Protection against Mucosal SHIV Challenge by Peptide and Helper-Dependent Adenovirus Vaccines

Adenovirus Type 6: Subtle Structural Distinctions from Adenovirus Type 5 Result in Essential Differences in Properties and Perspectives for Gene Therapy

Adenovirus vectors are the most frequently used agents for gene therapy, including oncolytic therapy and vaccine development. It’s hard to overestimate the value of adenoviruses during the COVID-19 pandemic as to date four out of four approved viral vector-based SARS-CoV-2 vaccines are developed on adenovirus platform. The vast majority of adenoviral vectors are based on the most studied human adenovirus type 5 (HAdV-C5), however, its immunogenicity often hampers the clinical translation of HAdV-C5 vectors. The search of less seroprevalent adenovirus types led to another species C adenovirus, Adenovirus type 6 (HAdV-C6). HAdV-C6 possesses high oncolytic efficacy against multiple cancer types and remarkable ability to induce the immune response towards carrying antigens. Being genetically very close to HAdV-C5, HAdV-C6 differs from HAdV-C5 in structure of the most abundant capsid protein, hexon. This leads to the ability of HAdV-C6 to evade the uptake by Kupffer cells as well as to distinct opsonization by immunoglobulins and other blood proteins, influencing the overall biodistribution of HAdV-C6 after systemic administration. This review describes the structural features of HAdV-C6, its interaction with liver cells and blood factors, summarizes the previous experiences using HAdV-C6, and provides the rationale behind the use of HAdV-C6 for vaccine and anticancer drugs developments.

Dose Effects of Recombinant Adenovirus Immunization in Rodents

Recombinant adenovirus type 5 (rAd) has been used as a vaccine platform against many infectious diseases and has been shown to be an effective vaccine vector. The dose of the vaccine varies significantly from study to study, making it very difficult to compare immune responses and vaccine efficacy. This study determined the immune correlates induced by serial dilutions of rAd vaccines delivered intramuscularly (IM) and intranasally (IN) to mice and rats. When immunized IM, mice had substantially higher antibody responses at the higher vaccine doses, whereas, the IN immunized mice showed a lower response to the higher rAd vaccine doses. Rats did not show dose-dependent antibody responses to increasing vaccine doses. The IM immunized mice and rats also showed significant dose-dependent T cell responses to the rAd vaccine. However, the T cell immunity plateaued in both mice and rats at 10⁹ and 10¹⁰ vp/animal, respectively. Additionally, the highest dose of vaccine in mice and rats did not improve the T cell responses. A final vaccine analysis using a lethal influenza virus challenge showed that despite the differences in the immune responses observed in the mice, the mice had very similar patterns of protection. This indicates that rAd vaccines induced dose-dependent immune responses, especially in IM immunized animals, and that immune correlates are not as predictive of protection as initially thought.

Comparison of systemic and mucosal immunization with replicating Single cycle Adenoviruses

HIV-1 infections occur during sexual contact at mucosal surfaces. Vaccines need to provide mucosal barrier protection and stimulate systemic immune responses to control HIV spread. Most vaccines are delivered by systemic immunization via intramuscular (IM) injection route. While this can drive systemic and mucosal immune responses, there are data show that mucosal immunization may be superior at driving responses at mucosal barriers. To explore this question, we immunized mice with replicating single-cycle adenovirus (SC Ad) vaccines expressing clade B HIV-1 envelope (Env) by intramuscular (IM), intranasal (IN), or intravaginal (IVAG) routes to compare vaccine responses. SC-Ads generated significant antibodies against Env after only a single immunization by the IN route, but not the other routes. These animals were boosted by the same route or by the mucosal IVAG routes. IM and IN primed animals generated strong antibody responses regardless of the boosting route. In contrast, IVAG primed animals failed to generate robust antibodies whether they were boosted by the IVAG or IM routes. These data suggest there may be benefits in first educating the immune system at mucosal sites during HIV vaccination. IN and IM prime-boost were then compared in Syrian hamsters which support SC-Ad DNA replication. In this case, IN immunization again was the only route that generated significant Env antibodies after a single immunization. Following a boost by IN or IM routes, IN primed animals had significantly higher antibody responses than the IM primed animals. Env antibodies could still be detected one year after immunization, but only in animals that received at least one mucosal IN immunization. These data suggest that there is merit in vaccination by mucosal routes.

