A replication competent adenovirus 5 host range mutant-simian immunodeficiency virus (SIV) recombinant priming/subunit protein boosting vaccine regimen induces broad, persistent SIV-specific cellular immunity to dominant and subdominant epitopes in Mamu-A*01 rhesus macaques

Current nonclinical approaches for immune assessments of immuno-oncology biotherapeutics

Harnessing the immune system to kill tumors has been revolutionary and, as a result, has had an enormous benefit for patients in extending life and resulting in effective cures in some. However, activation of the immune system can come at the cost of undesirable adverse events such as cytokine release syndrome, immune-related adverse events, on-target/off-tumor toxicity, neurotoxicity and tumor lysis syndrome, which are safety risks that can be challenging to assess non-clinically. This article provides a review of the biology and mechanisms that can result in immune-mediated adverse effects and describes industry approaches using in vitro and in vivo models to aid in the nonclinical safety risk assessments for immune-oncology modalities. Challenges and limitations of knowledge and models are also discussed.

Retargeting adenoviruses for therapeutic applications and vaccines

Adenoviruses (Ads) are robust vectors for therapeutic applications and vaccines, but their use can be limited by differences in their in vitro and in vivo pharmacologies. This review emphasizes that there is not just one Ad, but a whole virome of diverse viruses that can be used as therapeutics. It discusses that true vector targeting involves not only retargeting viruses, but importantly also detargeting the viruses from off-target cells.

Genetic Adjuvants in Replicating Single-Cycle Adenovirus Vectors Amplify Systemic and Mucosal Immune Responses against HIV-1 Envelope

Most infections occur at mucosal surfaces. Providing a barrier of protection at these surfaces may be a useful strategy to combat the earliest events in infection when there are relatively few pathogens to address. The majority of vaccines are delivered systemically by the intramuscular (IM) route. While IM vaccination can drive mucosal immune responses, mucosal immunization at intranasal (IN) or oral sites can lead to better immune responses at mucosal sites of viral entry. In macaques, IN immunization with replicating single-cycle adenovirus (SC-Ads) and protein boosts generated favorable mucosal immune responses. However, there was an apparent "distance effect" in generating mucosal immune responses. IN immunization generated antibodies against HIV envelope (env) nearby in the saliva, but weaker responses in samples collected from the distant vaginal samples. To improve on this, we tested here if SC-Ads expressing genetic adjuvants could be used to amplify antibody responses in distant vaginal samples when they are codelivered with SC-Ads expressing clade C HIV env immunogen. SC-Ads env 1157 was coadministered with SC-Ads expressing 4-1BBL, granulocyte macrophage colony-stimulating factor (GMCSF), IL-21, or Clostridoides difficile (C. diff.) toxin fragments by IN or IM routes. These data show that vaginal antibody responses were markedly amplified after a single immunization by the IN or IM routes, with SC-Ad expressing HIV env if this vaccine is complemented with SC-Ads expressing genetic adjuvants. Furthermore, the site and combination of adjuvants appear to "tune" these antibody responses towards an IgA or IgG isotype bias. Boosting these priming SC-Ad responses with another SC-Ad or with SOSIP native-like env proteins markedly amplifies env antibody levels in vaginal washes. Together, this data may be useful in informing the choice of route of delivery adenovirus and peptide vaccines against HIV-1.

Divergent HIV-1-Directed Immune Responses Generated by Systemic and Mucosal Immunization with Replicating Single-Cycle Adenoviruses in Rhesus Macaques

Most human immunodeficiency virus type 1 (HIV-1) infections begin at mucosal surfaces. Providing a barrier of protection at these may assist in combating the earliest events in infection. Systemic immunization by intramuscular (i.m.) injection can drive mucosal immune responses, but there are data suggesting that mucosal immunization can better educate these mucosal immune responses. To test this, rhesus macaques were immunized with replicating single-cycle adenovirus (SC-Ad) vaccines expressing clade B HIV-1 gp160 by the intranasal (i.n.) and i.m. routes to compare mucosal and systemic routes of vaccination. SC-Ad vaccines generated significant circulating antibody titers against Env after a single i.m. immunization. Switching the route of second immunization with the same SC-Ad serotype allowed a significant boost in these antibody levels. When these animals were boosted with envelope protein, envelope-binding antibodies were amplified 100-fold, but qualitatively different immune responses were generated. Animals immunized by only the i.m. route had high peripheral T follicular helper (pTfh) cell counts in blood but low Tfh cell counts in lymph nodes. Conversely, animals immunized by the i.n. route had high Tfh cell counts in lymph nodes but low pTfh cell counts in the blood. Animals immunized by only the i.m. route had lower antibody-dependent cellular cytotoxicity (ADCC) antibody activity, whereas animals immunized by the mucosal i.n. route had higher ADCC antibody activity. When these Env-immunized animals were challenged rectally with simian-human immunodeficiency virus (SHIV) strain SF162P3 (SHIVSF162P3), they all became infected. However, mucosally SC-Ad-immunized animals had lower viral loads in their gastrointestinal tracts. These data suggest that there may be benefits in educating the immune system at mucosal sites during HIV vaccination.IMPORTANCE HIV-1 infections usually start at a mucosal surface after sexual contact. Creating a barrier of protection at these mucosal sites may be a good strategy for to protect against HIV-1 infections. While HIV-1 enters at mucosa, most vaccines are not delivered here. Most are instead injected into the muscle, a site well distant and functionally different than mucosal tissues. This study tested if delivering HIV vaccines at mucosa or in the muscle makes a difference in the quality, quantity, and location of immune responses against the virus. These data suggest that there are indeed advantages to educating the immune system at mucosal sites with an HIV-1 vaccine.

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.

