Vaccination against lymphocytic choriomeningitis virus infection in MHC class II-deficient mice

Design and Immunological Validation of Macaca fascicularis Papillomavirus Type 3 Based Vaccine Candidates in Outbred Mice: Basis for Future Testing of a Therapeutic Papillomavirus Vaccine in NHPs

Persistent human papillomavirus (HPV) infections are causative for cervical neoplasia and carcinomas. Despite the availability of prophylactic vaccines, morbidity and mortality induced by HPV are still too high. Thus, an efficient therapy, such as a therapeutic vaccine, is urgently required. Herein, we describe the development and validation of Macaca fascicularis papillomavirus type 3 (MfPV3) antigens delivered via nucleic-acid and adenoviral vectors in outbred mouse models. Ten artificially fused polypeptides comprising early viral regulatory proteins were designed and optionally linked to the T cell adjuvant MHC-II-associated invariant chain. Transfected HEK293 cells and A549 cells transduced with recombinant adenoviruses expressing the same panel of artificial antigens proved proper and comparable expression, respectively. Immunization of outbred CD1 and OF1 mice led to CD8+ and CD4+ T cell responses against MfPV3 antigens after DNA- and adenoviral vector delivery. Moreover, in vivo cytotoxicity of vaccine-induced CD8+ T cells was demonstrated in BALB/c mice by quantifying specific killing of transferred peptide-pulsed syngeneic target cells. The use of the invariant chain as T cell adjuvant enhanced the T cell responses regarding cytotoxicity and in vitro analysis suggested an accelerated turnover of the antigens as causative. Notably, the fusion-polypeptide elicited the same level of T-cell responses as administration of the antigens individually, suggesting no loss of immunogenicity by fusing multiple proteins in one vaccine construct. These data support further development of the vaccine candidates in a follow up efficacy study in persistently infected Macaca fascicularis monkeys to assess their potential to eliminate pre-malignant papillomavirus infections, eventually instructing the design of an analogous therapeutic HPV vaccine.

MHC class II invariant chain-adjuvanted viral vectored vaccines enhances T cell responses in humans

Strategies to enhance the induction of high magnitude T cell responses through vaccination are urgently needed. Major histocompatibility complex (MHC) class II-associated invariant chain (Ii) plays a critical role in antigen presentation, forming MHC class II peptide complexes for the generation of CD4⁺ T cell responses. Preclinical studies evaluating the fusion of Ii to antigens encoded in vector delivery systems have shown that this strategy may enhance T cell immune responses to the encoded antigen. We now assess this strategy in humans, using chimpanzee adenovirus 3 and modified vaccinia Ankara vectors encoding human Ii fused to the nonstructural (NS) antigens of hepatitis C virus (HCV) in a heterologous prime/boost regimen. Vaccination was well tolerated and enhanced the peak magnitude, breadth, and proliferative capacity of anti-HCV T cell responses compared to non-Ii vaccines in humans. Very high frequencies of HCV-specific T cells were elicited in humans. Polyfunctional HCV-specific CD8⁺ and CD4⁺ responses were induced with up to 30% of CD3⁺CD8⁺ cells targeting single HCV epitopes; these were mostly effector memory cells with a high proportion expressing T cell activation and cytolytic markers. No volunteers developed anti-Ii T cell or antibody responses. Using a mouse model and in vitro experiments, we show that Ii fused to NS increases HCV immune responses through enhanced ubiquitination and proteasomal degradation. This strategy could be used to develop more potent HCV vaccines that may contribute to the HCV elimination targets and paves the way for developing class II Ii vaccines against cancer and other infections.

The potential of adenoviral vaccine vectors with altered antigen presentation capabilities

Introduction: Despite their appeal as vaccine vectors, adenoviral vectors are yet unable to induce protective immune responses against some weakly immunogenic antigens. Additionally, the maximum doses of adenovirus-based vaccines are limited by vector-induced toxicity, causing vector elimination and diminished immune responses against the target antigen. In order to increase immune responses to the transgene, while maintaining a moderate vector dose, new technologies for improved transgene presentation have been developed for adenoviral vaccine vectors.Areas covered: This review provides an overview of different genetic-fusion adjuvants that aim to improve antigen presentation in the context of adenoviral vector-based vaccines. The influence on both T cell and B cell responses are discussed, with a main focus on two technologies: MHC class II-associated invariant chain and virus-like-vaccines.Expert opinion: Different strategies have been tested to improve adenovirus-based vaccinations with varying degrees of success. The reviewed genetic adjuvants were designed to increase antigen processing and MHC presentation, or promote humoral immune responses with an improved conformational antigen display. While none of the introduced technologies is universally applicable, this review shall give an overview to identify potential improvements for future vaccination approaches.

