Adenovirus type 12 early region 1A proteins repress class I HLA expression in transformed human cells

Immune evasion by adenoviruses: a window into host-virus adaptation

Human adenoviruses (HAdVs) are widespread pathogens that cause a number of partially overlapping, species-specific infections associated with respiratory, urinary, gastrointestinal, and ocular diseases. The early 3 (E3) region of adenoviruses is highly divergent between different species, and it encodes a multitude of proteins with immunomodulatory functions. The study of genetic diversity in the E3 region offers a unique opportunity to gain insight into how the various HAdVs have evolutionarily adapted in response to the selection pressures exerted by host immune defenses. The objective of this review was to discuss subversion of host antiviral immune responses by HAdVs, with a focus on suppression of MHC class I antigen presentation, as a window into host-HAdV adaptation.

Adenovirus expressing β2-microglobulin recovers HLA class I expression and antitumor immunity by increasing T-cell recognition

Optimal tumor cell surface expression of human leukocyte antigen (HLA) class I molecules is essential for the presentation of tumor-associated peptides to T-lymphocytes. However, a hallmark of many types of tumor is the loss or downregulation of HLA class I expression associated with ineffective tumor antigen presentation to T cells. Frequently, HLA loss can be caused by structural alterations in genes coding for HLA class I complex, including the light chain of the complex, β2-microglobulin (β2m). Its best-characterized function is to interact with HLA heavy chain and stabilize the complex leading to a formation of antigen-binding cleft recognized by T-cell receptor on CD8+ T cells. Our previous study demonstrated that alterations in the β2m gene are frequently associated with cancer immune escape leading to metastatic progression and resistance to immunotherapy. These types of defects require genetic transfer strategies to recover normal expression of HLA genes. Here we characterize a replication-deficient adenoviral vector carrying human β2m gene, which is efficient in recovering proper tumor cell surface HLA class I expression in β2m-negative tumor cells without compromising the antigen presentation machinery. Tumor cells transduced with β2m induced strong activation of T cells in a peptide-specific HLA-restricted manner. Gene therapy using recombinant adenoviral vectors encoding HLA genes increases tumor antigen presentation and represents a powerful tool for modulation of tumor cell immunogenicity by restoration of missing or altered HLA genes. It should be considered as part of cancer treatment in combination with immunotherapy.

Computational analysis of human adenovirus serotype 18

The genome of the sole remaining unsequenced member of species A, human adenovirus type 18 (HAdV-A18), has been sequenced and analyzed. Members of species A are implicated as gastrointestinal pathogens and were shown to be tumorigenic in rodents. These whole genome and in silico proteome data are important as references for reexamining and integrating earlier work and observations based on lower resolution techniques, such as restriction enzyme digestion patterns, particularly for hypotheses based on pre-genomics data. Additionally, the genome of HAdV-A18 will also serve as reference for current studies examining the molecular evolution and origins of human and simian adenoviruses, particularly genome recombination studies. Applications of this virus as a potential vector for gene delivery protocols may be practical as data accumulate on this and other adenovirus genomes.

The N terminus of adenovirus type 12 E1A inhibits major histocompatibility complex class I expression by preventing phosphorylation of NF-kappaB p65 Ser276 through direct binding

The immune-escape strategy employed by human oncogenic adenovirus type 12 (Ad12) involves downregulation of major histocompatibility complex class I (MHC-I) transcription by disabling the transactivator NF-kappaB (p50/p65). This is accomplished by the Ad12 E1A protein (E1A-12), which prevents NF-kappaB from becoming phosphorylated by the protein kinase A catalytic subunit (PKAc). In this study, we examined the interactions between E1A-12 and NF-kappaB. Our data show that an E1A-12 mutant retaining the N-terminal 66 amino acids was as effective as the wild-type E1A-12 protein (266 amino acids) in binding p65, preventing phosphorylation of p65-Ser(276), and inhibiting transactivation. In contrast, the nontumorigenic adenovirus type 5 E1A protein (E1A-5) and other E1A-12 mutants lacking the N-terminal regions were severely defective in these activities. Further studies revealed that an N-terminal peptide consisting of residues 1 to 40 of E1A-12 was able to associate directly with p65 in vitro and prevent PKAc from phosphorylating p65-Ser(276). In the absence of the N terminus, there is an almost complete loss of E1A-12 binding to p65. These findings provide solid evidence for the role of the E1A-12 N terminus as an NF-kappaB binding domain. Significantly, this study indicates that the E1A-12 N terminus prevents PKAc from gaining access to p65 to account for Ser(276) hypophosphorylation. The E1A-12 N terminus interaction with p65 serves as a key explanation of how Ad12 downregulates MHC-I transcription and contributes to oncogenesis by escaping cytotoxic T lymphocytes.

c-MYC impairs immunogenicity of human B cells

Deregulation of c-myc expression through chromosomal translocation is essential in the pathogenesis of Burkitt's lymphoma (BL). A characteristic feature of BL cells, compared to Epstein-Barr Virus (EBV)-immortalized B cells, is their lack of immunogenicity. To study the contribution of EBV genes and of the c-MYC protein to this phenotype, we have generated a conditional B cell system in which the viral proliferation program and expression of c-myc can be regulated independently of each other. In cells proliferating due to exogenous c-myc overexpression, the cell surface phenotype, the pattern of proliferation in single cell suspension, and the immunological characteristics of BL cells could be completely recapitulated. Yet, it had remained open whether nonimmunogenicity is the default phenotype when EBNA2 and LMP1 are switched off, or whether c-MYC actively contributes to immunosuppression. We provide evidence also for the latter by showing that c-MYC down-regulates genes of the NF-kappaB and interferon pathway in a dose-dependent fashion. c-MYC acts at at least two different levels, the level of interferon induction as well as at the level of action of type I and type II interferons on their respective target promoters. c-MYC does not block the interferon pathway completely, it shifts the balance and increases the threshold of interferon induction and action.