Novel Concepts for HIV Vaccine Vector Design

The unprecedented challenges of developing effective vaccines against intracellular pathogens such as HIV, malaria, and tuberculosis have resulted in more rational approaches to vaccine development. Apart from the recent advances in the design and selection of improved epitopes and adjuvants, there are also ongoing efforts to optimize delivery platforms.

The unprecedented challenges of developing effective vaccines against intracellular pathogens such as HIV, malaria, and tuberculosis have resulted in more rational approaches to vaccine development. Apart from the recent advances in the design and selection of improved epitopes and adjuvants, there are also ongoing efforts to optimize delivery platforms. Viral vectors are the best-characterized delivery tools because of their intrinsic adjuvant capability, unique cellular tropism, and ability to trigger robust adaptive immune responses. However, a known limitation of viral vectors is preexisting immunity, and ongoing efforts are aimed at developing novel vector platforms with lower seroprevalence. It is also becoming increasingly clear that different vectors, even those derived from phylogenetically similar viruses, can elicit substantially distinct immune responses, in terms of quantity, quality, and location, which can ultimately affect immune protection. This review provides a summary of the status of viral vector development for HIV vaccines, with a particular focus on novel viral vectors and the types of adaptive immune responses that they induce.

IIIa deleted adenovirus as a single-cycle genome replicating vector

Replication competent adenovirus (RC-Ad) vectors mediate robust transgene expression by virtue of amplifying transgenes by replication but also put patients at a risk of frank adenovirus infection. In contrast, E1-deleted replication defective Ad (RD-Ad) vectors are safer but produce substantially less transgene product. To generate a robust, but safer adenoviral vector, we created a "single cycle" adenovirus (SC-Ad) vector that replicates its genome and transgene, but that does not cause adenovirus infections by deleting the capsid cement protein IIIa in low seroprevalence adenovirus serotype 6. Ad6-ΔIIIa can be produced in IIIa-expressing cell lines. In normal cells, Ad6-ΔIIIa replicates its genome and transgene but fails to package its DNA or form mature virus. SC-Ad and RC-Ad expressed transgenes hundreds of times higher than RD-Ad in human and mouse cells in vitro and in vivo in mice. These data suggest that SC-Ads may be safer amplifying vectors for vaccine and therapeutic applications.

Immune responses in macaques to a prototype recombinant adenovirus live oral human papillomavirus 16 vaccine

Immunization with human papillomavirus (HPV) L1 virus-like particles (VLPs) prevents infection with HPV. However, the expense and logistical demands of current VLP vaccines will limit their widespread use in resource-limited settings, where most HPV-induced cervical cancer occurs. Live oral adenovirus vaccines have properties that are well-suited for use in such settings. We have described a live recombinant adenovirus vaccine prototype that produces abundant HPV16 L1 protein from the adenovirus major late transcriptional unit and directs the assembly of HPV16 VLPs in tissue culture. Recombinant-derived VLPs potently elicit neutralizing antibodies in mice. Here, we characterize the immune response to the recombinant after dual oral and intranasal immunization of pigtail macaques, in which the virus replicates as it would in immunized humans. The immunization of macaques induced vigorous humoral responses to adenovirus capsid and nonstructural proteins, although, surprisingly, not against HPV L1. In contrast, immunization elicited strong T-cell responses to HPV VLPs as well as adenovirus virions. T-cell responses arose immediately after the primary immunization and were boosted by a second immunization with recombinant virus. T-cell immunity contributes to protection against a wide variety of pathogens, including many viruses. The induction of a strong cellular response by the recombinant indicates that live adenovirus recombinants have potential as vaccines for those agents. These studies encourage and will inform the continued development of viable recombinant adenovirus vaccines.

Prospects for oral replicating adenovirus-vectored vaccines

Orally delivered replicating adenovirus (Ad) vaccines have been used for decades to prevent adenovirus serotype 4 and 7 respiratory illness in military recruits, demonstrating exemplary safety and high efficacy. That experience suggests that oral administration of live recombinant Ads (rAds) holds promise for immunization against other infectious diseases, including those that have been refractory to traditional vaccination methods. Live rAds can express intact antigens from free-standing transgenes during replication in infected cells. Alternatively, antigenic epitopes can be displayed on the rAd capsid itself, allowing presentation of the epitope to the immune system both prior to and during replication of the virus. Such capsid-display rAds offer a novel vaccine approach that could be used either independently of or in combination with transgene expression strategies to provide a new tool in the search for protection from infectious disease.

Mucosal priming with a replicating-vaccinia virus-based vaccine elicits protective immunity to simian immunodeficiency virus challenge in rhesus monkeys

Mucosal surfaces are not targeted by most human immunodeficiency virus type 1 (HIV-1) vaccines, despite being major routes for HIV-1 transmission. Here we report a novel vaccination regimen consisting of a mucosal prime with a modified replicating vaccinia virus Tiantan strain (MVTT(SIVgpe)) and an intramuscular boost with a nonreplicating adenovirus strain (Ad5(SIVgpe)). This regimen elicited robust cellular immune responses with enhanced magnitudes, sustainability, and polyfunctionality, as well as higher titers of neutralizing antibodies against the simian immunodeficiency virus SIV(mac1A11) in rhesus monkeys. The reductions in peak and set-point viral loads were significant in most animals, with one other animal being protected fully from high-dose intrarectal inoculation of SIV(mac239). Furthermore, the animals vaccinated with this regimen were healthy, while ~75% of control animals developed simian AIDS. The protective effects correlated with the vaccine-elicited SIV-specific CD8(+) T cell responses against Gag and Pol. Our study provides a novel strategy for developing an HIV-1 vaccine by using the combination of a replicating vector and mucosal priming.

