Cutting edge: recombinant adenoviruses induce CD8 T cell responses to an inserted protein whose expression is limited to nonimmune cells

Analysis of polyclonal and monoclonal antibody to the influenza virus nucleoprotein in different oligomeric states

Influenza virus nucleoprotein (NP) is one of the most conserved influenza proteins. Both NP antigen and anti-NP antibodies are used as reagents in influenza diagnostic kits, with applications in both clinical practice and influenza zoonotic surveillance programs. Despite this, studies on the biochemical basis of NP diagnostic serology and NP epitopes are not as developed as for hemagglutinin (HA), the fast-evolving antigen which has been the critical component of current influenza vaccines. Here, we characterized the NP serology of mice, ferrets, and human sera and the immunogenic effects of NP antigen presented as different structural complexes. Furthermore, we show that the classical mouse anti-NP mAb HB65 could detect NP in some commercial influenza vaccines. MAb HB65 bound a linear epitope with nanomolar affinity. Our analysis suggests that linear NP epitopes paired with their corresponding characterized detection antibodies could aid in designing and improving diagnostic technologies for influenza viruses.

Analysis of polyclonal and monoclonal antibody to the influenza virus nucleoprotein in different oligomeric states

Influenza virus nucleoprotein (NP) is one of the most conserved influenza proteins. Both NP antigen and anti-NP antibodies are used as reagents in influenza diagnostic kits, with applications in both clinical practice, and influenza zoonotic surveillance programs. Despite this, studies on the biochemical basis of NP diagnostic serology and NP epitopes are not as developed as for hemagglutinin (HA), the fast-evolving antigen which has been the critical component of current influenza vaccines. Here, we characterized the NP serology of mice, ferret, and human sera and the immunogenic effects of NP antigen presented as different structural complexes. Furthermore, we show that a classical anti-NP mouse mAb HB65 could detect NP in some commercial influenza vaccines. MAb HB65 bound a linear epitope with nanomolar affinity. Our analysis suggests that linear NP epitopes paired with their corresponding characterized detection antibodies could aid in designing and improving diagnostic technologies for influenza virus.

Adenoviral Vector Vaccines Antigen Transgene

In the past decade adenovirus-based vaccines have progressed from preclinical studies, which universally showed the vectors’ high immunogenicity, to testing in humans. Clinical trials showed that adenovirus vectors are well tolerated by humans. They induce robust immune responses that can be expanded by booster immunization. The effect of preexisting neutralizing antibodies on vectors’ immunogenicity appears to be less severe than was observed in experimental animals and can readily be circumvented by using vectors to which most humans lack neutralizing antibodies. Additional clinical studies are needed to firmly establish the efficacy of adenoviral vector vaccines.

Differential immunogenicity between HAdV-5 and chimpanzee adenovirus vector ChAdOx1 is independent of fiber and penton RGD loop sequences in mice

Replication defective adenoviruses are promising vectors for the delivery of vaccine antigens. However, the potential of a vector to elicit transgene-specific adaptive immune responses is largely dependent on the viral serotype used. HAdV-5 (Human adenovirus C) vectors are more immunogenic than chimpanzee adenovirus vectors from species Human adenovirus E (ChAdOx1 and AdC68) in mice, though the mechanisms responsible for these differences in immunogenicity remain poorly understood. In this study, superior immunogenicity was associated with markedly higher levels of transgene expression in vivo, particularly within draining lymph nodes. To investigate the viral factors contributing to these phenotypes, we generated recombinant ChAdOx1 vectors by exchanging components of the viral capsid reported to be principally involved in cell entry with the corresponding sequences from HAdV-5. Remarkably, pseudotyping with the HAdV-5 fiber and/or penton RGD loop had little to no effect on in vivo transgene expression or transgene-specific adaptive immune responses despite considerable species-specific sequence heterogeneity in these components. Our results suggest that mechanisms governing vector transduction after intramuscular administration in mice may be different from those described in vitro.

Enhancement of recombinant adenovirus vaccine-induced primary but not secondary systemic and mucosal immune responses by all-trans retinoic acid

Vaccination is an important tool for enhancing immune responses against mucosal pathogens. Intramuscularly administered adenovirus (Ad) vectors have been demonstrated to be strong inducers of both systemic and mucosal immune responses. Further enhancement of immune responses following Ad vaccination is highly desirable. All-trans retinoic acid (ATRA), a biologically active vitamin A metabolite, has been explored as an adjuvant for primary immune responses following vaccination. In this study, we investigated the effect of ATRA on a heterologous Ad prime boost regimen. ATRA co-administration during priming increased mucosal and systemic antibody responses as well as mucosal but not systemic CD8(+) T cell responses. However, this effect was no longer apparent after boosting regardless of whether ATRA was administered at the time of priming, at the time of boosting, or at both immunizations. Our findings confirm ATRA as an adjuvant for primary immune responses and suggest that the adjuvant effect does not extend to secondary immune responses.

The rationale of vectored gene-fusion vaccines against cancer: evolving strategies and latest evidence

The development of vaccines that target tumor antigens in cancer has proven difficult. A major reason for this is that T cells specific for tumor self-antigens and neoantigens are eliminated or inactivated through mechanisms of tolerance. Antigen fusion strategies which increase the ability of vaccines to stimulate T cells that have escaped tolerance mechanisms, may have a particular potential as immunotherapies. This review highlights antigen fusion strategies that have been successful in stimulating the induction of T-cell immunity against cancer and counteracting tumor-associated tolerance. In preclinical studies, these strategies have shown to improve the potency of vectored vaccines through fusion of tumor antigen to proteins or protein domains that increase CD4+ T-cell help, CD8+ T-cell responses or both the CD4+ and CD8+ T-cell responses. However, in clinical trials such strategies seem to be less efficient when provided as a DNA vaccine. The first clinical trial using a viral vectored fusion-gene vaccine is expected to be tested as a partner in a heterologous prime-boost regimen directed against cervical cancer.

