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

Identification of genes differentially expressed as result of adenovirus type 5- and adenovirus type 12-transformation

BACKGROUND

Cells transformed by human adenoviruses (Ad) exhibit differential capacities to induce tumours in immunocompetent rodents; for example, Ad12-transformed rodent cells are oncogenic whereas Ad5-transformed cells are not. The E1A gene determines oncogenic phenotype, is a transcriptional regulator and dysregulates host cell gene expression, a key factor in both cellular transformation and oncogenesis. To reveal differences in gene expression between cells transformed with oncogenic and non-oncogenic adenoviruses we have performed comparative analysis of transcript profiles with the aim of identifying candidate genes involved in the process of neoplastic transformation.

RESULTS

Analysis of microarray data revealed that a total of 232 genes were differentially expressed in Ad12 E1- or Ad5 E1-transformed BRK cells compared to untransformed baby rat kidney (BRK) cells. Gene information was available for 193 transcripts and using gene ontology (GO) classifications and literature searches it was possible to assign known or suggested functions to 166 of these identified genes. A subset of differentially-expressed genes from the microarray was further examined by real-time PCR and Western blotting using BRK cells immortalised by Ad12 E1A or Ad5 E1A in addition to Ad12 E1- or Ad5 E1-transformed BRK cells. Up-regulation of RelA and significant dysregulation of collagen type I mRNA transcripts and proteins were found in Ad-transformed cells.

CONCLUSION

These results suggest that a complex web of cellular pathways become altered in Ad-transformed cells and that Ad E1A is sufficient for the observed dysregulation. Further work will focus on investigating which splice variant of Ad E1A is responsible for the observed dysregulation at the pathway level, and the mechanisms of E1A-mediated transcriptional regulation.

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.

Direct recognition by alphabeta cytolytic T cells of Hfe, a MHC class Ib molecule without antigen-presenting function

Crystallographic analysis of human Hfe has documented an overall structure similar to classical (class Ia) MHC molecules with a peptide binding groove deprived of ligand. Thus, to address the question of whether alphabeta T cells could recognize MHC molecules independently of bound ligands, we studied human and mouse Hfe interactions with T lymphocytes. We provide formal evidence of direct cytolytic recognition of human Hfe by mouse alphabeta T cell receptors (TCR) in HLA-A*0201 transgenic mice and that this interaction results in ZAP-70 phosphorylation. Furthermore, direct recognition of mouse Hfe molecules by cytotoxic T lymphocytes (CTLs) was demonstrated in DBA/2 Hfe knockout mice. These CTLs express predominantly two T cell antigen receptor alpha variable gene segments (AV6.1 and AV6.6). Interestingly, in wild-type mice we identified a subset of CD8+ T cells positively selected by Hfe that expresses the AV6.1/AV6.6 gene segments. T cell antigen receptor recognition of MHC molecules independently of bound ligand has potential general implications in alloreactivity and identifies in the Hfe case a cognitive link supporting the concept that the immune system could be involved in the control of iron metabolism.

Mouse HFE inhibits Tf-uptake and iron accumulation but induces non-transferrin bound iron (NTBI)-uptake in transformed mouse fibroblasts

Iron-uptake and storage are tightly regulated to guarantee sufficient iron for essential cellular processes and to prevent the production of damaging free radicals. A non-classical class I MHC molecule, the hemochromatosis factor (HFE), has been shown to regulate iron metabolism, potentially via its interaction with the transferrin receptor. Whereas, the effect of human HFE (hHFE) on transferrin/transferrin receptor association, as well as on transferrin receptor recycling and the level of cellular iron pools in various cell lines was analyzed, very little is known about the mouse HFE (mHFE) protein. In the following study, our aim was to analyze in more detail the function of mHFE. Surprisingly, we observed that over-expression of mHFE, but not of hHFE, in a mouse transformed cell line, results in a most significant inhibition of transferrin-uptake which correlated with apoptotic cell death. mHFE inhibited transferrin-uptake immediately following transfection and this inhibition persisted in the surviving stable transfectants. Concomitantly, cellular iron derived from transferrin-iron uptake was dramatically limited. The activation of a non-transferrin bound iron-uptake pathway that functions in the stable mHFE-transfected clones could explain their normal growth curves and survival. The hypothesis that iron starvation can induce iron-uptake by a novel transferrin-independent pathway is discussed.

