Replication-attenuated Human Adenoviral Type 4 vectors elicit capsid dependent enhanced innate immune responses that are partially dependent upon interactions with the complement system

Two Novel Adenovirus Vectors Mediated Differential Antibody Responses via Interferon-α and Natural Killer Cells

Recombinant adenovirus vectors have been widely used in vaccine development. To overcome the preexisting immunity of human adenovirus type 5 (Ad5) in populations, a range of chimpanzee or rare human adenovirus vectors have been generated. However, these novel adenovirus vectors mediate the diverse immune responses in the hosts. In this study, we explored the immune mechanism of differential antibody responses to SARS-CoV-2 S protein in mice immunized by our previously developed two novel simian adenovirus type 23 (Sad23L) and human adenovirus type 49 (Ad49L), and Ad5 vectored COVID-19 vaccines. Sad23L-nCoV-S and Ad5-nCoV-S vaccines induced the low level of interferon-α (IFN-α) and the high level of antigen-specific antibody responses in wild-type and IFN-α/β receptor defective (IFNAR-/-) C57 mice, while Ad49L-nCoV-S vaccine induced the high IFN-α and low antibody responses in C57 mice but the high antibody response in IFNAR-/- mice. In addition, the high antibody response was detected in natural killer (NK) cells-blocked but the low in follicular helper T (TFH) cells -blocked C57 mice immunized with Ad49L-nCoV-S vaccine. These results showed that Ad49L vectored vaccine stimulated IFN-α secretion to activate NK cells, and then reduced the number of TFH cells, generation center (GC) B cells and plasma cells, and subsequently reduced antigen-specific antibody production. The different novel adenovirus vectors could be selected for vaccine development according to the need for either humoral or cellular or both immune protections against a particular disease. IMPORTANCE Novel adenovirus vectors are an important antigen delivery platform for vaccine development. Understanding the immune diversity between different adenoviral vectors is critical to design the proper vaccine against an aim disease. In this study, we described the immune mechanism of Sad23L and Ad49L vectored vaccines for raising the equally high specific T cell response but the different level of specific antibody responses in mice. We found that Ad49L-vectored vaccine initiated the high IFN-α and activated NK cells to inhibit antibody response via downregulating the number of CD4+ TFH cells leading to the decline of GC B cells and plasma cells.

Prime-boost vaccination of mice and rhesus macaques with two novel adenovirus vectored COVID-19 vaccine candidates

ABSTRACTCOVID-19 vaccines are being developed urgently worldwide. Here, we constructed two adenovirus vectored COVID-19 vaccine candidates of Sad23L-nCoV-S and Ad49L-nCoV-S carrying the full-length gene of SARS-CoV-2 spike protein. The immunogenicity of two vaccines was individually evaluated in mice. Specific immune responses were observed by priming in a dose-dependent manner, and stronger responses were obtained by boosting. Furthermore, five rhesus macaques were primed with 5 × 10⁹ PFU Sad23L-nCoV-S, followed by boosting with 5 × 10⁹ PFU Ad49L-nCoV-S at 4-week interval. Both mice and macaques well tolerated the vaccine inoculations without detectable clinical or pathologic changes. In macaques, prime-boost regimen induced high titers of 10^3.16 anti-S, 10^2.75 anti-RBD binding antibody and 10^2.38 pseudovirus neutralizing antibody (pNAb) at 2 months, while pNAb decreased gradually to 10^1.45 at 7 months post-priming. Robust T-cell response of IFN-γ (712.6 SFCs/10⁶ cells), IL-2 (334 SFCs/10⁶ cells) and intracellular IFN-γ in CD4⁺/CD8⁺ T cell (0.39%/0.55%) to S peptides were detected in vaccinated macaques. It was concluded that prime-boost immunization with Sad23L-nCoV-S and Ad49L-nCoV-S can safely elicit strong immunity in animals in preparation of clinical phase 1/2 trials.

