Direct ex vivo kinetic and phenotypic analyses of CD8(+) T-cell responses induced by DNA immunization

Immuno-Enhancing Effect of Ginsenoside Rh2 Liposomes on Foot-and-Mouth Disease Vaccine

The adjuvant is essential for vaccines because it can enhance or directly induce a strong immune response associated with vaccine antigens. Ginsenoside Rh2 (Rh2) had immunomodulatory effects but was limited by poor solubility and hemolysis. In this study, Rh2 liposomes (Rh2-L) were prepared by ethanol injection methods. The Rh2-L effectively dispersed in a double emulsion adjuvant system to form a Water-in-Oil-in-Water (W/O/W) emulsion and had no hemolysis. The physicochemical properties of the adjuvants were tested, and the immune activity and auxiliary effects indicated by the Foot-and-Mouth disease (FMDV) antigen were evaluated. Compared with the mice vaccinated with the FMD vaccine prepared with the double emulsion adjuvant alone, those with the FMD vaccine prepared with the double emulsion adjuvant containing Rh2-L had significantly higher neutralizing antibody titer and splenocyte proliferation rates and showed higher cellular and humoral immune responses. The results demonstrated that Rh2-L could further enhance the immune effect of the double emulsion adjuvant against Foot-and-Mouth Disease.

Corosolic acid-modified lipid nanoparticles as delivery carriers for DNA vaccines against avian influenza

Cholesterol (CHOL) is essential for developing lipid nanoparticles (LNPs) for gene delivery because it enhances membrane fusion and improves the delivery efficiency of gene cargos. An attractive pDNA carrier, corosolic acid (CA)-modified lipid nanoparticles (CLNPs), was developed by replacing CHOL in LNPs to deliver pDNA at various ratios of nitrogen groups to phosphate groups (N/P). The resultant CLNPs with a higher CHOL/CA ratio exhibited similar mean particle size, zeta potential, and encapsulation efficiency to those of LNPs. In comparison with LNPs, CLNPs (CHOL:CA ratio = 2:1) achieved increased cellular uptake and enhanced transfection efficacy while maintaining low cytotoxicity. In vivo results from chicken experiments demonstrated that CLNPs encapsulating DNA vaccines against avian influenza at a N/P ratio of 3 could elicit similar-level humoral and cellular immune responses compared with those of LNPs at a higher N/P ratio, thereby suggesting the induction of desirable immune effects using less ionizable lipids. Our study provides a reference for further research on the application of CA in LNPs for gene delivery, and the development of novel delivery systems for DNA vaccines against avian influenza.

Evaluation of dendritic cell-targeting T7 phages as a vehicle to deliver avian influenza virus H5 DNA vaccine in SPF chickens

Introduction

There is a growing demand for effective technologies for the delivery of antigen to antigen-presenting cells (APCs) and their immune-activation for the success of DNA vaccines. Therefore, dendritic cell (DC)-targeting T7 phages were used as a vehicle to deliver DNA vaccine.

Methods

In this study, a eukaryotic expression plasmid pEGFP-C1-HA2-AS containing the HA2 gene derived from the avian H5N1 virus and an anchor sequence (AS) gene required for the T7 phage packaging process was developed. To verify the feasibility of phage delivery, the plasmid encapsulated in DC-targeting phage capsid through the recognition of AS was evaluated both in vitro and in vivo. The pEGFP-C1-HA2-AS plasmid could evade digestion by DNase I by becoming encapsulated into the phage particles and efficiently expressed the HA2 antigen in DCs with the benefit of DC-targeting phages.

Results

For chickens immunized with the DC-targeting phage 74 delivered DNA vaccine, the levels of IgY and IgA antibodies, the concentration of IFN-γ and IL-12 cytokines in serum, the proliferation of lymphocytes, and the percentage of CD4⁺/CD8⁺ T lymphocytes isolated from peripheral blood were significantly higher than chickens which were immunized with DNA vaccine that was delivered by non-DC-targeting phage or placebo (p<0.05). Phage 74 delivered one-fiftieth the amount of pEGFP-C1-HA2-AS plasmid compared to Lipofectin, however, a comparable humoral and cellular immune response was achieved. Although, the HA2 DNA vaccine delivered by the DC-targeting phage induced enhanced immune responses, the protection rate of virus challenge was not evaluated.

Conclusion

This study provides a strategy for development of a novel avian influenza DNA vaccine and demonstrates the potential of DC-targeting phage as a DNA vaccine delivery vehicle.

Dendrigraft poly-L-lysines delivery of DNA vaccine effectively enhances the immunogenic responses against H9N2 avian influenza virus infection in chickens

Biodegradable nanomaterials can protect antigens from degradation, promote cellular absorption, and enhance immune responses. We constructed a eukaryotic plasmid [pCAGGS-opti441-hemagglutinin (HA)] by inserting the optimized HA gene fragment of H9N2 AIV into the pCAGGS vector. The pCAGGS-opti441-HA/DGL was developed through packaging the pCAGGS-opti441-HA with dendrigraft poly-l-lysines (DGLs). DGL not only protected the pCAGGS-opti441-HA from degradation, but also exhibited high transfection efficiency. Strong cellular immune responses were induced in chickens immunized with the pCAGGS-opti441-HA/DGL. The levels of IFN-γ and IL-2, and lymphocyte transformation rate of the vaccinated chickens increased at the third week post the immunization. For the vaccinated chickens, T lymphocytes were activated and proliferated, the numbers of CD3+CD4+ and CD4+/CD8+ increased, and the chickens were protected completely against H9N2 AIV challenge. This study provides a method for the development of novel AIV vaccines, and a theoretical basis for the development of safe and efficient gene delivery carriers.

Chimeric Hemagglutinin-Based Influenza Virus Vaccines Induce Protective Stalk-Specific Humoral Immunity and Cellular Responses in Mice

The high variation of the influenza virus hemagglutinin (HA), particularly of its immunodominant head epitopes, makes it necessary to reformulate seasonal influenza virus vaccines every year. Novel influenza virus vaccines that redirect the immune response toward conserved epitopes of the HA stalk domain should afford broad and durable protection. Sequential immunization with chimeric HAs (cHAs) that express the same conserved HA stalk and distinct exotic HA heads has been shown to elicit high levels of broadly cross-reactive Abs. In the current mouse immunization studies, we tested this strategy using inactivated split virion cHA influenza virus vaccines (IIV) without adjuvant or adjuvanted with AS01 or AS03 to measure the impact of adjuvant on the Ab response. The vaccines elicited high levels of cross-reactive Abs that showed activity in an Ab-dependent, cell-mediated cytotoxicity reporter assay and were protective in a mouse viral challenge model after serum transfer. In addition, T cell responses to adjuvanted IIV were compared with responses to a cHA-expressing live attenuated influenza virus vaccine (LAIV). A strong but transient induction of Ag-specific T cells was observed in the spleens of mice vaccinated with LAIV. Interestingly, IIV also induced T cells, which were successfully recalled upon viral challenge. Groups that received AS01-adjuvanted IIV or LAIV 4 wk before the challenge showed the lowest level of viral replication (i.e., the highest level of protection). These studies provide evidence that broadly cross-reactive Abs elicited by cHA vaccination demonstrate Fc-mediated activity. In addition, cHA vaccination induced Ag-specific cellular responses that can contribute to protection upon infection.