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.

Human cytomegalovirus protein US2 interferes with the expression of human HFE, a nonclassical class I major histocompatibility complex molecule that regulates iron homeostasis

HFE is a nonclassical class I major histocompatibility complex (MHC) molecule that is mutated in the autosomal recessive iron overload disease hereditary hemochromatosis. There is evidence linking HFE with reduced iron uptake by the transferrin receptor (TfR). Using a panel of HFE and TfR monoclonal antibodies to examine human HFE (hHFE)-expressing cell lines, we demonstrate the expression of stable and fully glycosylated TfR-free and TfR-associated hHFE/beta2m complexes. We show that both the stability and assembly of hHFE complexes can be modified by the human cytomegalovirus (HCMV) viral protein US2, known to interfere with the expression of classical class I MHC molecules. HCMV US2, but not US11, targets HFE molecules for degradation by the proteasome. Whether this interference with the regulation of iron metabolism by a viral protein is a means of potentiating viral replication remains to be determined. The reduced expression of classical class I MHC and HFE complexes provides the virus with an efficient tool for altering cellular metabolism and escaping certain immune responses.

Organization and functional analysis of the mouse transporter associated with antigen processing 2 promoter

In accordance with the key role of MHC class I molecules in the adaptive immune response against viruses, they are expressed by most cells, and their expression can be enhanced by cytokines. The assembly and cell surface expression of class I complexes depend on a continuous peptide supply. The peptides are generated mainly by the proteasome and are transported to the endoplasmic reticulum by a peptide transport pump consisting of two subunits, TAP1 and TAP2. The proteasome low molecular weight polypeptide (2 and 7), as well as TAP (1 and 2) genes, are coordinately regulated and are induced by IFNs. Despite this coordinate regulation, examination of tumors shows that these genes can be discordantly down-regulated. In pursuing a molecular explanation for these observations, we have characterized the mouse TAP2 promoter region and 5'-flanking sequence. We show that the 5' untranslated regions of TAP2 genes have a characteristic genomic organization that is conserved in both the mouse and the human. The mouse TAP2 promoter belongs to a class of promoters that lack TATA boxes but contain a MED1 (multiple start site element downstream) sequence. Accordingly, transcription is initiated from multiple sites within a 100-nucleotide window. An IFN regulatory factor 1 (IRF1)/IRF2 binding site is located in this region and is involved in both basal and IRF1-induced TAP2 promoter activity. The implication of the extensive differences found among the promoters of class I heavy chain, low molecular weight polypeptide, and TAP genes, all encoding proteins involved in Ag presentation, is discussed.

Immune evasion by adenoviruses

Adenovirus is a human pathogen that infects mainly respiratory and gastrointestinal epithelia. While the pathology caused by this virus is generally not life threatening in immunocompetent individuals, there is a large literature describing its ability to establish a persistent infection. These persistent infections typically occur in apparently healthy individuals with no outward signs of disease. Such a long term and benign interaction between virus and immune system requires adenoviruses to dampen host antiviral effector mechanisms that would otherwise eliminate the virus and cause immune-mediated pathology to the host. Adenovirus devotes a significant portion of its genome to gene products whose sole function seems to be the modulation of host immune responses. This review focuses on what is currently understood about how these immunomodulatory mechanisms work and how they might play a role in maintaining the virus in a persistent state.