BK virus and human cancer: innocent until proven guilty

BK virus (BKV) is a polyomavirus that ubiquitously infects the human population. Following a typically subclinical primary infection, BKV establishes a life-long persistent infection in the kidney and urinary tract. BKV is known to reactivate and cause severe disease in immunosuppressed patients, particularly renal and bone marrow transplant patients. Infection of BKV in rodent animal models or cells in culture often results in tumor formation or transformation, respectively. When co-expressed with activated oncogenes, BKV large tumor antigen drives the transformation of primary human cells. An etiological role of BKV in human cancer, however, remains controversial. Multiple reports have demonstrated conflicting results in regards to the presence of BKV sequences and/or proteins in various tumor types. This review compiles the most recent findings of BKV detection in a number of human cancers. Due to the lack of conclusive causality data from these studies, there does not appear to be a definitive association between BKV and human cancers.

Induction of neuronal and tumor-related genes by adenovirus type 12 E1A

Adenovirus type 12 (Ad12) E1A protein (E1A-12) contains a unique 20-amino-acid spacer region between the second and third conserved regions. Substitution of a single amino acid in the spacer is able to abrogate Ad12 tumorigenesis. To investigate the function of the spacer, microarray analysis was performed on cells transformed by tumorigenic and nontumorigenic Ad12s that differ only by one amino acid in the spacer. Fewer than 0.8% of approximately 8,000 genes in the microarray exhibited differential expression of threefold and higher. Of these, more than half of the known genes with higher expression in the wild-type Ad12-transformed cells have neuronal-specific functions. Some of the other differentially expressed genes are involved in the regulation of the cell cycle, transcription, cell structure, and tumor invasiveness. Northern blot analyses of a subset of the neuronal genes, including Robo1, N-MYC, and alpha-internexin, confirmed their strong expression in multiple Ad12 tumorigenic cell lines. In contrast, these neuronal genes displayed only minor or negligible expression in cells transformed by spacer-mutated Ad12. Significantly, stable introduction of E1A-12 into nontumorigenic Ad5-transformed cells induced neuronal gene expression. We found that the neuron-restrictive silencer factor, which serves as a master repressor of neuronal genes, was inactivated in both Ad12- and Ad5-transformed cells via cytoplasmic retention, though only Ad12-transformed cells exhibited neuronal gene induction. Mutational analyses of the alpha-internexin promoter demonstrated that E1A-12-mediated neuronal gene induction further required the activation of neuronal promoter E-box elements. These results indicate that the spacer is involved in mediating neuronal and tumor-related genes.

BK virus as a cofactor in the etiology of prostate cancer in its early stages

Prostate cancer has been projected to cause almost 10% of all male cancer deaths in the United States in 2007. The incidence of mutations in the tumor suppressor genes Rb1 and p53, especially in the early stages of the disease, is low compared to those for other cancers. This has led to the hypothesis that a human virus such as BK virus (BKV), which establishes a persistent subclinical infection in the urinary tract and encodes oncoproteins that interfere with these tumor suppressor pathways, is involved. Previously, we detected BKV DNA in the epithelial cells of benign and proliferative inflammatory atrophy ducts of cancerous prostate specimens. In the present report, we demonstrate that BKV is present at a much lower frequency in noncancerous prostates. Additionally, in normal prostates, T-antigen (TAg) expression is observed only in specimens harboring proliferative inflammatory atrophy and prostatic intraepithelial neoplasia. We further demonstrate that the p53 gene from atrophic cells expressing TAg is wild type, whereas tumor cells expressing detectable nuclear p53 contain a mix of wild-type and mutant p53 genes, suggesting that TAg may inactivate p53 in the atrophic cells. Our results point toward a role for BKV in early prostate cancer progression.

Tumorigenic adenovirus type 12 E1A inhibits phosphorylation of NF-kappaB by PKAc, causing loss of DNA binding and transactivation

Human adenovirus type 12 (Ad12) E1A protein (E1A-12) is the key determinant of viral tumorigenesis. E1A-12 mediates major histocompatibility complex class I (MHC-I) shutoff by inhibiting the DNA binding of the transcriptional activator NF-kappaB (p50/p65) to the class I enhancer. This enables Ad12 tumorigenic cells to avoid class I recognition and lysis by cytotoxic T lymphocytes. In this study, we demonstrate that the phosphorylation of p50 and p65 by the catalytic subunit of protein kinase A (PKAc) is essential for NF-kappaB DNA binding and transactivation activity. Treatment with H89 and knockdown of PKAc in cells led to the inhibition of phosphorylation at p50 Ser(337) and p65 Ser(276) and loss of DNA binding by NF-kappaB. Importantly, NF-kappaB phosphorylation by PKAc was repressed by tumorigenic E1A-12, but not by nontumorigenic Ad5 E1A (E1A-5). The stable introduction of E1A-12 into Ad5 nontumorigenic cells resulted in a decrease in the phosphorylation of NF-kappaB, loss of NF-kappaB DNA binding, and the failure of NF-kappaB to activate a target promoter, as well as diminution of MHC-I transcription and cell surface expression. Significantly, the amount and enzymatic activity of PKAc were not altered in Ad12 tumorigenic cells relative to its amount and activity in nontumorigenic Ad5 cells. These results demonstrate that E1A-12 specifically prevents NF-kappaB from being phosphorylated by PKAc.