Stable antigen is most effective for eliciting CD8+ T-cell responses after DNA vaccination and infection with recombinant vaccinia virus in vivo

The induction of strong CD8(+) T-cell responses against infectious diseases and cancer has remained a major challenge. Depending on the source of antigen and the infectious agent, priming of CD8(+) T cells requires direct and/or cross-presentation of antigenic peptides on major histocompatibility complex (MHC) class I molecules by professional antigen-presenting cells (APCs). However, both pathways show distinct preferences concerning antigen stability. Whereas direct presentation was shown to efficiently present peptides derived from rapidly degraded proteins, cross-presentation is dependent on long-lived antigen species. In this report, we analyzed the role of antigen stability on DNA vaccination and recombinant vaccinia virus (VV) infection using altered versions of the same antigen. The long-lived nucleoprotein (NP) of lymphocytic choriomeningitis virus (LCMV) can be targeted for degradation by N-terminal fusion to ubiquitin or, as we show here, to the ubiquitin-like modifier FAT10. Direct presentation by cells either transfected with NP-encoding plasmids or infected with recombinant VV in vitro was enhanced in the presence of short-lived antigens. In vivo, however, the highest induction of NP-specific CD8(+) T-cell responses was achieved in the presence of long-lived NP. Our experiments provide evidence that targeting antigens for proteasomal degradation does not improve the immunogenicity of DNA vaccines and recombinant VVs. Rather, it is the long-lived antigen that is superior for the efficient activation of MHC class I-restricted immune responses in vivo. Hence, our results suggest a dominant role for antigen cross-priming in DNA vaccination and recombinant VV infection.

Manipulating Immune Tolerance with Micro-RNA Regulated Gene Therapy

The success of in vivo gene therapy greatly depends on the ability to control the immune response toward the therapeutic transgene. Over the last decade several vector-based and pharmacological approaches have been explored to control the immune-mediated clearance of transgene-expressing cells after viral delivery. One important outcome from these studies is the concept that expression of a transgene in tolerance-promoting organs, such as the liver and tolerogenic antigen-presenting cells, can help safeguard transgene-expressing cells from immune-mediated clearance. Gene therapists are now manipulating vectors to target naturally occurring tolerogenic properties of the body by: (i) incorporating tissue/cell specific promoters for targeted expression, (ii) using viral-capsid engineering to alter tropism and avoid pre-existing immunity, and (iii) regulating cell and activation dependent expression by including micro-RNA (miR) targets into expression cassettes. The combination of these three layers of vector regulation greatly enhances the targeting of tolerogenic cells and limits off-target expression of the transgene, which can lead to the induction of transgene-specific pathogenic effector T cells. In this review, we discuss the application of using miR transgene regulation to generate tolerogenic responses and speculate on possible mechanisms used by the liver to induce the transgene-specific regulatory T cells.

Induction of virus-specific cytotoxic T lymphocytes as a basis for the development of broadly protective influenza vaccines

There is considerable interest in the development of broadly protective influenza vaccines because of the continuous emergence of antigenic drift variants of seasonal influenza viruses and the threat posed by the emergence of antigenically distinct pandemic influenza viruses. It has been recognized more than three decades ago that influenza A virus-specific cytotoxic T lymphocytes recognize epitopes located in the relatively conserved proteins like the nucleoprotein and that they cross-react with various subtypes of influenza A viruses. This implies that these CD8+ T lymphocytes may contribute to protective heterosubtypic immunity induced by antecedent influenza A virus infections. In the present paper, we review the evidence for the role of virus-specific CD8+ T lymphocytes in protective immunity against influenza virus infections and discuss vaccination strategies that aim at the induction of cross-reactive virus-specific T-cell responses.

miRNA-mediated silencing in hepatocytes can increase adaptive immune responses to adenovirus vector-delivered transgenic antigens

Adenovirus vectors based on human serotype 5 can induce potent CD8 T cell responses to vector-encoded transgenic antigens. However, the individual contribution of different cell types expressing antigen upon adenovirus vector injection to the generation of antigen-directed adaptive immune responses is poorly understood so far. We investigated the role of hepatocytes, skeletal muscle, and hematopoietic cells for the induction of cellular and humoral immune responses by miRNA-mediated tissue-specific silencing of antigen expression. Using hepatitis B small surface antigen (HBsAg) as the vector-encoded transgene we show that adenovirus vector dissemination from an intramuscular (i.m.) injection site into the liver followed by HBsAg expression in hepatocytes can limit early priming of CD8 T cells and the generation of anti-HBsAg antibody responses. However, hepatocyte-specific miRNA122a-mediated silencing of HBsAg expression overcame these limitations. Early clonal expansion of K(b)/S(190-197)-specific CD8 T cells was significantly enhanced and improved polyfunctionality of CD8 T cells was found. Furthermore, miRNA122a-mediated antigen silencing induced significantly higher anti-HBsAg antibody titers allowing an up to 100-fold vector dose reduction. These results indicate that miRNA-mediated regulation of antigen expression in the context of adenovirus vectors can significantly improve transgene product-directed immune responses. This finding could be of interest for future adenovirus vaccine vector development.

Delivery route, MyD88 signaling and cross-priming events determine the anti-tumor efficacy of an adenovirus based melanoma vaccine

Adenovirus (Ad)-based vaccines are considered for cancer immunotherapy, yet, detailed knowledge on their mechanism of action and optimal delivery route for anti-tumor efficacy is lacking. Here, we compared the anti-tumor efficacy of an Ad-based melanoma vaccine after intradermal, intravenous, intranasal or intraperitoneal delivery in the B16F10 melanoma model. The intradermal route induced superior systemic anti-melanoma immunity which was MyD88 signaling-dependent. Predominant transduction of non-professional antigen-presenting cells at the dermal vaccination sites and draining lymph nodes, suggested a role for cross-presentation, which was confirmed in vitro. We conclude that the dermis provides an optimal route of entry for Ad-based vaccines for high-efficacy systemic anti-tumor immunization and that this immunization likely involves cross-priming events in the draining lymph nodes.