Adenovirus particles that display the Plasmodium falciparum circumsporozoite protein NANP repeat induce sporozoite-neutralizing antibodies in mice

Adenovirus particles can be engineered to display exogenous peptides on their surfaces by modification of viral capsid proteins, and particles that display pathogen-derived peptides can induce protective immunity. We constructed viable recombinant adenoviruses that display B-cell epitopes from the Plasmodium falciparum circumsporozoite protein (PfCSP) in the major adenovirus capsid protein, hexon. Recombinants induced high-titer antibodies against CSP when injected intraperitoneally into mice. Serum obtained from immunized mice recognized both recombinant PfCSP protein and P. falciparum sporozoites, and neutralized P. falciparum sporozoites in vitro. Replicating adenovirus vaccines have provided economical protection against adenovirus disease for over three decades. The recombinants described here may provide a path to an affordable malaria vaccine in the developing world.

Innovative approaches to develop prophylactic and therapeutic vaccines against HIV/AIDS

The acquired immunodeficiency syndrome (AIDS) emerged in the human population in the summer of 1981. According to the latest United Nations estimates, worldwide over 33 million people are infected with human immunodeficiency virus (HIV) and the prevalence rates continue to rise globally. To control the alarming spread of HIV, an urgent need exists for developing a safe and effective vaccine that prevents individuals from becoming infected or progressing to disease. To be effective, an HIV/AIDS vaccine should induce broad and long-lasting humoral and cellular immune responses, at both mucosal and systemic level. However, the nature of protective immune responses remains largely elusive and this represents one of the major roadblocks preventing the development of an effective vaccine. Here we summarize our present understanding of the factors responsible for resistance to infection or control of progression to disease in human and monkey that may be relevant to vaccine development and briefly review recent approaches which are currently being tested in clinical trials. Finally, the rationale and the current status of novel strategies based on nonstructural HIV-1 proteins, such as Tat, Nef and Rev, used alone or in combination with modified structural HIV-1 Env proteins are discussed.

Construction and immunogenicity of replication-competent adenovirus 5 host range mutant recombinants expressing HIV-1 gp160 of SF162 and TV1 strains

An HIV Env immunogen capable of eliciting broad immunity is critical for a successful vaccine. We constructed and characterized adenovirus 5 host range mutant (Ad5hr) recombinants encoding HIV(SF162) gp160 (subtype B) and HIV(TV1) gp160 (subtype C). Immunization of mice with one or both induced cellular immunity to subtype B and C peptides by ELISpot, and antibody responses with high binding titers to HIV Env of subtypes A, B, C, and E. Notably, Ad5hr-HIV(TV1) gp160 induced better cellular immunity than Ad5hr-HIV(SF162) gp160, either alone or following co-administration. Thus, the TV1 Env recombinant alone may be sufficient for eliciting immune responses against both subtype B and C envelopes. Further studies of Ad5hr-HIV(TV1) gp160 in rhesus macaques will evaluate the suitability of this insert for a future phase I clinical trial using a replication-competent Ad4 vector.

HIV-1 Tat-based vaccines: an overview and perspectives in the field of HIV/AIDS vaccine development

The HIV epidemic continues to represent one of the major problems worldwide, particularly in the Asia and Sub-Saharan regions of the world, with social and economical devastating effects. Although antiretroviral drugs have had a dramatically beneficial impact on HIV-infected individuals that have access to treatment, it has had a negligible impact on the global epidemic. Hence, the inexorable spreading of the HIV pandemic and the increasing deaths from AIDS, especially in developing countries, underscore the urgency for an effective vaccine against HIV/AIDS. However, the generation of such a vaccine has turned out to be extremely challenging. Here we provide an overview on the rationale for the use of non-structural HIV proteins, such as the Tat protein, alone or in combination with other HIV early and late structural HIV antigens, as novel, promising preventative and therapeutic HIV/AIDS vaccine strategies.

A new genetic vaccine platform based on an adeno-associated virus isolated from a rhesus macaque

We created a hybrid adeno-associated virus (AAV) from two related rhesus macaque isolates, called AAVrh32.33, and evaluated it as a vaccine carrier for human immunodeficiency virus type 1 (HIV-1) and type A influenza virus antigens. The goal was to overcome the limitations of vaccines based on other AAVs, which generate dysfunctional T-cell responses and are inhibited by antibodies found in human sera. Injection of a Gag-expressing AAVrh32.33 vector into mice resulted in a high-quality CD8(+) T-cell response. The resulting Gag-specific T cells express multiple cytokines at high levels, including interleukin-2, with many having memory phenotypes; a subsequent boost with an adenovirus vector yielded a brisk expansion of Gag-specific T cells. A priming dose of AAVrh32.33 led to high levels of Gag antibodies, which exceed levels found after injection of adenovirus vectors. Importantly, passive transfer of pooled human immunoglobulin into mice does not interfere with the efficacy of AAVrh32.33 expressing nucleoproteins from influenza virus, as measured by protection to a lethal dose of influenza virus, which is consistent with the very low seroprevalence to this virus in humans. Studies of macaques with vectors expressing gp140 from HIV-1 (i.e., with AAVrh32.33 as the prime and simian adenovirus type 24 as the boost) demonstrated results similar to those for mice with high-level and high-quality CD8(+) T-cell responses to gp140 and high-titered neutralizing antibodies to homologous HIV-1. The biology of this novel AAV hybrid suggests that it should be a preferred genetic vaccine carrier, capable of generating robust T- and B-cell responses.