Local Antigen Encounter Is Essential for Establishing Persistent CD8+ T-Cell Memory in the CNS

While the brain is considered an immune-privileged site, the CNS may nevertheless be the focus of immune mediated inflammation in the case of infection and certain autoimmune diseases, e.g., multiple sclerosis. As in other tissues, it has been found that acute T-cell infiltration may be followed by establishment of persistent local T-cell memory. To improve our understanding regarding the regulation of putative tissue resident memory T (Trm) cells in CNS, we devised a new model system for studying the generation of Trm cells in this site. To this purpose, we exploited the fact that the CNS may be a sanctuary for adenoviral infection, and to minimize virus-induced disease, we chose replication-deficient adenoviruses for infection of the CNS. Non-replicating adenoviruses are known to be highly immunogenic, and our studies demonstrate that intracerebral inoculation causes marked local T-cell recruitment, which is followed by persistent infiltration of the CNS parenchyma by antigen specific CD8⁺ T cells. Phenotypical analysis of CNS infiltrating antigen specific CD8⁺ T cells was consistent with these cells being Trms. Regarding the long-term stability of the infiltrate, resident CD8⁺ T cells expressed high levels of the anti-apoptotic molecule Bcl-2 as well as the proliferation marker Ki-67 suggesting that the population is maintained through steady homeostatic proliferation. Functionally, memory CD8⁺ T cells from CNS matched peripheral memory cells with regard to capacity for ex vivo cytotoxicity and cytokine production. Most importantly, our experiments revealed a key role for local antigen encounter in the establishment of sustained CD8⁺ T-cell memory in the brain. Inflammation in the absence of cognate antigen only led to limited and transient infiltration by antigen specific CD8⁺ T cells. Together these results indicate that memory CD8⁺ T cells residing in the CNS predominantly mirror previous local infections and immune responses to local autoantigens.

Development of a Molecular Adjuvant to Enhance Antigen-Specific CD8+ T Cell Responses

Despite promising progress in malaria vaccine development, an efficacious subunit vaccine against P. falciparum remains to be licensed and deployed. This study aimed to improve on the immunogenicity of the leading liver-stage vaccine candidate (ChAd63-MVA ME-TRAP), known to confer protection by eliciting high levels of antigen-specific CD8⁺ T cells. We previously showed fusion of ME-TRAP to the human MHC class II invariant chain (Ii) could enhance CD8⁺ T cell responses in non-human primates, but did not progress to clinical testing due to potential risk of auto-immunity by vaccination of humans with a self-antigen. Initial immunogenicity analyses of ME-TRAP fused to subdomains of the Ii showed that the Ii transmembrane domain alone can enhance CD8⁺ T cell responses. Subsequently, truncated Ii sequences with low homology to human Ii were developed and shown to enhance CD8⁺ T cell responses. By systematically mutating the TM domain sequence, multimerization of the Ii chain was shown to be important for immune enhancement. We subsequently identified several proteins from a variety of microbial pathogens with similar characteristics, that also enhance the CD8⁺ T cell response and could therefore be used in viral vector vaccines when potent cell mediated immunity is required.

Mucosal Vaccination with Heterologous Viral Vectored Vaccine Targeting Subdominant SIV Accessory Antigens Strongly Inhibits Early Viral Replication

Conventional HIV T cell vaccine strategies have not been successful in containing acute peak viremia, nor in providing long-term control. We immunized rhesus macaques intramuscularly and rectally using a heterologous adenovirus vectored SIV vaccine regimen encoding normally weakly immunogenic tat, vif, rev and vpr antigens fused to the MHC class II associated invariant chain. Immunizations induced broad T cell responses in all vaccinees. Following up to 10 repeated low-dose intrarectal challenges, vaccinees suppressed early viral replication (P = 0.01) and prevented the peak viremia in 5/6 animals. Despite consistently undetectable viremia in 2 out of 6 vaccinees, all animals showed evidence of infection induced immune responses indicating that infection had taken place. Vaccinees, with and without detectable viremia better preserved their rectal CD4 + T cell population and had reduced immune hyperactivation as measured by naïve T cell depletion, Ki-67 and PD-1 expression on T cells. These results indicate that vaccination towards SIV accessory antigens vaccine can provide a level of acute control of SIV replication with a suggestion of beneficial immunological consequences in infected animals of unknown long-term significance.