Allele- and Locus-Specific Recognition of Class I MHC Molecules by the Immunomodulatory E3-19K Protein from Adenovirus

The E3-19K protein from human adenoviruses (Ads) retains class I MHC molecules in the endoplasmic reticulum. As a consequence, the cell surface expression of class I molecules is suppressed, allowing Ads to evade immune surveillance. Using native gel electrophoresis, gel filtration chromatography, and surface plasmon resonance, we show that a soluble form of the Ad type 2 (Ad2) E3-19K protein associates with HLA-A and -B molecules; equilibrium dissociation constants were in the nanomolar range and approximately 2.5-fold higher affinity for HLA-A (-A*0201, -A*0301, -A*1101, -A*3301, and -Aw*6801) relative to HLA-B (-B*0702 and -B*0801) molecules. Among the alleles of the HLA-A locus examined, HLA-A*3101 associated approximately 15-fold less avidly with soluble E3-19K. Soluble E3-19K interacted only very weakly with HLA-Cw*0304, and no interaction with HLA-Cw*0401 could be detected under identical conditions. Site-directed mutagenesis and flow cytometry demonstrated that MHC residue 56 plays a critical role in the association and endoplasmic reticulum retention of HLA-A molecules by E3-19K. This delineates the spatial environment around residue 56 as a putative E3-19K interaction surface on class I molecules. Overall, our data imply that a link may exist between host genetic factors and the susceptibility of individuals to Ad infections.

E1A is the component of the MHC class I enhancer complex that mediates HDAC chromatin repression in adenovirus-12 tumorigenic cells

In adenovirus-12 tumorigenic cells, the viral E1A-12 protein mediates transcriptional down-regulation of the major histocompatibility complex (MHC) class I genes by targeting the class I enhancer. Here, we demonstrate by a combination of antisense and chromatin immunoprecipitation (ChIP) analysis that E1A-12 is a physical component of the class I enhancer repression complex, known to comprise COUP-TFII and histone deacetylase 1 (HDAC1). Significantly, E1A antisense was shown to co-eliminate E1A-12 as well as HDAC1 and HDAC8, but not HDAC3, from the enhancer repression complex. Consistent with elimination of HDAC1 and HDAC8, E1A antisense also resulted in a dramatic increase in histone acetylation, a hallmark of transcriptionally active chromatin. Importantly, MHC class I antigen expression was restored on the surface of E1A antisense-transfected cells. These results demonstrate that E1A-12 is associated with the MHC class I complex and apparently mediates class I transcriptional down-regulation by enacting chromatin repression through HDAC1 and HDAC8.

Differential expression of LFA-3, Fas and MHC Class I on Ad5- and Ad12-transformed human cells and their susceptibility to lymphokine-activated killer (LAK) cells

Adenovirus (Ad) E1A is a potent oncogene and has been shown to deregulate the expression of a large number of cellular genes leading to cellular transformation. Here we have analysed the expression of several immunomodulatory molecules on the surface of a set of human cell lines transformed with either Ad12 or Ad5. Human cells transformed with Ad12 demonstrated reduced expression of cell surface LFA-3, Fas and MHC class I when compared to Ad5-transformed cells. Furthermore, Ad12-transformed human cell lines demonstrated greater susceptibility to lysis by lymphokine-activated killer (LAK) cells, compared to Ad5-transformed human cell lines. In contrast, previous studies with rodent cells showed that both Ad5- and Ad12-transformed rat cells were susceptible to LAK cells. Thus, transformation of human cells with Ad5 or Ad12 results in differences in the expression of immunomodulatory molecules on the cell surface and differential recognition of these virus-transformed cells by immune effector cells.

Viral interference with MHC class I antigen presentation pathway: the battle continues

CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in the defense against viral infections. In general, CD8+ CTLs recognize antigenic peptides in the context of the major histocompatibility complex (MHC) class I molecule. The MHC class I molecules are expressed on almost all the nucleated cells in the body. The trimolecular complex consisting of the class I heavy chain, beta2-microglobulin and the peptide are generated by the MHC class I antigen presentation pathway. This pathway is designed to sample the intracellular milieu and present the information to the CTLs trafficking the area. This rigorous sampling of intracellular environment enables the CTLs to quickly identify and eliminate the cells that synthesize non-self proteins as a result of a viral infection. Many viruses, including several viruses of veterinary importance, have evolved astounding strategies to interfere with the MHC class I antigen presentation pathway, as a means of evading the CTL response of the host. This review focuses on the diverse mechanisms of viral evasion of the MHC class I antigen presentation pathway with particular emphasis on viruses of veterinary importance.