Single-cycle adenovirus vectors in the current vaccine landscape

INTRODUCTION

Traditional inactivated and protein vaccines generate strong antibodies, but struggle to generate T cell responses. Attenuated pathogen vaccines generate both, but risk causing the disease they aim to prevent. Newer gene-based vaccines drive both responses and avoid the risk of infection. While these replication-defective (RD) vaccines work well in small animals, they can be weak in humans because they do not replicate antigen genes like more potent replication-competent (RC) vaccines. RC vaccines generate substantially stronger immune responses, but also risk causing their own infections. To circumvent these problems, we developed single-cycle adenovirus (SC-Ad) vectors that amplify vaccine genes, but that avoid the risk of infection. This review will discuss these vectors and their prospects for use as vaccines.

AREAS COVERED

This review provides a background of different types of vaccines. The benefits of gene-based vaccines and their ability to replicate antigen genes are described. Adenovirus vectors are discussed and compared to other vaccine types. Replication-defective, single-cycle, and replication-competent Ad vaccines are compared.

EXPERT COMMENTARY

The potential utility of these vaccines are discussed when used against infectious diseases and as cancer vaccines. We propose a move away from replication-defective vaccines towards more robust replication-competent or single-cycle vaccines.

Gene therapy with helper-dependent adenoviral vectors: lessons from studies in large animal models

Helper-dependent adenoviral vectors (HDAd) are deleted of all viral genes and they can efficiently transduce a wide variety of dividing and non-dividing cells to mediate high transgene expression levels. Unlike early generation adenoviral vectors, the absence of viral genes in HDAd results in long-term transgene expression without chronic toxicity and permits a large cloning capacity of 36 kb. Moreover, HDAd genomes exist extra-chromosomally thus minimizing the risks of germline transmission and insertional mutagenesis. For these reasons, HDAd offers tremendous potential for in vivo gene therapy. This chapter reviews preclinical studies using HDAd in large animal models to assess safety and efficacy in a wide variety of gene therapy applications.

Intranasal Vaccination Affords Localization and Persistence of Antigen-Specific CD8⁺ T Lymphocytes in the Female Reproductive Tract

Immunization strategies generating large numbers of antigen-specific T cells in the female reproductive tract (FRT) can provide barrier protection against sexually-transmitted pathogens, such as the human immunodeficiency virus (HIV) and human papillomaviruses (HPV). The kinetics and mechanisms of regulation of vaccine-induced adaptive T cell-mediated immune responses in FRT are less well defined. We present here evidence for intranasal delivery of the model antigen ovalbumin (OVA) along with alpha-galactosylceramide adjuvant as a protein vaccine to induce significantly higher levels of antigen-specific effector and memory CD8⁺ T cells in the FRT, relative to other systemic and mucosal tissues. Antibody blocking of the CXCR3 receptor significantly reduced antigen-specific CD8⁺ T cells subsequent to intranasal delivery of the protein vaccine suggesting an important role for the CXCR3 chemokine-receptor signaling for T cell trafficking. Further, intranasal vaccination with an adenoviral vector expressing OVA or HIV-1 envelope was as effective as intramuscular vaccination for generating OVA- or ENV-specific immunity in the FRT. These results support the application of the needle-free intranasal route as a practical approach to delivering protein as well as DNA/virus vector-based vaccines for efficient induction of effector and memory T cell immunity in the FRT.

Comparison of the Life Cycles of Genetically Distant Species C and Species D Human Adenoviruses Ad6 and Ad26 in Human Cells

Our understanding of adenovirus (Ad) biology is largely extrapolated from human species C Ad5. Most humans are immune to Ad5, so lower-seroprevalence viruses like human Ad6 and Ad26 are being tested as therapeutic vectors. Ad6 and Ad26 differ at the DNA level by 34%. To better understand how this might impact their biology, we examined the life cycle of the two viruses in human lung cells in vitro. Both viruses infected A549 cells with similar efficiencies, executed DNA replication with identical kinetics within 12 h, and began killing cells within 72 h. While Ad6-infected cells remained adherent until death, Ad26-infected cells detached within 12 h of infection but remained viable. Next-generation sequencing (NGS) of mRNA from infected cells demonstrated that viral transcripts constituted 1% of cellular mRNAs within 6 h and 8 to 16% within 12 h. Quantitative PCR and NGS revealed the activation of key early genes at 6 h and transition to late gene activation by 12 h by both viruses. There were marked differences in the balance of E1A and E1B activation by the two viruses and in the expression of E3 immune evasion mRNAs. Ad6 was markedly more effective at suppressing major histocompatibility complex class I (MHC I) display on the cell surface and in evading TRAIL-mediated apoptosis than was Ad26. These data demonstrate shared as well as divergent life cycles in these genetically distant human adenoviruses. An understanding of these differences expands the knowledge of alternative Ad species and may inform the selection of related Ads for therapeutic development.