Characterization of a Species E Adenovirus Vector as a Zika virus vaccine

The development of a safe and efficacious Zika virus (ZIKV) vaccine remains a global health priority. In our previous work, we developed an Adenovirus vectored ZIKV vaccine using a low-seroprevalent human Adenovirus type 4 (Ad4-prM-E) and compared it to an Ad5 vector (Ad5-prM-E). We found that vaccination with Ad4-prM-E leads to the development of a strong anti-ZIKV T-cell response without eliciting significant anti-ZIKV antibodies, while vaccination with Ad5-prM-E leads to the development of both anti-ZIKV antibody and T-cell responses in C57BL/6 mice. However, both vectors conferred protection against ZIKV infection in a lethal challenge model. Here we continued to characterize the T-cell biased immune response observed in Ad4 immunized mice. Vaccination of BALB/c mice resulted in immune correlates similar to C57BL/6 mice, confirming that this response is not mouse strain-specific. Vaccination with an Ad4 expressing an influenza hemagglutinin (HA) protein resulted in anti-HA T-cell responses without the development of significant anti-HA antibodies, indicating this unique response is specific to the Ad4 serotype rather than the transgene expressed. Co-administration of a UV inactivated Ad4 vector with the Ad5-prM-E vaccine led to a significant reduction in anti-ZIKV antibody development suggesting that this serotype-specific immune profile is capsid-dependent. These results highlight the serotype-specific immune profiles elicited by different Adenovirus vector types and emphasize the importance of continued characterization of these alternative Ad serotypes.

New Insights to Adenovirus-Directed Innate Immunity in Respiratory Epithelial Cells

The nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) family of transcription factors is a key component of the host innate immune response to infectious adenoviruses and adenovirus vectors. In this review, we will discuss a regulatory adenoviral protein encoded by early region 3 (E3) called E3-RIDα, which targets NFκB through subversion of novel host cell pathways. E3-RIDα down-regulates an EGF receptor signaling pathway, which overrides NFκB negative feedback control in the nucleus, and is induced by cell stress associated with viral infection and exposure to the pro-inflammatory cytokine TNF-α. E3-RIDα also modulates NFκB signaling downstream of the lipopolysaccharide receptor, Toll-like receptor 4, through formation of membrane contact sites controlling cholesterol levels in endosomes. These innate immune evasion tactics have yielded unique perspectives regarding the potential physiological functions of host cell pathways with important roles in infectious disease.

Genetic strategy to decrease complement activation with adenoviral therapies

Background

A major obstacle to using recombinant adenoviral vectors in gene therapy is the natural ability of human adenovirus to activate the classical and alternate complement pathways. These innate immune responses contribute to hepatic adenoviral uptake following systemic delivery and enhance the humoral immune responses associated with adenoviral infection.

Methods

A recombinant Ad5 vector was genetically modified to display a peptide sequence (“rH17d’”), a known inhibitor of the classical complement pathway. The replication-defective vectors Ad5.HVR2-rH17d’ and Ad5.HVR5-rH17d’ were constructed by engineering the rH17d’ peptide into the hypervariable region (HVR)-2 or HVR5 of their major capsid protein hexon. Control Ad5 vectors were created by incorporation of a 6-histidine (His₆)-insert in either HVR2 or HVR5 (Ad5.HVR2-His₆ and Ad5.HVR5-His₆, respectively). All vectors encoded CMV promoter-controlled firefly luciferase (Luc). The four vectors were evaluated in TIB76 mouse liver cells and immunocompetent mice to compare infectivity and liver sequestration, respectively.

Results

In vitro studies demonstrated that preincubation of all the Ad5 vectors with fresh serum significantly increased their gene transfer relative to preincubation with PBS except Ad5.HVR5-rH17d’, whose infectivity of liver cells showed no serum-mediated enhancement. In line with that, mice injected with Ad5.HVR2-rH17d’ or Ad5.HVR5-rH17d’ showed significantly lower luciferase expression levels in the liver as compared to the respective control vectors, whereas efficiency of tumor transduction by rH17d’ and His₆ vectors following their intratumoral injection was similar.