Vaccination with Ad5 vectors expands Ad5-specific CD8 T cells without altering memory phenotype or functionality

Background

Adenoviral (Ad) vaccine vectors represent both a vehicle to present a novel antigen to the immune system as well as restimulation of immune responses against the Ad vector itself. To what degree Ad-specific CD8⁺ T cells are restimulated by Ad vector vaccination is unclear, although such knowledge would be important as vector-specific CD8⁺ T cell expansion could potentially further limit Ad vaccine efficacy beyond Ad-specific neutralizing antibody alone.

Methodology/Principal Findings

Here we addressed this issue by measuring human Adenovirus serotype 5 (Ad5)-specific CD8⁺ T cells in recipients of the Merck Ad5 HIV-1 vaccine vector before, during, and after vaccination by multicolor flow cytometry. Ad5-specific CD8⁺ T-cells were detectable in 95% of subjects prior to vaccination, and displayed primarily an effector-type functional profile and phenotype. Peripheral blood Ad5-specific CD8⁺ T-cell numbers expanded after Ad5-HIV vaccination in all subjects, but differential expansion kinetics were noted in some baseline Ad5-neutralizing antibody (Ad5 nAb) seronegative subjects compared to baseline Ad5 nAb seropositive subjects. However, in neither group did vaccination alter polyfunctionality, mucosal targeting marker expression, or memory phenotype of Ad5-specific CD8⁺ T-cells.

Conclusions

These data indicate that repeat Ad5-vector administration in humans expands Ad5-specific CD8⁺ T-cells without overtly affecting their functional capacity or phenotypic properties. This is a secondary analysis of samples collected during the 016 trial. Results of the Merck 016 trial safety and immunogenicity have been previously published in the journal of clinical infectious diseases [1].

Trial Registration

ClinicalTrials.gov NCT00849680 [NCT00849680]

Dendritic cell migration limits the duration of CD8+ T-cell priming to peripheral viral antigen

CD8(+) T cells (T(CD8(+))) play a crucial role in immunity to viruses. Antiviral T(CD8(+)) are initially activated by recognition of major histocompatibility complex (MHC) class I-peptide complexes on the surface of professional antigen-presenting cells (pAPC). Migration of pAPC from the site of infection to secondary lymphoid organs is likely required during a natural infection. Migrating pAPC can be directly infected with virus or may internalize antigen derived from virus-infected cells. The use of experimental virus infections to assess the requirement for pAPC migration in initiation of T(CD8(+)) responses has proven difficult to interpret because injected virus can readily drain to secondary lymphoid organs without the need for cell-mediated transport. To overcome this ambiguity, we examined the generation of antigen-specific T(CD8(+)) after immunization with recombinant adenoviruses that express antigen driven by skin-specific or ubiquitous promoters. We show that the induction of T(CD8(+)) in response to tissue-targeted antigen is less efficient than the response to ubiquitously expressed antigen and that the resulting T(CD8(+)) fail to clear all target cells pulsed with the antigenic peptide. This failure to prime a fully functional T(CD8(+)) response results from a reduced period of priming to peripherally expressed antigen versus ubiquitously expressed antigen and correlated with a brief burst of pAPC migration from the skin, a requirement for induction of the response to peripheral antigen. These results indicate that a reduced duration of pAPC migration after virus infection likely reduces the amplitude of the T(CD8(+)) response, allowing persistence of the peripheral virus.

Targets for the induction of protective immunity against influenza a viruses

The current pandemic caused by the new influenza A(H1N1) virus of swine origin and the current pandemic threat caused by the highly pathogenic avian influenza A viruses of the H5N1 subtype have renewed the interest in the development of vaccines that can induce broad protective immunity. Preferably, vaccines not only provide protection against the homologous strains, but also against heterologous strains, even of another subtype. Here we describe viral targets and the arms of the immune response involved in protection against influenza virus infections such as antibodies directed against the hemagglutinin, neuraminidase and the M2 protein and cellular immune responses directed against the internal viral proteins.

Yearly influenza vaccinations: a double-edged sword?

Yearly vaccination against seasonal influenza viruses is recommended for certain individuals at high risk of complications associated with influenza. It has been recommended in some countries, including the USA, that all children aged 6-59 months are vaccinated against seasonal influenza. However, it has been shown-mainly in animals-that infection with influenza A viruses can induce protective immunity to influenza A viruses of other unrelated subtypes. This so-called heterosubtypic immunity does not provide full protection, but can limit virus replication and reduce morbidity and mortality of the host. This type of immunity might be relevant to human beings when a new subtype of influenza A virus is introduced into the population, such as the new influenza A H1N1 virus responsible for the present influenza pandemic and the highly pathogenic avian influenza H5N1 viruses that are causing an ever increasing number of human infections with high mortality rates. Preventing infection with seasonal influenza viruses by vaccination might prevent the induction of heterosubtypic immunity to pandemic strains, which might be a disadvantage to immunologically naive people-eg, infants.

New insights on adenovirus as vaccine vectors

Adenovirus (Ad) vectors were initially developed for treatment of genetic diseases. Their usefulness for permanent gene replacement was limited by their high immunogenicity, which resulted in rapid elimination of transduced cells through induction of T and B cells to antigens of Ad and the transgene product. The very trait that excluded their use for sustained treatment of genetic diseases made them highly attractive as vaccine carriers. Recently though results showed that Ad vectors based on common human serotypes, such as serotype 5, may not be ideal as vaccine carriers. A recently conducted phase 2b trial, termed STEP trial, with an AdHu5-based vaccine expressing antigens of human immunodeficiency virus 1 (HIV-1) not only showed lack of efficacy in spite of the vaccine's immunogenicity, but also suggested an increased trend for HIV acquisition in individuals that had circulating AdHu5 neutralizing antibodies prior to vaccination. Alternative serotypes from humans or nonhuman primates (NHPs), to which most humans lack pre-existing immunity, have been vectored and may circumvent the problems encountered with the use of AdHu5 vectors in humans. In summary, although Ad vectors have seen their share of setbacks in recent years, they remain viable tools for prevention or treatment of a multitude of diseases.