IMPORTANCE

A burgeoning number of adenoviruses (Ads) are being harnessed as therapeutics, yet the biology of these viruses is generally extrapolated from Ad2 and Ad5. Here, we are the first to compare the transcriptional programs of two genetically distant Ads by mRNA next-generation sequencing (NGS). Species C Ad6 and Ad26 are being pursued as lower-seroprevalence Ad vectors but differ at the DNA level by 34%. Head-to-head comparison in human lung cells by NGS revealed that the two viruses generally conform to our general understanding of the Ad transcriptional program. However, fine mapping revealed subtle and strong differences in how these two viruses execute these programs, including differences in the balance of E1A and E1B mRNAs and in E3 immune evasion genes. This suggests that not all adenoviruses behave like Ad2 and Ad5 and that they may have unique strategies to infect cells and evade the immune system.

Enhancement of Mucosal Immunogenicity of Viral Vectored Vaccines by the NKT Cell Agonist Alpha-Galactosylceramide as Adjuvant

Gene-based vaccination strategies, specifically viral vectors encoding vaccine immunogens are effective at priming strong immune responses. Mucosal routes offer practical advantages for vaccination by ease of needle-free administration, and immunogen delivery at readily accessible oral/nasal sites to efficiently induce immunity at distant gut and genital tissues. However, since mucosal tissues are inherently tolerant for induction of immune responses, incorporation of adjuvants for optimal mucosal vaccination strategies is important. We report here the effectiveness of alpha-galactosylceramide (α-GalCer), a synthetic glycolipid agonist of natural killer T (NKT) cells, as an adjuvant for enhancing immunogenicity of vaccine antigens delivered using viral vectors by mucosal routes in murine and nonhuman primate models. Significant improvement in adaptive immune responses in systemic and mucosal tissues was observed by including α-GalCer adjuvant for intranasal immunization of mice with vesicular stomatitis virus vector encoding the model antigen ovalbumin and adenoviral vectors expressing HIV env and Gag antigens. Activation of NKT cells in systemic and mucosal tissues along with significant increases in adaptive immune responses were observed in rhesus macaques immunized by intranasal and sublingual routes with protein or adenovirus vectored antigens when combined with α-GalCer adjuvant. These results support the utility of α-GalCer adjuvant for enhancing immunogenicity of mucosal vaccines delivered using viral vectors.

Generation and evaluation of clade C simian-human immunodeficiency virus challenge stocks

The development of a panel of mucosally transmissible simian-human immunodeficiency virus (SHIV) challenge stocks from multiple virus clades would facilitate preclinical evaluation of candidate HIV-1 vaccines and therapeutics. The majority of SHIV stocks that have been generated to date have been derived from clade B HIV-1 env sequences from viruses isolated during chronic infection and typically required serial animal-to-animal adaptation for establishing mucosal transmissibility and pathogenicity. To capture essential features of mucosal transmission of clade C viruses, we produced a series of SHIVs with early clade C HIV-1 env sequences from acutely HIV-1-infected individuals from South Africa. SHIV-327c and SHIV-327cRM expressed env sequences that were 99.7 to 100% identical to the original HIV-1 isolate and did not require in vivo passaging for mucosal infectivity. These challenge stocks infected rhesus monkeys efficiently by both intrarectal and intravaginal routes, replicated to high levels during acute infection, and established chronic setpoint viremia in 13 of 17 (76%) infected animals. The SHIV-327cRM challenge stock was also titrated for both single, high-dose intrarectal challenges and repetitive, low-dose intrarectal challenges in rhesus monkeys. These SHIV challenge stocks should facilitate the preclinical evaluation of vaccines and other interventions aimed at preventing clade C HIV-1 infection.

IMPORTANCE

We describe the development of two related clade C SHIV challenge stocks. These challenge stocks should prove useful for preclinical testing of vaccines and other interventions aimed at preventing clade C HIV-1 infection.

