p70, which is directly sensed by Ag-specific CD8(+) T cells, resulting in increased IL-2Rα expression. Second, Th cells provide CD8(+) T cells with IL-2, thereby enhancing their survival. Thus, Th cells are at the center of an important communication loop with a central role for IL-2/IL-2R and bioactive IL-12.     

Effector CD4+ T cell expression signatures and immune-mediated disease associated genes

Genome-wide association studies (GWAS) in immune-mediated diseases have identified over 150 associated genomic loci. Many of these loci play a role in T cell responses, and regulation of T cell differentiation plays a critical role in immune-mediated diseases; however, the relationship between implicated disease loci and T cell differentiation is incompletely understood. To further address this relationship, we examined differential gene expression in na?ve human CD4+ T cells, as well as in in vitro differentiated Th1, memory Th17-negative and Th17-enriched CD4+ T cells subsets using microarray and RNASeq. We observed a marked enrichment for increased expression in memory CD4+ compared to na?ve CD4+ T cells of genes contained among immune-mediated disease loci. Within memory T cells, expression of disease-associated genes was typically increased in Th17-enriched compared to Th17-negative cells. Utilizing RNASeq and promoter methylation studies, we identified a differential regulation pattern for genes solely expressed in Th17 cells (IL17A and CCL20) compared to genes expressed in both Th17 and Th1 cells (IL23R and IL12RB2), where high levels of promoter methylation are correlated to near zero RNASeq levels for IL17A and CCL20. These findings have implications for human Th17 celI plasticity and for the regulation of Th17-Th1 expression signatures. Importantly, utilizing RNASeq we found an abundant isoform of IL23R terminating before the transmembrane domain that was enriched in Th17 cells. In addition to molecular resolution, we find that RNASeq provides significantly improved power to define differential gene expression and identify alternative gene variants relative to microarray analysis. The comprehensive integration of differential gene expression between cell subsets with disease-association signals, and functional pathways provides insight into disease pathogenesis.     

Interleukin‐17 family cytokines in protective immunity against infections: role of hematopoietic cell‐derived and non‐hematopoietic cell‐derived interleukin‐17s

Interleukin‐17 family cytokines, consisting of six members, participate in immune response in infections and autoimmune and inflammatory diseases. The prototype cytokine of the family, IL‐17A, was originally identified from CD4+ T cells which are now termed Th17 cells. Later, IL‐17A‐producing cells were expanded to include various hematopoietic cells, namely CD8+ T cells (Tc17), invariant NKT cells, γδ T cells, non‐T non‐B lymphocytes (termed type 3 innate lymphoid cells) and neutrophils. Some IL‐17 family cytokines other than IL‐17A are also expressed by CD4+ T cells: IL‐17E by Th2 cells and IL‐17F by Th17 cells. IL‐17A and IL‐17F induce expression of pro‐inflammatory cytokines to induce inflammation and anti‐microbial peptides to kill pathogens, whereas IL‐17E induces allergic inflammation. However, the functions of other IL‐17 family cytokines have been unclear. Recent studies have shown that IL‐17B and IL‐17C are expressed by epithelial rather than hematopoietic cells. Interestingly, expression of IL‐17E and IL‐17F by epithelial cells has also been reported and epithelial cell‐derived IL‐17 family cytokines shown to play important roles in immune responses to infections at epithelial sites. In this review, we summarize current information on hematopoietic cell‐derived IL‐17A and non‐hematopoietic cell‐derived IL‐17B, IL‐17C, IL‐17D, IL‐17E and IL‐17F in infections and propose functional differences between these two categories of IL‐17 family cytokines.     

CD4+ T-Cell Help Is Required for Effective CD8+ T Cell-Mediated Resolution of Acute Viral Hepatitis in Mice

Cytotoxic CD8+ T cells are essential for the control of viral liver infections, such as those caused by HBV or HCV. It is not entirely clear whether CD4+ T-cell help is necessary for establishing anti-viral CD8+ T cell responses that successfully control liver infection. To address the role of CD4+ T cells in acute viral hepatitis, we infected mice with Lymphocytic Choriomeningitis Virus (LCMV) of the strain WE; LCMV-WE causes acute hepatitis in mice and is cleared from the liver by CD8+ T cells within about two weeks. The role of CD4+ T-cell help was studied in CD4+ T cell-lymphopenic mice, which were either induced by genetic deficiency of the major histocompatibility (MHC) class II transactivator (CIITA) in CIITA−/− mice, or by antibody-mediated CD4+ cell depletion. We found that CD4+ T cell-lymphopenic mice developed protracted viral liver infection, which seemed to be a consequence of reduced virus-specific CD8+ T-cell numbers in the liver. Moreover, the anti-viral effector functions of the liver-infiltrating CD8+ T cells in response to stimulation with LCMV peptide, notably the IFN-γ production and degranulation capacity were impaired in CIITA−/− mice. The impaired CD8+ T-cell function in CIITA−/− mice was not associated with increased expression of the exhaustion marker PD-1. Our findings indicate that CD4+ T-cell help is required to establish an effective antiviral CD8+ T-cell response in the liver during acute viral infection. Insufficient virus control and protracted viral hepatitis may be consequences of impaired initial CD4+ T-cell help.     