Correlation of vaccine-elicited systemic and mucosal nonneutralizing antibody activities with reduced acute viremia following intrarectal simian immunodeficiency virus SIVmac251 challenge of rhesus macaques

Cell-mediated immunity and neutralizing antibodies contribute to control of human immunodeficiency virus/simian immunodeficiency virus (HIV/SIV) infection, but the role of nonneutralizing antibodies is not defined. Previously, we reported that sequential oral/oral or intranasal/oral (I/O) priming with replication-competent adenovirus type 5 host range mutant (Ad5hr)-SIV recombinants, followed by intramuscular envelope protein boosting, elicited systemic and mucosal cellular immunity and exhibited equivalent, significant reductions of chronic viremia after rectal SIV(mac251) challenge. However, I/O priming gave significantly better control of acute viremia. Here, systemic and mucosal humoral immunity were investigated for potential correlates with the acute challenge outcome. Strong serum binding but nonneutralizing antibody responses against SIV(mac251) were induced in both groups. Antibody responses appeared earlier and overall were higher in the I/O group. Reduced acute viremia was significantly correlated with higher serum binding titer, stronger antibody-dependent cellular cytotoxicity activity, and peak prechallenge and 2-week-postchallenge antibody-dependent cell-mediated viral inhibition (ADCVI). The I/O group consistently displayed greater anti-envelope immunoglobulin A (IgA) antibody responses in bronchoalveolar lavage and a stronger rectal anti-envelope IgA anamnestic response 2 weeks postchallenge. Pre- and postchallenge rectal secretions inhibited SIV transcytosis across epithelial cells. The inhibition was significantly higher in the I/O group, although a significant correlation with reduced acute viremia was not reached. Overall, the replicating Ad5hr-SIV priming/envelope boosting approach elicited strong systemic and mucosal antibodies with multiple functional activities. The pattern of elevated immune responses in the I/O group is consistent with its better control of acute viremia mediated, at least in part, by ADCVI activity and transcytosis inhibition.

Replicating adenovirus vector prime/protein boost strategies for HIV vaccine development

BACKGROUND

In recent years the HIV vaccine field introduced a number of promising vaccine candidates into human clinical trials.

OBJECTIVE

To briefly discuss the advances made in vaccine development and HIV pathogenesis and give an overview of the body of work our lab has generated in multiple animal models on replication-competent Adenovirus recombinant vaccines.

METHODS

Emphasis is placed on comparative examination of vaccine components, routes of immunization and challenge models using replicating Adenovirus vectors.

RESULTS/CONCLUSION

The findings make the case that replicating Adenovirus vectors are superior in priming multiple arms of the immune system, and in conjunction with protein boosting, have resulted in dramatic protective efficacy leading to their advancement to Phase I trials. Implications of the recent halting of the Merck Ad5-HIV Phase IIb clinical trial of our vaccine approach and other vectored vaccines are discussed.

Sequential priming with simian immunodeficiency virus (SIV) DNA vaccines, with or without encoded cytokines, and a replicating adenovirus-SIV recombinant followed by protein boosting does not control a pathogenic SIVmac251 mucosal challenge

Previously, combination DNA/nonreplicating adenovirus (Ad)- or poxvirus-vectored vaccines have strongly protected against SHIV(89.6P), DNAs expressing cytokines have modulated immunity elicited by DNA vaccines, and replication-competent Ad-recombinant priming and protein boosting has strongly protected against simian immunodeficiency virus (SIV) challenge. Here we evaluated a vaccine strategy composed of these promising components. Seven rhesus macaques per group were primed twice with multigenic SIV plasmid DNA with or without interleukin-12 (IL-12) DNA or IL-15 DNA. After a multigenic replicating Ad-SIV immunization, all groups received two booster immunizations with SIV gp140 and SIV Nef protein. Four control macaques received control DNA plasmids, empty Ad vector, and adjuvant. All vaccine components were immunogenic, but the cytokine DNAs had little effect. Macaques that received IL-15-DNA exhibited higher peak anti-Nef titers, a more rapid anti-Nef anamnestic response postchallenge, and expanded CD8(CM) T cells 2 weeks postchallenge compared to the DNA-only group. Other immune responses were indistinguishable between groups. Overall, no protection against intrarectal challenge with SIV(mac251) was observed, although immunized non-Mamu-A*01 macaques as a group exhibited a statistically significant 1-log decline in acute viremia compared to non-Mamu-A*01 controls. Possible factors contributing to the poor outcome include administration of cytokine DNAs to sites different from the Ad recombinants (intramuscular and intratracheal, respectively), too few DNA priming immunizations, a suboptimal DNA delivery method, failure to ensure delivery of SIV and cytokine plasmids to the same cell, and instability and short half-life of the IL-15 component. Future experiments should address these issues to determine if this combination approach is able to control a virulent SIV challenge.

Adenovirus vector induced innate immune responses: impact upon efficacy and toxicity in gene therapy and vaccine applications

Extensively characterized, modified, and employed for a variety of purposes, adenovirus (Ad) vectors are generally regarded as having great potential by many applied virologists who wish to manipulate and use viral biology to achieve beneficial clinical outcomes. Despite widespread functional prominence and utility (i.e., Ad-based clinical trials have begun to progress to critical Phase III levels, it has recently become apparent that investigations regarding the innate immune response to Ads may reveal not only reasons behind previous failures, but also reveal novel insights that will allow for safer, more efficacious uses of this important gene transfer platform. Insights gained by the exploration of Ad induced innate immune responses will likely be most important to the fields of vaccine development, since Ad-based vaccines are regarded as one of the more promising vaccine platforms in development today. Adenovirus is currently known to interact with several different extracellular, intracellular, and membrane-bound innate immune sensing systems. Past and recent studies involving manipulation of the Ad infectious cycle as well as use of different mutants have shed light on some of the initiation mechanisms underlying Ad induced immune responses. More recent studies using microarray-based analyses, genetically modified cell lines and/or mouse mutants, and advanced generation Ad vectors have revealed important new insights into the scope and mechanism of this cellular defensive response. This review is an attempt to synthesize these studies, update Ad biologists to the current knowledge surrounding these increasingly important issues, as well as highlight areas where future research should be directed. It should also serve as a sobering reality to researchers exploring the use of any gene transfer vector, as to the complexities potentially involved when contemplating use of such vectors for human applications.