Determination of specific CD4 and CD8 T cell epitopes after AAV2- and AAV8-hF.IX gene therapy

The application of AAV2 or AAV8 vectors for delivery of human coagulation factor IX (hF.IX) is a promising gene therapy for hemophilia B. One major limitation of this therapy is the development of antibodies and a cytotoxic T lymphocyte (CTL) response against both the vector capsid and the transgene. We determined the class I and class II MHC peptide epitopes for AAV2, AAV8, and hF.IX after administration of AAV-2-hF.IX or AAV8-hF.IX in H2(b) (C57BL/6), H2(d) (BALB/c), or H2(k) (C3H) strains of mice. The results indicate that the AAV2 capsid peptide AA(373-381), the AAV8 capsid peptide AA(50-58), and the hF.IX transgene peptide AA(311-319) can elicit a CTL response as indicated by an IFN-gamma ELISPOT assay and an in vivo CTL assay. Furthermore, a strong H2(k) MHC II-restricted Th1 response can be elicited in C3H mice by the AAV8 capsid peptide AA(126-140) and the hF.IX peptide AA(108-122), whereas a strong Th2 response can be elicited by the AAV2 peptide AA(475-489). These results show that specific CTL responses are generated to both AAV capsid epitopes and hF.IX epitopes after injection of AAV-hF.IX, and MHC class II epitopes derived from AAV-hF.IX promote development of either Th1 or Th2 cells.

Redundancy renders the glycoprotein 96 receptor scavenger receptor A dispensable for cross priming in vivo

CD8(+) T cells (T(CD8+)) differentiate into effector cells following recognition of specific peptide-major histocompatibility complex (MHC) class I complexes (pMHC-I) on the surface of professional APCs (pAPCs), such as dendritic cells. Antigenic pMHC-I can be generated from two spatially distinct sources. The direct presentation pathway involves generation of peptide from protein substrate synthesized within the cell that is presenting the pMHC-I. Alternatively, the cross presentation pathway involves presentation of antigen that is not synthesized within the presenting cell, but is derived from exogenous proteins synthesized within other donor cells. The mechanisms by which cross presentation of exogenous antigens occur in vivo remain controversial. The C-type lectin scavenger receptor A (SR-A) has been implicated in a number of potential cross presentation pathways, including the presentation of peptide bound to heat shock proteins, such as glycoprotein 96 (gp96), and the transfer of pMHC-I from a donor cell to the pAPC. We demonstrate here that initiation of T(CD8+) responses is normal in mice lacking SR-A, and that the redundancy of ligand binding exhibited by the SR family is likely to be an important mechanism that ensures cross presentation in vivo. These observations emphasize the requirement to target multiple receptors and antigen-processing pathways during the rational design of vaccines aimed at eliciting protective T(CD8+).

Steady state dendritic cells present parenchymal self-antigen and contribute to, but are not essential for, tolerization of naive and Th1 effector CD4 cells

Bone marrow-derived APC are critical for both priming effector/memory T cell responses to pathogens and inducing peripheral tolerance in self-reactive T cells. In particular, dendritic cells (DC) can acquire peripheral self-Ags under steady state conditions and are thought to present them to cognate T cells in a default tolerogenic manner, whereas exposure to pathogen-associated inflammatory mediators during the acquisition of pathogen-derived Ags appears to reprogram DCs to prime effector and memory T cell function. Recent studies have confirmed the critical role of DCs in priming CD8 cell effector responses to certain pathogens, although the necessity of steady state DCs in programming T cell tolerance to peripheral self-Ags has not been directly tested. In the current study, the role of steady state DCs in programming self-reactive CD4 cell peripheral tolerance was assessed by combining the CD11c-diphtheria toxin receptor transgenic system, in which DC can be depleted via treatment with diphtheria toxin, with a TCR-transgenic adoptive transfer system in which either naive or Th1 effector CD4 cells are induced to undergo tolerization after exposure to cognate parenchymally derived self-Ag. Although steady state DCs present parenchymal self-Ag and contribute to the tolerization of cognate naive and Th1 effector CD4 cells, they are not essential, indicating the involvement of a non-DC tolerogenic APC population(s). Tolerogenic APCs, however, do not require the cooperation of CD4(+)CD25(+) regulatory T cells. Similarly, DC were required for maximal priming of naive CD4 cells to vaccinia viral-Ag, but priming could still occur in the absence of DC.

Understanding presentation of viral antigens to CD8+ T cells in vivo: the key to rational vaccine design

CD8+ T cells play a critical role in antiviral immunity by exerting direct antiviral activity against infected cells. Because of their ability to recognize all types of viral proteins, they offer the promise of providing broad immunity to viruses that evade humoral immunity by varying their surface proteins. Consequently, there is considerable interest in developing vaccines that elicit effective antiviral T(CD8+) responses. Generating optimal vaccines ultimately requires rational design based on detailed knowledge of how T(CD8+) are activated in vivo under natural circumstances. Here we review recent progress obtained largely by in vivo studies in mice to understand the mechanistic basis for activation of naive T(CD8+) in virus infections. These studies point the way to detailed understanding and provide some key information for vaccine development, although much remains to be learned to enable truly rational vaccine design.