T cell activation triggers reversible inosine-5'-monophosphate dehydrogenase assembly

T cell-mediated adaptive immunity requires naïve, unstimulated T cells to transition from a quiescent metabolic state into a highly proliferative state upon T cell receptor engagement. This complex process depends on transcriptional changes mediated by Ca²⁺-dependent NFAT signaling, mTOR-mediated signaling and increased activity of the guanine nucleotide biosynthetic inosine-5'-monophosphate (IMP) dehydrogenase 1 and 2 enzymes (IMPDH1 and IMPDH2, hereafter IMPDH). Inhibitors of these pathways serve as potent immunosuppressants. Unexpectedly, we discovered that all three pathways converge to promote the assembly of IMPDH protein into micron-scale macromolecular filamentous structures in response to T cell activation. Assembly is post-transcriptionally controlled by mTOR and the Ca²⁺ influx regulator STIM1. Furthermore, IMPDH assembly and catalytic activity were negatively regulated by guanine nucleotide levels, suggesting a negative feedback loop that limits biosynthesis of guanine nucleotides. Filamentous IMPDH may be more resistant to this inhibition, facilitating accumulation of the higher GTP levels required for T cell proliferation.

Self in vivo production of a synthetic biological drug CTLA4Ig using a minicircle vector

Cytotoxic T lymphocyte-associated antigen 4 immunoglobulin fusion protein (CTLA4Ig, abatacept) is a B7/CD28 costimulation inhibitor that can ward off the immune response by preventing the activation of naïve T cells. This therapeutic agent is administered to patients with autoimmune diseases such as rheumatoid arthritis. Its antiarthritic efficacy is satisfactory, but the limitations are the necessity for frequent injection and high cost. Minicircles can robustly express the target molecule and excrete it outside the cell as an indirect method to produce the protein of interest in vivo. We inserted the sequence of abatacept into the minicircle vector, and by successful in vivo injection the host was able to produce the synthetic protein drug. Intravenous infusion of the minicircle induced spontaneous production of CTLA4Ig in mice with collagen-induced arthritis. Self-produced CTLA4Ig significantly decreased the symptoms of arthritis. Injection of minicircle CTLA4Ig regulated Foxp3(+) T cells and Th17 cells. Parental and mock vectors did not ameliorate arthritis or modify the T cell population. We have developed a new concept of spontaneous protein drug delivery using a minicircle vector. Self in vivo production of a synthetic protein drug may be useful when biological drugs cannot be injected because of manufacturing or practical problems.

Plasmid DNA vaccines against cancer: cytotoxic T-lymphocyte induction against tumor antigens

In recent years, a number of tumor vaccination strategies have been developed. Most of these rely on the identification of tumor antigens that can be recognized by the immune system. DNA vaccination represents one such approach for the induction of both humoral and cellular immune responses against tumor antigens. Studies in animal models have demonstrated the feasibility of utilizing DNA vaccination to elicit protective antitumor immune responses. However, most tumor antigens expressed by cancer cells in humans are weakly immunogenic, and therefore require the development of strategies to potentiate DNA vaccine efficacy in the clinical setting. This review focuses on recent advances in understanding of the immunology of DNA vaccines, as well as strategies used to increase DNA vaccine potency with respect to cytotoxic T-lymphocyte activity.

A BCR/ABL-hIL-2 DNA vaccine enhances the immune responses in BALB/c mice

The use of a DNA vaccine encoding the BCR/ABL fusion gene is thought to be a promising approach for patients with chronic myeloid leukemia (CML) to eradicate minimal residual disease after treatment with chemotherapy or targeted therapy. In this study, our strategy employs genetic technology to create a DNA vaccine encoding the BCR/ABL fusion and human interleukin-2 (hIL-2) genes. The successfully constructed plasmids BCR/ABL-pIRES-hIL-2, BCR/ABL-pIRES, and pIRES-hIL-2 were delivered intramuscularly to BALB/c mice at 14-day intervals for three cycles. The transcription and expression of the BCR/ABL and hIL-2 genes were found in the injected muscle tissues. The interferon-γ (IFN-γ) serum levels were increased, and the splenic CD4⁺/CD8⁺ T cell ratio was significantly decreased in the BCR/ABL-pIRES-hIL-2-injected mice. Furthermore, specific antibodies against K562 cells could be detected by indirect immunofluorescence. These results indicate that a DNA vaccine containing BCR/ABL and hIL-2 together may elicit increased in vivo humoral and cellular immune responses in BALB/c mice.

An emerging role for Serine Protease Inhibitors in T lymphocyte immunity and beyond

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmo-regulation, tissue re-modeling and immunity to infection. T lymphocytes are required for adaptive cell mediated immunity and serine proteases are not only important for effector function but also homeostatic regulation of cell numbers. Serine Protease Inhibitors (Serpins) are the physiological regulators of serine proteases activity. In this review, I will discuss the role of serpins in controlling the recognition of antigen, effector function and homeostatic control of T lymphocytes through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This will be discussed in the context of the T lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. The potent anti-apoptotic properties of serpins can also work against adaptive cell immunity by protecting viruses and tumors from eradication by cytotoxic T cells (CTL). Recent insights from knock-out mouse models demonstrate that these serpins also are required for hematological progenitor cells and so are critical for the development of lineages other than T lymphocytes. Given the emerging role of serpins in multiple aspects of lymphocyte immunity and blood development I will review the progress to date in developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Development of a new hydrogen peroxide–based vaccine platform

Safe and effective vaccines are crucial for maintaining public health and reducing the global burden of infectious disease. Here we introduce a new vaccine platform that uses hydrogen peroxide (H(2)O(2)) to inactivate viruses for vaccine production. H(2)O(2) rapidly inactivates both RNA and DNA viruses with minimal damage to antigenic structure or immunogenicity and is a highly effective method when compared with conventional vaccine inactivation approaches such as formaldehyde or β-propiolactone. Mice immunized with H(2)O(2)-inactivated lymphocytic choriomeningitis virus (LCMV) generated cytolytic, multifunctional virus-specific CD8(+) T cells that conferred protection against chronic LCMV infection. Likewise, mice vaccinated with H(2)O(2)-inactivated vaccinia virus or H(2)O(2)-inactivated West Nile virus showed high virus-specific neutralizing antibody titers and were fully protected against lethal challenge. Together, these studies demonstrate that H(2)O(2)-based vaccines are highly immunogenic, provide protection against a range of viral pathogens in mice and represent a promising new approach to future vaccine development.

Adjuvant effect of HER-2/neu-specific adenoviral vector stimulating CD8⁺ T and natural killer cell responses on anti-HER-2/neu antibody therapy for well-established breast tumors in HER-2/neu transgenic mice