Conclusions

Displaying a complement-inhibiting peptide on the Ad5 capsid surface by genetic modification of the hexon protein could be a suitable strategy for reducing Ad5 liver tropism (Ad5 sequestration by liver), which may be applicable to other gene therapy vectors with natural liver tropism.

Bcl-2 Overexpression Improves Survival and Efficacy of Neural Stem Cell-Mediated Enzyme Prodrug Therapy

Tumor-tropic neural stem cells (NSCs) can be engineered to localize gene therapies to invasive brain tumors. However, like other stem cell-based therapies, survival of therapeutic NSCs after transplantation is currently suboptimal. One approach to prolonging cell survival is to transiently overexpress an antiapoptotic protein within the cells prior to transplantation. Here, we investigate the utility and safety of this approach using a clinically tested, v-myc immortalized, human NSC line engineered to contain the suicide gene, cytosine deaminase (CD-NSCs). We demonstrate that both adenoviral- and minicircle-driven expression of the antiapoptotic protein Bcl-2 can partially rescue CD-NSCs from transplant-associated insults. We further demonstrate that the improved CD-NSC survival afforded by transient Bcl-2 overexpression results in decreased tumor burden in an orthotopic xenograft glioma mouse model following administrations of intracerebral CD-NSCs and systemic prodrug. Importantly, no evidence of CD-NSC transformation was observed upon transient overexpression of Bcl-2. This research highlights a critical need to develop clinically relevant strategies to improve survival of therapeutic stem cell posttransplantation. We demonstrate for the first time in this disease setting that improving CD-NSC survival using Bcl-2 overexpression can significantly improve therapeutic outcomes.

Cell-mediated enzyme prodrug cancer therapies

Cell-directed gene therapy is a promising new frontier for the field of targeted cancer therapies. Here we discuss the current pre-clinical and clinical use of cell-mediated enzyme prodrug therapy (EPT) directed against solid tumors and avenues for further development. We also discuss some of the challenges encountered upon translating these therapies to clinical trials. Upon sufficient development, cell-mediated enzyme prodrug therapy has the potential to maximize the distribution of therapeutic enzymes within the tumor environment, localizing conversion of prodrug to active drug at the tumor sites thereby decreasing off-target toxicities. New combinatorial possibilities are also promising. For example, when combined with viral gene-delivery vehicles, this may result in new hybrid vehicles that attain heretofore unmatched levels of therapeutic gene expression within the tumor.

Exploitation of viral properties for intracellular delivery

Nanotechnology is an expanding area of study with potentially pivotal applications in a discipline as medicine where new biomedical active molecules or strategies are continuously developing. One of the principal drawbacks for the application of new therapies is the difficulty to cross membranes that represent the main physiological barrier in our body and in all living cells. Membranes are selectively permeable and allow the selective internalization of substances; generally, they form a highly impermeable barrier to most polar and charged molecules, and represent an obstacle for drug delivery, limiting absorption to specific routes and mechanisms. Viruses provide attracting suggestions for the development of targeted drug carriers as they have evolved naturally to deliver their genomes to host cells with high fidelity.

A detailed understanding of virus structure and their mechanisms of entry into mammalian cells will facilitate the development and analysis of virus‐based materials for medical applications. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.

CD46-mediated transduction of a species D adenovirus vaccine improves mucosal vaccine efficacy

The high levels of preexisting immunity against Adenovirus type 5 (Ad5) have deemed Ad5 unusable for translation as a human vaccine vector. Low seroprevalent alternative viral vectors may be less impacted by preexisting immunity, but they may also have significantly different phenotypes from that of Ad5. In this study we compare species D Ads (26, 28, and 48) to the species C Ad5. In vitro transduction studies show striking differences between the species C and D viruses. Most notably, Ad26 transduced human dendritic cells much more effectively than Ad5. In vivo imaging studies showed strikingly different transgene expression profiles. The Ad5 virus was superior to the species D viruses in BALB/c mice when delivered intramuscularly. However, the inverse was true when the viruses were delivered mucosally via the intranasal epithelia. Intramuscular transduction was restored in mice that ubiquitously expressed human CD46, the primary receptor for species D viruses. We analyzed both species C and D Ads for their ability to induce prophylactic immunity against influenza in the CD46 transgenic mouse model. Surprisingly, the species D vaccines again failed to induce greater levels of protective immunity as compared with the species C Ad5 when delivered intramuscularly. However, the species D Ad vaccine vector, Ad48, induced significantly greater protection as compared with Ad5 when delivered mucosally via the intranasal route in CD46 transgenic mice. These data shed light on the complexities between the species and types of Ad. Our findings indicate that more research will be required to identify the mechanisms that play a key role in the induction of protective immunity induced by species D Ad vaccines.

