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

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.

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.

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.

Adenovirus subversion of immune surveillance, apoptotic and growth regulatory pathways: a model for tumorigenesis

The adenovirus system provides a novel model for evaluating the roles of multiple factors involved in tumour progression. In common with other DNA tumour viruses, adenovirus employs a variety of strategies to evade immune surveillance and perturbs cellular apoptotic and growth regulatory pathways to ensure efficient replication of progeny virions. Such subversion of cellular networks is also found in tumour cells. The mechanism behind the avoidance of immune surveillance and the extent of cellular network interference achieved by adenovirus is still being uncovered and is predicted to have ramifications for the design of cancer therapeutics.

Identification of specific cellular genes up-regulated late in adenovirus type 12 infection

The infection of human cells by adenoviruses leads to a gradual reduction in the activity of host cell functions while viral gene expression progresses in a regulated way. We used the DNA microarray technique to determine the transcriptional activity profiles of cellular genes upon infection with adenovirus type 12 (Ad12). The microarray data were validated by quantitative real-time PCR for genes which showed significant alterations after Ad12 infection. At 12 h postinfection, there is a striking up-regulation between 10- and 30-fold in the expression of the G1P2, IFIT1, and IFIT2 cellular immune response genes compared to mock-infected cells. At later stages of infection, when the majority of regulated cellular genes has been turned down, a limited number of cellular genes exhibit increased activities by factors of 3 or less. These genes belong to the signal transduction or transcriptional regulator classes or are active in protein degradation, like ANPEP, an aminopeptidase. The SCD and CYP2S1 genes function in lipid metabolism. The eucaryotic translation initiation factor 4 is up-regulated, and one of the major histocompatibility complex genes is diminished in activity. For two of the genes, one up-regulated (CTSF gene) and one down-regulated (CYR61 gene), alterations in gene activity were confirmed at the protein level by Western blotting experiments. Increased genetic activity of cellular genes late in adenovirus infection has not been reported previously and demonstrates that Ad12 has a sustained control of host cell gene expression well into the late phase of infection.

Evasion of the immune system by adenoviruses

Human adenoviruses (Ads) have the ability to transform primary cells, and certain Ads, the subgenus A adenoviruses such as Ad12, induce tumours in immunocompetent rodents. The oncogenic phenotype of the subgenus A adenoviruses is determined by the viral E1A oncogene. In order to generate tumours, Ad12-transformed cells must evade the cellular immune system of the host. Ad12 E1A gene products mediate transcriptional repression of several genes in the major histocompatibility complex (MHC) involved in antigen processing and presentation, resulting in evasion of cytotoxic T lymphocyte (CTL) killing of transformed cells. In this review, the molecular mechanisms of E1A-mediated transcriptional repression of MHC gene expression are described. In addition, evasion of natural killer (NK) cell killing by Ad-transformed cells is also considered.

E1A-Based Determinants of Oncogenicity in Human Adenovirus Groups A and C

A broad spectrum of genetic and molecular investigations carried out with group C, Ad2 and Ad5, and with group A, Ad12, have shown that early region1 (E1) gene products are sufficient for complete transformation of rodent cells in vitro by these viruses. During the past quarter century, the processes by which E1A proteins, in cooperation with E1B proteins, perturb the cell cycle and induce the transformed phenotype, have become well defined. Somewhat less understood is the basis for the differential oncogenicity of these two groups of viruses, and the processes by which the E1A proteins of Ad12 induce a tumorigenic phenotype in transformants resulting from infection of cells in vivo and in vitro. In this chapter we review previous findings and present new evidence which demonstrates that Ad12 E1A possesses two or more independent functions enabling it to induce tumors. One of these functions lies in its capacity to repress transcription of MHC class I genes, allowing the tumor cells to avoid lysis by cytotoxic T lymphocytes. We have shown that class I repression is mediated through increased binding of repressor COUP-TF and decreased binding of NF-kB to the class I enhancer. In addition to mediating immune escape, E1A also determines the susceptibility of transformants to Natural Killer (NK) cell lysis, and in this case, also, Ad12 transformants are not susceptible. By using Ad12 mutants containing chimeric E1A Ad12-Ad5 genes, point mutations, or a specific deletion, we have shown that the unique spacer region of Ad12 E1A is an oncogenic determinant, but is not required for transformation in vitro. Given that the E1A regions responsible for class I repression are first exon encoded, we have examined a set of cell lines transformed by these altered viruses, and have found that while they display greatly reduced tumorigenicity, they maintain a wildtype capacity to repress class I transcription. Whether the spacer contributes to NK evasion remains unresolved. Lastly, we discuss the properties of the Ad2/Ad5 E1A C-terminal negative modulator of tumorigenicity, and examine the effects on transformation, tumor induction and transformant tumorigenicity, when the Ad5 negative modulator is placed by chimeric construction in Ad12 E1A.