The human polyomavirus BK: Potential role in cancer

In human cancer, a role has been suggested for the human polyomavirus BK, primarily associated with tubulointerstitial nephritis and ureteric stenosis in renal transplant recipients, and with hemorrhagic cystitis in bone marrow transplant (BMT) recipients. After the initial infection, primarily unapparent and without clinical signs, the virus disseminates and establishes a persistent infection in the urinary tract and lymphocytes. There is correlative evidence regarding potential role of polyomavirus BK in cancer. In fact, the BK virus (BKV) DNA (complete genome and/or subgenomic fragments containing the early region) is able to transform embryonic fibroblasts and cells cultured from kidney and brain of hamster, mouse, rat, rabbit, and monkey. Nevertheless, transformation of human cells by BKV is inefficient and often abortive. Evidence supporting a possible role for BKV in human cancer has accumulated slowly in recent years, after the advent of polymerase chain reaction (PCR). BKV is known to commonly establish persistent infections in people and to be excreted in the urine by individuals who are asymptomatic, complicating the evaluation of its potential role in development of human cancer. Therefore, there is no certain proof that human polyomavirus BK directly causes the cancer in humans or acts as a cofactor in the pathogenesis of some types of human cancer.

Oncogenic transformation by BK virus and association with human tumors

BK virus (BKV), a human polyomavirus closely related to JC virus and Simian Virus 40, is ubiquitous in human populations worldwide. After primary infection, BKV establishes a lifelong latent infection in many organs. BKV transforms rodent cells to the neoplastic phenotype and is highly oncogenic in rodents. This review considers the oncogenic potential of BKV in humans and its possible involvement in human tumors. BKV sequences and T antigen (Tag) are detected in several types of human neoplasms, although the viral load is generally low, with less than one copy of the viral genome per cell. The possible causative role of BKV in human oncogenesis rests on the ability of BKV Tag to inactivate the functions of tumor suppressor proteins p53 and pRB family as well as on its ability to induce chromosomal aberrations in human cells. A 'hit and run' mechanism and secretion of paracrine growth factors by BKV Tag-positive cells, recruiting into proliferation neighboring and distant cells, are discussed as possible BKV pathogenic elements in human oncogenesis.

Chromatin repression by COUP-TFII and HDAC dominates activation by NF-kappaB in regulating major histocompatibility complex class I transcription in adenovirus tumorigenic cells

In adenovirus type 12 transformed cells, the down-regulation of MHC class I transcription contributes to the tumorigenic phenotype and is solely mediated by Ad12 E1A. Previous in vitro studies with class I enhancer sequences have indicated that there is an increased binding of repressor COUP-TFII and its associated HDAC and a decreased binding of activator NF-kappaB. In this study, we used chromatin immunoprecipitation (ChIP) assay in order to determine in vivo whether these proteins regulate class I transcription by affecting chromatin. The ChIP assay revealed that there is lack of chromatin histone acetylation in the region of the class I enhancer in Ad12-transformed cells. This is regulated by histone deacetylation as it was further demonstrated in vivo that COUP-TFII and HDAC are associated with the class I enhancer chromatin. In agreement with in vitro studies, NF-kappaB could be recruited to the class I enhancer following induction by TNF-alpha. However, this enhancer-bound NF-kappaB failed to up-regulate class I expression because the class I enhancer chromatin remained repressed as a result of histone deacetylation by HDAC in association with COUP-TFII. Thus, we have demonstrated for the first time that repression of chromatin through histone deacetylation is a major mechanism in down-regulating class I transcription in Ad12-transformed cells. Finally, Ad12 E1A, a non-DNA binding protein, was shown to be present in the natural protein complex bound to the class I enhancer.

In adenovirus type 12 tumorigenic cells, major histocompatibility complex class I transcription shutoff is overcome by induction of NF-kappaB and relief of COUP-TFII repression

The surface levels of major histocompatibility complex class I antigens are diminished on tumorigenic adenovirus type 12 (Ad12)-transformed cells, enabling them to escape from immunosurveillant cytotoxic T lymphocytes (CTLs). This is due to the down-regulation of the class I transcriptional enhancer, in which there is strong binding of the repressor COUP-TFII and lack of binding of the activator NF-kappaB. Even though NF-kappaB (p65/p50) translocates to the nuclei of Ad12-transformed cells, it fails to bind to DNA efficiently due to the hypophosphorylation of the p50 subunit. In this study, tumor necrosis factor alpha (TNF-alpha) and interleukin 1beta (IL-1beta) were shown to promote degradation of the NF-kappaB cytoplasmic inhibitor IkappaBalpha and permit the nuclear translocation of a phosphorylated form of NF-kappaB that is capable of binding DNA. Interestingly, when Ad12-transformed cells were treated with TNF-alpha or IL-1beta, class I gene transcription substantially increased when transcriptional repression by COUP-TFII was blocked. This indicates that in cytokine-treated Ad12-transformed cells, COUP-TFII is able to repress activation of class I transcription by newly nucleus-localized NF-kappaB. Our results suggest that Ad12 likely employs a "fail-safe" mechanism to ensure that the transcription of class I genes remains tightly repressed under various physiological conditions, thus providing tumorigenic Ad12-transformed cells with a means of escaping CTL recognition and lysis.

COUP-TFII Is Up-regulated in Adenovirus Type 12 Tumorigenic Cells and Is a Repressor of MHC Class I Transcription

Down-regulation of the MHC class I enhancer in tumorigenic Ad12 cells is associated with strong binding of COUP-TF and negligible binding of activator NF-kappaB. By comparison, in nontumorigenic Ad5 cells, class I expression is high due to negligible binding of COUP-TF and strong binding of NF-kappaB. Here, we show that COUP-TFII, but not COUP-TFI, is expressed in Ad12-transformed cells. The dramatically stronger DNA binding of COUP-TFII to the class I enhancer in Ad12- compared to Ad5-transformed cells correlates with higher COUP-TFII promoter activity and higher levels of COUP-TFII mRNA and protein. Significantly, NF-kappaB p50/p52 double-knockout cells enabled us to demonstrate directly that COUP-TFII can completely repress both nonactivated and NF-kappaB-activated MHC class I transcription.

