Hildegund C. J. Ertl

Adenoviruses as vaccine vectors.

n n Adenoviruses have transitioned from tools for gene replacement therapy to bona fide vaccine delivery vehicles. They are attractive vaccine vectors as they induce both innate and adaptive immune responses in mammalian hosts. Currently, adenovirus vectors are being tested as subunit vaccine systems for numerous infectious agents ranging from malaria to HIV-1. Additionally, they are being explored as vaccines against a multitude of tumor-associated antigens. In this review we describe the molecular biology of adenoviruses as well as ways the adenovirus vectors can be manipulated to enhance their efficacy as vaccine carriers. We describe methods of evaluating immune responses to transgene products expressed by adenoviral vectors and discuss data on adenoviral vaccines to a selected number of pathogens. Last, we comment on the limitations of using human adenoviral vectors and provide alternatives to circumvent these problems. This field is growing at an exciting and rapid pace, thus we have limited our scope to the use of adenoviral vectors as vaccines against viral pathogens.n n

A Simian Replication-Defective Adenoviral Recombinant Vaccine to HIV-1 Gag

In animal models, E1-deleted human adenoviral recombinants of the serotype 5 (AdHu5) have shown high efficacy as vaccine carriers for different Ags including those of HIV-1. Humans are infected by common serotypes of human adenovirus such as AdHu5 early in life and a significant percentage has high levels of neutralizing Abs to these serotypes, which will very likely impair the efficacy of recombinant vaccines based on the homologous virus. To circumvent this problem, a novel replication-defective adenoviral vaccine carrier based on an E1-deleted recombinant of the chimpanzee adenovirus 68 (AdC68) was developed. An AdC68 construct expressing a codon-optimized, truncated form of gag of HIV-1 induces CD8+ T cells to gag in mice which at the height of the immune response encompass nearly 20% of the entire splenic CD8+ T cell population. The vaccine-induced immune response provides protection to challenge with a vaccinia gag recombinant virus. Induction of transgene-specific CD8+ T cells and protection against viral challenge elicited by the AdC68 vaccines is not strongly inhibited in animals preimmune to AdHu5 virus. However, the response elicited by the AdHu5 vaccine is greatly attenuated in AdHu5 preimmune animals.

Management of Rabies in Humans

Rabies is a fatal disease in humans, and, to date, the only survivors of the disease have received rabies vaccine before the onset of illness. The approach to management of the rabies normally should be palliative. In unusual circumstances, a decision may be made to use an aggressive approach to therapy for patients who present at an early stage of clinical disease. No single therapeutic agent is likely to be effective, but a combination of specific therapies could be considered, including rabies vaccine, rabies immunoglobulin, monoclonal antibodies, ribavirin, interferon-alpha, and ketamine. Corticosteroids should not be used. As research advances, new agents may become available in the future for the treatment of human rabies.

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.

Human Immunodeficiency Virus Type 1-Specific Immune Responses in Primates upon Sequential Immunization with Adenoviral Vaccine Carriers of Human and Simian Serotypes

ABSTRACT Two triple immunization vaccine regimens with adenoviral vectors with E1 deleted expressing Gag of human immunodeficiency virus type 1 were tested for induction of T- and B-cell-mediated-immune responses in mice and in nonhuman primates. The vaccine carriers were derived from distinct serotypes of human and simian adenoviruses that fail to elicit cross-neutralizing antibodies expected to dampen the effect of booster immunizations. Both triple immunization regimens induced unprecedented frequencies of gamma interferon-producing CD8+ T cells to Gag in mice and monkeys that remained remarkably stable over time. In addition, monkeys developed Gag-specific interleukin-2-secreting T cells, presumably belonging to the CD4+ T-cell subset, and antibodies to both Gag and the adenoviral vaccine carriers.

Dendritic Cell Maturation, but Not CD8+ T Cell Induction, Is Dependent on Type I IFN Signaling during Vaccination with Adenovirus Vectors

To understand how vaccines initiate adaptive immune responses, it is necessary to study how they interact with APCs such as dendritic cells (DCs). In this study, we analyzed interactions between recombinant adenovirus (Ad) vectors and mouse DCs. Mouse bone marrow-derived DCs transduced with Ad vectors produced type I IFN, which promoted the maturation of both transduced and bystander DCs. DCs transduced with a vector derived from a chimpanzee Ad serotype (AdC68) produced more type I IFN and matured more efficiently compared with DCs transduced with a vector derived from a human Ad serotype (AdHu5). Both vectors stimulated type I IFN production independently of viral transcription, replication, and TLR signaling. However, each vector induced type I IFN through distinct pathways; whereas AdHu5 vectors required phosphoinositide-3-OH kinase for type I IFN induction, AdC68 vectors did not. Both vectors induced strong transgene product-specific CD8+ T cell responses in wild-type mice. DCs isolated from mice that have a defect in type I IFN signaling failed to undergo full maturation after Ad vaccination, but surprisingly, these mice mounted strong transgene product-specific CD8+ T cell responses. In these mice, we were able to detect a small number of transduced DCs that expressed high levels of costimulatory molecules, and these DCs were able to stimulate transgene product-specific CD8+ T cells. Thus, type I IFN signaling is an important component of Ad-mediated DC maturation but is dispensable during the generation of transgene product-specific CD8+ T cell responses.