Comparative evaluation of oral and intranasal priming with replication-competent adenovirus 5 host range mutant (Ad5hr)-simian immunodeficiency virus (SIV) recombinant vaccines on immunogenicity and protective efficacy against SIV(mac251)

Oral, replication-competent Ad-HIV vaccines are advancing to human trials. Previous evaluation of protective efficacy in non-human primates has primarily followed upper respiratory tract administrations. Here we compared sequential oral (O/O) versus intranasal/oral (I/O) priming of rhesus macaques with Ad5 host range mutant-SIV recombinants expressing SIV env/rev, gag, and nef genes followed by boosting with SIV gp120 protein. Cellular immune responses in PBMC were stronger and more frequent after I/O administration. Both groups developed mucosal immunity, including memory cells in bronchial alveolar lavage, and gut-homing receptors on PBMC. Following intrarectal SIV(mac251) challenge, both groups exhibited equivalent, significant protection and robust post-challenge cellular immunity. Our results illustrate the promise of oral replication-competent Ad-recombinant vaccines. Pre-challenge PBMC ELISPOT and proliferative responses did not predict protection in the O/O group, highlighting the need for simple, non-invasive methods to reliably assess mucosal immunity.

Seroprevalence of neutralizing antibodies to adenovirus type 5 among children in India: implications for recombinant adenovirus-based vaccines

We determined the levels of adenovirus 5 (Ad5) neutralizing antibodies in children in India less than 2 years of age. The results clearly show an age-dependent increase in Ad5-specific immunity, with 7- to 12-month-old children having the lowest levels of Ad5 immunity. This opens up the scope for the use of recombinant Ad5-based vaccines in this age group.

Comparison of intranasal with targeted lymph node immunization using PR8-Flu ISCOM adjuvanted HIV antigens in macaques

The rapidly spreading HIV epidemic requires a vaccine that elicits potent mucosal immunity to halt or slow transmission. Induction of these responses will depend on the use of appropriate adjuvants and targeting of the mucosal immune system. Previously, immune stimulating complexes (ISCOM) have shown great potency as adjuvant in the induction of mucosal responses in mice and systemic responses in non-human primates. In this study, HIV formulated in PR8-Flu ISCOM adjuvant was applied to immunize rhesus macaques against HIV; targeting the mucosa either via intranasal (IN) application or via targeted lymph node immunization (TLNI). While, strong systemic, HIV specific, cytokine, lymphoproliferative, and antibody responses were induced via the TLNI route, the IN application generated only low responses. Furthermore, all four animals immunized via TLNI developed vaginal IgA antibodies against gp120. In conclusion, in contrast to what has been demonstrated in mice, the IN application of PR8-Flu ISCOM did not induce strong immune responses in rhesus macaques unlike those immunized by the TLNI route.

Immune response in rhesus macaques after mixed modality immunisations with DNA, recombinant adenovirus and recombinant gp120 from human immunodeficiency virus type 1

The establishment of effective regimens for a vaccine against human immunodeficiency virus type 1 (HIV-1) is urgently needed. In the present study we have produced HIV-1 gp120 from a vaccine-relevant primary R5 isolate in recombinant vaccinia (rVV)-infected Vero cells. We have investigated the effect of boosting with this protein in mixed modality immunisations of rhesus macaques following different immunisation. As reported earlier, animals were primed with codon-optimised HIV-1(BX08)env DNA delivered as plasmid or as replication-deficient recombinant human adenovirus type 5 (rAd5), which both induced specific antibody and cellular immune responses (1). Boosting with rAd5 temporarily had increased the anti-gp120 antibody titres approximately 1 log (rAd5+rAd5) or 3 log (DNA+rAd5) (1). However, secondary rAd5 boosting showed less effect due to the induced vector-specific immunity. To further boost the antibody response, the rgp120(BX08) was injected with Quadri A saponin adjuvant. The protein boosting resulted in a 1-2 log antibody increase and also boosting of the cell-mediated immune response. Neutralising antibodies to the heterologous HIV-1(MN) were detected; however, neutralising antibodies to the primary HIV-1(Bx08) isolate were seen only transiently after rAd5 but not the rgp120 immunisation. It is concluded that the rgp120(Bx08) reagent from rVV-infected Vero cells is functional and immunogenic in macaques, inducing both antibody and cellular immunity. The rgp120(Bx08) is a relevant model antigen that may be used to boost antibody and cellular immunity in mixed modality vaccine regimens against HIV-1 in higher animals.

Evaluation of passively transferred, nonneutralizing antibody-dependent cellular cytotoxicity-mediating IgG in protection of neonatal rhesus macaques against oral SIVmac251 challenge

Previously, Ab-dependent cellular cytotoxicity (ADCC) was significantly correlated with reduced acute viremia upon intrarectal SIVmac251 challenge of immunized rhesus macaques. To directly assess ADCC protective efficacy, six neonatal macaques were infused s.c. with immune IgG (220 mg/kg) purified from the immunized animals and positive for ADCC and Ab-dependent cell-mediated viral inhibition (ADCVI) activities. Six neonates received control IgG. The neonates were challenged twice orally with 10(5) 50% inhibiting tissue culture-infective dose of SIVmac251 2 days post-IgG infusion. At challenge, plasma of neonates that received immune IgG did not neutralize SIVmac251 but had geometric mean ADCC titers of 48,130 and 232,850 against SIVmac251 -infected and gp120-coated targets, respectively. Peak ADCVI activity varied from 62 to 81%. ADCC activity declined with the 2-wk IgG half-life but was boosted at wk 4, together with de novo ADCC-mediating Abs in controls, by postchallenge viremia. ADCVI activity was similarly induced. No protection, assessed by viral burdens, CD4 counts, and time to euthanasia was observed. Possible factors contributing to the discrepancy between the previous correlation and lack of protection here include: the high oral challenge dose compared with the 400-fold lower intrarectal dose; the challenge route with regard to viral dissemination and distribution of infused IgG; insufficient NK effector activity and/or poor functionality in newborns; insufficient immune IgG; and the possibility that the previous correlation of ADCC with protection was augmented by cellular immune responses also present at challenge. Future studies should explore additional challenge routes in juvenile macaques using higher amounts of potent IgG preparations.