Identification of genes associated with adenovirus 12 tumorigenesis by microarray

A total of 242 genes were shown to be differentially expressed between haplotypically matched tumorigenic adenovirus 12 (Ad12) and nontumorigenic Ad5-transformed cells using a microarray containing 8734 cDNAs. Eighty-seven of the differentially expressed genes have known roles that include signal transduction, cell growth and proliferation, transcription regulation, protease, and immune functions. The remaining differentially expressed genes are represented by EST cDNAs which have functions that are either completely unknown or proposed, based on sequence similarity to known genes. A subset of 22 differentially expressed genes from the microarray was further examined by Northern blot analyses to verify the identification of new genes associated with Ad12 tumorigenesis. Growth factor receptor binding protein 10 (Grb10) and protease nexin 1 (PN-1) were overexpressed in all of the tumorigenic Ad12-transformed cells examined, whereas expression of these genes was negligible in all of the nontumorigenic Ad5-transformed cells. By contrast, other genes including B cell translocation gene 2 (BTG2) were shown to be significantly up-regulated in Ad5-transformed cells as compared to Ad12-transformed cells.

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.

Immunomodulatory functions encoded by the E3 transcription unit of adenoviruses

Persistent viruses have evolved multiple strategies to escape the host immune system. One important prerequisite for efficient viral reproduction in the face of an ongoing immune response is prevention of premature lysis of infected cells. A number of viruses achieve this goal by interfering with antigen presentation and recognition of infected cells by cytotoxic T cells (CTL). Another viral strategy aims to block apoptosis triggered by host defense mechanisms. Both types of strategies seem to be realized by human adenoviruses (Ads). The early transcription unit E3 of Ads encodes proteins that inhibit antigen presentation by MHC class I molecules as well as apoptosis induced by tumor necrosis factor alpha (TNF-alpha) and Fas ligand (FasL). Here, we will describe the organization of the E3 regions of different Ad subgroups and compare the structure and functions of the known immunomodulatory E3 proteins.

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.

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 Regulation of Murine H-2Dd Expression by Activation Transcription Factor 1 and cAMP Response Element Binding Protein

Resistance to radiation leukemia virus (RadLV)-induced leukemia is correlated with an increase in H-2Dd expression on the thymocyte surface. It has been shown that elevated H-2Dd expression on infected thymocytes is a result of elevated mRNA transcription and that the transcriptional increase is correlated with elevated levels of a DNA binding activity, H-2 binding factor 1 (H-2 BF1), which recognizes the 5′-flanking sequence (5′-TGACGCG-3′) of the H-2Dd gene. Recently, it has been shown that the activation transcription factor 1 (ATF-1) homodimer is one form of the H-2 BF1 complex. Here we demonstrate that the cAMP response element binding protein (CREB) homodimer and the heterodimer of CREB/ATF-1 also recognize the cis regulatory motif and are two additional forms of the H-2 BF1 complex. The levels of mRNA encoding ATF-1 and CREB were both increased in RadLV-infected thymocytes that showed increased levels of H-2 mRNA. Also, all three H-2 BF1 binding activities, ATF-1 homodimer, CREB homodimer, and ATF-1/CREB heterodimer, were increased in RadLV-infected thymocytes that expressed high levels of H-2Dd Ag on the cell surface. Transfection experiments demonstrated that ATF-1 and CREB activated a reporter plasmid containing the H-2 BF1 motif. These observations strongly suggest that both ATF-1 and CREB are involved in the regulation of H-2 gene expression following RadLV infection of mouse thymocytes.