Infectious hematopoietic necrosis virus matrix protein inhibits host-directed gene expression and induces morphological changes of apoptosis in cell cultures

Infectious hematopoietic necrosis virus (IHNV) infection in tissue culture cells has previously been shown to result in the shutdown of host protein synthesis, cell rounding, and cell death. We report here an investigation of the cytopathogenicity of the viral phosphoprotein (P or M1), matrix (M or M2), and nonvirion (NV) proteins in cultured fish cells. The expression of M alone potently inhibited reporter gene expression from a viral and an interferon (IFN)-inducible promoter, whereas P and NV did not produce a similar effect. Northern blot analysis further revealed a reduction in the steady-state level of reporter mRNA when the M gene was cotransfected into cells; conversely, M mRNA was not drastically reduced in the same cells. By immunofluorescence confocal microscopy, fragmented nuclei were found in some cells expressing M protein but not in cells expressing P, NV, or beta-galactosidase protein. Electron microscopy revealed the morphological changes associated with apoptosis in the M-transfected cells. Furthermore, IHNV infection was shown to produce DNA "laddering" in cultured cells. Taken together, these data suggested at least two functions for M protein in an IHNV infection: down regulation of host transcription and the induction of programmed cell death. In the course of these experiments, we also discovered that NV expression was associated with cell rounding, the first biological effect on cells to be attributed to the NV gene.

Association of histone deacetylase with COUP-TF in tumorigenic Ad12-transformed cells and its potential role in shut-off of MHC class I transcription

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) is an orphan nuclear receptor that represses transcription of many genes. In adenovirus type 12 (Ad12) transformed cells, a high level of binding activity of COUP-TF to the major histocompatibility complex (MHC) class I enhancer correlates with the down-regulation of class I transcription, which, in turn, contributes to tumorigenesis. The mechanism by which COUP-TF represses transcription has yet to be elucidated. Here we show that COUP-TF represses transcription through its association with histone deacetylase. This was demonstrated using reciprocal binding assays that determined that the interaction between COUP-TF and histone deacetylase requires the COUP-TF C-terminal repression domain. Moreover, a histone deacetylase enzymatic activity was found to be associated with COUP-TF in Ad12-transformed cells. Transfection experiments further revealed that exogenous histone deacetylase facilitates transcriptional repression by COUP-TF. Also, supershift assays suggest that the transcriptional corepressor N-CoR, which is known to associate with histone deacetylases, is a part of the COUP-TF complex bound to the MHC class I enhancer R2 site. Finally, we provide evidence that inhibition of histone deacetylases relieves the repression of MHC class I expression in Ad12-transformed cells. Taken together these results support the notion that deacetylation of histones, mediated through COUP-TF, serves to down-regulate MHC class I transcription in Ad12-transformed cells.

Reduced phosphorylation of p50 is responsible for diminished NF-kappaB binding to the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells

Reduced cell surface levels of major histocompatibility complex class I antigens enable adenovirus type 12 (Ad12)-transformed cells to escape immunosurveillance by cytotoxic T lymphocytes (CTL), contributing to their tumorigenic potential. In contrast, nontumorigenic Ad5-transformed cells harbor significant cell surface levels of class I antigens and are susceptible to CTL lysis. Ad12 E1A mediates down-regulation of class I transcription by increasing COUP-TF repressor binding and decreasing NF-kappaB activator binding to the class I enhancer. The mechanism underlying the decreased binding of nuclear NF-kappaB in Ad12-transformed cells was investigated. Electrophoretic mobility shift assay analysis of hybrid NF-kappaB dimers reconstituted from denatured and renatured p50 and p65 subunits from Ad12- and Ad5-transformed cell nuclear extracts demonstrated that p50, and not p65, is responsible for the decreased ability of NF-kappaB to bind to DNA in Ad12-transformed cells. Hypophosphorylation of p50 was found to correlate with restricted binding of NF-kappaB to DNA in Ad12-transformed cells. The importance of phosphorylation of p50 for NF-kappaB binding was further demonstrated by showing that an NF-kappaB dimer composed of p65 and alkaline phosphatase-treated p50 from Ad5-transformed cell nuclear extracts could not bind to DNA. These results suggest that phosphorylation of p50 is a key step in the nuclear regulation of NF-kappaB in adenovirus-transformed cells.