HIV vaccines: can mucosal CD4 T cells be protected?

PURPOSE OF REVIEW

The aim of this article is to understand the significance of protecting the mucosal tissue compartment during acute HIV infection, and to describe the current efforts towards this goal.

RECENT FINDINGS

The mucosa is the primary route of HIV transmission, and serves as a major site for viral dissemination leading to a massive destruction of the memory CD4 T cell compartment. This destruction is mediated as a consequence of direct viral infection and occurs in all the tissues of the body suggesting that once infection explodes out of the mucosal tissues memory CD4 T cells at all other sites are very rapidly infected and destroyed.

SUMMARY

The enrichment of highly susceptible CD4 targets in mucosal tissues suggests that the immune system will need to be in a state of high alert to contain infection once HIV crosses the mucosal barrier. This will require the generation and maintenance of strong vaccine-induced neutralizing antibodies and CD8 T cell responses in mucosal tissues. Given the challenges of inducing neutralizing antibodies, current efforts are focused on developing a T cell based vaccine that can contain the spread of HIV infection. Developing a T cell based vaccine is hampered by the lack of any predictive correlates of protection. In the absence of such correlates, protection can be measured by the extent to which mucosal CD4 T cells are preserved. Preservation of mucosal CD4 T cells will have a significant impact on disease course and long-term outcome.

Durable protection of rhesus macaques immunized with a replicating adenovirus-SIV multigene prime/protein boost vaccine regimen against a second SIVmac251 rectal challenge: role of SIV-specific CD8+ T cell responses

Previously, priming with replication-competent adenovirus-SIV multigenic vaccines and boosting with envelope subunits strongly protected 39% of rhesus macaques against rectal SIV(mac251) challenge. To evaluate protection durability, eleven of the protected and two SIV-infected unimmunized macaques that controlled viremia were re-challenged rectally with SIV(mac251). Strong protection was observed in 8/11 vaccinees, including two exhibiting <50 SIV RNA copies. Decreased viremia compared to naïve controls was observed in the other three. The SIV-infected unimmunized macaques modestly controlled viremia but exhibited CD4 counts < or =200, unlike the protected macaques. Durable protection was associated with significantly increased SIV-specific ELISPOT responses and lymphoproliferative responses to p27 at re-challenge. After CD8 depletion, 2 of 8 re-challenged, protected vaccinees maintained <50 SIV RNA copies; SIV RNA emerged in 6. Re-appearance of CD8 cells and restoration of SIV-specific cellular immunity coincided with viremia suppression. Overall, cellular immunity induced by vaccination and/or low-level, inapparent viremia post-first SIV(mac251) challenge, was associated with durable protection against re-challenge.

Oral delivery of replication-competent adenovirus vectors is well tolerated by SIV- and SHIV-infected rhesus macaques

Although replication-competent adenovirus (Ad) vectors are promising in AIDS vaccine design, their safety in immune compromised hosts is unknown. To initially address this question, enteric-coated tablets containing a replicating Ad vector were orally administered to SHIV- and SIV-infected rhesus macaques with normal, intermediate or low CD4 T cell counts and stable disease. The vector was detected within a week after tablet administration in stools of all animals but not in nasal secretions, indicating no spread of virus to the upper respiratory tract. CD4 T cell counts and viral loads remained stable in all animals and no signs of fever, weight loss, or other clinical symptoms of Ad-induced disease were observed during 10 weeks of follow-up. Oral delivery of the replicating Ad vector was safe and well tolerated by SHIV- and SIV-infected hosts. Oral enteric-coated tablets may prove safe for administering replicating Ad-vectored vaccines in areas with high HIV prevalence.

Enhanced immunity and protective efficacy against SIVmac251 intrarectal challenge following ad-SIV priming by multiple mucosal routes and gp120 boosting in MPL-SE

Previously, 39% of rhesus macaques primed orally, intranasally, and intratracheally with adenovirus (Ad)-simian immunodeficiency virus (SIV) recombinants and boosted with gp120 in monophosphoryl lipid A-stable emulsion (MPL-SE) remained aviremic or cleared or controlled viremia at the threshold of detection following SIV(mac251) intrarectal challenge (Study B). In contrast, no macaques primed orally and intranasally with Ad-SIV recombinants and boosted with gp120 in Quillaja Saponaria-21 exhibited undetectable viremia post-challenge (Study A). We conducted a detailed comparison of the studies to elucidate the effect of different vaccine regimens on induced immunity associated with the different challenge outcomes. Quantitative viral load comparisons were statistically analyzed. All immune responses were assessed at identical timepoints post-immunization, and cellular immunity was re-evaluated on cryopreserved cells from Study B macaques to match Study A data acquired with frozen cells. Study B exhibited greater protective efficacy, increased levels of p11C and p54m tetramer positive cells and a trend toward enhanced interferon-gamma secreting cells in response to Env and Gag peptides, modestly enhanced serum neutralizing antibodies, and greater positivity in anti-gp120 rectal IgA and IgG antibodies. Study A macaques exhibited greater positivity in salivary IgA anti-gp120 antibodies. Thus, the vaccine regimen using oral-intranasal-intratracheal priming and protein boosting in MPL-SE was superior, eliciting greater protective efficacy against pathogenic SIV(mac251) and enhanced SIV-specific immunity, systemically and at rectal sites. The mechanism(s) by which binding antibodies, lacking neutralizing activity against the primary challenge virus, may contribute to protection requires further study.