Gene therapy to manipulate effector T cell trafficking to tumors for immunotherapy

Strategies that generate tumor Ag-specific effector cells do not necessarily cure established tumors. We hypothesized that the relative efficiency with which tumor-specific effector cells reach the tumor is critical for therapy. We demonstrate in this study that activated T cells respond to the chemokine CCL3, both in vitro and in vivo, and we further demonstrate that expression of CCL3 within tumors increases the effector T cell infiltrate in those tumors. Importantly, we show that adenoviral gene transfer to cause expression of CCL3 within B16ova tumors in vivo increases the efficacy of adoptive transfer of tumor-specific effector OT1 T cells. We additionally demonstrate that such therapies result in endogenous immune responses to tumor Ags that are capable of protecting animals against subsequent tumor challenge. Strategies that modify the "visibility" of tumors have the potential to significantly enhance the efficacy of both vaccine and adoptive transfer therapies currently in development.

Rapid Functional Exhaustion and Deletion of CTL following Immunization with Recombinant Adenovirus

Replication-deficient adenoviruses (recombinant adenovirus (rec-AdV)) expressing different transgenes are widely used vectors for gene therapy and vaccination. In this study, we describe the tolerization of transgene-specific CTL following administration of beta-galactosidase (beta gal)-recombinant adenovirus (Ad-LacZ). Using MHC class I tetramers to track beta gal-specific CTL, we found that a significant expansion of beta gal-specific CTL was restricted to a very narrow dose range. Functional analysis revealed that adenovirus-induced beta gal-specific CTL produced only very low amounts of effector cytokines and were unable to exhibit cytolytic activity in a 51Cr release assay. Furthermore, Ad-LacZ vaccination failed to efficiently clear established beta gal-positive tumors. The impaired function of Ad-LacZ-induced CTL correlated with the presence of persisting beta gal Ag in the liver. A further increase in the peripheral Ag load by injection of Ad-LacZ into SM-LacZ transgenic mice which express beta gal as self-Ag exclusively in peripheral nonlymphoid organs, resulted in the physical deletion of beta gal-specific CTL. Our results indicate first that CTL deletion in the course of adenoviral vaccination is preceded by their functional impairment and second, that the outcome of rec-AdV vaccination depends critically on the Ag load in peripheral tissues.

Prospects and problems of gene therapy: an update

The gene therapy approach can vary from delivering extra copies of a gene, through modifications of a genome using the properties of ribozymes or chimeraplasts, to injection of modified cells. For the treatment of genetic deficits the ultimate goal would be the repair of the mutated gene in the target tissue(s). The techniques required for such an approach are emerging, albeit slowly. Therefore, delivery of an extra copy of a normal gene in a specific vector remains the predominant approach. Moreover, this method finds wider applications in gene therapy relating to disorders other than heritable defects, e.g., malignancies, cardiovascular diseases and infections. The major and most intensive areas of research are: i) vectors and delivery methods, ii) regulation of transgene expression and iii) stability of expression. Targeting of the therapeutic gene is being accomplished by using viral vectors or non-viral delivery systems, either ex vivo or in vivo. The choice of vectors and delivery routes depends on the nature of the target cells and the required levels and stability of expression. Although there have been the first positive clinical results and significant technical achievements over the past 2 years, there are still obstacles to the development of effective clinical products and these remain largely unchanged. The most important barriers are the low levels and stability of expression and immune responses to vectors and/or gene products. The safety aspects of gene therapy have become painfully evident with the first death conclusively linked to gene therapy. The progress in AAV and lentiviral vectors, improved regulation of transgene expression and advances in stem cell technology are among the recent most exciting developments.

Viral vectors for inducing CD8+T cell responses

CD8(+) T cells (T(CD8+)) can mediate protective immunity to intracellular pathogens and tumours. Viruses generate strong T(CD8+) responses and, therefore, represent attractive vectors for generating vaccines aimed at producing T(CD8+)-mediated protective immunity. This review will examine the immunological properties of viruses that make them good candidates as vaccine vectors, as well as the manipulations of both vector and antigen that may be required to produce an effective vaccine. The areas addressed include virus infection of dendritic cells in vivo, stimulation of the innate immune response via intracellular and extracellular pattern recognition receptors, the effect of antigenic form on the pathways of antigen presentation and the requirement for elimination of viral genes that target various aspects of the innate and adaptive immune response.

Cutting edge: cross-presentation of cell-associated antigens to MHC class I molecule is regulated by a major transcription factor for heat shock proteins

The ability for the professional APC to cross-present Ag to MHC class I from parenchymal cells is essential for priming as well as tolerance of CD8+ T cells against intracellular Ags. Since cross-presentations of non-cell-associated free Ags are inefficient, the roles of molecular chaperones or heat shock proteins (HSPs) in chaperoning Ags to APCs have been postulated. We herein genetically addressed this hypothesis using mice that were defective of heat shock factor 1 (Hsf1), a major transcription factor for HSPs. Hsf1(-/-) mice have a decreased expression of several HSPs including HSP90 and HSP70. Using multiple Ag systems, we demonstrated that cross-priming of Ag-specific CD8+ T cells was inefficient when Ag expression was restricted to Hsf1(-/-) non-APCs. Our study provides the first genetic evidence for the roles of Hsf1 in regulating cross-presentation of MHC class I-associated Ags.

Pathways for antigen cross presentation

Dendritic cells (DCs) have the unique ability to capture cellular tissue antigens, and to present them on MHC class I molecules to antigen-specific CD8(+) T lymphocytes after migration to the draining lymph nodes. This process, called "cross presentation" can lead either to the tolerization or activation of antigen-specific CD8(+) T cells. Antigen capture is believed to occur by phagocytosis of antigen-bearing dead cells. Recent studies suggest that the antigen transferred from the phagocytosed cell to the DC during cross presentation is a proteasome substrate, rather than a proteasomal degradation product. In most cases, the formation of the peptide-MHC class I complexes in DCs requires the export of protein antigens from phagosomes to the cytosol, where they undergo proteasomal degradation. The resulting peptides are then translocated by TAP to the lumen of a cross presentation-loading compartment, for association to MHC class I under the control of chaperones and oxido-reductases. This loading compartment may be either the endoplasmic reticulum (ER) or a mix phagosome-ER compartment. MHC class I egress from the loading compartment to cell surface remains to be analyzed.