The role of mouse adenovirus type 1 early region 1A in acute and persistent infections in mice

Mouse adenovirus type 1 (MAV-1) early region 1A (E1A) viral mutants were used to determine the importance of this region in pathogenesis and establishment of a persistent infection in the natural host. Lethal dose analysis with adult male Swiss outbred mice revealed a significant reduction in virulence for all of the E1A mutants. During acute infections with 10(5) PFU of virus, an E1A null mutant, pmE109, was found in the same organs (brain, spleen, and spinal cord) and the same cell types (endothelial cells and mononuclear cells in lymphoid tissue) as wild-type virus. Another null mutant, pmE112, was detected in the same organs but in lower numbers. However, when mice were given a lower dose, 1 PFU, pmE109 and pmE112 reached none of the target organs analyzed by 14 days postinfection (p.i.). The absence of E1A did not hinder the ability of MAV-1 to establish a persistent infection. Viral nucleic acid was detected by PCR amplification or in situ hybridization in the kidneys, brains, spleens, and prefemoral lymph nodes of mice infected with wild-type or mutant virus up to 55 weeks p.i. The brain, spleen, and lymph node are recognized sites of acute viral infection but are previously unrecognized sites for MAV-1 persistence. Evidence for the potential reactivation of persistent MAV-1 infections is also presented.

Molecular basis of immune evasion strategies by adenoviruses

Human adenoviruses have provided valuable insights into virus-host interactions at the clinical and experimental levels. In addition to the medical importance of adenoviruses in acute infections and the ability of the virus to persist in the host, adenovirus-based recombinants are being developed as potential vaccine vectors. It is now clear that adenoviruses employ various strategies to modulate the innate and the adaptive host immune defences. Adenovirus genome-coded products that interact with the immune response of the host have been identified, and to a large extent the molecular mechanisms of their functions have been revealed. Such knowledge will no doubt influence our approach to the areas of viral pathogenesis, vaccine development and immune modulation for disease management.

Evidence for the involvement of a nuclear NF-kappa B inhibitor in global down-regulation of the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells

Diminished expression of major histocompatibility complex class I antigens on the surface of adenovirus type 12 (Ad12)-transformed cells contributes to their high tumorigenic potential by enabling them to escape immune recognition by cytotoxic T lymphocytes. This low class I antigen expression is due to a block in class I transcription, which is mediated by Ad12 E1A. Genetic analysis has shown that the class I enhancer is the target for transcriptional down-regulation. In this study, we show that the ability of the R1 element of the class I enhancer to stimulate transcription is greatly reduced in Ad12-transformed cells. The loss of functional activity by the R1 element was attributed to loss of binding by the NF-kappa B p50-p65 heterodimer. NF-kappa B binding appears to be blocked within the nucleus rather than at the level of nuclear translocation. Significantly, NF-kappa B binding activity could be recovered from the nuclear extracts of Ad12-transformed cells following detergent treatment, suggesting that the block is mediated through a nuclear inhibitor present in the Ad12-transformed cells. These results, taken together with the fact that the R2 element of the class I enhancer exhibits strong binding to the transcriptional repressor COUP-TF, suggest that the class I enhancer is globally down-regulated in Ad12-transformed cells.

Selective mechanisms utilized by persistent and oncogenic viruses to interfere with antigen processing and presentation

Cell-mediated immunity is effective against cells harboring active virus replication, and is critical for the elimination of ongoing infections, regression of virus-associated tumors, and reducing or preventing the reactivation of persistent viruses. The capacity of persistent and oncogenic viruses to maintain a long-term relationship with their host presupposes viral mechanisms for circumventing antiviral defenses. By suppressing the expression of molecules associated with antigen processing and presentation, viruses abrogate the major immune mechanism that deals with the elimination of infected and tumor cells. This is accomplished either by transcriptional downregulation of genes encoding class I MHC antigens, peptide transporter molecules, and the proteasome-associated LMP subunits, or by interfering with transport of class I molecules to the cell surface. In some cases viruses shut off the expression of most viral proteins during latency or express mainly nonimmunogenic or antagonistic peptide epitopes. This review describes selective mechanisms utilized by viruses for interference with antigen processing and presentation, and addresses their significance for in vivo viral persistence and tumor progression.

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Adenovirus 12-mediated down-regulation of the major histocompatibility complex (MHC) class I promoter: identification of a negative regulatory element responsive to Ad12 E1A

In highly oncogenic adenovirus (Ad) 12-transformed cells, major histocompatibility complex (MHC) class I gene expression is down-regulated by the products of the viral E1A oncogene at the level of initiation of transcription. However, class I gene expression is unaltered or elevated in non-oncogenic Ad2- or Ad5-transformed cells. These changes in class I expression may permit Ad12-transformed cells to escape host immune surveillance and elicit tumour formation. Here we show that the 2kb of 5' flanking region of the mouse H-2Kb class I gene is sufficient to mediate down-regulation of transcription driven from homologous or heterologous (HSV thymidine kinase) basal promoter elements in cells expressing Ad12 E1A, but not in Ad2 E1A-expressing cells. Deletion analysis of the 2kb region showed that sequences from -1.18 to -1.44kb (relative to the cap site) were a target for Ad12 E1A-mediated transcriptional down-regulation. Deletion of this entire region from the 2kb flanking sequence of the H-2Kb gene abolished Ad12 E1A-mediated down-regulation of transcription. Computer analysis of the -1.18 to -1.44kb sequence identified two 6/7bp matches with the AP-1 transcription factor consensus sequence and two matches with the pig MHC class I PD1 repressor element. Gel retardation analysis using overlapping DNA fragments derived from the -1.18 to -1.44kb sequence revealed several DNA:protein complexes formed using nuclear extract derived from Ad12-, but not from Ad2- or Ad5-transformed cells. Some of these DNA:protein complexes were also present, but at lower levels, in nuclear extracts from untransformed rat cells suggesting the possible involvement of cellular factors in the mechanism of down-regulation mediated by Ad12 E1A. A binding site for the AP-1 factor failed to compete for protein binding to fragments within the -1.18 to -1.44 sequence, while the PD1 site competed for binding only in the -1.15 to -1.23 region. These results indicate that novel factors (as well as a previously identified class I repressor, PD1) may be involved in Ad12 E1A-mediated down-regulation of MHC class I transcription.