Heterologous prime/boost immunizations of rhesus monkeys using chimpanzee adenovirus vectors.

Pre-existing immunity to human adenovirus serotype 5 (AdHu5) has been shown to suppress the immunogenicity of recombinant Ad5 (rAdHu5) vector-based vaccines for human immunodeficiency virus type 1 (HIV-1) in both preclinical studies and clinical trials. As a potential solution to this problem we developed adenovirus vaccine vectors of chimpanzee origin. In the present study we assessed the immunogenicity of various chimpanzee adenovirus vectors in a prime/boost regimen to HIV-1 envelope and SIV Gag-Pol in rhesus monkeys and their ability to protect against pathogenic viral challenge. Although rAdHu5-primed monkeys had higher magnitude T cell responses than rAdC7 or rAdC68 prior to challenge, the rAdC7-rAdC1/C5 and rAdHu5-rAdC1/C5 immunizations resulted in comparable magnitude recall cellular immune responses and comparable level of control of viremia post-challenge.

Two distinct activation states of plasmacytoid dendritic cells induced by influenza virus and CpG 1826 oligonucleotide

There is growing evidence that plasmacytoid dendritic cells (pDC) are involved in the innate recognition of various microbes. However, the precise consequences of pathogen recognition on pDC activation and function are incompletely understood. Using a novel transgenic mouse model that facilitates the isolation of highly pure pDC populations, we found that influenza virus PR/8, a TLR7 ligand, and CpG 1826 oligonucleotide, a TLR9 ligand, induced surprisingly divergent activation programs in these cells. pDC stimulated with PR/8 produced large amounts of type I IFNs, and CpG 1826‐stimulated pDC expressed higher levels of costimulatory molecules and proinflammatory cytokines and induced stronger proliferation of T cells. Transcriptome analysis uncovered the differential regulation in pDC of 178 and 1577 genes by PR/8 and CpG 1826, respectively. These differences may relate to the activation of discrete signaling pathways, as evidenced by distinct ERK1/2 and p38 MAPK phosphorylation kinetics. Finally, pDC isolated ex vivo during PR/8 infection or after i.v. CpG 1826 injection resembled their in vitro counterparts, corroborating that these cells can adopt specialized phenotypes in vivo. Thus, pDC display remarkable functional flexibility, which emphasizes their versatile functions in antimicrobial immunity and inflammatory processes.

Mucosally Delivered E1-Deleted Adenoviral Vaccine Carriers Induce Transgene Product-Specific Antibody Responses in Neonatal Mice

E1-deleted adenoviral vectors of the human serotype 5 (AdHu5) and the chimpanzee serotype 68 (AdC68) expressing the rabies virus glycoprotein (rab.gp) were tested for induction of transgene product-specific Abs upon intranasal or oral immunization of newborn mice. Both vectors induced Abs to rabies virus that could be detected in serum and from mucosal secretions. Serum rabies virus neutralizing Ab titers sufficed to protect neonatally vaccinated mice against a subsequent challenge with rabies virus. The efficacy of the AdHu5rab.gp vector given orally to newborn mice born to AdHu5 virus-immune dams was not impaired by maternally transferred Abs to the vaccine carrier.

Superior In vivo Transduction of Human Hepatocytes Using Engineered AAV3 Capsid

Adeno-associated viral (AAV) vectors are currently being tested in multiple clinical trials for liver-directed gene transfer to treat the bleeding disorders hemophilia A and B and metabolic disorders. The optimal viral capsid for transduction of human hepatocytes has been under active investigation, but results across various models are inconsistent. We tested in vivo transduction in humanized mice. Methods to quantitate percent AAV transduced human and murine hepatocytes in chimeric livers were optimized using flow cytometry and confocal microscopy with image analysis. Distinct transduction efficiencies were noted following peripheral vein administration of a self-complementary vector expressing a gfp reporter gene. An engineered AAV3 capsid with two amino acid changes, S663V+T492V (AAV3-ST), showed best efficiency for human hepatocytes (~3-times, ~8-times, and ~80-times higher than for AAV9, AAV8, and AAV5, respectively). AAV5, 8, and 9 were more efficient in transducing murine than human hepatocytes. AAV8 yielded the highest transduction rate of murine hepatocytes, which was 19-times higher than that for human hepatocytes. In summary, our data show substantial differences among AAV serotypes in transduction of human and mouse hepatocytes, are the first to report on AAV5 in humanized mice, and support the use of AAV3-based vectors for human liver gene transfer.