In conclusion, our studies demonstrate that a vaccine encoding subdominant antigens not normally associated with virus control can exert a significant impact on acute peak viremia.

•

Mucosal heterologousvirus-vectored vaccine used with MHC class II associated invariant chain linked SIV accessory antigens

•

Mucosal vaccination targeting subdominant antigens delay SIV mac251 replication in rhesus macaques.

•

Longterm reduction of immune hyperactivation following SIV infection of vaccinated macaques.

Mucosal immunization is used with heterologous viral vectors and a genetic adjuvant to raise responses against poorly immunogenic SIV antigens. Following repeated low-dose challenge we observed delayed establishment of chronic phase viremia and reduced immune hyperactivation 6 months after established infection. Vaccination was found to strongly reduce viremia at early, but not late time points, after detected infection and in 2 out of 6 animals infection could only observed as virus induced T cell responses. Subdominant antigen vaccines may thus be used to delay SIV mac251 infection and can enable control of chronic viremia in a minority of cases.

PB1 as a potential target for increasing the breadth of T-cell mediated immunity to Influenza A

Recently, we showed that combined intranasal and subcutaneous immunization with a non-replicating adenoviral vector expressing NP of influenza A, strain PR8, induced long-standing protection against a range of influenza A viruses. However, H-2^b mice challenged with an influenza A strain mutated in the dominant NP₃₆₆ epitope were not efficiently protected. To address this problem, we envision the use of a cocktail of adenovectors targeting different internal proteins of influenza A virus. Consequently, we investigated the possibility of using PB1 as a target for an adenovector-based vaccine against influenza A. Our results showed that PB1 is not as immunogenic as the NP protein. However, by tethering PB1 to the murine invariant chain we were able to circumvent this problem and raise quite high numbers of PB1-specific CD8⁺ T cells in the circulation. Nevertheless, mice immunized against PB1 were not as efficiently protected against influenza A challenge as similarly NP-vaccinated animals. The reason for this is not a difference in the quality of the primed cells, nor in functional avidity. However, under similar conditions of immunization fewer PB1-specific cells were recruited to the airways, and surface expression of the dominant PB1 peptide, PB1₇₀₃, was less stable than in the case of NP₃₆₆.

Immediate Dysfunction of Vaccine-Elicited CD8+ T Cells Primed in the Absence of CD4+ T Cells

CD4(+) T cell help is critical for optimal CD8(+) T cell memory differentiation and maintenance in many experimental systems. In addition, many reports have identified reduced primary CD8(+) T cell responses in the absence of CD4(+) T cell help, which often coincides with reduced Ag or pathogen clearance. In this study, we demonstrate that absence of CD4(+) T cells at the time of adenovirus vector immunization of mice led to immediate impairments in early CD8(+) T cell functionality and differentiation. Unhelped CD8(+) T cells exhibited a reduced effector phenotype, decreased ex vivo cytotoxicity, and decreased capacity to produce cytokines. This dysfunctional state was imprinted within 3 d of immunization. Unhelped CD8(+) T cells expressed elevated levels of inhibitory receptors and exhibited transcriptomic exhaustion and anergy profiles by gene set enrichment analysis. Dysfunctional, impaired effector differentiation also occurred following immunization of CD4(+) T cell-deficient mice with a poxvirus vector. This study demonstrates that following priming with viral vectors, CD4(+) T cell help is required to promote both the expansion and acquisition of effector functions by CD8(+) T cells, which is accomplished by preventing immediate dysfunction.

Vaccine Targeting of Subdominant CD8+ T Cell Epitopes Increases the Breadth of the T Cell Response upon Viral Challenge, but May Impair Immediate Virus Control