Approximately one third of patients with advanced human epidermal growth factor receptor 2 (HER-2)/neu-positive breast cancer respond to trastuzumab monotherapy, a humanized anti-HER-2/neu antibody. However, de novo and acquired antibody resistance is one of the major limitations of trastuzumab therapy warranting the search for other therapeutic strategies. One of the most remarkable features of adenovirus (AdV)-based vaccine is its ability to induce exceptionally high and sustained frequencies of transgene product-specific CD8(+) T-cell responses. In this study, we constructed two recombinant AdVs (AdV(OVA) and AdV(HER-2)) expressing ovalbumin (OVA) and HER-2/neu, and assessed AdV-induced antigen-specific cellular immune responses and preventive/therapeutic antitumor immunity. We demonstrate that AdV(OVA) stimulates efficient OVA-specific CD8(+) cytotoxic T lymphocyte (CTL) and natural killer responses, leading to preventive long-term immunity against OVA-expressing BL6-10ova melanoma in wild-type C56BL/6 mice. We further demonstrate that AdV(HER-2) stimulates HER-2/neu-specific CD8(+) CTL responses, leading to a significant reduction in breast carcinogenesis in transgenic FVBneuN mice (P<0.05), but has little therapeutic effect on pre-existing Tg1-1 tumor even at early stage (15 mm(3)). In contrast, the anti-HER-2/neu antibody therapy is capable of completely inhibiting Tg1-1 tumor growth at early stage, but fails to eradicate well-established Tg1-1 breast tumor (100 mm(3)). Interestingly, a combinatorial immunotherapy of anti-HER-2/neu antibody with AdV(HER-2) vaccine was capable of curing 4 of 10 studied mice bearing well-established Tg1-1 breast tumors and significantly delaying in death of the remaining six tumor-bearing mice (P<0.05). Taken together, our results suggest an adjuvant effect of AdV(HER-2) on anti-HER-2/neu antibody therapy for well-established breast tumor in transgenic FVBneuN mice, and this combinatorial immunotherapy of trastuzumab with AdV(HER-2) vaccine may be used as a new therapeutic strategy for treatment of advanced HER-2/neu-positive breast cancer.

A highly optimized DNA vaccine confers complete protective immunity against high-dose lethal lymphocytic choriomeningitis virus challenge

Protection against infection is the hallmark of immunity and the basis of effective vaccination. For a variety of reasons there is a great demand to develop new, safer and more effective vaccine platforms. In this regard, while 'first-generation' DNA vaccines were poorly immunogenic, new genetic 'optimization' strategies and the application of in vivo electroporation (EP) have dramatically boosted their potency. We developed a highly optimized plasmid DNA vaccine that expresses the lymphocytic choriomeningitis virus (LCMV) nucleocapsid protein (NP) and evaluated it using the LCMV challenge model, a gold standard for studying infection and immunity. When administered intramuscularly with EP, robust NP-specific cellular and humoral immune responses were elicited, the magnitudes of which approached those following acute LCMV infection. Furthermore, these responses were capable of providing 100% protection against a high-dose, normally lethal virus challenge. This is the first non-infectious vaccine conferring complete protective immunity up to 8 weeks after vaccination and demonstrates the potential of 'next-generation' DNA vaccines.

Serine protease inhibitors and cytotoxic T lymphocytes

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmoregulation, tissue remodeling, and immunity to infection. Cytotoxic T lymphocytes (CTLs) are required for adaptive cell-mediated immunity to intracellular pathogens by killing infected cells and through the development of memory T cells. Serine proteases not only allow a CTL to kill but also impose homeostatic control on CTL number. Serine protease inhibitors (serpins) are the physiological regulators of serine proteases' activity. In this review, I discuss the role of serpins in controlling the recognition of antigen, effector function, and homeostatic control of CTLs through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This view is discussed in the context of the T-lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. Given the important role serpins play in CTL immunity, I discuss the potential for developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Essential role for TLR9 in prime but not prime-boost plasmid DNA vaccination to activate dendritic cells and protect from lethal viral infection

One of the requirements for efficient vaccination against infection is to achieve the best combination of an adequate adjuvant with the antigenic information to deliver. Although plasmid DNA is a promising tool bearing the unique potential to activate humoral and cellular immunity, an actual challenge is to increase plasmid immunogenicity in human vaccination protocols in which efficacy has proven rather limited. Previous work showed that the bacterial DNA backbone of the plasmid has potent adjuvant properties because it contains CpG motifs that are particular activating nucleotidic sequences. Among TLRs, which are key sensors of microbial products, TLR9 can detect CpG motifs and confer activation of APCs, such as dendritic cells. However, whether the immunogenic properties of plasmid DNA involve TLR9 signaling has not been clearly established. In the current study, we demonstrate that TLR9 determines the effectiveness of vaccination against lethal lymphocytic choriomeningitis virus infection using plasmid DNA in a prime, but not prime-boost, vaccination regimen. Furthermore, we provide evidence that the presence of TLR9 in dendritic cells is necessary for effective and functional priming of virus-specific CD8+ T cells upon plasmid exposure in vitro or single-dose vaccination in vivo. Therefore, at single or low vaccine doses that are often used in human-vaccination protocols, CpG/TLR9 interactions participate in the immunogenicity of plasmid DNA. These results suggest that the TLR9 signaling pathway is involved in the efficacy of plasmid vaccination; therefore, it should remain a focus in the development or amelioration of vaccines to treat infections in humans.

Fusion of a viral antigen to invariant chain leads to augmented T-cell immunity and improved protection in gene-gun DNA-vaccinated mice

It has recently been demonstrated that a recombinant replication-deficient human adenovirus 5 (Ad5) vector expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) fused to the p31 invariant (Ii) chain confers broad, long-lasting T-cell immunity that completely protects C57BL/6 mice against lethal peripheral challenge. The current study questioned whether the same strategy, i.e. linkage of GP to an Ii chain, could be applied to a naked DNA vaccine. Following gene-gun immunization with the linked construct (DNA–IiGP), GP-specific CD4+ T cells could not be detected by flow cytometry. However, inclusion of the Ii chain augmented the priming of GP-specific CD8+ T cells directed towards both immunodominant (GP33–41) and subdominant (GP276–286 and GP92–101) epitopes, and vaccination with DNA–IiGP conferred significantly improved protection against systemic LCMV infection compared with the unlinked construct. In contrast, substantial protection against peripheral challenge was not observed. Additional experiments with T-cell subset-depleted or perforin-deficient mice revealed that virus control in vaccinated mice depends critically on cytotoxic CD8+ T cells. Finally, priming with the naked DNA vaccine was shown to augment the immune response raised by subsequent immunization with the Ad5 vector. In conclusion, this study showed that the immunoenhancing effect of Ii chain linkage is not limited to the Ad5 vector, but is also relevant with a DNA platform. Furthermore, given the fact that the Ii chain enhances the presentation of more than one epitope, this suggests that Ii-chain-based DNA vaccines may be promising candidates for various heterologous prime–boost regimes.

The immunogenicity of adenovirus vectors limits the multispecificity of CD8 T-cell responses to vector-encoded transgenic antigens

We investigated whether the immunogenicity of antigens delivered by recombinant E1-deleted adenovirus (Ad) is impaired by the concomitant priming of specific immunity to protein antigens of the vector. A comparative evaluation of the immunogenicity of hepatitis B surface antigen (HBsAg or S) or ovalbumin (OVA) was carried out in mice injected with either the antigen-encoding Ad vector or a corresponding plasmid DNA vaccine. Recombinant Ad, but not the plasmid DNA vaccine, induced long lasting, specific CD8 T-cell immunity to immunodominant epitopes of the two antigens. In contrast, the HBsAg-encoding pCI/S DNA, but not the Ad/S vaccine, was shown to prime CD8 T-cell responses to subdominant HBsAg epitopes. Ad/S-primed CD8 T-cell responses to immunodominant epitopes of vector-encoded or capsid-delivered Ad proteins apparently suppressed CD8 T-cell priming to subdominant HBsAg epitopes. In B-cell-deficient mice, the established, Ad-specific T-cell immunity induced by vaccination with an irrelevant Ad vector impaired the priming of HBsAg-specific CD8 T-cell responses by Ad/S. It is clear, therefore, that a T-cell immunity specific for Ad proteins (either delivered with the Ad capsid or transcribed from the Ad genome) is efficiently primed by vaccination with Ad vectors, and can limit the immunogenicity (particularly of subdominant epitopes) of Ad vector-encoded transgenic antigens.

