Novel role for E4 region genes in protection of adenovirus vectors from lysis by cytotoxic T lymphocytes

Induction of cancer-specific cell death by the adenovirus E4orf4 protein

The adenovirus E4orf4 protein is a multifunctional viral regulator that contributes to temporal regulation of the progression of viral infection. When expressed alone, outside the context of the virus, E4orf4 induces p53-independent cell-death in transformed cells. Oncogenic transformation of primary cells in tissue culture sensitizes them to cell killing by E4orf4, indicating that E4orf4 research may have implications for cancer therapy. It has also been reported that E4orf4 induces a caspase-independent, non-classical apoptotic pathway, which maintains crosstalk with classical caspase-dependent pathways. Furthermore, several E4orf4 activities in the nucleus and in the cytoplasm and various protein partners contribute to cell killing by this viral protein. In the following chapter I summarize the current knowledge of the unique mode of E4orf4-induced cell death and its underlying mechanisms. Although several explanations for the cancer-specificity of E4orf4-induced toxicity have been proposed, a better grasp of the mechanisms responsible for E4orf4-induced cell death is required to elucidate the differential sensitivity of normal and cancer cells to E4orf4.

Effects of the deletion of early region 4 (E4) open reading frame 1 (orf1), orf1-2, orf1-3 and orf1-4 on virus-host cell interaction, transgene expression, and immunogenicity of replicating adenovirus HIV vaccine vectors

The global health burden engendered by human immunodeficiency virus (HIV)-induced acquired immunodeficiency syndrome (AIDS) is a sobering reminder of the pressing need for a preventative vaccine. In non-human primate models replicating adenovirus (Ad)-HIV/SIV recombinant vaccine vectors have been shown to stimulate potent immune responses culminating in protection against challenge exposures. Nonetheless, an increase in the transgene carrying capacity of these Ad vectors, currently limited to approximately 3000 base pairs, would greatly enhance their utility. Using a replicating, E3-deleted Ad type 5 host range mutant (Ad5 hr) encoding full-length single-chain HIVBaLgp120 linked to the D1 and D2 domains of rhesus macaque CD4 (rhFLSC) we systematically deleted the genes encoding early region 4 open reading frame 1 (E4orf1) through E4orf4. All the Ad-rhFLSC vectors produced similar levels of viral progeny. Cell cycle analysis of infected human and monkey cells revealed no differences in virus-host interaction. The parental and E4-deleted viruses expressed comparable levels of the transgene with kinetics similar to Ad late proteins. Similar levels of cellular immune responses and transgene-specific antibodies were elicited in vaccinated mice. However, differences in recognition of Ad proteins and induced antibody subtypes were observed, suggesting that the E4 gene products might modulate antibody responses by as yet unknown mechanisms. In short, we have improved the transgene carrying capacity by one thousand base pairs while preserving the replicability, levels of transgene expression, and immunogenicity critical to these vaccine vectors. This additional space allows for flexibility in vaccine design that could not be obtained with the current vector and as such should facilitate the goal of improving vaccine efficacy. To the best of our knowledge, this is the first report describing the effects of these E4 deletions on transgene expression and immunogenicity in a replicating Ad vector.

Transcription mapping and characterization of proteins produced from early region 4 of porcine adenovirus type 3

The early region 4 (E4) of porcine adenovirus 3 (PAdV-3) was characterized by Northern blot, rapid amplification of cDNA ends (RACE), RT-PCR and cDNA sequence analysis. Northern blot analysis revealed three different classes of transcripts, which appeared and peaked at different times post-infection. The RT-PCR, RACE and cDNA sequence analysis identified nine major E4 transcripts, all of which shared a 107-bp 5' leader sequence and a 126-bp 3' terminus. These transcripts have one to three introns removed. Interestingly, of the nine major transcripts, there was one fusion transcript of ORFp1 and ORFp7 (ORFp1/7), which codes for a protein of 119 amino acids. All transcripts initiated at nucleotide 33740 of the PAdV-3 genome. To identify proteins, rabbit antiserum was prepared using a bacterial fusion protein encoding p2, p3, p4 or p7 proteins. Serum against p2, p3 and p4 immunoprecipitated proteins of 13.5, 13.6 and 15.3 kDa, respectively, in in-vitro transcribed and translated mRNA and in PAdV-3-infected cells. Serum against p7 immunoprecipitated a protein of 19.8 kDa in in-vitro transcription and translation analysis but recognized two proteins of 19.8 kDa (encoded by ORFp7) and 14 kDa (encoded by the fusion transcript ORF1/7) in PAdV-3-infected cells. The protein encoded by ORFp2 was localized in the nucleus of PAdV-3-infected cells. The proteins encoded by ORFp3 and ORFp7\ORFp1/7 were detected in the cytoplasm of PAdV-3-infected cells. However, the protein encoded by ORFp4 was observed both in the cytoplasm and nucleus of PAdV-3-infected cells.