Adenovirus-Based Vectors for the Development of Prophylactic and Therapeutic Vaccines

Emerging and reemerging infectious diseases as well as cancer pose great global health impacts on the society. Vaccines have emerged as effective treatments to prevent or reduce the burdens of already developed diseases. This is achieved by means of activating various components of the immune system to generate systemic inflammatory reactions targeting infectious agents or diseased cells for control/elimination. DNA virus-based genetic vaccines gained significant attention in the past decades owing to the development of DNA manipulation technologies, which allowed engineering of recombinant viral vectors encoding sequences for foreign antigens or their immunogenic epitopes as well as various immunomodulatory molecules. Despite tremendous progress in the past 50 years, many hurdles still remain for achieving the full clinical potential of viral-vectored vaccines. This chapter will present the evolution of vaccines from “live” or “attenuated” first-generation agents to recombinant DNA and viral-vectored vaccines. Particular emphasis will be given to human adenovirus (Ad) for the development of prophylactic and therapeutic vaccines. Ad biological properties related to vaccine development will be highlighted along with their advantages and potential hurdles to be overcome. In particular, we will discuss (1) genetic modifications in the Ad capsid protein to reduce the intrinsic viral immunogenicity, (2) antigen capsid incorporation for effective presentation of foreign antigens to the immune system, (3) modification of the hexon and fiber capsid proteins for Ad liver de-targeting and selective retargeting to cancer cells, (4) Ad-based vaccines carrying “arming” transgenes with immunostimulatory functions as immune adjuvants, and (5) oncolytic Ad vectors as a new therapeutic approach against cancer. Finally, the combination of adenoviral vectors with other non-adenoviral vector systems, the prime/boost strategy of immunization, clinical trials involving Ad-based vaccines, and the perspectives for the field development will be discussed.

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.

Short-term supplementation with active hexose correlated compound improves the antibody response to influenza B vaccine

Administration of bioactive nutritional supplements near or at the time of immunization has been a recent approach to stimulate human immune response to vaccination. Active hexose correlated compound (AHCC), a mushroom extract, has been shown to protect mice against lethal primary influenza infection. Moreover, when AHCC was administered pre-vaccination in mice, they showed improved protection from lethal avian flu infection when compared to mice vaccinated alone. In this study, we hypothesized that AHCC will also improve the immune responses of healthy individuals to influenza vaccine. A randomized controlled study was performed with 30 healthy adults to evaluate the effects of AHCC supplementation on the immune response to the 2009-2010 seasonal influenza vaccine. Blood was drawn pre-vaccination and 3 wk post-vaccination. Immediately post-vaccination, the AHCC group began supplementation with AHCC (3 g/d). Flow cytometric analysis of lymphocyte subpopulations revealed that AHCC supplementation increased NKT cells (P < .1), and CD8 T cells (P < .05) post-vaccination compared to controls. Analysis of antibody production 3 weeks post-vaccination revealed that AHCC supplementation significantly improved protective antibody titers to influenza B, while the improvement was not significant in the control group. Overall, our study showed that AHCC supplementation improved some lymphocyte percentages and influenza B antibody titers over the control. Future studies are required to determine the kinetics of AHCC supplementation to improve the overall response to influenza vaccination.