Modulation of antigen processing and presentation by persistent virus infections and in tumors

Cell-mediated immunity is effective against cells harboring active virus replication and is critical for the elimination of ongoing infections, opposing tumor progression, and reducing or preventing the reactivation of persistent viruses and tumor metastasis. The capacity of persistent viruses and tumor cells to maintain a long-term relationship with their host presupposes mechanisms for circumventing antiviral or antitumor defenses. By suppressing the expression of molecules associated with antigen processing and presentation, abrogation of the major immune mechanism that deals with the elimination of infected and transformed cells is achieved. This is accomplished in tumors predominantly by transcriptional downregulation of genes encoding class I major histocompatibility complex antigens, peptide transporter molecules, and the proteasome-associated low molecular mass protease subunits, and in cells expressing viral proteins by interfering with peptide transport and the assembly/transport of class I complexes. In addition, virus-infected cells and selected tumor cells express mainly nonimmunogenic or antagonistic peptide epitopes. This review describes mechanisms used by viruses and in transformed cells for interference with antigen processing and presentation and addresses their significance for in vivo viral persistence and tumor progression.

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 loss of H-2Ds antigen on a murine B lymphoma due to a post-transcriptional block in expression

The major histocompatibility (MHC) class I antigens are coordinately expressed in most cells. However, some tumors or virus-infected cells lack expression of one MHC class I antigen, while expression of the other MHC class I antigens is unaffected. We previously described the selective expression of MHC class I antigens on a B-cell lymphoma from SJL/J mice called RCS5. This tumor expresses H-2Ks, but has lost cell surface expression of H-2Ds. To understand the mechanism responsible for the selective loss of H-2Ds on the cell surface, we analysed H-2Ds mRNA and protein in the RCS5 tumor. Here we report that H-2Ds mRNA was expressed in RCS5, but H-2Ds protein was not detected in cell lysates. To determine whether the H-2Ds mRNA from RCS5 was able to direct the synthesis of H-2Ds protein, we performed cDNA cloning, in vitro translation and gene transfer experiments using a cell line related to RCS5 (cRCS-X). Our results indicated that the inhibition of H-2Ds expression in cRCS-X occurred after transcription of a non-defective H-2Ds mRNA. Furthermore, H-2Ds antigen expression was restored in cRCS-X using a retroviral vector to express the recombinant H-2Ds cDNA. These results indicate that the inhibition of H-2Ds expression could be overcome either by out competing an inhibitor that functions in trans or by removing cis-acting regulatory sequences from the endogenous H-2Ds mRNA.

Identification of the mouse cytomegalovirus genomic region affecting major histocompatibility complex class I molecule transport