As a result of the difficulties in making efficient vaccines against genetically unstable viruses such as HIV, it has been suggested that future vaccines should preferentially target subdominant epitopes, the idea being that this should allow a greater breadth of the induced T cell response and, hence, a greater efficiency in controlling escape variants. However, to our knowledge the evidence supporting this concept is limited at best. To improve upon this, we used the murine lymphocytic choriomeningitis virus model and adenoviral vectors to compare a vaccine expressing unmodified Ag to a vaccine expressing the same Ag without its immunodominant epitope. We found that removal of the dominant epitope allowed the induction of CD8(+) T cell responses targeting at least two otherwise subdominant epitopes. Importantly, the overall magnitude of the induced T cell responses was similar, allowing us to directly compare the efficiency of these vaccines. Doing this, we observed that mice vaccinated with the vaccine expressing unmodified Ag more efficiently controlled an acute viral challenge. In the course of a more chronic viral infection, mice vaccinated using the vaccine targeting subdominant epitopes caught up with the conventionally vaccinated mice, and analysis of the breadth of the CD8(+) T cell response revealed that this was notably greater in the former mice. However, under the conditions of our studies, we never saw any functional advantage of this. This may represent a limitation of our model, but clearly our findings underscore the importance of carefully weighing the pros and cons of changes in epitope targeting before any implementation.

Targeting of non-dominant antigens as a vaccine strategy to broaden T-cell responses during chronic viral infection

In this study, we compared adenoviral vaccine vectors with the capacity to induce equally potent immune responses against non-dominant and immunodominant epitopes of murine lymphocytic choriomeningitis virus (LCMV). Our results demonstrate that vaccination targeting non-dominant epitopes facilitates potent virus-induced T-cell responses against immunodominant epitopes during subsequent challenge with highly invasive virus. In contrast, when an immunodominant epitope was included in the vaccine, the T-cell response associated with viral challenge remained focussed on that epitope. Early after challenge with live virus, the CD8+ T cells specific for vaccine-encoded epitopes, displayed a phenotype typically associated with prolonged/persistent antigenic stimulation marked by high levels of KLRG-1, as compared to T cells reacting to epitopes not included in the vaccine. Notably, this association was lost over time in T cells specific for the dominant T cell epitopes, and these cells were fully capable of expanding in response to a new viral challenge. Overall, our data suggests a potential for broadening of the antiviral CD8+ T-cell response by selecting non-dominant antigens to be targeted by vaccination. In addition, our findings suggest that prior adenoviral vaccination is not likely to negatively impact the long-term and protective immune response induced and maintained by a vaccine-attenuated chronic viral infection.

Co-expression of tumor antigen and interleukin-2 from an adenoviral vector augments the efficiency of therapeutic tumor vaccination

We have previously shown that for the majority of antigens, adenoviral vaccines expressing the target antigen fused to the MHC associated invariant chain (Ii) induce an accelerated, augmented, and prolonged transgene-specific CD8(+) T-cell response. Here we describe a new adenoviral vaccine vector approach where the target antigen fused to Ii is expressed from the adenoviral E1 region and IL-2 is expressed from the E3 region. Immunization of mice with this new vector construct resulted in an augmented primary effector CD8(+) T-cell response. Furthermore, in a melanoma model we observed significantly prolonged tumor control in vaccinated wild type (WT) mice. The improved tumor control required antigen-specific cells, since no tumor control was observed, unless the melanoma cells expressed the vaccine targeted antigen. We also tested our new vaccine in immunodeficient (CD80/86 deficient) mice. Following vaccination with the IL-2 expressing construct, these mice were able to raise a delayed but substantial CD8(+) T-cell response, and to control melanoma growth nearly as efficaciously as similarly vaccinated WT mice. Taken together, these results demonstrate that current vaccine vectors can be improved and even tailored to meet specific demands: in the context of therapeutic vaccination, the capacity to promote an augmented effector T-cell response.

Priming of CD8 T cells by adenoviral vectors is critically dependent on B7 and dendritic cells but only partially dependent on CD28 ligation on CD8 T cells

Adenoviral vectors have long been forerunners in the development of effective CD8 T cell-based vaccines; therefore, it is imperative that we understand the factors controlling the induction of robust and long-lasting transgene-specific immune responses by these vectors. In this study, we investigated the organ sites, molecules, and cell subsets that play a critical role in the priming of transgene-specific CD8 T cells after vaccination with a replication-deficient adenoviral vector. Using a human adenovirus serotype 5 (Ad5) vector and genetically engineered mice, we found that CD8(+) and/or CD103(+) dendritic cells in the draining lymph node played a critical role in the priming of Ad5-induced CD8 T cell responses. Moreover, we found that CD80/86, but not CD28, was essential for efficient generation of both primary effectors and memory CD8 T cells. Interestingly, the lack of CD28 expression resulted in a delayed primary response, whereas memory CD8 T cells generated in CD28-deficient mice appeared almost normal in terms of both phenotype and effector cytokine profile, but they exhibited a significantly reduced proliferative capacity upon secondary challenge while retaining immediate in vivo effector capabilities: in vivo cytotoxicity and short-term in vivo protective capacity. Overall, our data point to an absolute requirement for professional APCs and the expression of the costimulatory molecules CD80/86 for efficient CD8 T cell priming by adenoviral vectors. Additionally, our results suggest the existence of an alternative receptor for CD80/86, which may substitute, in part, for CD28.