Replicating rather than nonreplicating adenovirus-human immunodeficiency virus recombinant vaccines are better at eliciting potent cellular immunity and priming high-titer antibodies

A major challenge in combating the human immunodeficiency virus (HIV) epidemic is the development of vaccines capable of inducing potent, persistent cellular immunity and broadly reactive neutralizing antibody responses to HIV type 1 (HIV-1). We report here the results of a preclinical trial using the chimpanzee model to investigate a combination vaccine strategy involving sequential priming immunizations with different serotypes of adenovirus (Ad)/HIV-1(MN)env/rev recombinants and boosting with an HIV envelope subunit protein, oligomeric HIV(SF162) gp140deltaV2. The immunogenicities of replicating and nonreplicating Ad/HIV-1(MN)env/rev recombinants were compared. Replicating Ad/HIV recombinants were better at eliciting HIV-specific cellular immune responses and better at priming humoral immunity against HIV than nonreplicating Ad-HIV recombinants carrying the same gene insert. Enhanced cellular immunity was manifested by a greater frequency of HIV envelope-specific gamma interferon-secreting peripheral blood lymphocytes and better priming of T-cell proliferative responses. Enhanced humoral immunity was seen in higher anti-envelope binding and neutralizing antibody titers and better induction of antibody-dependent cellular cytotoxicity. More animals primed with replicating Ad recombinants mounted neutralizing antibodies against heterologous R5 viruses after one or two booster immunizations with the mismatched oligomeric HIV-1(SF162) gp140deltaV2 protein. These results support continued development of the replicating Ad-HIV recombinant vaccine approach and suggest that the use of replicating vectors for other vaccines may prove fruitful.

Priming with plasmid DNAs expressing interleukin-12 and simian immunodeficiency virus gag enhances the immunogenicity and efficacy of an experimental AIDS vaccine based on recombinant vesicular stomatitis virus

Of the various approaches being developed as prophylactic HIV vaccines, those based on a heterologous plasmid DNA prime, live vector boost vaccination regimen appear especially promising in the nonhuman primate/simian-human immunodeficiency virus (SHIV) challenge model. In this study, we sought to determine whether a series of intramuscular priming immunizations with a plasmid DNA vaccine expressing SIVgag p39, in combination with plasmid expressed rhesus IL-12, could effectively enhance the immunogenicity and postchallenge efficacy of two intranasal doses of recombinant vesicular stomatitis virus (rVSV)-based vectors expressing HIV-1 env 89.6P gp160 and SIVmac239 gag p55 in rhesus macaques. In macaques receiving the combination plasmid DNA prime, rVSV boost vaccination regimen we observed significantly increased SIVgag- specific cell-mediated and humoral immune responses and significantly lower viral loads postintravenous SHIV89.6P challenge relative to macaques receiving only the rVSV vectored immunizations. In addition, the plasmid DNA prime, rVSV boost vaccination regimen also tended to increase the preservation of peripheral blood CD4+ cells and reduce the morbidity and mortality associated with SHIV89.6P infection. An analysis of immune correlates of protection after SHIV89.6P challenge revealed that the prechallenge SHIV-specific IFN-gamma ELISpot response elicited by vaccination and the ability of the host to mount a virus-specific neutralizing antibody response postchallenge correlated with postchallenge clinical outcome. The correlation between vaccine-elicited cell-mediated immune responses and an improved clinical outcome after SHIV challenge provides strong justification for the continued development of a cytokine-enhanced plasmid DNA prime, rVSV vector boost immunization regimen for the prevention of HIV infection.

Adenoviral Vectors for Mucosal Vaccination Against Infectious Diseases

Adenoviral vector has been extensively studied as a vaccine platform because of its ability to induce potent cellular and humoral immunity. One main advantage of adenoviral vectors is their natural tropism for mucosal surfaces, which makes them ideal for the purpose of mucosal vaccination against pathogens that preferentially initiate infection at the mucosal site. The current understanding of mucosal immunity suggests that mucosal vaccination is far superior to parenteral vaccination in protecting mucosal surfaces. Mucosal vaccination is particularly relevant to those infections for which parenteral immunization strategies have failed to confer protection. This review examines the use of adenoviral vector at mucosal sites for infectious disease against which the current vaccination strategies have been unsuccessful in eliciting protection. Data from animal models have suggested that adenoviral vectors are effective in protecting against infections caused by HIV, herpes simplex virus and Mycobacterium tuberculosis. We believe that these encouraging results will lead to further evaluation in clinical trials in the near future.

Vaccine-elicited antibodies mediate antibody-dependent cellular cytotoxicity correlated with significantly reduced acute viremia in rhesus macaques challenged with SIVmac251

Effector cells armed with Abs can eliminate virus-infected target cells by Ab-dependent cellular cytotoxicity (ADCC), an immune mechanism that has been largely overlooked in HIV vaccine development. Here, we show that a prime/boost AIDS vaccine approach elicits potent ADCC activity correlating with protection against SIV in rhesus macaques (Macacca mulatta). Priming with replicating adenovirus type 5 host range mutant-SIV recombinants, followed by boosting with SIV gp120, elicited Abs with ADCC activity against SIV(mac251)-infected cells. In vitro ADCC activity correlated with in vivo reduced acute viremia after a mucosal challenge with pathogenic SIV. Our findings expose ADCC activity as an immune correlate that may be relevant in the rational design of an efficacious vaccine against HIV.

Protection against mucosal simian immunodeficiency virus SIV(mac251) challenge by using replicating adenovirus-SIV multigene vaccine priming and subunit boosting

Whereas several recent AIDS vaccine strategies have protected rhesus macaques against a pathogenic simian/human immunodeficiency virus (SHIV)(89.6P) challenge, similar approaches have provided only modest, transient reductions in viral burden after challenge with virulent, pathogenic SIV, which is more representative of HIV infection of people. We show here that priming with replicating adenovirus recombinants encoding SIV env/rev, gag, and/or nef genes, followed by boosting with SIV gp120 or an SIV polypeptide mimicking the CD4 binding region of the envelope, protects rhesus macaques from intrarectal infection with the highly pathogenic SIV(mac251). Using trend analysis, significant reductions in acute-phase and set point viremia were correlated with anti-gp120 antibody and cellular immune responses, respectively. Within immunization groups exhibiting significant protection, a subset (39%) of macaques have exhibited either no viremia, cleared viremia, or controlled viremia at the threshold of detection, now more than 40 weeks postchallenge. This combination prime-boost strategy, utilizing replication competent adenovirus, is a promising alternative for HIV vaccine development.