Induction of polyclonal CD8+ T cell activation and effector function by Pertussis toxin

Pertussis toxin (PTX) has pronounced adjuvant activity and strongly enhances innate and adaptive immune responses, including increased antibody production and Th1/Th2 cytokine production. Adjuvant effects of PTX on Th1 and Th2 cells are primarily mediated via CD80/86 costimulation via enhanced expression of these molecules by APCs. However, it has remained unresolved whether PTX modulates the expression of costimulatory and inhibitory molecules on CD4+ and CD8+ T cells. To address this question, we determined the expression kinetics of CD28, CTLA-4, and CD40L on spleen CD4+ and CD8+ T cells after incubation with PTX. The results show that PTX upregulated the expression of CD28 by CD8+ T cells, but not by CD4+ T cells. In contrast, the expression of CTLA-4 and CD40L was not substantially altered on CD4+ or CD8+ T cells. CD28 upregulation by CD8+ T cells was paralleled by upregulation of CD69 and the induction of IFN-γ, Granzyme B (GrB), and IL-17. CD8+ T cell activation and cytokine production could be substantially blocked with anti-CD80 and CD86 antibodies, consistent with CD28 mediated signaling. Treatment of highly purified CD8+ T cells with PTX resulted in upregulation of CD28 and CD69, and production of IFN-γ. Incubation with CD28 mAb further enhanced this effect, suggesting that PTX has direct effects on CD8+ T cells which are enhanced by CD80/86-mediated costimulation provided by APCs.     

T8 cell regulation of human B cell responsiveness: regulatory influences of CD45RA+ and CD45RA- T8 cell subsets

The immunoregulatory functions of human T8 cell subpopulations defined by mAb to the CD45RA molecule (2H4) were examined. Both CD45RA+ and CD45RA- T8 cells that had been treated with mitomycin C provided help for the production of immunoglobulins by B cells in cultures stimulated with immobilized mAb to CD3 (64.1). In contrast, both CD45RA+ and CD45RA- T8 cells that had not been treated with mitomycin C suppressed B cell responses in anti-CD3-stimulated cultures, although CD45RA+ T8 cells were more effective in this regard. Interleukin 2 (IL2) enhanced suppression by anti-CD3-activated CD45RA- T8 cells, whereas suppression by CD45RA+ T8 cells was almost maximal and not as much increased by IL2. The differentiation into suppressor-effector cells in this system appeared to involve the production of IL2, but not the production of interferon (INF)-gamma. Thus, CD45RA+ T8 cells produced higher amounts of IL2 but lower amounts of IFN-gamma than CD45RA- T8 cells in anti-CD3-stimulated cultures. Moreover, addition of mAb to the p55 component of IL2 receptor (anti-Tac) inhibited the generation of suppressor activity from CD45RA+ and CD45RA- T8 cells. The pattern and magnitude of suppression of B cell responses by CD45RA+ and CD45RA- T4 cells were similar to that by CD45RA+ and CD45RA- T8 cells in this system. Finally, preactivated CD45RA+ T8 cells that had lost CD45RA expression suppressed the B cell responses as effectively as fresh CD45RA+ T8 cells. The results indicate that both CD45RA+ and CD45RA- T8 cells can help or suppress B cell responses. More importantly, the data suggest that the suppressor-effector function of human T cells may rather be related with the stages of the post-thymic differentiation as evidenced by the expression of the CD45RA molecule than represent the fully differentiated T cell subsets, such as T4 and T8 cells. In addition, the CD45RA molecule appeared not to be involved in the suppressor-effector function, but to determine the stage of post-thymic differentiation.     

Characterization of porcine T lymphocytes and their immune response against viral antigens.

T lymphocytes play a central role in the antigen-specific immune response against various pathogens. To detect and to characterize porcine T lymphocytes, monoclonal antibodies (mAb) against leukocyte differentiation antigens had been raised and classified for their specificity. Analyses of porcine T lymphocytes with specific mAb against CD4 and CD8 differentiation antigens revealed differences in the composition of the porcine T-lymphocyte population compared to other species. In addition to the known subpopulations, CD4+CD8- T helper cells and CD4-CD8+ cytolytic T lymphocytes, extra-thymic CD4+CD8+ T lymphocytes and a substantial proportion of CD2-CD4-CD8- T cell receptor (TcR)-gamma delta+ T cells could be detected in swine. Functional analyses of porcine T-lymphocyte subpopulations revealed the existence of two T-helper cell fractions with the phenotype CD4+CD8- and CD4+CD8+. Both were reactive in primary immune responses in vitro, whereas only cells derived from the CD4+CD8+ T-helper-cell subpopulation were able to respond to recall antigen in a secondary immune response. With regard to T lymphocytes with cytolytic activities, two subsets within the CD4-CD8+ T-cell subpopulation could be defined by the expression of CD6 differentiation antigens: CD6- cells which showed spontaneous cytolytic activity and CD6+ MHC I-restricted cytolytic T lymphocytes including virus-specific cytolytic T lymphocytes. These results enable now a detailed view into the porcine T-cell population and the reactivity of specific T cells involved in the porcine immune response against pathogens. Furthermore this knowledge offers the possibility to investigate specific interactions of porcine T lymphocytes with virus-specific epitopes during vaccination and viral infections.     