Effects of IL-7 on memory CD8 T cell homeostasis are influenced by the timing of therapy in mice

IL-7 is integral to the generation and maintenance of CD8(+) T cell memory, and insufficient IL-7 is believed to limit survival and the persistence of memory CD8(+) T cells. Here, we show that during the mouse T cell response to lymphocytic choriomeningitis virus, IL-7 enhanced the number of memory CD8(+) T cells when its administration was restricted to the contraction phase of the response. Likewise, IL-7 administration during the contraction phase of the mouse T cell response to vaccinia virus or a DNA vaccine potentiated antigen-specific CD8(+) memory T cell proliferation and function. Qualitatively, CD8(+) T cells from IL-7-treated mice exhibited superior recall responses and improved viral control. IL-7 treatment during the memory phase stimulated a marked increase in the number of memory CD8(+) T cells, but the effects were transient. IL-7 therapy during contraction of the secondary CD8(+) T cell response also expanded the pool of memory CD8(+) T cells. Collectively, our studies show differential effects of IL-7 on memory CD8(+) T cell homeostasis and underscore the importance of the timing of IL-7 therapy to effectively improve CD8(+) T cell memory and protective immunity. These findings may have implications in the clinical use of IL-7 as an immunotherapeutic agent to bolster vaccine-induced CD8(+) T cell memory.

Localization of CD8+ cells specific for hepatitis B virus surface protein in the liver of immunized mice

DNA plasmids are potent inducers of long-lasting antigen-specific CTL responses. Little is known about the distribution of antigen-specific CD8+ T cells in the lymphoid tissue and the non-lymphoid tissue after DNA immunization. HBsAg-specific CD8+ T cells in peripheral blood mononuclear cells, spleen, lymph nodes, and the liver of Balb/c mice have been quantified after injection with a DNA plasmid expressing the major S protein of hepatitis B virus (HBV). The kinetics of CD8+ T-cell responses in the circulation were measured after priming and boosting, showing that antigen-specific CD8+ T cells undergo first expansion and then decline to a sustainable level in the circulation, although the frequencies of HBsAg-specific CD8+ T cells in the circulation were lower than for the spleen. The greater frequencies of HBsAg-specific CD8+ T cells were found in the liver, whereas the largest numbers of antigen-specific CD8+ T cells were found in the spleen. By day 100 after priming, HBsAg-specific CD8+ T cells were still detected in the circulation, the spleen and the liver. After boosting with the same plasmid DNA immunogen, HBsAg-specific CD8+ T cells proliferated quickly and vigorously. By 150 days after boosting, HBsAg-specific memory CD8+ T cells were sustained at higher levels than those recorded after the first, primary injection, both in the spleen and the liver: anti-HBs antibody-secreting plasma cells persisted in the bone marrow and in the spleen, consistent with the detection of anti-HBs antibodies detected in the blood. These findings indicate that DNA immunization has considerable potential for inducing specific T cell responses in the liver and offers a strategy for the development of post-exposure immunotherapy against persistent hepatitis B infections.

Direct interferon-gamma signaling dramatically enhances CD4+ and CD8+ T cell memory

Studies in IFN-gamma-deficient mice suggest that the delivery of IFN-gamma to CD8(+) T cells early in virus infection programs their eventual contraction, thereby reducing the abundance of CD8(+) memory T cells. In this study, we show that such mice fail to completely eliminate virus infection and that, when evaluated without the confounding factor of persisting Ag, both CD4(+) and CD8(+) T cells undergo profound contraction when they are unable to receive IFN-gamma signals. Furthermore, the abundance of CD4(+) and CD8(+) memory cells that express the IFN-gamma receptor is approximately 100-fold higher than cells lacking this molecule. Thus, direct IFN-gamma signaling is not required for T cell contraction during virus infection, and it enhances, rather than suppresses, the development of virus-specific CD4(+) and CD8(+) T cell memory.

Priming of CD8+ T-cell responses after DNA immunization is impaired in TLR9- and MyD88-deficient mice

The plasmid DNA vaccine not only provides expression of the antigen in vivo, but also activates cells of the innate immune system via unmethylated CpG-containing DNA sequences that are recognized by Toll like receptor 9 (TLR9). The requirement of such immunostimulatory activity for induction of CD8+ T-cell responses after DNA immunization is still controversial. In the present study we assessed induction of CD8+ T-cell responses against an immunodominant H-2D(b)-restricted epitope of human prostate-specific antigen in C57Bl/6 (wild-type), TLR9- and MyD88-deficient mice. A single DNA immunization resulted in efficient priming of CD8+ T responses in wild-type mice but not in TLR9- or MyD88-deficient mice. However, priming of CD8+ T cell responses was observed in TLR9-deficient but not in MyD88-deficient mice after multiple DNA immunizations. Moreover, induction of CD8+ T cell responses in TLR9-deficient mice was dependent on the presence of endotoxin contamination in plasmid DNA preparations. Collectively, these results demonstrate that TLR9-dependent immunostimulatory activity of plasmid DNA is essential for priming of CD8+ T-cell responses and that other bacterial compounds present in plasmid DNA preparations and acting via MyD88-dependent pathway could provide alternative signals necessary for priming of CD8+ T cells.

Rapid and sustained CD4+ T-cell-independent immunity from adenovirus-encoded vaccine antigens

Many novel vaccine strategies rely on recombinant viral vectors for antigen delivery, and adenovirus vectors have emerged among the most potent of these. In this report, we have compared the immune response induced through priming with adenovirus vector-encoded full-length viral protein to that elicited with an adenovirus-encoded minimal epitope covalently linked to β 2-microglobulin. We demonstrate that the β 2-microglobulin-linked epitope induced an accelerated and augmented CD8+ T-cell response. Furthermore, the immunity conferred by vaccination with β 2-microglobulin-linked lymphocytic choriomeningitis virus (LCMV)-derived epitopes was long-lived and protective. Notably, in contrast to full-length protein, the response elicited with the β 2-microglobulin-linked LCMV-derived epitope was CD4+ T-cell independent. Furthermore, virus-specific CD8+ T cells primed in the absence of CD4+ T-cell help were sustained in the long term and able to expand and control a secondary challenge with LCMV. Our results demonstrate that modifications to the antigen used in adenovirus vaccines may be used to improve the induced T-cell response. Such a strategy for CD4+ T-cell-independent immunity from adenovirus vectors offers prospects for vaccination against opportunistic pathogens in AIDS patients and possibly immunotherapy in chronic virus infections.

Adenoviral vectors persist in vivo and maintain activated CD8+ T cells: implications for their use as vaccines

CD8(+) T cell-numbers rapidly expand and then contract after exposure to their cognate antigen. Here we show that the sustained frequencies of transgene product-specific CD8(+) T cells elicited by replication-defective adenovirus vectors are linked to persistence of low levels of transcriptionally active adenovirus vector genomes at the site of inoculation, in liver, and lymphatic tissues. Continuously produced small amounts of antigen maintain fully active effector CD8(+) T cells, while also allowing for their differentiation into central memory cells. The long-term persistence of adenoviral vectors may be highly advantageous for their use as vaccines against pathogens for which T-cell-mediated protection requires both fully activated T cells for immediate control of virus-infected cells and central memory CD8(+) T cells that, because of their higher proliferative capacity, may be suited best to eliminate cells infected by pathogens that escaped the initial wave of effector T cells.