Evaluation of combination DNA/replication-competent Ad-SIV recombinant immunization regimens in rhesus macaques

Combination vaccine regimens in which priming with recombinant DNA is followed by boosting with recombinant viral vectors have been shown in previous studies to effectively enhance cellular immunity. However, no information exists concerning possible synergy of the cellular immune response when DNA immunization is followed by administration of a recombinant vector able to replicate. As our approach makes use of replication-competent Ad HIV and SIV recombinants, we performed a pilot experiment in six rhesus macaques in which we compared immunogenicity resulting from priming with one or two DNA recombinants encoding the SIVsmH4 env and rev genes with that elicited by a single replication-competent Ad5hr-SIV env/rev priming immunization. All macaques were subsequently administered an Ad5hr-SIV env/rev booster immunization followed by two immunizations with SIV gp120 protein. The choice of the env gene as target immunogen allowed comparison of induced cellular immune responses as well as binding and neutralizing antibodies elicited in serum and mucosal secretions. We report here that all immunized monkeys developed strong cellular immunity to the SIV envelope as shown by secretion of interferon-gamma, lysis of envelope-expressing target cells, and/or proliferation in response to gp120 or inactivated SIV. Similarly, all macaques developed anti-gp120 binding antibodies and neutralizing antibodies in serum and IgG and IgA binding antibodies in mucosal secretions. We did not observe consistently enhanced immune responses in any immunization group. We conclude that two sequential immunizations with the same replication-competent Ad5hr-SIV recombinant is as effective as priming with one or two recombinant DNA vaccines followed by a single Ad5hrSIV recombinant immunization.

Induction of CD8+T Cells to an HIV-1 Antigen through a Prime Boost Regimen with Heterologous E1-Deleted Adenoviral Vaccine Carriers

E1-deleted adenoviral recombinants most commonly based on the human serotype 5 (AdHu5) have been shown thus far to induce unsurpassed transgene product-specific CD8(+) T cell responses. A large percentage of the adult human population carries neutralizing Abs due to natural exposures to AdHu5 virus. To circumvent reduction of the efficacy of adenovirus (Ad) vector-based vaccines by neutralizing Abs to the vaccine carrier, we developed E1-deleted adenoviral vaccine carriers based on simian serotypes. One of these carriers, termed AdC68, expressing a codon-optimized truncated form of gag of HIV-1 was shown previously to induce a potent transgene product-specific CD8(+) T cell response in mice. We constructed a second chimpanzee adenovirus vaccine vector, termed AdC6, also expressing the truncated gag of HIV-1. This vector, which belongs to a different serotype than the AdC68 virus, induces high frequencies of gag-specific CD8(+) T cells in mice including those pre-exposed to AdHu5 virus. Generation of an additional E1-deleted adenoviral vector of chimpanzee origin allows for sequential booster immunizations with heterologous vaccine carriers. In this study, we show that such heterologous prime boost regimens based on E1-deleted adenoviral vectors of different serotypes expressing the same transgene product are highly efficient in increasing the transgene product-specific CD8(+) T cell response. They are equivalent to sequential vaccinations with an E1-deleted Ad vector followed by booster immunization with a poxvirus vector and they surpass regimens based on DNA vaccine prime followed by a recombinant adenoviral vector boost.

Potent, persistent induction and modulation of cellular immune responses in rhesus macaques primed with Ad5hr-simian immunodeficiency virus (SIV) env/rev, gag, and/or nef vaccines and boosted with SIV gp120

Immunity elicited by multicomponent vaccines delivered by replication-competent Ad5hr-simian immunodeficiency virus (SIV) recombinants was systematically investigated. Rhesus macaques were immunized mucosally at weeks 0 and 12 with Ad5hr-SIV(smH4) env/rev, with or without Ad5hr-SIV(mac239) gag or Ad5hr-SIV(mac239) nef, or with all three recombinants. The total Ad5hr dosage was comparably adjusted among all animals with empty Ad5hr-DeltaE3 vector. The macaques were boosted with SIV gp120 in monophosphoryl A-stable emulsion adjuvant at 24 and 36 weeks. Controls received Ad5hr-DeltaE3 vector or adjuvant only. By ELISPOT analysis, all four SIV gene products elicited potent cellular immune responses that persisted 42 weeks post-initial immunization. Unexpectedly, modulation of this cellular immune response was observed among macaques receiving one, two, or three Ad5hr-SIV recombinants. Env responses were significantly enhanced throughout the immunization period in macaques immunized with Ad5hr-SIV env/rev plus Ad5hr-SIV gag and tended to be higher in macaques that also received Ad5hr-SIV nef. Macaques primed with all three recombinants displayed significant down-modulation in numbers of gamma interferon (IFN-gamma)-secreting cells specific for SIV Nef, and the Env- and Gag-specific responses were also diminished. Modulation of antibody responses was not observed. Down-modulation was seen only during the period of Ad5hr-recombinant priming, not during subunit boosting, although SIV-specific IFN-gamma-secreting cells persisted. The effect was not attributable to Ad5hr replication differences among immunization groups. Vaccine delivery via replication-competent live vectors, which can persistently infect new cells and continuously present low-level antigen, may be advantageous in overcoming competition among complex immunogens for immune recognition. Effects of current multicomponent vaccines on individual immune responses should be evaluated with regard to future vaccine design.