MHC class I heavy chain mRNA must exceed a threshold level for the reconstitution of cell surface expression of class I MHC complexes in cells transformed by the highly oncogenic adenovirus 12

In primary embryonal fibroblasts from transgenic mice expressing H-2(b) genes and a miniature swine class I transgene (PD1), transformation with adenovirus 12 results in suppression of assembly and cell surface expression of all class I complexes. Cell surface expression of PD1 can be recovered by transfecting the cells with peptide transporter genes. However, reconstitution of the H-2Kb gene expression requires, in addition, a 2-fold increase in the steady state level of the H-2Kb mRNA that can be attained by treatment of the cells with interferons or by transfecting them with the H-2Kb gene. A detailed analyses of the biogenesis of class I molecules has revealed the steady state expression of free class I heavy chains that are not converted into conformed complexes even when peptide transporter genes are overexpressed. The fact that class I complex assembly seems to be highly inefficient in certain cell lines might be a major in vivo obstacle for the elimination of transformed or virus-infected cells by cytotoxic T lymphocytes, especially in view of the fact that the level of class I gene transcription is often down-regulated in cancer cells and/or that assembly of class I major histocompatibility complexes can be subverted by virus-encoded proteins.

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.

Adenovirus E1a interferes with expression of vaccinia viral genes

The 12S and 13S cDNAs of the oncogene E1a encoded by the early region of adenovirus 12 (Ad12) were overexpressed using the T7/encephalomyocarditis (EMC)/vaccinia hybrid expression system. The E1a proteins were stable for at least 12 h in monkey epithelial BSC1 cells. The E1a proteins were recognized by a rabbit polyclonal antibody and displayed phosphorylation patterns similar to those displayed by the E1a proteins expressed in Ad12-transformed cells. Expression of E1a proteins by recombinant vaccinia virus led to inhibition of vaccinia viral protein synthesis which was observed as soon as 6 h after infection. This suppression was mediated by both the 12S and the 13S products of Ad12E1a and to a somewhat lesser extent by the 13S product of Ad2E1a. The inhibition of vaccinia virus gene expression resulted in enhanced survival of vaccinia virus-infected cells. These results suggest that the proteins encoded by the E1a sequester a viral or a cellular product(s) that is essential for the expression of vaccinia virus-encoded genes.

Synthesis and turnover of beta2-microglobulin in Ad12-transformed cells defective in assembly and transport of class I major histocompatibility complex molecules

In primary embryonal fibroblasts from transgenic mice expressing H-2 genes and a miniature swine class I transgene (PD1), transformation with the highly oncogenic Ad12 results in a reduction in peptide transporter and proteasome-associated (LMP2 and LMP7) gene expression, and suppression in transport and cell surface expression of all class I antigens. The selective suppression in transport of H-2 (but not of PD1) molecules in cells reconstituted for the expression of peptide transporter and LMP genes implied that an additional factor(s) is involved in the assembly of class I complexes. Here we show that the beta2m, H-2Db, and H-2Kb genes are transcribed and translated in Ad12-transformed cells. However, unlike normal and E1Ad5-transformed cells, in which beta2m is either secreted unbound or bound to class I heavy chains, in Ad12-transformed cells significant amounts of beta2m are retained in the cell bound to the membrane, but free of class I heavy chains. This abnormal turnover of beta2m in the Ad12-transformed cells suggests the existence of a novel beta2m-binding molecule(s) that sequesters beta2m, and this process may provide a mechanism by which transformation with Ad12 may subvert class I complex formation.

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.

Lack of effect of mouse adenovirus type 1 infection on cell surface expression of major histocompatibility complex class I antigens

It has been proposed that adenoviruses establish and maintain persistent infections by reducing the class I major histocompatibility complex-associated presentation of viral antigens to cytotoxic T lymphocytes, leading to ineffective cell-mediated immunity and impaired clearance of infected cells (W.S.M. Wold and L. R. Gooding, Virology 184:1-8, 1991). Early region 3 of human adenovirus types 2 and 5 encodes a 19-kDa glycoprotein that associates with the class I major histocompatibility complex (MHC) antigens in the endoplasmic reticulum and prevents their maturation and transport to the cell surface. Early region 1A of human adenovirus type 12 encodes a protein that inhibits class I MHC mRNA production at the transcriptional or posttranscriptional processing level. Unlike human adenovirus infections, however, mouse adenovirus type 1 (MAV-1) infection of a variety of cell types did not affect the surface expression of 10 different mouse class I MHC allotypes. MAV-1-infected cells also regenerated cell surface class I MHC antigens following proteolytic removal as efficiently as mock-infected cells. The ability of cells to present antigen to class I MHC (Kb)-ovalbumin-specific T-cell hybridoma cells was likewise unaltered by MAV-1 infection. Thus, the ability of MAV-1 to persist cannot be explained by the model of reduced class I MHC-associated antigen presentation proposed for human adenoviruses.