Cross-presentation, dendritic cell subsets, and the generation of immunity to cellular antigens

Cross-presentation involves the uptake and processing of exogenous antigens within the major histocompatibility complex (MHC) class I pathway. This process is primarily performed by dendritic cells (DCs), which are not a single cell type but may be divided into several distinct subsets. Those expressing CD8alpha together with CD205, found primarily in the T-cell areas of the spleen and lymph nodes, are the major subset responsible for cross-presenting cellular antigens. This ability is likely to be important for the generation of cytotoxic T-cell immunity to a variety of antigens, particularly those associated with viral infection, tumorigenesis, and DNA vaccination. At present, it is unclear whether the CD8alpha-expressing DC subset captures antigen directly from target cells or obtains it indirectly from intermediary DCs that traffic from peripheral sites. In this review, we examine the molecular basis for cross-presentation, discuss the role of DC subsets, and examine the contribution of this process to immunity, with some emphasis on DNA vaccination.

Protection of weaned pigs by vaccination with human adenovirus 5 recombinant viruses expressing the hemagglutinin and the nucleoprotein of H3N2 swine influenza virus

Swine influenza virus (SIV), subtype H3N2, is a recent reassortant virus that emerged in 1998 in North American swine causing severe respiratory and reproductive disease. In this study, two replication-defective adenovirus recombinants were developed as potential vaccines against H3N2 influenza viruses. Three groups of 3-week-old pigs (10 pigs per group) were vaccinated intramuscularly (IM) with the recombinants; one group was vaccinated with the recombinant adenovirus expressing the influenza virus H3 hemagglutinin (HA) protein, one group was vaccinated with the recombinant adenovirus expressing the nucleoprotein (NP), and one group was vaccinated with both recombinants in a mixture. Two additional control groups (10 pigs per group) were included in the animal trial. One control group was challenged with a virulent H3N2 field strain and one control group remained unchallenged. The results showed that pigs in the groups given the recombinant adenovirus expressing HA alone and HA plus NP developed high levels of virus-specific hemagglutination-inhibition (HI) antibody by 4 weeks post vaccination. Pigs in the group vaccinated with both recombinant viruses in a mixture were completely protected. Complete protection was shown by the lack of nasal shedding of virus following challenge and by the lack of lung lesions at 1 week following the challenge infection. Thus, replication-incompetent adenovirus vaccines given simultaneously to pigs are efficacious for SIV and have the additional advantage over commercial vaccines that suckling piglets have no pre-existing maternally-derived antibody to block early life vaccination.

Loading of MHC class I and II presentation pathways by exogenous antigens: a quantitative in vivo comparison

The MHC class I pathway is usually fueled by endogenous Ags, while exogenous Ags reach the MHC class II pathway. Although exogenous epitopes may also enter the MHC class I pathway, quantification of the efficiency of the process has remained a difficult task. In an attempt of such a quantification, we directly compared the amount of exogenous virus-like particles required for induction of cytotoxic T cell responses by cross-priming with the amount of virus-like particles required for induction of Th cell responses by the conventional route of MHC class II loading as an internal standard. Surprisingly, we found that cross-presentation of peptides derived from exogenous Ags on MHC class I molecules is of only marginally lower efficiency ( approximately 1- to 10-fold) than the classical MHC class II pathway in vitro and in vivo. Thus, Ag quantities required for cross-presentation and cross-priming are similar to those required for fueling the MHC class II pathway.

Peyer's patch dendritic cells process viral antigen from apoptotic epithelial cells in the intestine of reovirus-infected mice

We explored the role of Peyer's patch (PP) dendritic cell (DC) populations in the induction of immune responses to reovirus strain type 1 Lang (T1L). Immunofluorescence staining revealed the presence of T1L structural (σ1) and nonstructural (σNS) proteins in PPs of T1L-infected mice. Cells in the follicle-associated epithelium contained both σ1 and σNS, indicating productive viral replication. In contrast, σ1, but not σNS, was detected in the subepithelial dome (SED) in association with CD11c⁺/CD8α⁻/CD11b^lo DCs, suggesting antigen uptake by these DCs in the absence of infection. Consistent with this possibility, PP DCs purified from infected mice contained σ1, but not σNS, and PP DCs from uninfected mice could not be productively infected in vitro. Furthermore, σ1 protein in the SED was associated with fragmented DNA by terminal deoxy-UTP nick-end labeling staining, activated caspase-3, and the epithelial cell protein cytokeratin, suggesting that DCs capture T1L antigen from infected apoptotic epithelial cells. Finally, PP DCs from infected mice activated T1L-primed CD4⁺ T cells in vitro. These studies show that CD8α⁻/CD11b^lo DCs in the PP SED process T1L antigen from infected apoptotic epithelial cells for presentation to CD4⁺ T cells, and therefore demonstrate the cross-presentation of virally infected cells by DCs in vivo during a natural viral infection.

Recombinant adeno-associated virus as delivery vector for gene therapy--a review

Recombinant adeno-associated virus (rAAV) is one of the most promising delivery vectors for gene therapy, due to its nonpathogenic property, nonimmunogenecity to host, and broad cell and tissue tropisms. This article summarizes the biological characteristics of AAV; the procedures to prepare, purify, and characterize the rAAV for gene therapy applications; and some of the clinical trials utilizing rAAV as delivery vehicles. Also discussed are the current efforts to modify rAAV to change its tropism, the application of different promoters to accommodate specific transgene expression, and the strategy to expand its capacity.