Sublingual administration of a helper-dependent adenoviral vector expressing the codon-optimized soluble fusion glycoprotein of human respiratory syncytial virus elicits protective immunity in mice

Sublingual (s.l.) immunization has been described as a convenient and safe way to induce mucosal immune responses in the respiratory and genital tracts. We constructed a helper-dependent adenoviral (HDAd) vector expressing a condon-optimized soluble fusion glycoprotein (sFsyn) of respiratory syncytial virus (HDAd-sFsyn) and explored the potential of s.l. immunization with HDAd-sFsyn to stimulate immune responses in the respiratory mucosa. The RSV specific systemic and mucosal immune responses were generated in BALB/c mice, and the serum IgG with neutralizing activity was significantly elevated after homologous boost with s.l. application of HDAd-sFsyn. Humoral immune responses could be measured even 14weeks after a single immunization. Upon challenge, s.l. immunization with HDAd-sFsyn displayed an effective protection against RSV infection. These findings suggest that s.l. administration of HDAd-sFsyn acts as an effective and safe mucosal vaccine against RSV infection, and may be a useful tool in the prevention of RSV infection.

Vaccines within vaccines: the use of adenovirus types 4 and 7 as influenza vaccine vectors

Adenovirus Types 4 and 7 (Ad4 and Ad7) are associated with acute respiratory distress (ARD). In order to prevent widespread Ad-associated ARD (Ad-ARD) the United States military immunizes new recruits using a safe and effective lyophilized wildtype Ad4 and Ad7 delivered orally in an enteric-coated capsule. We cloned Ad4 and Ad7 and modified them to express either a GFP-Luciferase (GFPLuc) fusion gene or a centralized influenza H1 hemagglutinin (HA1-con). BALB/c mice were injected with GFPLuc expressing viruses intramuscularly (i.m.) and intranasally (i.n.). Ad4 induced significantly higher luciferase expression levels as compared with Ad7 by both routes. Ad7 transduction was restored using a human CD46+ transgenic mouse model. Mice immunized with serial dilutions of viruses expressing the HA1-con influenza vaccine gene were challenged with 100 MLD 50 of influenza virus. Ad4 protected BALB/c mice at a lower dose by i.m. immunization as compared with Ad7. Unexpectedly, there was no difference in protection by i.n. immunization. Although Ad7 i.m. transduction was restored in CD46+ transgenic mice, protection against influenza challenge required even higher doses as compared with the BALB/c mice. However, Ad7 i.n. immunized CD46+ transgenic mice were better protected as compared with Ad4. Interestingly, the restoration of Ad7 transduction in CD46+ mice did not increase vaccine efficacy and indicates that Ad7 may transduce a different subset of cells through alternative receptors in the absence of CD46. These data indicate that both Ad4 and Ad7 can effectively induce anti-H1N1 immunity against a heterologous challenge using a centralized H1 gene. Future studies in non-human primates or human clinical trials will determine the overall effectiveness of Ad4 and Ad7 as vaccines for influenza.

Comparison of systemic and mucosal immunization with helper-dependent adenoviruses for vaccination against mucosal challenge with SHIV

Most HIV-1 infections are thought to occur at mucosal surfaces during sexual contact. It has been hypothesized that vaccines delivered at mucosal surfaces may mediate better protection against HIV-1 than vaccines that are delivered systemically. To test this, rhesus macaques were vaccinated by intramuscular (i.m.) or intravaginal (ivag.) routes with helper-dependent adenoviral (HD-Ad) vectors expressing HIV-1 envelope. Macaques were first immunized intranasally with species C Ad serotype 5 (Ad5) prior to serotype-switching with species C HD-Ad6, Ad1, Ad5, and Ad2 vectors expressing env followed by rectal challenge with CCR5-tropic SHIV-SF162P3. Vaccination by the systemic route generated stronger systemic CD8 T cell responses in PBMC, but weaker mucosal responses. Conversely, mucosal immunization generated stronger CD4 T cell central memory (Tcm) responses in the colon. Intramuscular immunization generated higher levels of env-binding antibodies, but neither produced neutralizing or cytotoxic antibodies. After mucosal SHIV challenge, both groups controlled SHIV better than control animals. However, more animals in the ivag. group had lower viral set points than in in the i.m. group. These data suggest mucosal vaccination may have improve protection against sexually-transmitted HIV. These data also demonstrate that helper-dependent Ad vaccines can mediate robust vaccine responses in the face of prior immunity to Ad5 and during four rounds of adenovirus vaccination.