LMP-associated proteolytic activities and TAP-dependent peptide transport for class 1 MHC molecules are suppressed in cell lines transformed by the highly oncogenic adenovirus 12

Expression of class I major histocompatibility complex antigens on the surface of cells transformed by adenovirus 12 (Ad12) is generally very low, and correlates with the in vivo oncogenicity of this virus. In primary embryonal fibroblasts (H-2b) that express transgenic swine class I antigen (PD1), Ad12-mediated transformation results in inhibition in transport of newly synthesized class I molecules, as well as significant reduction in transporter associated with antigen presentation (TAP) gene expression. In this report we show that reexpression of TAP molecules either by stable transfection of mouse TAP genes or by infection with recombinant vaccinia viruses expressing human TAP genes, only partially reconstitutes the expression and transport of the class I molecules. Further analysis of Ad12-transformed cells revealed that the expression of both LMP2 and LMP7, but not of other proteasome complex components, was downregulated, resulting in altered proteolytic activities of the 20S proteasomes. Reconstitution of both TAP and LMP expression resulted in complete restoration of PD1 cell surface expression and enhanced expression of the endogenous H-2D(b) molecules encoded by recombinant vaccinia viruses, in reconstituted Ad12-transformed cells, efficient transport of H-2 class I molecules could only be achieved by treatment of the cells with gamma-interferon. These data suggest that an additional factor(s) that is interferon-regulated plays a role in the biosynthetic pathway of the class I complex, and that its function is deficient in this cell system. Thus, Ad12 viral transformation appears to suppress the expression of multiple genes that are important for antigen processing and presentation, which allows such transformed cells to escape immune surveillance. This coordinate downregulation of immune response genes must likely occur through their use of common regulatory elements.

Functional domain analysis of interferon consensus sequence binding protein (ICSBP) and its association with interferon regulatory factors

Interferon consensus sequence binding protein (ICSBP) is a member of the interferon regulatory factor (IRF) family of proteins that include IRF-1, IRF-2, and ISGF3gamma which share sequence similarity at the putative DNA binding domain (DBD). ICSBP is expressed exclusively in cells of the immune system and acts as a repressor of interferon consensus sequence (ICS) containing promoters that can be alleviated by interferons. In this communication, we have searched for functional domains of ICSBP by dissecting the DBD from the repression activity. The putative DBD of ICSBP (amino acids 1-121) when fused in frame to the transcriptional activation domain of the herpes simplex VP16 (ICSBP-VP16) is a very strong activator of ICS-containing promoters. In addition, ICSBP-VP16 fusion construct transfected into adenovirus (Ad) 12 transformed cells enabled cell surface expression of major histocompatibility complex class I antigens as did treatment with interferon. On the other hand, the DBD of the yeast transcriptional activator GAL4 was fused in frame to a truncated ICSBP in which the DBD was impaired resulting in a chimeric construct GAL4-ICSBP. This construct is capable of repressing promoters containing GAL4 binding sites. Thus, ICSBP contains at least two independent domains: a DBD and a transcriptional repressor domain. Furthermore, we have tested possible interactions between ICSBP and IRFs. The chimeric construct GAL4-ICSBP inhibited the stimulated effect of IRF-1 on a reporter gene, implying for a possible interaction between IRF-1 and ICSBP. Electromobility shift assays, demonstrated that ICSBP can associate with IRF-2 or IRF-1 in vitro as well as in vivo. Thus, ICSBP contains a third functional domain that enables the association with IRFs. These associations are probably important for the fine balance between positive and negative regulators involved in the interferon-mediated signal transduction pathways in cells of the immune system.

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.