Immunization with non-replicating E. coli minicells delivering both protein antigen and DNA protects mice from lethal challenge with lymphocytic choriomeningitis virus

In the midst of new investigations into the mechanisms of both delivery and protection of new vaccines and vaccine carriers, it has become clear that immunization with delivery mechanisms that do not involve living, replicating organisms are vastly preferred. In this report, non-replicating bacterial minicells simultaneously co-delivering the nucleoprotein (NP) of lymphocytic choriomeningitis virus (LCMV) and the corresponding DNA vaccine were tested for the ability to generate protective cellular immune responses in mice. It was found that good protection (89%) was achieved after intramuscular administration, moderate protection (31%) was achieved after intranasal administration, and less protection (7%) was achieved following gastric immunization. These results provide a solid foundation on which to pursue the use of bacterial minicells as a non-replicating vaccine delivery platform.

The role of virus-induced regulatory T cells in immunopathology

In recent years, regulatory T cells have received increased attention for their role in immune responses to microbial infections. The list of microbial pathogens associated with regulatory T cell responses is growing rapidly and includes bacteria, viruses, parasites, and fungi. As the biology of regulatory T cells is revealed, we are discovering that their induction during infection is a normal aspect of immunity, necessary to limit collateral damage from inflammatory responses and aggressive immunological effectors. Thus, these cells play a critical role in maintaining the delicate balance between preventing immunopathology and allowing the immune response to clear infections. While generally successful, there are notable exceptions where regulatory T cell-mediated suppression appears to be responsible for allowing certain viruses to establish and maintain a persistent state. In this review, we will discuss our current understanding of what virus-induced regulatory T cells are, how they are induced, and what mechanisms they use to suppress immunity. The complex role of Tregs in regulating immunity to viral infections, and the consequences their activity has on disease is illustrated by a review of specific viral infections including hepatitis C virus and human immunodeficiency virus.

Dissecting the components of the humoral immune response elicited by DNA vaccines

Although DNA vaccines appear to be efficient at inducing strong cellular immune responses, a number of questions remain regarding their ability to induce humoral immunity. The essential components for generating an antibody response include B and T cell recognition of antigen, subsequent activation, clonal expansion of each lymphocyte type and migration of T cells into B cell follicles to provide help, all leading to germinal centre formation and antibody production. We have employed a double adoptive transfer system based on ovalbumin (OVA)-specific CD4+ DO11.10 T cells and hen egg lysozyme (HEL)-specific MD4 B cells to assess all of these parameters in the context of DNA vaccination in vivo. We find that vaccination with DNA constructs expressing an OVA-HEL gene fusion (encoding contiguous T and B cell epitopes) can induce T cell activation, clonal expansion and migration into B cell follicles accompanied by B cell activation, blastogenesis, expansion and antibody production. These findings show that DNA vaccination can induce all of the components required for humoral immunity and also provide a system for in depth analysis of factors that influence the development of antibody responses. Such strategies may facilitate the rational design of vaccines capable of inducing effective humoral immunity.

Identification of an immunodominant H-2D(b)-restricted CTL epitope of human PSA

BACKGROUND

Human prostate specific antigen (PSA) is expressed selectively in prostate epithelium and is a potential target for the immunotherapy against prostate cancer. Various PSA-based vaccines have been reported to induce cytotoxic T lymphocyte (CTL) responses in animal models. Here, we present the identification and validation of an immunodominant CTL epitope of PSA in C57Bl/6 mice (H-2(b)).

METHODS

PSA-specific CTLs were induced by immunization with a plasmid expressing PSA. Epitope specificity of the CTLs was determined by their reactivity against a panel of C-terminus truncated or mutated PSA proteins and use of bioinformatical prediction with the SYFPEITHI algorithm.

RESULTS

The majority of PSA-specific CTLs were directed against a single H-2D(b) restricted epitope corresponding to the amino acid residues 65-74 (HCIRNKSVIL) of the protein. The CTLs had similar functional avidity against two putative H-2D(b) binding peptides: a 9-aa-long psa65-73 (HCIRNKSVI) and a 10-aa-long psa65-74 (HCIRNKSVIL).

CONCLUSIONS

We demonstrate that the psa65-73 peptide can be used for reactivation of PSA-specific CTLs in vitro and ex vivo, and H-2D(b) pentamers assembled with this peptide are an efficient tool for monitoring of PSA-specific CTL responses after DNA vaccination.

Evaluating the immunogenicity and protective efficacy of a DNA vaccine encoding Lassa virus nucleoprotein

Several viruses in the Arenavirus genus of the family Arenaviridae cause severe, often fatal, hemorrhagic fever. One such virus, Lassa virus (LV), is a frequent cause of disease in Africa, and survivors often are left with substantial neurological impairment. The feasibility of protective immunization against LV infection, and the associated disease, has been demonstrated in animal models, using recombinant vaccinia viruses to deliver Lassa proteins. Circumstantial evidence implicates cellular immunity in this Lassa-induced protection, but this has not been confirmed. Here, we describe DNA vaccines that encode LV proteins. A single inoculation of a plasmid encoding full-length Lassa nucleoprotein (LNP) can induce CD8(+) T cell responses in mice and can protect against challenge with two arenaviruses, lymphocytic choriomeningitis virus (LCMV) and Pichinde virus (PV). A DNA minigene vaccine encoding a 9 amino acid sequence from LNP also induces CD8(+) T cells and protects against arenavirus challenge, thus confirming prior speculation that protective cellular immunity is induced by LV proteins.

In vivo compartmentalization of functionally distinct, rapidly responsive antigen-specific T-cell populations in DNA-immunized or Salmonella enterica serovar Typhimurium-infected mice

The location and functional properties of antigen-specific memory T-cell populations in lymphoid and nonlymphoid compartments following DNA immunization or infection with Salmonella were investigated. Epitope-specific CD8+ -T-cell expansion and retention during the memory phase were analyzed for DNA-immunized mice by use of a 5-h peptide restimulation assay. These data revealed that epitope-specific gamma interferon (IFN-gamma)-positive CD8+ T cells occur at higher frequencies in the spleen, liver, and blood than in draining or peripheral lymph nodes during the expansion phase. Moreover, this distribution is maintained into long-term memory. The location and function of both CD4+ and CD8+ Salmonella-specific memory T cells in mice who were given a single dose of Salmonella enterica serovar Typhimurium was also quantitated by an ex vivo restimulation with bacterial lysate to detect the total Salmonella-specific memory pool. Mice immunized up to 6 months previously with S. enterica serovar Typhimurium had bacterium-specific CD4+ T cells that were capable of producing IFN-gamma or tumor necrosis factor alpha (TNF-alpha) at each site analyzed. Similar findings were observed for CD8+ T cells that were capable of producing IFN-gamma, while a much lower frequency and more restricted distribution were associated with TNF-alpha-producing CD8+ T cells. This study is the first to assess the frequencies, locations, and functions of both CD4+ and CD8+ memory T-cell populations in the same Salmonella-infected individuals and demonstrates the organ-specific functional compartmentalization of memory T cells after Salmonella infection.