Enhanced vaccine-induced CD8+ T cell responses to malaria antigen ME-TRAP by fusion to MHC class ii invariant chain

The orthodox role of the invariant chain (CD74; Ii) is in antigen presentation to CD4+ T cells, but enhanced CD8+ T cells responses have been reported after vaccination with vectored viral vaccines encoding a fusion of Ii to the antigen of interest. In this study we assessed whether fusion of the malarial antigen, ME-TRAP, to Ii could increase the vaccine-induced CD8+ T cell response. Following single or heterologous prime-boost vaccination of mice with a recombinant chimpanzee adenovirus vector, ChAd63, or recombinant modified vaccinia virus Ankara (MVA), higher frequencies of antigen-specific CD4+ and CD8+ T cells were observed, with the largest increases observed following a ChAd63-MVA heterologous prime-boost regimen. Studies in non-human primates confirmed the ability of Ii-fusion to augment the T cell response, where a 4-fold increase was maintained up to 11 weeks after the MVA boost. Of the numerous different approaches explored to increase vectored vaccine induced immunogenicity over the years, fusion to the invariant chain showed a consistent enhancement in CD8+ T cell responses across different animal species and may therefore find application in the development of vaccines against human malaria and other diseases where high levels of cell-mediated immunity are required.

Fusion of HCV nonstructural antigen to MHC class II-associated invariant chain enhances T-cell responses induced by vectored vaccines in nonhuman primates

Despite viral vectors being potent inducers of antigen-specific T cells, strategies to further improve their immunogenicity are actively pursued. Of the numerous approaches investigated, fusion of the encoded antigen to major histocompatibility complex class II-associated invariant chain (Ii) has been reported to enhance CD8(+) T-cell responses. We have previously shown that adenovirus vaccine encoding nonstructural (NS) hepatitis C virus (HCV) proteins induces potent T-cell responses in humans. However, even higher T-cell responses might be required to achieve efficacy against different HCV genotypes or therapeutic effect in chronically infected HCV patients. In this study, we assessed fusion of the HCV NS antigen to murine and human Ii expressed by the chimpanzee adenovirus vector ChAd3 or recombinant modified vaccinia Ankara in mice and nonhuman primates (NHPs). A dramatic increase was observed in outbred mice in which vaccination with ChAd3 expressing the fusion antigen resulted in a 10-fold increase in interferon-γ(+) CD8(+) T cells. In NHPs, CD8(+) T-cell responses were enhanced and accelerated with vectors encoding the Ii-fused antigen. These data show for the first time that the enhancement induced by vector vaccines encoding li-fused antigen was not species specific and can be translated from mice to NHPs, opening the way for testing in humans.

Detection of Foreign Antigen-specific CD4(+)Foxp3(+) Regulatory T Cells by MHC Class II Tetramer and Intracellular CD154 Staining

The unrestricted population of CD4(+)Foxp3(+) regulatory T (Treg) cells, which have been known to control the expression of autoimmune diseases and protective immunity to inflammatory reactions, has led to greater appreciation of functional plasticity. Detecting and/or isolating Ag-specific CD4(+)Foxp3(+) Tregs at the single cell level are required to study their function and plasticity. In this study, we established and compared both MHC class II tetramer and intracellular CD154 staining, in order to detect CD4(+)Foxp3(+) Treg specific for foreign Ag in acute and chronic infections with lymphocytic choriomeningitis virus (LCMV). Our results revealed that MHC class II tetramer staining showed a lower detection rate of LCMV GP66-77-specific CD4(+) T cells because most of MHC class II tetramers were unbound and unstable when combined staining was performed with intracellular cytokines. In contrast, intracellular CD154 staining was revealed to be easier and simple for detecting LCMV GP66-77-specific CD4(+) T cells, compared to MHC class II tetramer staining. Subsequently, we employed intracellular CD154 staining to detect LCMV GP66-77-specific CD4(+)Foxp3(+) Tregs using Foxp3(GFP) knock-in mouse, and found that LCMV GP66-77-specific CD4(+)Foxp3(+) Tregs and polyclonal CD4(+)Foxp3(+) Tregs showed differential expansion in mice infected with LCMV Arms or Cl13 at acute (8 and 13 days pi) and chronic phases (35 days pi). Therefore, our results provide insight into the valuable use of intracellular CD154 staining to detect and characterize foreign Ag-specific CD4(+)Foxp3(+) Treg in various models.