An oncolytic adenovirus enhanced for toll-like receptor 9 stimulation increases antitumor immune responses and tumor clearance

Oncolytic viruses represent a multifaceted tool for cancer treatment. In addition to specific killing of cancer cells (oncolysis), these agents also provide danger signals prompting the immune system to stimulate an antitumor immune response. To increase adenovirus adjuvancy, we engineered the genome of Ad5D24 by inserting 18 immunostimulatory islands (Ad5D24-CpG). The toxicity and immunogenicity profile of Ad5D24-CpG showed that the safety of the maternal virus was retained. The efficacy of the CpG-enriched virus was assessed in a xenograft model of lung cancer where a significant increase in antitumor effect was seen in comparison with controls. When the experiment was repeated in animal depleted of natural killer (NK) cells, Ad5D24-CpG lost its advantage. The same was seen when Toll-like receptor (TLR)9 was blocked systemically. In a syngeneic model of melanoma (B16-OVA), we observed a significant increase of OVA-specific T cells and a decrease of activation of myeloid-derived suppressor cells in Ad5D24-CpG-treated mice. In conclusion, we have generated the first genetically modified oncolytic adenovirus backbone able to enhance TLR9-stimulation for increased antitumor activity.

Immunogenicity when utilizing adenovirus serotype 4 and 5 vaccines expressing circumsporozoite protein in naïve and adenovirus (Ad5) immune mice

Background

Induction of potent long lasting effector T cell responses against liver stage malaria antigens strongly correlates with protection from malaria. While Adenovirus serotype 5 (Ad5) based malaria vaccine platforms have the ability to induce potent effector T cell responses against transgenes, high rates of pre-existing Ad5 immunity in malaria endemic regions has prompted study of alternative Ad serotype based malaria vaccines as replacements for Ad5 based malaria vaccines. The research described in this article examines the utility of alternative serotype adenovirus serotype 4 (Ad4) expressing a sporozoite surface protein (circumsporozoite protein (CSP)) (Ad4-CSP) to induce immune responses against CSP. The immunogenicity of Ad4-CSP was also tested in homologous and heterologous prime boost vaccinations in both Ad5 naïve and Ad5 immune backgrounds as compared to use of Ad5-CSP.

Results

In Ad5 naïve animals, use of Ad4-CSP priming vaccinations followed by boosting with Ad5-CSP (Ad4-CSP/Ad5-CSP) maximally increased the numbers of CSP specific cytokine secreting cytotoxic T cells relative to repeated use of Ad5-CSP. The Ad4-CSP/Ad5-CSP regimen also induced equivalent levels of CSP specific cell killing as did homologous prime-boost vaccinations with Ad5-CSP, despite stimulating lower numbers of CSP specific cytotoxic T cells. Priming with Ad4-CSP followed by a homologous boost resulted in significantly less CSP specific humoral responses than any other vaccination regimen tested in Ad naïve animals. In Ad5 immune animals, addition of Ad4-CSP in homologous or heterologous prime boost resulted in inductions of higher CSP specific responses than animals repeatedly vaccinated with Ad5-CSP alone. However, the observed responses were well below those observed in similarly treated Ad naïve mice.

Conclusions

While the Ad4-CSP/Ad5-CSP and Ad5-CSP/Ad5-CSP vaccination regimens resulted in equivalent CSP specific killing in Ad naïve animals, Ad4-CSP/Ad5-CSP achieved this result with a lower percentage of CSP specific CD8⁺ T cells and a higher number of IFNγ secreting cells, suggesting that the Ad4-CSP/Ad5-CSP vaccination regimen elicits more efficient cytotoxic T cells. In Ad5 immune animals use of Ad4-CSP improved CSP specific immune responses as compared to repeated use of Ad5-CSP, but could not achieve the levels of immunogenicity observed when the same vaccine regimens were used in Ad naïve animals. These data indicate the existence of some level of immunological cross-reactivity between these two adenovirus subgroups. Based on these results, it is suggested that future studies should undertake similarly stringent analyses of alternative Ad serotypes to establish their effectiveness as replacements for Ad5.