IL-23 induces stronger sustained CTL and Th1 immune responses than IL-12 in hepatitis C virus envelope protein 2 DNA immunization

IL-23 is a heterodimeric cytokine consisting of p19 and the p40 subunit of IL-12. IL-23 has been shown to possess IL-12-like biological activities, but is different in its capacity to stimulate memory T cells in vitro. In this study, we investigated whether IL-23 could influence envelope protein 2 (E2)-specific cell-mediated immunity induced by immunization of hepatitis C virus E2 DNA. We found that IL-23 induced long-lasting Th1 and CTL immune responses to E2, which are much stronger than IL-12-mediated immune responses. Interestingly, IL-23N220L, an N-glycosylation mutant showing reduced expression of excess p40 without changing the level of IL-23, exhibited a higher ratio of IFN-gamma- to IL-4-producing CD4(+) T cell frequency than did wild-type IL-23, suggesting a negative regulatory effect of p40 on Th1-prone immune response induced by IL-23. These data suggest that IL-23, particularly IL-23N220L, would be an effective adjuvant of DNA vaccine for the induction of durable Ag-specific T cell immunity.

A murine model of type 2 autoimmune hepatitis: Xenoimmunization with human antigens

Autoimmune hepatitis (AIH) is characterized by an immune-mediated injury of the hepatic parenchyma of unknown pathogenesis. Type 2 AIH is identified by the presence of anti-liver-kidney microsomes type 1 (anti-LKM1) and anti-liver cytosol type 1 (anti-LC1) autoantibodies. The current study shows that a murine model of AIH can be generated by DNA immunization against type 2 AIH self-antigens (P450 2D6 and formiminotransferase-cyclodeaminase). A pCMV plasmid containing the N-terminal region of mouse CTLA-4 and the antigenic region of human CYP2D6 (672-1,377 bp) and human formiminotransferase cyclodeaminase (FTCD; 1,232-1,668 bp) was used for DNA immunization of C57BL/6 female mice. Immunized mice showed elevated levels of alanine aminotransferase (ALT), with peaks at 4 and 7 months postinjection. Periportal, portal, and intralobular liver inflammatory infiltrates were observed at histology. Mainly CD4+ lymphocytes, but also CD8+ and B lymphocytes, were found in the liver. Cytotoxic-specific T cells were found in both the liver and spleen of these animals. Mice developed anti-LKM1 and anti-LC1 antibodies of immunoglobulin G2 (IgG2) subclass, against specific mouse autoantigens. The ALT levels correlated with both the presence of anti-LKM1/anti-LC1 antibodies and the presence of liver necroinflammation. In conclusion, in mice, DNA immunization against human autoantigens breaks tolerance and induces an autoimmune liver disease. Molecular mimicry between foreign and self-antigens explains the liver injury. This model of AIH resembles human type 2 AIH and will be helpful for the study of its pathogenesis.

Detailed analysis of the CD8+ T-cell response following adenovirus vaccination

We examined CD8(+) T-cell expansion and function following intramuscular immunization with a recombinant adenovirus. This study has identified a number of properties which may explain the strong immunogenicity of adenovirus vectors: (i) the ability to deliver large amounts of antigen into the lymphoid tissues, (ii) the ability to induce rapid expansion and migration of CD8(+) T cells throughout the lymphatics, and (iii) the ability to produce a sustained, high-level CD8(+) T-cell response.

Codelivery of a DNA vaccine and a protein vaccine with aluminum phosphate stimulates a potent and multivalent immune response

The study explores the possibility of efficiently codelivering DNA vaccines and protein-based vaccines by formulation with aluminum phosphate (AlPO4). When mixed with aluminum adjuvants, plasmid DNA bound tightly onto aluminum hydroxide [Al(OH)3] but not to AlPO4. Different doses of DNA vaccines formulated with AlPO4 [but not Al(OH)3] induced enhanced humoral responses and supported priming of MHC class I restricted cellular immunity. Different proteins mixed with the plasmid DNA vaccine in the AlPO4 formulation did not impair its immunogenicity. Coinjection of two different vaccine-relevant antigens in the same AlPO4 formulation, one as a DNA vaccine and the other as a recombinant protein, elicited polyvalent, humoral, and cellular immune responses to all antigens delivered. The isotype profiles of the induced humoral responses and the cytokine profiles of the specifically primed T cell responses indicated that the combined vaccines supported copriming of Th1- and Th2-biased as well as balanced responses. These findings indicate that the AlPO4 adjuvant, a widely accepted adjuvant in human vaccination practice, can be used to combine protein- and DNA-based vaccination to prime an enhanced and balanced specific immunity.

Neonates mount robust and protective adult-like CD8(+)-T-cell responses to DNA vaccines

Neonates are thought to mount less vigorous adaptive immune responses than adults to antigens and infectious agents. This concept has led to a delay in the administration of many currently available vaccines until late infancy or early childhood. It has recently been shown that vaccines composed of plasmid DNA can induce both humoral and cell-mediated antimicrobial immunity when administered within hours of birth. In most of these studies, immune responses were measured weeks or months after the initial vaccination, and it is therefore questionable whether the observed responses were actually the result of priming of splenocytes within the neonatal period. Here we show that DNA vaccination at birth results in the rapid induction of antigen-specific CD8(+) T cells within neonatal life. Analyses of T-cell effector functions critical for the resolution of many viral infections revealed that neonatal and adult CD8(+) T cells produce similar arrays of cytokines. Furthermore, the avidities of neonatal and adult CD8(+) T cells for peptide and the rapidity with which they upregulate cytokine production after recall encounters with antigen are similar. Protective immunity against the arenavirus lymphocytic choriomeningitis virus, which is mediated by CD8(+) cytotoxic T cells, is also rapidly acquired within the neonatal period. Collectively these data imply that, at least in the case of CD8(+) T cells, neonates are not as immunodeficient as previously supposed and that DNA vaccines may be an effective and safe means of providing critical cell-mediated antiviral immunity extremely early in life.

DNA vaccination breaks tolerance for a neo-self antigen in liver: a transgenic murine model of autoimmune hepatitis

Understanding the pathogenesis of autoimmune hepatitis requires an animal model in which chronic progressive immune injury develops spontaneously or with minimal manipulations. The new transgenic mouse model proposed in this study is based on the hypothesis that infectious agents have the potential to initiate autoreactivity through molecular mimicry. A transgenic mouse expressing lymphocytic choriomeningitis virus nucleoprotein (NP) in a H-2(b) background developed liver injury when vaccinated with plasmids expressing NP as an intracellular or a secretory protein. Coinjection of plasmids coding for NP and IL-12 facilitated the induction of a Th1 phenotype as detected by a specific B lymphocyte response characterized by a predominance of IgG2 subclass anti-NP Abs. CTLs activated in peripheral lymphoid organs by DNA vaccination migrated to the periportal and lobular areas of the liver. Their presence was associated with a significant degree of cytolysis, as evidenced by elevated transaminases several weeks after immunization. As activated specific T lymphocytes proliferated in the periphery and caused cytolysis of target cells, this study suggests that autoimmune hepatitis can be triggered by molecular mimicry, and that local injury may not be essential to initiate autoreactivity in the liver.