Gene expression changes induced by a recombinant E1-/E3- adenovirus type 5 vector in human mammary epithelial cells

OBJECTIVES

Adenoviral vectors are used in transferring exogenous genes to a variety of cells and tissue types both in vitro and in vivo. Gene expression changes induced by an E1/E3-defective adenovirus vector have been studied in human mammary epithelial cells by comparing the gene expression profile in infected and uninfected cells.

METHODS

The human mammary epithelial cell line HB2 was infected with an E1/E3-defective adenovirus type 5 vector. Total RNA was extracted from infected and uninfected cells 24 and 72 h after infection and subjected to microarray analysis using the Affymetrix U133A genomic chip system. Semiquantitative RT-PCR confirmed the regulation of genes observed by microarray analysis.

RESULTS

The microarray analysis showed 24 and 95 transcripts to be regulated 24 and 72 h after infection, respectively. A relatively high number of genes involved in innate and inflammatory host immune responses, including interleukin-8, interleukin-6, NF-kappaB(2), RELB and fos, were induced. As expected from an E1-defective virus, changes in the expression of genes involved in the G1-S transition and in the activation of cell proliferation were not detected.

CONCLUSION

Our study provides insight into the host transcriptional response following transduction of an adenoviral vector into mammary epithelial cells.

Subversion of host defense mechanisms by adenoviruses

Adenoviruses (Ads) cause acute and persistent infections. Alike the much more complex herpesviruses, Ads encode numerous immunomodulatory functions. About a third of the viral genome is devoted to counteract both the innate and the adaptive antiviral immune response. Immediately upon infection, E1A blocks interferon-induced gene expression and the VA-RNA inhibits interferon-induced PKR activity. At the same time, E1A reprograms the cell for DNA synthesis and induces the intrinsic cellular apoptosis program that is interrupted by E1B/19K and E1B/55K proteins, the latter inhibits p53-mediated apoptosis. Most other viral stealth functions are encoded by a separate transcription units, E3. Several E3 products prevent death receptor-mediated apoptosis. E3/14.7K seems to interfere with the cytolytic and pro-inflammatory activities of TNF while E3/10.4K and 14.5K proteins remove Fas and TRAIL receptors from the cell surface by inducing their degradation in lysosomes. These and other functions that may afect granule-mediated cell death might drastically limit lysis by NK cells and cytotoxic T cells (CTL). Moreover, Ads interfere with recognition of infected cell by CTL. The paradigmatic E3/19K protein subverts antigen presentation by MHC class I molecules by inhibiting their transport to the cell surface. In concert, these viral countermeasures ensure prolonged survival in the infected host and, as a consequence, facilitate transmission. Elucidating the molecular mechanisms of Ad-mediated immune evasion has stimulated corresponding research on other viruses. This knowledge will also be instrumental for designing better vectors for gene therapy and vaccination, and may lead to a more rational treatment of life-threatening Ad infections, e.g. in transplantation patients.

Adenovirus and adeno-associated virus vectors

Recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) are among the most extensively used vectors in gene therapy studies to date. These two vectors share some similar features such as a broad host range and ability to infect both proliferating and quiescent cells. However, they also possess their own unique set of properties that render them particularly attractive for gene therapy applications. rAd vectors can accommodate larger inserts, mediate transient but high levels of protein expression, and can be easily produced at high titers. Development of gutted rAd vectors has further increased the cloning capacity of these vectors. The gaining popularity of rAAV use in gene therapy can be attributed to its lack of pathogenicity and added safety due to its replication defectiveness, and its ability to mediate long-term expression in a variety of tissues. Site-specific integration, as occurs with wild-type AAV, will be a unique and valuable feature if incorporated into rAAV vectors, further improving their safety. This paper describes these properties of rAd and rAAV vectors, and discusses further development and vector improvements that continue to extend the utility of these vectors, such as cell retargeting by capsid modification, differential transduction by use of serotypes, and extension of the cloning capacity of rAAV vectors by dual vector heterodimerization.