Malaria vaccines: focus on adenovirus based vectors

Protection against malaria through vaccination is known to be achievable, as first demonstrated over 30 years ago. Vaccination via repeated bites with Plasmodium falciparum infected and irradiated mosquitoes provided short lived protection from malaria infection to these vaccinees. Though this method still remains the most protective malaria vaccine to date, it is likely impractical for widespread use. However, recent developments in sub-unit malaria vaccine platforms are bridging the gap between high levels of protection and feasibility. The current leading sub-unit vaccine, RTS,S (which consists of a fusion of a portion of the P. falciparum derived circumsporozoite protein to the Hepatitis B surface antigen), has demonstrated the ability to induce protection from malaria infection in up 56% of RTS,S vaccinees. Though encouraging, these results may fall short of protection levels generally considered to be required to achieve eradication of malaria. Therefore, the use of viral vectored vaccine platforms has recently been pursued to further improve the efficacy of malaria targeted vaccines. Adenovirus based vaccine platforms have demonstrated potent anti-malaria immune responses when used alone, as well when utilized in heterologous prime boost regimens. This review will provide an update as to the current advancements in malaria vaccine development, with a focus on the use of adenovirus vectored malaria vaccines.

Improving adenovirus based gene transfer: strategies to accomplish immune evasion

Adenovirus (Ad) based gene transfer vectors continue to be the platform of choice for an increasing number of clinical trials worldwide. In fact, within the last five years, the number of clinical trials that utilize Ad based vectors has doubled, indicating growing enthusiasm for the numerous positive characteristics of this gene transfer platform. For example, Ad vectors can be easily and relatively inexpensively produced to high titers in a cGMP compliant manner, can be stably stored and transported, and have a broad applicability for a wide range of clinical conditions, including both gene therapy and vaccine applications. Ad vector based gene transfer will become more useful as strategies to counteract innate and/or pre-existing adaptive immune responses to Ads are developed and confirmed to be efficacious. The approaches attempting to overcome these limitations can be divided into two broad categories: pre-emptive immune modulation of the host, and selective modification of the Ad vector itself. The first category of methods includes the use of immunosuppressive drugs or specific compounds to block important immune pathways, which are known to be induced by Ads. The second category comprises several innovative strategies inclusive of: (1) Ad-capsid-display of specific inhibitors or ligands; (2) covalent modifications of the entire Ad vector capsid moiety; (3) the use of tissue specific promoters and local administration routes; (4) the use of genome modified Ads; and (5) the development of chimeric or alternative serotype Ads. This review article will focus on both the promise and the limitations of each of these immune evasion strategies, and in the process delineate future directions in developing safer and more efficacious Ad-based gene transfer strategies.

Overcoming pre-existing adenovirus immunity by genetic engineering of adenovirus-based vectors

Adenovirus (Ad)-based vectors offer several benefits showing their potential for use in a variety of vaccine applications. Recombinant Ad-based vaccines possess potent immunogenic potential, capable of generating humoral and cellular immune responses to a variety of pathogen-specific antigens expressed by the vectors. Ad5 vectors can be readily produced, allowing for usage in thousands of clinical trial subjects. This is now coupled with a history of safe clinical use in the vaccine setting. However, traditional Ad5-based vaccines may not be generating optimal antigen-specific immune responses, and generate diminished antigen-specific immune responses when pre-existing Ad5 immunity is present. These limitations have driven initiation of several approaches to improve the efficacy of Ad-based vaccines, and/or allow modified vaccines to overcome pre-existing Ad immunity. These include: generation of chemically modified Ad5 capsids; generation of chimeric Ads; complete replacement of Ad5-based vaccine platforms with alternative (human and non-human origin) Ad serotypes, and Ad5 genome modification approaches that attempt to retain the native Ad5 capsid, while simultaneously improving the efficacy of the platform as well as minimizing the effect of pre-existing Ad immunity. Here we discuss recent advances in- and limitations of each of these approaches, relative to their abilities to overcome pre-existing Ad immunity.