Robust recall and long-term memory T-cell responses induced by prime-boost regimens with heterologous live viral vectors expressing human immunodeficiency virus type 1 Gag and Env proteins

We investigated long-term memory and recall cellular immune responses to human immunodeficiency virus type 1 (HIV-1) Env and Gag proteins elicited by recombinant vesicular stomatitis viruses (VSVs) expressing Env and Gag. More than 7 months after a single vaccination with VSV-Env, approximately 6% of CD8(+) splenocytes stained with major histocompatibility complex class I tetramers containing the Env p18-I10 immunodominant peptide and showed a memory phenotype (CD44(Hi)). The level of tetramer-positive cells in memory was about 14% of the peak primary response. Recall responses elicited in these mice 5 days after boosting with a heterologous recombinant vaccinia virus expressing HIV-1 Env showed that 40 to 45% of CD8(+) splenocytes were tetramer positive and activated (CD62L(Lo)), and these cells produced gamma interferon after stimulation with Env peptide, indicating that they were functional. Five months after the boost, the long-term memory cell population (tetramer positive, CD44(Hi)) constituted 30% of the CD8(+) splenocytes. Recall responses to HIV-1 Gag were examined in mice primed with VSV recombinants expressing HIV-1 Gag protein and boosted with a vaccinia virus recombinant expressing Gag. Using this protocol, we found that approximately 40% of CD8(+) splenocytes were activated (CD62L(Lo)) and specific for a Gag immunodominant peptide (tetramer positive). The high-level Gag recall response elicited by the vaccinia virus-Gag was greater than that obtained by boosting with a VSV-Gag vector with a different VSV glycoprotein. The corresponding levels of CD44(Hi) memory cells were also higher long after boosting with vaccinia virus-Gag than after boosting with a glycoprotein exchange VSV-Gag. Our results show that VSV vectors elicit high-level memory CTL responses and that these can be amplified as much as six- to sevenfold using a heterologous boosting vector.

HER2 peptide-specific CD8(+) T cells are proportionally detectable long after multiple DNA vaccinations

We prepared a plasmid encoding 147 amino acid residues from the N terminus of c-erbB-2/HER2/neu (HER2), which included both a cytotoxic T lymphocyte (CTL) epitope (HER2p63) and a helper epitope (HER2p1), using the mammalian expression vector pCAGGS-New (pCAGGS147HER2). In a parallel analysis with a Tetramer assay and CTL assay, good specificity and sensitivity of a quantitative enzyme-linked immunospot (ELISPOT) assay to detect functional HER2p63-specific CD8(+) T cells were demonstrated after intramuscular immunization of pCAGGS147HER2. In an ELISPOT assay for HER2p63, spots of IFN gamma-producing cells were first detected 10 days after the first immunization, and additional immunizations increased the number of spots. HER2p63-specific CD8(+) T cells were detected over a period of more than 10 months after the last immunization. In hosts receiving more than three immunizations, surprisingly high numbers of specific CD8(+) T cells were persistently detectable. HER2 protein-specific antibodies of IgG class with dominance of IgG2a remain detectable 6 months after single or multiple immunizations. The antibodies however, were not reactive with cell surface HER2 antigens. Total suppression of tumor growth was observed when syngeneic HER2(+) tumor cells (2 x 10(6)) were injected subcutaneously 14 days after a single immunization with pCAGGS147HER2. Furthermore, the number of pulmonary metastases decreased significantly when DNA vaccination was initiated on the day of, or 3 days after, intravenous injection (1 x 10(6) cells).

DNA vaccines as a tool for analysing the protective immune response against rhabdoviruses in rainbow trout

DNA vaccines based on the glycoprotein genes of the salmonid rhabdoviruses VHSV and IHNV have been demonstrated to be very efficient in inducing a protective immune response against the respective diseases in rainbow trout. Nanogram doses of plasmid DNA delivered by intramuscular injection are sufficient to induce high levels of immunity in fingerling-size fish, whereas larger fish require more vaccine for protection. The protection is long lasting and, more surprisingly, is partly established already 4 days post vaccination. The early protection involves cross-protective anti-viral defence mechanisms, while the long duration immunity is highly specific. The nature of these immune response mechanisms is discussed and it is suggested that the efficacy of the vaccines is related to their ability to activate the innate immune system as it is activated by live virus.

Immunodominance in virus-induced CD8(+) T-cell responses is dramatically modified by DNA immunization and is regulated by gamma interferon

The phenomenon whereby the host immune system responds to only a few of the many possible epitopes in a foreign protein is termed immunodominance. Immunodominance occurs not only during microbial infection but also following vaccination, and clarification of the underlying mechanism may permit the rational design of vaccines which can circumvent immunodominance, thereby inducing responses to all epitopes, dominant and subdominant. Here, we show that immunodominance affects DNA vaccines and that the effects can be avoided by the simple expedient of epitope separation. DNA vaccines encoding isolated dominant and subdominant epitopes induce equivalent responses, confirming a previous demonstration that coexpression of dominant and subdominant epitopes on the same antigen-presenting cell (APC) is central to immunodominance. We conclude that multiepitope DNA vaccines should comprise a cocktail of plasmids, each with its own epitope, to allow maximal epitope dispersal among APCs. In addition, we demonstrate that subdominant responses are actively suppressed by dominant CD8(+) T-cell responses and that gamma interferon (IFN-gamma) is required for this suppression. Furthermore, priming of CD8(+) T cells to a single dominant epitope results in strong suppression of responses to other normally dominant epitopes in immunocompetent mice, in effect rendering these epitopes subdominant; however, responses to these epitopes are increased 6- to 20-fold in mice lacking IFN-gamma. We suggest that, in agreement with our previous observations, IFN-gamma secretion by CD8(+) T cells is highly localized, and we propose that its immunosuppressive effect is focused on the APC with which the dominant CD8(+) T cell is in contact.

Strong CD8 T-cell responses following coimmunization with plasmids expressing the dominant pp89 and subdominant M84 antigens of murine cytomegalovirus correlate with long-term protection against subsequent viral challenge

We previously showed that intradermal immunization with plasmids expressing the murine cytomegalovirus (MCMV) protein IE1-pp89 or M84 protects against viral challenge and that coimmunization has a synergistic protective effect (C. S. Morello, L. D. Cranmer, and D. H. Spector, J. Virol. 74:3696-3708, 2000). Using an intracellular gamma interferon cytokine staining assay, we have now characterized the CD8+ T-cell response after DNA immunization with pp89, M84, or pp89 plus M84. The pp89- and M84-specific CD8+ T-cell responses peaked rapidly after three immunizations. DNA immunization and MCMV infection generated similar levels of pp89-specific CD8+ T cells. In contrast, a significantly higher level of M84-specific CD8+ T cells was elicited by DNA immunization than by MCMV infection. Fusion of ubiquitin to pp89 enhanced the CD8+ T-cell response only under conditions where vaccination was suboptimal. Three immunizations with either pp89, M84, or pp89 plus M84 DNA also provided significant protection against MCMV infection for at least 6 months, with the best protection produced by coimmunization. A substantial percentage of antigen-specific CD8+ T cells remained detectable, and they responded rapidly to the MCMV challenge. These results underscore the importance of considering antigens that do not appear to be highly immunogenic during infection as DNA vaccine candidates.

Activating CTL precursors to reveal CTL function without skewing the repertoire by in vitro expansion

Detection of the functional CD8(+) CTL response usually requires in vitro restimulation. The differences between the CD8(+) CTL repertoire in freshly isolated precursor cells and CD8(+) CTL after short-term in vitro expansion have been generally assumed to be minimal, but have never been defined experimentally. Using staining with P18-I10/H-2D(d) tetramers and monoclonal antibodies (mAb) against Vbeta, we show the surprising result that there was significant skewing of the CD8(+) CTL repertoire after just 7 days of stimulation. In contrast, we found that overnight incubation of precursor cells with peptide allows the functional assessment of CD8(+) CTL (which cannot be detected ex vivo from freshly isolated cells) without changing the absolute number of antigen-specific CTL as measured by tetramer staining or the repertoire of TCR analyzed with mAb. This study affords a better understanding of the differences between the ex vivo and in vitro stimulated CTL repertoire, and provides an approach to reveal a more faithful representation of the functional in vivo CTL response without skewing of the repertoire of T cells detected.