CD154 and IL-2 signaling of CD4+ T cells play a critical role in multiple phases of CD8+ CTL responses following adenovirus vaccination

Adenoviral (AdV) vectors represent most commonly utilized viral vaccines in clinical studies. While the role of CD8⁺ cytotoxic T lymphocyte (CTL) responses in mediating AdV-induced protection is well understood, the involvement of CD4⁺ T cell-provided signals in the development of functional CD8⁺ CTL responses remain unclear. To explore CD4⁺ T helper signals required for AdVova-stimulated CTL responses, we established an adoptive transfer system by transferring CD4⁺ T cells derived from various knock out and transgenic mice into wild-type and/or CD4-deficient animals, followed by immunizing with recombinant ovalbumin (OVA)-expressing AdVova vector. Without CD4⁺ T help, both primary and memory CTL responses were greatly reduced in this model, and were associated with increased PD-1 expression. The provision of OVA-specific CD4⁺ T help in CD4⁺ T cell-deficient mice restored AdVova-induced primary CTL responses, and supported survival and recall responses of AdVova-stimulated memory CTLs. These effects were specifically mediated by CD4⁺ T cell-produced IL-2 and CD154 signals. Adoptive transfer of “helped” or “unhelped” effector and memory CTLs into naïve CD4⁺ T cell-deficient or -sufficient mice also revealed an additional role for polyclonal CD4⁺ T cell environment in the survival of AdVova-stimulated CTLs, partially explaining the extension of CTL contraction phase. Finally, during recall responses, CD4⁺ T cell environment, particularly involving memory CD4⁺ T cells, greatly enhanced expansion of memory CTLs. Collectively, our data strongly suggest a critical role for CD4⁺ T help in multiple phases of AdV-stimulated CTL responses, and could partially explain certain failures in AdV-based immunization trials targeting malignant tumors and chronic diseases that are often associated with compromised CD4⁺ T cell population and function.

Pre-existing vector immunity does not prevent replication deficient adenovirus from inducing efficient CD8 T-cell memory and recall responses

Adenoviral vectors have shown a great potential for vaccine development due to their inherent ability to induce potent and protective CD8 T-cell responses. However, a critical issue regarding the use of these vectors is the existence of inhibitory immunity against the most commonly used Ad5 vector in a large part of the human population. We have recently developed an improved adenoviral vaccine vector system in which the vector expresses the transgene tethered to the MHC class II associated invariant chain (Ii). To further evaluate the potential of this system, the concept of pre-existing inhibitory immunity to adenoviral vectors was revisited to investigate whether the inhibition previously seen with the Ad5 vector also applied to the optimized vector system. We found this to be the case, and antibodies dominated as the mechanism underlying inhibitory vector immunity. However, presence of CD8 T cells directed against epitopes in the adenoviral vector seemed to correlate with repression of the induced response in re-vaccinated B-cell deficient mice. More importantly, despite a repressed primary effector CD8 T-cell response in Ad5-immune animals subjected to vaccination, memory T cells were generated that provided the foundation for an efficient recall response and protection upon subsequent viral challenge. Furthermore, the transgene specific response could be efficiently boosted by homologous re-immunization. Taken together, these studies indicate that adenoviral vectors can be used to induce efficient CD8 T-cell memory even in individuals with pre-existing vector immunity.

Engineered Viruses as Vaccine Platforms

Many viruses have been investigated for the development of genetic vaccines and the ideal ones must be endowed with many properties, such as the quality and the quantity of the immunological response induced against the encoded antigens, safety and production on a large scale basis. Viral based vaccines must also deal with the potential problem of the pre-existing antivector immunity. Several viral vaccine vectors have emerged to date, all of them having relative advantages and limits depending on the proposed application. Recent successes reflect diverse improvements such as development of new adenovirus serotypes and prime-boost regimes. This chapter describes the features of four viral vector systems based on poxviruses, adenoviruses, alphaviruses and lentiviruses and recent results following their use with a particular emphasis on clinical research, highlighting the challenges and successes.