Tumorigenicity of adenovirus-transformed rodent cells is influenced by at least two regions of adenovirus type 12 early region 1A

Chimeric adenovirus type 5 (Ad5)/Ad12 early region 1A (E1A) genes were used to transform primary baby rat kidney cells in cooperation with Ad12 E1B, and the resulting cell lines were assayed for tumorigenicity in syngeneic rats. It was found that lines were nontumorigenic when transformed by hybrid E1A genes consisting of the amino-terminal 80 amino acids from Ad12 including conserved region 1 (CR1), with the remaining portion from Ad5. In contrast, cell lines transformed by hybrids containing Ad12 E1A sequences from the amino terminus to the leftmost border of CR3 or beyond were tumorigenic. To extend these results, sequences spanning CR2 and CR3 of Ad5 E1A were replaced with the homologous regions of Ad12 E1A and additional transformed cell lines were established. These lines were weakly-to-moderately tumorigenic, suggesting that Ad12 E1A sequences between CR2 and CR3 may be involved in tumorigenicity but are not the sole factors influencing it. Interestingly, examination of an E1A sequence alignment indicated that the region between CR2 and CR3 of Ad12 E1A is also conserved in the corresponding sequence of simian adenovirus type 7, which, like Ad12, is highly oncogenic. This region is characterized by the presence of a stretch of several alanine residues and is similar to a motif present in a number of proteins with transcriptional repression activity. The possibility that this region may influence tumorigenicity by means of a transcriptional regulatory mechanism is discussed.

Down-regulation of the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells is accompanied by an increase in factor binding

In transformed cells, the E1A gene of adenovirus type 12 (Ad12) represses transcription of class I genes of the major histocompatibility complex. The tumorigenic potential of Ad12-transformed cells correlates with this diminished class I expression. In contrast, the E1A gene of the nontumorigenic Ad5 does not affect class I expression. We show here that a transfected reporter chloramphenicol acetyltransferase plasmid driven by an H-2K promoter (-1049 bp) was expressed at much lower levels in Ad12- than in Ad5-transformed mouse cells. Analysis of mutant constructs revealed that only 83 bp of H-2 DNA, consisting of the enhancer juxtaposed to the basal promoter, was sufficient for this differential expression. Whereas the H-2 basal promoter alone was somewhat less active in Ad12-transformed cells, the H-2 TATA box itself did not appear to be important. The H-2 enhancer proved to be the principal element in Ad12 E1A-mediated repression, since (i) substitution of the H-2 enhancer by simian virus 40 enhancers overcame the repression, and (ii) when juxtaposed to either its native or heterologous basal promoters, the H-2 enhancer was functional in Ad5- but not Ad12-transformed cells. Mobility shift assays showed that there is a DNA-binding activity to the 5' site (R2 element) of the enhancer that is significantly higher in Ad12- than in Ad5-transformed cells. These results suggest that decreased class I enhancer activity in Ad12-transformed cells may, at least in part, be due to the higher levels of an enhancer-specific factor, possibly acting as a repressor.

Transcriptional and posttranscriptional regulation of class I major histocompatibility complex genes following transformation with human adenoviruses

Transformation of rodent cells by human adenoviruses is a well-established model system for studying the expression, regulation, and function of class I antigens. In this report, we demonstrate that the highly oncogenic adenovirus type 12 operates at the transcriptional and posttranscriptional levels in regulating the activity of major histocompatibility complex class I genes and products in transformed cells. Adenovirus type 12 suppresses the cell surface expression of class I antigens in most cell lines. Nevertheless, in a number of cell lines suppression is the result of reduction in the amount of stable specific mRNA, while in another group of cell lines suppression involves interference with processing of a posttranscriptional product. The two mechanisms operate both for the endogenous H-2 genes and for a miniature swine class I transgene that is expressed in the cells.

Expression and interactions of human adenovirus oncoproteins

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Expression of HLA class I antigens in human tumors and their involvement in tumor growth

A decreased expression of major histocompatibility complex (MHC) class I antigens is a common feature of many experimental and human tumors and can often be correlated with malignancy grade. In fact, reduction of class I antigens is associated in most tumors with an enhanced ability to elude immune surveillance. Loss of HLA-A,B,C antigens ranges from a decrease in the percentage of A,B,C-positive cells to selective loss of particular antigens and total loss of class I molecule expression. In man, this has been documented in melanomas, carcinomas, lymphomas, neuroblastoma and acute leukemias. The reduction in membrane antigens is generally associated with a parallel fall in immunoprecipitable intracellular proteins and the corresponding mRNAs in the absence of structural changes in the coding genes. The literature concerning the above mentioned topics is reviewed and discussed.