Immunity to adenovirus and adeno-associated viral vectors: implications for gene therapy

Viral vectors have provided effective methods for in vivo gene delivery for therapeutic purposes. The ability of viruses to infect a wide variety of cell types in vivo has been exploited for several applications, such as liver, lung, muscle, brain, eye and many others. Immune responses directed towards the viral capsids and the transgene products have severely affected the ability of these vectors to induce long-term gene expression. This paper reviews the influence of viral vectors on antigen-presenting cells (APC), which are central to the induction of innate as well as adaptive immune responses. In this respect, we have focused on adenovirus and adeno-associated viruses because of the polar responses these vector systems induce in vivo. While adenovirus vector can induce significant inflammatory responses, adeno-associated viral vectors are characterized by their inability to consistantly induce immune responses to the transgene product. Understanding the mechanism of infection, transduction and activation of APC by viral vectors will provide strategies to develop safe vectors and prevent immune responses in gene therapies.

CD4+ T cells pass through an effector phase during the process of in vivo tolerance induction

An important process in the generation of tolerance to peripheral self-Ags is the induction of unresponsiveness in mature specific T cells. Although the end stage of this process, termed anergy, is well defined, the pathway by which naive T cells become anergic remains to be elucidated. Using an in vivo self-tolerance model, we demonstrate that CD4(+) T cells pass through a significant effector stage on their way to an anergic state. This stage is characterized by production of effector cytokines, provision of help for CD8(+) T cells, and induction of in vivo pathology within organs that express cognate Ag. These results suggest that the initial activation stage in T cell tolerance is similar to that seen in memory induction. They also suggest that autoimmune pathology can result during the natural process of tolerance induction rather than requiring that tolerance be broken.

Viral interference with antigen presentation

CD8+ T cells play an important role in immunity to viruses. Just how important these cells are is demonstrated by the evolution of viral strategies for blocking the generation or display of peptide-major histocompatibility complex class I complexes on the surfaces of virus-infected cells. Here, we focus on viral interference with antigen presentation; in particular we consider the importance (and difficulty) of establishing the evolutionary significance (that is, the ability to enhance viral transmission) of viral gene products that interfere with antigen presentation in vitro.

CD4+ and CD8+ mediated cellular immune response to recombinant influenza nucleoprotein

The stimulatory properties of soluble recombinant influenza nucleoprotein (NP) on purified CD4(+) and CD8(+) T cells from young and elderly individuals were studied. Recombinant influenza NP failed to induce proliferation of resting CD4(+) and CD8(+) T cells in the absence of IL-2. Addition of small amounts of IL-2, however, led to strong proliferation of resting CD4(+) and CD8(+) T cells from young and elderly donors. NP-reactive CD4(+) and CD8(+) T cell lines from both age groups grew equally well under long-term culture conditions. T cell lines raised to live influenza virus could recognize recombinant influenza NP and showed a substantial proliferative response. Stimulation of CD8(+) T cells is presumably due to cross-presentation, as EBV-transformed MHC class I-positive cell lines, which are incapable of antigen processing, stimulated live influenza virus-reactive CD8(+) T cell lines when loaded with NP-derived immunodominant peptides but not following loading with the whole NP molecule. Vaccines containing recombinant influenza NP might confer cross-protective immunity and could therefore be especially useful in cases of major epidemics or pandemics.

Noncoding RNA danger motifs bridge innate and adaptive immunity and are potent adjuvants for vaccination

The adaptive immune response is triggered by recognition of T and B cell epitopes and is influenced by "danger" motifs that act via innate immune receptors. This study shows that motifs associated with noncoding RNA are essential features in the immune response reminiscent of viral infection, mediating rapid induction of proinflammatory chemokine expression, recruitment and activation of antigen-presenting cells, modulation of regulatory cytokines, subsequent differentiation of Th1 cells, isotype switching, and stimulation of cross-priming. The heterogeneity of RNA-associated motifs results in differential binding to cellular receptors, and specifically impacts the immune profile. Naturally occurring double-stranded RNA (dsRNA) triggered activation of dendritic cells and enhancement of specific immunity, similar to selected synthetic dsRNA motifs. Based on the ability of specific RNA motifs to block tolerance induction and effectively organize the immune defense during viral infection, we conclude that such RNA species are potent danger motifs. We also demonstrate the feasibility of using selected RNA motifs as adjuvants in the context of novel aerosol carriers for optimizing the immune response to subunit vaccines. In conclusion, RNA-associated motifs produced during viral infection bridge the early response with the late adaptive phase, regulating the activation and differentiation of antigen-specific B and T cells, in addition to a short-term impact on innate immunity.

Direct priming and cross-priming contribute differentially to the induction of CD8+ CTL following exposure to vaccinia virus via different routes

To explore the relative importance of direct presentation vs cross-priming in the induction of CTL responses to viruses and viral vectors, we generated a recombinant vaccinia vector, vUS11, expressing the human CMV (HCMV) protein US11. US11 dislocates most allelic forms of human and murine MHC class I heavy chains from the lumen of the endoplasmic reticulum into the cytosol, where they are degraded by proteasomes. Expression of US11 dramatically decreased the presentation of viral Ag and CTL recognition of infected cells in vitro without significantly reducing total cell surface MHC class I levels. However, because US11 is an endoplasmic reticulum resident membrane protein, it cannot block presentation by non-infected cells that take up Ag through the cross-priming pathway. We show that the expression of US11 strongly inhibits the induction of primary CD8(+) CTLs when the infection occurs via the i.p. or i.v. route, demonstrating that direct priming is critical for the induction of CTL responses to viral infections introduced via these routes. This effect is less dramatic following i.m. infection and is minimal after s.c. or intradermal infection. Thus, classic MHC class I Ag presentation and cross-priming contribute differentially to the induction of CD8(+) CTLs following exposure to vaccinia virus via different routes.