Intranasal immunization with a helper-dependent adenoviral vector expressing the codon-optimized fusion glycoprotein of human respiratory syncytial virus elicits protective immunity in BALB/c mice

Background

Human respiratory syncytial virus (RSV) is a serious pediatric pathogen of the lower respiratory tract. Currently, there is no clinically approved vaccine against RSV infection. Recent studies have shown that helper-dependent adenoviral (HDAd) vectors may represent effective and safe vaccine vectors. However, viral challenge has not been investigated following mucosal vaccination with HDAd vector vaccines.

Methods

To explore the role played by HDAd as an intranasally administered RSV vaccine vector, we constructed a HDAd vector encoding the codon optimized fusion glycoprotein (Fsyn) of RSV, designated HDAd-Fsyn, and delivered intranasally HDAd-Fsyn to mice.

Results

RSV-specific humoral and cellular immune responses were generated in BALB/c mice, and serum IgG with neutralizing activity was significantly elevated after a homologous boost with intranasal (i.n.) application of HDAd-Fsyn. Humoral immune responses could be measured even 14 weeks after a single immunization. Immunization with i.n. HDAd-Fsyn led to effective protection against RSV infection on challenge.

Conclusion

The results indicate that HDAd-Fsyn can induce powerful systemic immunity against subsequent i.n. RSV challenge in a mouse model and is a promising candidate vaccine against RSV infection.

High-capacity adenoviral vectors circumvent the limitations of ΔE1 and ΔE1/ΔE3 adenovirus vectors to induce multispecific transgene product-directed CD8 T-cell responses

BACKGROUND

The ability to induce cytotoxic T lymphocyte (CTL) responses that are multispecific is considered to comprise an essential feature for an efficacious genetic vaccine against many pathogens including HIV and hepatitis C virus. ΔE1Ad vectors are promising vectored vaccines but have been shown to induce antigen-specific CTLs with only limited multispecificity. In the present study, we investigated the applicability of gene-deleted high-capacity adenovirus (HC-Ad) vectors and focused on the induction of multispecific CTL responses.

METHODS

We generated Δ E1 and HC-Ad vectors expressing hepatitis B virus small surface antigen (HBsAg). We comparatively analyzed the CTL profiles against various transgene product- and vector-derived epitopes in several mouse strains and HBsAg- and vector-directed antibody responses.

RESULTS

HC-Ad vectors efficiently induced multispecific HBsAg-directed CTLs. By contrast, ΔE1Ad vectors mainly primed CTLs against one immunodominant epitope of HBsAg. This absence of multispecific CTL responses correlated with the induction of CTLs against viral epitopes generated by de novo expression of Ad genes from the ΔE1Ad vector. However, Ad-specific CTLs induced in trans did not impair HC-AdS-induced multispecific CTL responses against HBsAg. Finally, HC-Ad vectors also induced higher HBsAg antibody titers compared to ΔE1Ad vectors.

CONCLUSIONS

De novo expression of viral genes from ΔE1Ad vector genomes restricts the multispecificity of transgene product-specific CTLs by immunodominance effects. HC-Ad vectors devoid of Ad genes are favorable for the induction of both multispecific CD8 T-cell responses and high antibody responses. Our results suggest the deletion of Ad genes as an important means for developing potent Ad-based vectored vaccines.

Molecularly cloned SHIV-CN97001: a replication-competent, R5 simian/human immunodeficiency virus containing env of a primary Chinese HIV-1 clade C isolate

BACKGROUND

The increasing prevalence of human immunodeficiency virus type 1 (HIV-1) subtype C infection worldwide calls for efforts to develop a relevant animal model for evaluating AIDS candidate vaccines. In China, the prevalent HIV strains comprise a circulating recombinant form, BC (CRF07_BC), in which the envelope belongs to subtype C.

METHODS

To evaluate potential AIDS vaccines targeting Chinese viral strains in non-human primate models, we constructed a simian/human immunodeficiency virus (SHIV) carrying most of the envelope sequence of a primary HIV-1 clade C strain isolated from an HIV-positive intravenous drug user from YunNan province in China. Furthermore, to determine whether in vivo adaptation would enhance the infectivity of SHIV-CN97001, the parental infectious strain was serially passaged through eight Chinese rhesus macaques.

RESULTS

Infection of six Chinese rhesus macaques with SHIV-CN97001 resulted in a low level of viremia and no significant alteration in CD4+ T-lymphocyte counts. However, the hallmarks of SHIV infectivity developed gradually, as shown by the increasingly elevated peak viremia with each passage.