Induction of shock after intravenous injection of adenovirus vectors: a critical role for platelet-activating factor

Innate immune responses are a major barrier to safe systemic gene therapy with adenovirus (Ad) vectors. We show that intravenous (IV) injection of rats with Ad5 vectors causes a novel rapid shock reaction that involves hypotension, hemoconcentration, tissue edema, and vasocongestion, with notable pathology in the pancreas and the gastrointestinal system. We show for the first time that this reaction is dependent on platelet-activating factor (PAF), a lipid signaling molecule that is a known shock inducer. Ad upregulated PAF within 5 minutes in vivo, and antagonists of the PAF receptor were able to prevent Ad-induced shock. Ad upregulated PAF via the reticuloendothelial system (RES), because splenectomy or depletion of phagocytes blocked the ability of Ad to induce both PAF and shock. Rats were considerably more sensitive to Ad-induced shock than were mice, but PAF mediated shock in both species. Other Ad-induced innate immune responses such as cytokine induction and thrombocytopenia were not mediated by PAF. In summary, systemic IV injection of Ad stimulates the RES to upregulate PAF within a matter of minutes, which results in shock. The identification of this novel pathway suggests strategies to improve the safety of systemic gene therapy with Ad vectors.

Adenovirus activates complement by distinctly different mechanisms in vitro and in vivo: indirect complement activation by virions in vivo

Understanding innate immunity is key to improving the safety of adenovirus (Ad) vectors for systemic gene therapy. Ad has been shown to activate complement in vitro, but activation of complement after Ad injection in vivo has not been directly measured. Using complement protein C3a as a marker of complement activation, we show that types 2 and 5 human Ads cause rapid complement activation after intravenous injection in mice. Unexpectedly, the mechanisms in vivo were different than those in vitro. Antibodies were critical for the activation of complement by Ad in vitro, but antibodies were not required in vivo. The classical pathway was required in vitro, whereas complement activation in vivo involved both classical and nonclassical pathways as well as the reticuloendothelial system. Remarkably, the entry-deficient Ad mutant ts1 was completely unable to activate complement in vivo even though it was fully able to activate complement in vitro. This result demonstrates that the complement system senses intravenously injected Ad primarily by detecting the effects of Ad on cells rather than through direct interaction of complement with virions. Encouragingly, shielding Ad with polyethylene glycol was effective at reducing complement activation both in vitro and in vivo. In summary, intravenously injected Ad rapidly activates complement through multiple pathways, but these pathways are different than those identified by in vitro studies. In vitro studies are poorly predictive of in vivo mechanisms because Ad virions activate complement through indirect mechanisms in vivo.

TRIF, and TRIF-interacting TLRs differentially modulate several adenovirus vector-induced immune responses

The use of Adenovirus (Ad)-based vectors has proven to be a useful platform for the development of gene therapy and vaccine protocols. The immunological mechanisms underlying these properties need to be identified and understood to foster safer, more efficacious use of this important gene transfer platform. Our recent studies have confirmed an important role for MyD88 dependent toll-like receptor (TLR) pathways as mediators of these responses. In this study, we confirm that TLR3, TLR4 and TRIF (TIR-domain-containing adapter-inducing interferon-beta) can also have augmentative or inhibitory roles during Ad-induced immune responses. Importantly, our studies revealed that TLR4 acts to suppress several aspects of the Ad-induced innate immune response, a finding not previously reported for this TLR in any model system. In addition, using MyD88 and TRIF double knockout mice, we demonstrate that the MyD88 and TRIF adaptor proteins can play either additive or redundant roles in mediating certain aspects of Ad vector-induced innate and adaptive immune responses. Furthering this complexity, our model system strongly suggests that non-TLR based systems must not only exist, but also have a significant role to play during Ad vector-mediated induction of adaptive immune responses.

Optimization of vaccine responses with an E1, E2b and E3-deleted Ad5 vector circumvents pre-existing anti-vector immunity

Recombinant serotype 5 adenovirus (Ad5) vectors lacking E1 expression induce robust immune responses against encoded transgenes in preclinical models, but have muted responses in human trials due to wide spread pre-existing anti-adenovirus immunity. Attempts to circumvent Ad5 specific immunity by using alternative serotypes or modifying capsid components have not yielded profound clinical improvement. To address this issue, we explored a novel alternative strategy, specifically reducing the expression of structural Ad5 genes by creating E1 and E2b deleted recombinant Ad5 vectors. Our data demonstrate that [E1−, E2b−]vectors retaining the Ad5 serotype are potent immunogens in pre-clinical models despite the presence of significant Ad5 specific immunity, in contrast to [E1−] vectors. These preclinical studies with E1 and E2b deleted recombinant Ad5 vectors suggest that anti-Ad immunity will no longer be a limiting factor and that clinical trials to evaluate their performance are warranted.