Viral persistence alters CD8 T-cell immunodominance and tissue distribution and results in distinct stages of functional impairment

Chronic viral infections often result in ineffective CD8 T-cell responses due to functional exhaustion or physical deletion of virus-specific T cells. However, how persisting virus impacts various CD8 T-cell effector functions and influences other aspects of CD8 T-cell dynamics, such as immunodominance and tissue distribution, remains largely unknown. Using different strains of lymphocytic choriomeningitis virus (LCMV), we compared responses to the same CD8 T-cell epitopes during acute or chronic infection. Persistent infection led to a disruption of the normal immunodominance hierarchy of CD8 T-cell responses seen following acute infection and dramatically altered the tissue distribution of LCMV-specific CD8 T cells in lymphoid and nonlymphoid tissues. Most importantly, CD8 T-cell functional impairment occurred in a hierarchical fashion in chronically infected mice. Production of interleukin 2 and the ability to lyse target cells in vitro were the first functions compromised, followed by the ability to make tumor necrosis factor alpha, while gamma interferon production was most resistant to functional exhaustion. Antigen appeared to be the driving force for this loss of function, since a strong correlation existed between the viral load and the level of exhaustion. Further, epitopes presented at higher levels in vivo resulted in physical deletion, while those presented at lower levels induced functional exhaustion. A model is proposed in which antigen levels drive the hierarchical loss of different CD8 T-cell effector functions during chronic infection, leading to distinct stages of functional impairment and eventually to physical deletion of virus-specific T cells. These results have implications for the study of human chronic infections, where similar T-cell deletion and functional dysregulation has been observed.     

Combining Regulatory T Cell Depletion and Inhibitory Receptor Blockade Improves Reactivation of Exhausted Virus-Specific CD8+ T Cells and Efficiently Reduces Chronic Retroviral Loads

Chronic infections with human viruses, such as HIV and HCV, or mouse viruses, such as LCMV or Friend Virus (FV), result in functional exhaustion of CD8⁺ T cells. Two main mechanisms have been described that mediate this exhaustion: expression of inhibitory receptors on CD8⁺ T cells and expansion of regulatory T cells (Tregs) that suppress CD8⁺ T cell activity. Several studies show that blockage of one of these pathways results in reactivation of CD8⁺ T cells and partial reduction in chronic viral loads. Using blocking antibodies against PD-1 ligand and Tim-3 and transgenic mice in which Tregs can be selectively ablated, we compared these two treatment strategies and combined them for the first time in a model of chronic retrovirus infection. Blocking inhibitory receptors was more efficient than transient depletion of Tregs in reactivating exhausted CD8⁺ T cells and reducing viral set points. However, a combination therapy was superior to any single treatment and further augmented CD8⁺ T cell responses and resulted in a sustained reduction in chronic viral loads. These results demonstrate that Tregs and inhibitory receptors are non-overlapping factors in the maintenance of chronic viral infections and that immunotherapies targeting both pathways may be a promising strategy to treat chronic infectious diseases.     

Plasmid DNA vaccine encoding prostatic acid phosphatase is effective in eliciting autologous antigen-specific CD8+ T cells

Prostatic acid phosphatase (PAP) is a prostate cancer tumor antigen and a prostate-specific protein shared by rats and humans. Previous studies indicated that Copenhagen rats immunized with a recombinant vaccinia virus expressing human PAP (hPAP) developed PAP-specific cytotoxic T cells (CTL) with cross reactivity to rat PAP (rPAP) and evidence of prostate inflammation. Viral delivery of vaccine antigens is an active area of clinical investigation. However, a potential difficulty with viral-based immunizations is that immune responses elicited to the viral vector might limit the possibility of multiple immunizations. In this paper, we investigate the ability of another genetic immunization method, a DNA vaccine encoding PAP, to elicit antigen-specific CD8+ T cell immune responses. Specifically, Lewis rats were immunized with either a plasmid DNA-based (pTVG-HP) or vaccinia-based (VV-HP) vaccine each encoding hPAP. We determined that rats immunized with a DNA vaccine encoding hPAP developed a Th1-biased immune response as indicated by proliferating PAP-specific CD4+ and CD8+ cells and IFNγ production. Rats immunized with vaccinia virus encoding PAP did not develop a PAP-specific response unless boosted with a heterologous vaccination scheme. Most importantly, multiple immunizations with a DNA vaccine encoding the rat PAP homologue (pTVG-RP) could overcome peripheral self-tolerance against rPAP and generate a Th1-biased antigen-specific CD4+ and CD8+ T cell response. Overall, DNA vaccines provide a safe and effective method of generating prostate antigen-specific T cell responses. These findings support the investigation of PAP-specific DNA vaccines in human clinical trials.