Mouse cytomegalovirus (MCMV) functions expressed at the beginning of the early phase of the viral replication cycle interfere with the major histocompatibility complex (MHC) class I-restricted pathway of antigen presentation (M. J. Reddehase, M. R. Fibi, G. M. Keil, and U. H. Koszinowski, J. Virol. 60:1125-1129, 1986; M. Del Val, K. Münch, M. J. Reddehase, and U. H. Koszinowski, Cell 58:305-315, 1989). Nascent MHC class I heavy chains associate with beta 2-microglobulin and peptide, but the assembled trimolecular complex is retained in the endoplasmatic reticulum/cis-Golgi compartment (M. Del Val, H. Hengel, H. Häcker, U. Hartlaub, T. Ruppert, P. Lucin, and U. H. Koszinowski, J. Exp. Med. 176:729-738, 1992). To locate the responsible genomic region, the cytoplasmic retention of MHC class I molecules after injection of MCMV DNA was tested. The function was mapped to the HindIII E fragment. A recombinant MCMV deletion mutant delta MS94.5 lacking 15.8 kb in HindIII-E was constructed. Restoration of MHC class I molecule maturation and recognition of antigenic peptides by cytolytic T lymphocytes during the first hours of the early phase in mutant virus-infected cells proved the correct location to a 6.8-kb region in the HindIII E fragment. At later stages of the early phase, membrane-resident MHC class I molecules and cytolytic T lymphocyte recognition disappeared in delta MS94.5 mutant virus-infected cells. These results demonstrate that more than one early-gene function of MCMV affects the MHC class I pathway of antigen presentation. The redundant MHC class I-reactive functions target the transport of MHC class I molecules at different steps.

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.

Characterization of mature and immature RadLV-induced thymic T-cell lines for tumorigenesis and MHC-class-I gene expression

Class-I-MHC molecules are divided into class-Ia molecules, which play major roles in recognition of virus-infected cells, graft rejection and immune responses against tumors, and class-Ib molecules, which are less polymorphic and may be responsible for presenting unique classes of peptides. Our report characterizes RadLV-induced thymic T-cell lines that differ both in their tumorigenic potential and in the level of protein for class-Ia and TL genes. The PD1.1 cell line is CD4-CD8+ and expresses relatively high levels of class-I as compared with the CD4+CD8+ PD1.2 cell line. These class-I-expression levels correlate with thymocytes and splenic T cells of the same phenotype, except that normal cells fail to express TL3b. Interferon-treated PD1.2 cells demonstrate significantly lower levels of class-I expression than do interferon-treated PD1.1 cells, and were shown to contain large amounts of degraded class-I mRNA, at least some of which was TL in origin. These RNA products were not detected in PD1.1 cells, suggesting the existence of a mechanism controlling cell-specific and gene-specific mRNA stability. Such RadLV-induced cell lines provide a means for obtaining stage-specific T cells, which can be used for studying the regulation of class-I gene expression during T-cell differentiation, as well as factors that differentially regulate class-Ia and class-Ib expression and are potentially useful for studying T-cell differentiation in general.

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.

Transcriptional regulation of class-I major histocompatibility complex genes transformed in murine cells is mediated by positive and negative regulatory elements

The expression of class-I major histocompatibility complex (MHC) antigens on the surface of cells transformed by adenovirus 12 (Ad12) is generally very low or absent; a phenotype that correlates with the high tumorigenicity of these cell lines. In primary mouse embryonal fibroblasts (MEF) from class-I transgenic mice (PD1 transgenic mice), Ad12-mediated transformation results in down-regulation of both endogenous genes and the transgene. Functional analysis of class-I regulatory elements revealed that the suppression of a class-I promoter is mediated by two negative regulatory elements, one of which functions specifically in Ad12-transformed cells. In addition, Ad12-transformed cells produce only minute amounts of the nuclear factors that bind to the major class-I enhancer, RI (region I or H2TF1). A silencer element derived from the 5' region of the miniature swine class-I gene (PD1) is capable of competing for the binding of nuclear factors to a second enhancer, RII (region II or CREII), that is located upstream from RI in the class-I regulatory element (CRE). Based on these results, we propose that down-regulation of class-I genes in Ad12-transformed cells is mediated mainly by negative regulators.