CD4(+) T cells induced by a DNA vaccine: immunological consequences of epitope-specific lysosomal targeting

Our previous studies have shown that targeting DNA vaccine-encoded major histocompatibility complex class I epitopes to the proteasome enhanced CD8(+) T-cell induction and protection against lymphocytic choriomeningitis virus (LCMV) challenge. Here, we expand these studies to evaluate CD4(+) T-cell responses induced by DNA immunization and describe a system for targeting proteins and minigenes to lysosomes. Full-length proteins can be targeted to the lysosomal compartment by covalent attachment to the 20-amino-acid C-terminal tail of lysosomal integral membrane protein-II (LIMP-II). Using minigenes encoding defined T-helper epitopes from lymphocytic choriomeningitis virus, we show that the CD4(+) T-cell response induced by the NP(309-328) epitope of LCMV was greatly enhanced by addition of the LIMP-II tail. However, the immunological consequence of lysosomal targeting is not invariably positive; the CD4(+) T-cell response induced by the GP(61-80) epitope was almost abolished when attached to the LIMP-II tail. We identify the mechanism which underlies this marked difference in outcome. The GP(61-80) epitope is highly susceptible to cleavage by cathepsin D, an aspartic endopeptidase found almost exclusively in lysosomes. We show, using mass spectrometry, that the GP(61-80) peptide is cleaved between residues F(74) and K(75) and that this destroys its ability to stimulate virus-specific CD4(+) T cells. Thus, the immunological result of lysosomal targeting varies, depending upon the primary sequence of the encoded antigen. We analyze the effects of CD4(+) T-cell priming on the virus-specific antibody and CD8(+) T-cell responses which are mounted after virus infection and show that neither response appears to be accelerated or enhanced. Finally, we evaluate the protective benefits of CD4(+) T-cell vaccination in the LCMV model system; in contrast to DNA vaccine-induced CD8(+) T cells, which can confer solid protection against LCMV challenge, DNA vaccine-mediated priming of CD4(+) T cells does not appear to enhance the vaccinee's ability to combat viral challenge.

Enhancing T cell activation and antiviral protection by introducing the HIV-1 protein transduction domain into a DNA vaccine

Protein transduction domains (PTD), which can transport proteins or peptides across biological membranes, have been identified in several proteins of viral, invertebrate, and vertebrate origin. Here, we evaluate the immunological and biological consequences of including PTD in synthetic peptides and in DNA vaccines that contain CD8(+) T cell epitopes from lymphocytic choriomeningitis virus (LCMV). Synthetic PTD-peptides did not induce detectable CD8(+) T cell responses. However, fusion of an open reading frame encoding a PTD to an epitope minigene caused transfected tissue culture cells to stimulate epitope-specific T cells much more effectively. Kinetic studies indicated that the epitope reached the surface of transfected cells more rapidly and that the number of transfected cells needed to stimulate T cell responses was reduced by 35- to 50-fold when compared to cells transfected with a standard minigene plasmid. The mechanism underlying the effect of PTD linkage is not clear, but transit of the PTD-attached epitope from transfected cells to nontransfected cells (cross presentation) seemed to play, at most, a minimal role. Mice immunized once with the plasmid encoding the PTD-linked epitope showed a markedly accelerated CD8(+) T cell response and, unlike mice immunized with a standard plasmid, were completely protected against a normally lethal LCMV challenge administered only 8 days post-immunization.

Two overlapping subdominant epitopes identified by DNA immunization induce protective CD8(+) T-cell populations with differing cytolytic activities

Subdominant CD8(+) T-cell responses contribute to control of several viral infections and to vaccine-induced immunity. Here, using the lymphocytic choriomeningitis virus model, we demonstrate that subdominant epitopes can be more reliably identified by DNA immunization than by other methods, permitting the identification, in the virus nucleoprotein, of two overlapping subdominant epitopes: one presented by L(d) and the other presented by K(d). This subdominant sequence confers immunity as effective as that induced by the dominant epitope, against which >90% of the antiviral CD8(+) T cells are normally directed. We compare the kinetics of the dominant and subdominant responses after vaccination with those following subsequent viral infection. The dominant CD8(+) response expands more rapidly than the subdominant responses, but after virus infection is cleared, mice which had been immunized with the "dominant" vaccine have a pool of memory T cells focused almost entirely upon the dominant epitope. In contrast, after virus infection, mice which had been immunized with the "subdominant" vaccine retain both dominant and subdominant memory cells. During the acute phase of the immune response, the acquisition of cytokine responsiveness by subdominant CD8(+) T cells precedes their development of lytic activity. Furthermore, in both dominant and subdominant populations, lytic activity declines more rapidly than cytokine responsiveness. Thus, the lysis(low)-cytokine(competent) phenotype associated with most memory CD8(+) T cells appears to develop soon after antigen clearance. Finally, lytic activity differs among CD8(+) T-cell populations with different epitope specificities, suggesting that vaccines can be designed to selectively induce CD8(+) T cells with distinct functional attributes.

Using recombinant coxsackievirus B3 to evaluate the induction and protective efficacy of CD8+ T cells during picornavirus infection

Coxsackievirus B3 (CVB3) is a common human pathogen that has been associated with serious diseases including myocarditis and pancreatitis. To better understand the effect of cytotoxic T-lymphocyte (CTL) responses in controlling CVB3 infection, we have inserted well-characterized CTL epitopes into the CVB3 genome. Constructs were made by placing the epitope of interest upstream of the open reading frame encoding the CVB3 polyprotein, separated by a poly-glycine linker and an artificial 3Cpro/3CDpro cleavage site. This strategy results in the foreign protein being translated at the amino- terminus of the viral polyprotein, from which it is cleaved prior to viral assembly. In this study, we cloned major histocompatibility complex class I-restricted CTL epitopes from lymphocytic choriomeningitis virus (LCMV) into recombinant CVB3 (rCVB3). In vitro, rCVB3 growth kinetics showed a 1- to 2-h lag period before exponential growth was initiated, and peak titers were approximately 1 log unit lower than for wild-type virus. rCVB3 replicated to high titers in vivo and caused severe pancreatitis but minimal myocarditis. Despite the high virus titers, rCVB3 infection of naive mice failed to induce a strong CD8+ T-cell response to the encoded epitope; this has implications for the proposed role of "cross-priming" during virus infection and for the utility of recombinant picornaviruses as vaccine vectors. In contrast, rCVB3 infection of LCMV-immune mice resulted in direct ex vivo cytotoxic activity against target cells coated with the epitope peptide, demonstrating that the rCVB3-encoded LCMV-specific epitope was expressed and presented in vivo. The preexisting CD8+ memory T cells could limit rCVB replication; compared to naive mice, infection of LCMV-immune mice with rCVB3 resulted in approximately 50-fold-lower virus titers in the heart and approximately 6-fold-lower virus titers in the pancreas. Although the inserted CTL epitope was retained by rCVB3 through several passages in tissue culture, it was lost in an organ-specific manner in vivo; a substantial proportion of viruses from the pancreas retained the insert, compared to only 0 to 1.8% of myocardial viruses. Together, these results show that expression of heterologous viral proteins by recombinant CVB3 provides a useful model for determining the mechanisms underlying the immune response to this viral pathogen.