Downregulation of peptide transporter genes in cell lines transformed with the highly oncogenic adenovirus 12

The expression of class I major histocompatibility complex antigens on the surface of cells transformed by adenovirus 12 (Ad12) is generally very low, and correlates with the high oncogenicity of this virus. In primary embryonal fibroblasts from transgenic mice that express both endogenous H-2 genes and a miniature swine class I gene (PD1), Ad12-mediated transformation results in suppression of cell surface expression of all class I antigens. Although class I mRNA levels of PD1 and H-2Db are similar to those in nonvirally transformed cells, recognition of newly synthesized class I molecules by a panel of monoclonal antibodies is impaired, presumably as a result of inefficient assembly and transport of the class I molecules. Class I expression can be partially induced by culturing cells at 26 degrees C, or by coculture of cells with class I binding peptides at 37 degrees C. Analysis of steady state mRNA levels of the TAP1 and TAP2 transporter genes for Ad12-transformed cell lines revealed that they both are significantly reduced, TAP2 by about 100-fold and TAP1 by 5-10-fold. Reconstitution of PD1 and H-2Db, but not H-2Kb, expression is achieved in an Ad12-transformed cell line by stable transfection with a TAP2, but not a TAP1, expression construct. From these data it may be concluded that suppressed expression of peptide transporter genes, especially TAP2, in Ad12-transformed cells inhibits cell surface expression of class I molecules. The failure to fully reconstitute H-2Db and H-2Kb expression indicates that additional factors are involved in controlling class I gene expression in Ad12-transformed cells. Nevertheless, these results suggest that suppression of peptide transporter genes might be an important mechanism whereby virus-transformed cells escape immune recognition in vivo.

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.

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.

Deletion mutation analysis of the adenovirus type 2 E3-gp19K protein: identification of sequences within the endoplasmic reticulum lumenal domain that are required for class I antigen binding and protection from adenovirus-specific cytotoxic T lymphocytes

Adenovirus E3-gp19K is a transmembrane glycoprotein, localized in the endoplasmic reticulum (ER), which forms a complex with major histocompatibility complex (MHC) class I antigens and retains them in the ER, thereby preventing cytolysis by cytotoxic T lymphocytes (CTL). The ER lumenal domain of gp19K, residues 1 to 107, is known to be sufficient for binding to class I antigens; the transmembrane and cytoplasmic ER retention domains are located at residues ca. 108 to 127 and 128 to 142, respectively. To identify more precisely which gp19K regions are involved in binding to class I antigens, we constructed 13 in-frame virus deletion mutants (4 to 12 amino acids deleted) in the ER lumenal domain of gp19K, and we analyzed the ability of the mutant proteins to form a complex with class I antigens, retain them in the ER, and prevent cytolysis by adenovirus-specific CTL. All mutant proteins except one (residues 102 to 107 deleted) were defective for these properties, indicating that the ability of gp19K to bind to class I antigens is highly sensitive to mutation. All mutant proteins were stable and were retained in the ER. Sequence comparisons among adenovirus serotypes reveal that the ER lumenal domain of gp19K consists of a variable region (residues 1 to 76) and a conserved region (residues 77 to 98). We show, using the mutant proteins, that the gp19K-specific monoclonal antibody Tw1.3 recognizes a noncontiguous epitope in the variable region and that disruption of the variable region by deletion destroys the epitope. The monoclonal antibody and class I antigen binding results, together with the serotype sequence comparisons, are consistent with the idea that the ER lumenal domain of gp19K has three subdomains that we have termed the ER lumenal variable domain (residues 1 to ca. 77 to 83), the ER lumenal conserved domain (residues ca. 84 to 98), and the ER lumenal spacer domain (residues 99 to 107). We suggest that the ER lumenal variable domain of gp19K has a specific tertiary structure that is important for binding to the polymorphic alpha 1 and alpha 2 domains of class I heavy (alpha) chains. We suggest that the ER lumenal conserved domain of gp19K may interact with some conserved protein, perhaps the highly conserved alpha 3 domain of class I heavy chains. Finally, the ER lumenal spacer domain may allow the ER lumenal variable and conserved domains to extend out from the ER membrane so that they can interact with class I heavy chains.