Co-operation in cell transformation between BK virus and the human c-Harvey-ras oncogene

Early-passage hamster embryo cells were transformed by recombinant DNA molecules containing BK virus (BKV) early-region gene and either the activated c-Ha-ras oncogene (pBK/c-rasA) or the normal c-Ha-ras proto-oncogene (pBK/c-rasN). The recombinant DNAs had a greater transforming ability and converted hamster cells to a more malignant phenotype than the single genes transfected separately. pBK/c-rasA was significantly more powerful than pBK/c-rasN in conferring to cells all the characteristics of transformation. Transfected DNA sequences were integrated mostly as single insertions into cellular DNA. Specific c-Ha-ras and BKV transcripts as well as c-Ha-ras p21 and BKV T antigen were detected in transformed cells. Although stimulation of c-Ha-ras expression by BKV enhancers cannot be excluded in recombinants, super-transfection and co-transfection experiments in hamster embryo cells and pre-neoplastic cell lines showed that BKV early-region and c-Ha-ras co-operate in transformation by contributing separate and independent functions.

Viral gene inhibition of class I major histocompatibility antigen expression: not a general mechanism governing the tumorigenicity of adenovirus type 2-, adenovirus type 12-, and simian virus 40-transformed Syrian hamster cells

The association between the level of class I major histocompatibility (MHC) antigen expression and the tumorigenic phenotype was determined for cells from a series of 15 lines of adenovirus type 2 (Ad2)-, Ad12-, and simian virus 40 (SV40)-transformed hamster cells and 16 lines of cells established from hamster tumors induced by SV40 mutants. These cells range from nontumorigenic to highly tumorigenic in both syngeneic and allogeneic adult hamsters. The Ad2-transformed cells--cells that were nontumorigenic in syngeneic adult hamsters--expressed either high levels or low levels of class I MHC antigens. The SV40-transformed cells--cells transformed in vitro that produced tumors with equal efficiency in both syngeneic and allogeneic adult hamsters--or cells derived from SV40-induced tumors expressed very high levels of class I MHC antigens. The Ad12-transformed cells uniformly expressed low levels of class I MHC antigens; these cells produced tumors 200- to 1,000-fold less efficiently in allogeneic adult hamsters than in syngeneic adult hamsters and produced tumors with about the same efficiency in immunoimmature newborns and immunocompetent syngeneic adult hamsters. We conclude that the expression of either high levels or low levels of class I MHC antigens is, at most, a minor factor in the differences observed among these adenovirus- and SV40-transformed cells in their tumor-inducing capacity in naive, immunocompetent hamsters.

Negative and positive regulation of human leukocyte antigen class I gene transcription in K562 leukemia cells

The mechanism of transcriptional activation of human leukocyte antigen class I genes by gamma interferon and 5-azacytidine was studied in K562 human leukemia cells. Nuclear run-on transcription assays with various protein and RNA synthesis inhibitors yield evidence for both stimulation of a positive regulatory factor and inhibition of an mRNA that codes for a labile repressor. A novel mechanism is proposed to explain how 5-azacytidine can activate repressed genes without affecting DNA methylation.

Alternative mechanisms for activation of human immunodeficiency virus enhancer in T cells

The expression of human immunodeficiency virus (HIV) after T cell activation is regulated by NF-kappa B, an inducible DNA-binding protein that stimulates transcription. Proteins encoded by a variety of DNA viruses are also able to activate expression from the HIV enhancer. To determine how this activation occurs, specific genes from herpes simplex virus type 1 and adenovirus that activate HIV in T lymphoma cells have been identified. The cis-acting regulatory sequences in the HIV enhancer that mediate their effect have also been characterized. The relevant genes are those for ICP0-an immediate-early product of herpes simplex virus type 1-and the form of E1A encoded by the 13S messenger RNA of adenovirus. Activation of HIV by adenovirus E1A was found to depend on the TATA box, whereas herpesvirus ICP0 did not work through a single defined cis-acting element. These findings suggest multiple pathways that can be used to bypass normal cellular activation of HIV, and they raise the possibility that infection by herpes simplex virus or adenovirus may directly contribute to the activation of HIV in acquired immunodeficiency syndrome by mechanisms independent of antigenic stimulation in T cells.

Negative and positive regulation of human leukocyte antigen class I gene transcription in K562 leukemia cells

The mechanism of transcriptional activation of human leukocyte antigen class I genes by gamma interferon and 5-azacytidine was studied in K562 human leukemia cells. Nuclear run-on transcription assays with various protein and RNA synthesis inhibitors yield evidence for both stimulation of a positive regulatory factor and inhibition of an mRNA that codes for a labile repressor. A novel mechanism is proposed to explain how 5-azacytidine can activate repressed genes without affecting DNA methylation.

The level of expression of class-I MHC antigens in adenovirus-transformed human cell lines

The level of expression of the class-I major histocompatibility (MHC) antigen was determined in a series of human embryo cell lines transformed with either adenovirus 12 (Ad 12) early region I (EI) or adenovirus 5 (Ad 5) EI DNA or with combinations of Ad 12 early region IA (EIA) or Ad 2 EIA with activated N-ras DNA. MHC class-I antigen expression was greatly reduced in all Ad 12 transformants, compared to primary cells. Expression was also reduced in the Ad 5 cell lines transformed with EIA and EIB DNA, but levels were near normal in those lines with only EIA DNA present. Amounts of MHC class-I antigen on the cell surface, as determined by RIA and FACS analysis, generally reflected total cellular levels as determined by Western blotting. Expression of beta 2 microglobulin was also much reduced in those cell lines with low levels of MHC class-I antigen. The treatment of primary cells and all the transformants with human gamma-interferon resulted in increased expression of HLA on the cell surface. Infection of primary human cells with Ad 12 or with a series of Ad 12 mutants did not have any effect on the MHC class-I antigens present.