The role of adenovirus E4orf4 protein in viral replication and cell killing

It has only been within the last few years that insights have been gained into the remarkable diversity of functions of the adenovirus early transcription region 4 (E4) products. The polypeptide encoded by E4 open reading frame 4 (E4orf4) has emerged as an enigmatic product. Although it accomplishes certain functions that propel viral replication, it has also been shown to be highly toxic, an effect that could dampen the infectious cycle, but that also might serve to facilitate release of viral progeny. When expressed alone, E4orf4 induces a novel form of p53-independent apoptosis in cancer cells but not in normal human cells, thus making it of potential use in cancer gene therapy. In addition, knowledge of its mechanism of action, especially with regard to its interaction with protein phosphatase 2A (PP2A), could provide insights to develop new small molecule anti-cancer drugs. Thus future studies on E4orf4 should be both informative and potentially valuable therapeutically. In this study we review the current status of knowledge on E4orf4.

Molecular regulation and biological function of adenovirus early genes: the E4 ORFs

Over the past few years there have been a number of interesting advances in our understanding of the functions encoded by the adenovirus early transcription unit 4 (Ad E4). A large body of recent data demonstrates that E4 proteins encompass an unexpectedly diverse collection of functions required for efficient viral replication. E4 gene products operate through a complex network of protein interactions with key viral and cellular regulatory components involved in transcription, apoptosis, cell cycle control and DNA repair, as well as host cell factors that regulate cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as nuclear bodies (NBs) or PML oncogenic domains (PODs). As understood at present, some of the lytic functions overlap with roles in oncogenic transformation of primary mammalian cells. These observations, together with findings that E4 proteins substantially affect cell toxicity and the immune response of the host have profound implications for the development of Ad vectors for gene therapy. In this article we will summarize recent findings regarding the diverse functions of E4 gene products in the context of earlier work. We will emphasize the interaction of E4 proteins with cellular and viral interaction partners, the role of these interactions for lytic virus growth and how these interactions may contribute to viral oncogenesis. Finally, we will discuss their role in Ad vector and adeno-associated virus infections.

Adenoviruses activate human dendritic cells without polarization toward a T-helper type 1-inducing subset

Human monocyte-derived dendritic cells (DC) infected with recombinant adenoviruses (rAd) are promising candidate vaccines for inducing protective immunity against pathogens and tumors. However, since some viruses are known to negatively affect DC function, it is important to investigate the interactions between rAd and DC. We now show that infection by rAd enhances the immunostimulatory capacity of immature human monocyte-derived DC through the upregulation of the costimulatory molecules CD80, CD86, and CD40 and the major histocompatibility complex class I and II molecules. Although rAd infection fails to induce the secretion of interleukin-12 (IL-12) and only marginally induces the expression of the DC maturation marker CD83, it acts in synergy with CD40 triggering in rendering DC fully mature. rAd-infected DC triggered through CD40 produce more IL-12 and are more efficient in eliciting T-helper type 1 responses than DC activated by CD40 triggering only. rAd lacking one or more of the early regions, E1, E2A, E3, and E4, which play an important role in virus-host cell interactions are equally capable of DC activation. Efficient DC infection requires a high multiplicity of infection (>1,000), a fact which can be attributed to the absence of the coxsackievirus and adenovirus receptor on this cell type. Despite the poor ability of DC to be infected by rAd, which may be improved by targeting rAd to alternative DC surface molecules, DC infected with all currently tested rAd constitute potent immunostimulators. Our study provides new insights into the interactions between two highly promising vaccine components, rAd and DC, and indicates that their combination into one vaccine may be very advantageous for the stimulation of T-cell immunity.

Adenovirus vectors for gene delivery

Recent endeavors in the development of adenovirus as a gene vector have focused on the modification of virus tropism, the accommodation of larger genes, and the increase in stability and control of transgene expression. Whereas partial or total deletions of viral genes increase the cloning capacity and partly reduce the cellular immune response, control of the humoral response, which often precludes efficient readministration, remains a challenge.

E4ORF3 requirement for achieving long-term transgene expression from the cytomegalovirus promoter in adenovirus vectors

Analysis of transgene expression under the control of the cytomegalovirus (CMV) promoter from adenovirus vectors in which the E4 region was modified indicated that E4ORF3 is required for long-term expression in the murine lung. CMV promoter truncation led to the persistence of expression in the absence of E4, thus eliminating the ORF3 requirement.