De novo methylation of an MHC class I transgene following transformation with human adenoviruses is not correlated with its altered expression

The biological importance of class I histocompatibility antigens in a large variety of immune mechanisms is widely recognized, and their role in tumor rejection has been proven in several experimental tumor systems. Reduced expression of class I antigens, which is correlated with enhanced tumorigenicity, was shown in these systems to be mainly the result of transcriptional down-regulation. Mouse embryonal fibroblasts expressing H-2 antigens and the product of a miniature swine class I transgene, transformed by adenovirus 12, exhibit low levels of all class I antigens on the cell surface. Half of the cell lines demonstrate a suppressed level of class I mRNAs. Cell lines derived from transformation with the early region of adenovirus 5 express a high level of class I antigens. DNAs from adenovirus-transformed cells are extensively hypermethylated both in the 5' and the coding regions of the transgene compared to DNAs from immortalized cell lines and primary embryonal fibroblasts. Nevertheless, hypermethylation of these sequences is not correlated with mRNA level or cell-surface expression of the transgene product. Treatment of the transformed cells with high concentration of 5-azacytidine (5 Aza-C) induced merely a minor enhancement in the expression of class I mRNAs and class I antigens. Thus, this system is a perfect example of where viral transformation is associated with induced methylation of a class I gene, but hypermethylation does not affect its expression. The role of de novo methylation of genes in this system might be associated with transformation, or generation of mutations in CpG-rich sequences.

Virus strategies for evasion of the host response to infection

The attack on viruses and virus-infected cells by the mammalian immune system has provided considerable selective pressure for viruses that have evolved vigorous countermeasures to pre-empt, neutralize or evade this host attack. These countermeasures are astonishingly diverse, and their study imparts fundamental information about immunology and the mechanisms enabling viruses to survive and cause disease.

Down regulation of murine MHC class I expression by bovine herpesvirus 1

The objective of this study was to investigate the effect(s) of bovine herpesvirus 1 (BHV-1) infection on the expression of MHC class I molecules in murine fibroblasts. L-cells were infected with BHV-1 at a multiplicity of infection (m.o.i.) of 10 plaque forming units (PFU) per cell, and the expression of MHC class I molecules was analyzed by flow cytometry and immunoprecipitation. Temporal studies revealed a reduction in class I expression beginning at 8 h post infection (p.i.) which reached a maximum between 10 to 16 h p.i. The loss of class I expression was restored in the presence of phosphonoacetic acid (30 micrograms/ml), a late herpesviral protein synthesis inhibitor. However, addition of cycloheximide, a total protein synthesis inhibitor (100 micrograms/ml), did not result in any difference in class I expression between virus-infected and mock-infected cells. These results suggest that the reduced class I expression is a direct consequence of BHV-1 infection, and that the late viral gene product(s) may be involved in this process. Similar phenomena may occur in natural BHV-1 infection in cattle, and this may be one of the mechanisms of immune suppression by BHV-1.

Negative regulation of the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells via a retinoic acid response element

In cells transformed by the highly oncogenic adenovirus type 12 (Ad12), the viral E1A proteins mediate transcriptional repression of the major histocompatibility class I genes. In contrast, class I transcription is not reduced in cells transformed by the nononcogenic Ad5. The decreased rate of class I transcription is, at least in part, the result of a reduced major histocompatibility complex class I enhancer activity in Ad12-transformed cells and correlates with an increase in the levels of a DNA-binding activity to the R2 element of the enhancer (R. Ge, A. Kralli, R. Weinmann, and R. P. Ricciardi, J. Virol. 66:6969-6978, 1992). Employing transient transfection assays, we now provide direct evidence that the R2 element can confer repression in Ad12- but not Ad5-transformed cells. Repression by R2 was observed only in the presence of the positive enhancer element R1 and was dependent on (i) the number of the R2 elements and (ii) the relative arrangement of R2 and R1 elements. The putative R2-binding repressor protein, R2BF, was similar in molecular weight and binding specificity to members of the thyroid hormone/retinoic acid (RA) receptor family. RA treatment abrogated the R2-mediated repression in Ad12-transformed cells and had no effect on the activity of R2/R1-containing promoters in Ad5-transformed cells. These results are consistent with the presence of an R2-binding repressor in Ad12-transformed cells. In the absence of RA, the repressor compromises enhancer activity by interfering with the activity of the positive cis element R1. RA treatment of Ad12-transformed cells may render the repressor inactive.