CONCLUSION

These findings establish that the R5-tropic SHIV-CN97001/Chinese rhesus macaque model should be very useful for the evaluation of HIV-1 subtype C vaccines in China.

Advances and future challenges in adenoviral vector pharmacology and targeting

Adenovirus is a robust vector for therapeutic applications, but its use is limited by our understanding of its complex in vivo pharmacology. In this review we describe the necessity of identifying its natural, widespread, and multifaceted interactions with the host since this information will be crucial for efficiently redirecting virus into target cells. In the rational design of vectors, the notion of overcoming a sequence of viral "sinks" must be combined with re-targeting to target populations with capsid as well as shielding the vectors from pre-existing or toxic immune responses. It must also be noted that most known adenoviral pharmacology is deduced from the most commonly used serotypes, Ad5 and Ad2. However, these serotypes may not represent all adenoviruses, and may not even represent the most useful vectors for all purposes. Chimeras between Ad serotypes may become useful in engineering vectors that can selectively evade substantial viral traps, such as Kupffer cells, while retaining the robust qualities of Ad5. Similarly, vectorizing other Ad serotypes may become useful in avoiding immunity against Ad5 altogether. Taken together, this research on basic adenovirus biology will be necessary in developing vectors that interact more strategically with the host for the most optimal therapeutic effect.

Characterization of species C human adenovirus serotype 6 (Ad6)

Adenovirus serotype (Ad5) is the most studied Ad. Ad1, 2, and 6 are also members of species C Ad and are presumed to have biologies similar to Ad5. In this work, we have compared the ability of Ad1, 2, 5, and 6 to infect liver and muscle after intravenous and intramuscular injection. We found that Ad6 was surprisingly the most potent at liver gene delivery and that Ad1 and Ad2 were markedly weaker than Ad5 and 6. To understand these differences, we sequenced the Ad6 genome. This revealed that the Ad6 fiber protein is surprisingly three shaft repeats shorter than the others which may explain differences in virus infectivity in vitro, but not in the liver. Comparison of hexon hypervariable regions (HVRs) suggests that the higher transduction by Ad5 and 6 as compared to Ad1 and 2 may be related to differences in charge and length.

Helper-Dependent Adenoviral Vectors

Helper-dependent adenoviral vectors are devoid of all viral coding sequences, possess a large cloning capacity, and can efficiently transduce a wide variety of cell types from various species independent of the cell cycle to mediate long-term transgene expression without chronic toxicity. These non-integrating vectors hold tremendous potential for a variety of gene transfer and gene therapy applications. Here, we review the production technologies, applications, obstacles to clinical translation and their potential resolutions, and the future challenges and unanswered questions regarding this promising gene transfer technology.

Current progress in the development of a prophylactic vaccine for HIV-1

Since its discovery and characterization in the early 1980s as a virus that attacks the immune system, there has been some success for the treatment of human immunodeficiency virus-1 (HIV-1) infection. However, due to the overwhelming public health impact of this virus, a vaccine is needed urgently. Despite the tireless efforts of scientist and clinicians, there is still no safe and effective vaccine that provides sterilizing immunity. A vaccine that provides sterilizing immunity against HIV infection remains elusive in part due to the following reasons: 1) degree of diversity of the virus, 2) ability of the virus to evade the hosts' immunity, and 3) lack of appropriate animal models in which to test vaccine candidates. There have been several attempts to stimulate the immune system to provide protection against HIV-infection. Here, we will discuss attempts that have been made to induce sterilizing immunity, including traditional vaccination attempts, induction of broadly neutralizing antibody production, DNA vaccines, and use of viral vectors. Some of these attempts show promise pending continued research efforts.

Gene therapy with helper-dependent adenoviral vectors: current advances and future perspectives

Recombinant Adenoviral vectors represent one of the best gene transfer platforms due to their ability to efficiently transduce a wide range of quiescent and proliferating cell types from various tissues and species. The activation of an adaptive immune response against the transduced cells is one of the major drawbacks of first generation Adenovirus vectors and has been overcome by the latest generation of recombinant Adenovirus, the Helper-Dependent Adenoviral (HDAd) vectors. HDAds have innovative features including the complete absence of viral coding sequences and the ability to mediate high level transgene expression with negligible chronic toxicity. This review summarizes the many aspects of HDAd biology and structure with a major focus on in vivo gene therapy application and with an emphasis on the unsolved issues that these vectors still presents toward clinical application.