Transient pretreatment with glucocorticoid ablates innate toxicity of systemically delivered adenoviral vectors without reducing efficacy

More than 300 human clinical trials utilize recombinant adenoviruses (rAds) as a gene transfer vector, confirming that rAds continue to be of high clinical interest. A primary weakness of rAds is their known propensity to trigger an innate, proinflammatory immune response rapidly after high-dose, systemic administration. In this study, we investigated what affects that pre-emptive treatment with anti-inflammatory glucocorticoids might have upon Ad vector-triggered inflammatory immune responses. We found that a simple pretreatment regimen with Dexamethasone (DEX) can significantly reduce most Ad-induced innate immune responses. DEX prevented rAd induction of systemic cytokine/chemokine releases in a dose-dependent fashion, with higher dosages preventing rAd induction of acute thrombocytopenia, endothelial cell activation, proinflammatory gene induction, and leukocyte infiltration into transduced organs. Transient glucocorticoid pretreatment also significantly reduced rAd-induced adaptive immune responses, including a decreased induction of Ad-neutralizing antibodies (NAbs). Importantly, use of DEX did not reduce the efficacy of rAd-mediated gene transduction nor rAd-derived transgene expression. Our results demonstrate that a simple, pre-emptive and transient glucocorticoid pretreatment is a viable approach to reduce rAd-associated acute toxicities that currently limit the use of Ad vectors in systemic clinical applications.

A preliminary and comparative evaluation of a novel Ad5 [E1-, E2b-] recombinant-based vaccine used to induce cell mediated immune responses

Adenovirus vectors have been shown to be highly effective as vaccine platforms capable of inducing both humoral and cell mediated immune (CMI) responses. An Ad serotype 5 vector containing unique deletions in the E2b region (Ad5 [E1-, E2b-]) has been reported to have several advantages over conventional Adenovirus serotype 5 (Ad5) vectors deleted in only the E1 region (Ad5 [E1-]), including increased carrying capacity and diminished viral late gene expression. Here, we evaluated a novel Ad5 [E1-, E2b-] vector utilizing the E.C7 cell line for viral packaging. Its' effectiveness as a potential vaccine platform as compared to the currently utilized Ad5 [E1-]-based platform was assessed in both Ad5 naïve and Ad5 immune mice. We employed the HIV-1 Gag gene as the antigenic transgene expressed by the novel vector. Cellular expression of the Gag was confirmed by Western Blot analysis. Dose response studies using three intradermal immunizations of 10(7) to 10(10) virus particles (VP) of each construct revealed that immunization with 10(10)VP resulted in the maximum immunological response. Multiple immunizations of Ad naïve BALB/c mice with an Ad5 [E1-, E2b]-gag vaccine resulted in higher ELISpot CMI responses as compared to mice immunized with an Ad5 [E1-]-gag vaccine. More importantly, multiple immunizations of Ad5 immune BALB/c mice with an Ad5 [E1-, E2b]-gag vaccine resulted in significant increases in ELISpot CMI responses when compared to Ad5 immune mice vaccinated with an Ad5 [E1-]-gag vector. Preliminary studies in three Ad5 immune non-human primates (NHP) demonstrated that vaccination with Ad5 [E1-, E2b-]-gag-induced elevated levels of interferon-gamma and IL-2 secreting lymphocytes as assessed by ELISpot assays. These studies indicate that the novel Ad5 [E1-, E2b-] viral vector can be utilized as a potential vaccine platform to induce elevated CMI responses as compared to current generation Ad5 [E1-] viral vectors even in the presence of pre-existing Ad5 immunity.