Inhibitory effects of cytomegalovirus proteins US2 and US11 point to contributions from direct priming and cross-priming in induction of vaccinia virus-specific CD8(+) T cells

The extent to which naive CD8(+) CTLs (T(CD8)(+)) are primed by APCs presenting endogenous Ags (direct priming) or Ags acquired from other infected cells (cross-priming) is a critical topic in basic and applied immunology. To examine the contribution of direct priming in the induction of VV-specific T(CD8)(+), we generated recombinant vaccinia viruses that express human CMV proteins (US2 and US11) that induce the destruction of newly synthesized MHC class I molecules. Expression of US2 or US11 was associated with a 24-63% decrease in numbers of primary or secondary VV-specific T(CD8)(+) responding to i.p. infection. Using HPLC-isolated peptides from VV-infected cells, we show that US2 and US11 selectively inhibit T(CD8)(+) responses to a subset of immunogenic VV determinants. Moreover, VV-US2 and lysates from VV-infected histoincompatible cells elicit T(CD8)(+) specific for a similar subset of VV determinants. These findings indicate that US2 and US11 can function in vivo to interfere with the activation of virus-specific T(CD8)(+). Furthermore, they suggest that 1) both cross-priming and direct priming contribute significantly to the generation of VV-specific T(CD8)(+), 2) the sets of immunogenic vaccinia virus determinants generated by cross-priming and direct priming are not completely overlapping, and 3) cross-priming overrides the effects of cis-acting viral interference with the class I Ag presentation pathway.

CD4 cell priming and tolerization are differentially programmed by APCs upon initial engagement

Bone marrow-derived APCs present both parenchymal-self and pathogen-derived Ags in a manner that elicits either T cell tolerization or immunity, respectively. To study the parameters that confer tolerogenic vs immunogenic APC function we used an adoptive transfer system in which naive TCR transgenic hemagglutinin (HA)-specific CD4(+) T cells are either tolerized upon encountering HA expressed constitutively as a parenchymal self-Ag (self-HA) or primed to express effector function upon encountering transiently expressed vaccinia-derived HA (viral-HA). When the duration of viral-HA presentation was extended for the period required to elicit tolerization toward self-HA, CD4 cell tolerization to viral-HA did not occur. Furthermore, CD4 cells exhibited both phenotypic as well as functional differences during early stages of tolerization and priming, suggesting that these divergent differentiation processes are programmed soon after the initial APC-CD4 cell interaction. When mice expressing self-HA were infected with an irrelevant vaccinia, CD4 cell tolerization still occurred, indicating that priming vs tolerization cannot be explained by pathogen-induced third parties (i.e., non-APCs) that act directly on CD4 cells. Taken together, these results suggest that CD4 cell tolerization to parenchymal self-Ags and priming to pathogen-derived Ags are initiated by functionally distinct APCs.

Visualizing priming of virus-specific CD8+ T cells by infected dendritic cells in vivo

The rational design of vaccines that elicit CD8+ T cell responses requires knowledge of the identity of the antigen-presenting cell (APC), the location and time of presentation and the nature of the antigen presented by the APC. Here we address these questions for an antigen encoded by a recombinant vaccinia virus. We found that, following local infection, vaccinia virus infected macrophages and dendritic cells in draining lymph nodes. However, only the dendritic cells presented antigen to naïve CD8+ T cells, as determined by direct visualization of sectioned nodes by confocal microscopy. Presentation occurred as rapidly as 6 h after inoculation and quickly declined in parallel with the number of infected cells present in the nodes. These data provide direct evidence that virus-infected APCs prime naïve CD8+ T cells in vivo.

Efficacy of recombinant adenovirus as vector for allergen gene therapy in a mouse model of type I allergy

DNA-based immunization represents an attractive alternative approach to the current treatment of allergic diseases by specific immunotherapy with allergen extracts. In this study, we used a replication-deficient adenovirus vector (AdCMV), to examine the in vivo efficacy of preventive and therapeutic genetic immunization in a mouse model of type I allergy. Primary immunization with a recombinant adenovirus expressing the model antigen beta-galactosidase (AdCMV-(beta)gal) induced a Th1 immune response (predominance of IgG2a antibodies, high frequency of IFN-gamma producing T cells) and large numbers of cytotoxic T lymphocytes. Prophylactic vaccination with AdCMV-(beta)gal abolished the production of specific IgE following subsequent immunization with (beta)gal-protein, and skewed the Th2-biased immune response to a Th1-orientated response. In contrast, therapeutic administration of AdCMV-(beta)gal after priming with (beta)gal-protein neither significantly inhibited ongoing IgE production nor modulated a manifest Th2 immune response. Thus, allergen gene transfer via recombinant adenovirus represents an effective method to establish protection against the development of allergic disorders, but does not qualify as a therapeutic tool to interfere with ongoing high IgE production.

Immune evasion by muscle-specific gene expression in dystrophic muscle

Muscle tissue from Duchenne muscular dystrophy patients and the Dmd(mdx/mdx) (hereafter referred to as mdx) mouse is characterized by an abundance of necrotic myofibers and infiltrating macrophages. Both features may provide additional stimulus to the immune response directed against novel antigens, such as those delivered by gene therapy vectors. It has previously been shown that the immune evasion achieved by adeno-associated virus in healthy muscle fails in one model of muscular dystrophy. Here, we examined the immune response to adenoviral vectors and their transgenes in normal and mdx mice. We found that mdx mouse muscles contain 20 times more macrophages and 7 times more dendritic cells than healthy muscles. This higher professional antigen-presenting cell content results in a stronger immune response to antigens that can be directly presented by those cells, including viral antigens and constitutively expressed transgene products. However, we did not detect a significant immune response to beta-galactosidase expressed specifically in muscle, even at high expression levels. This result suggests that cross-presentation is not more effective in mdx mouse muscle, and that targeted vectors and tissue-specific promoters may be useful tools for evasion of the immune response in dystrophic muscle.