Tumorigenicity of cells transformed by adenovirus type 12 by evasion of T-cell immunity

A comparative review of adenovirus A12 and C5 oncogenes

Oncogenic viruses contribute to 15% of global human cancers. To achieve that, virus-encoded oncoproteins deregulate cellular transcription, antagonize common cellular pathways, and thus drive cell transformation. Notably, adenoviruses were the first human viruses proven to induce cancers in diverse animal models. Over the past decades, human adenovirus (HAdV)-mediated oncogenic transformation has been pivotal in deciphering underlying molecular mechanisms. Key adenovirus oncoproteins, encoded in early regions 1 (E1) and 4 (E4), co-ordinate these processes. Among the different adenovirus species, the most extensively studied HAdV-C5 displays lower oncogenicity than HAdV-A12. A complete understanding of the different HAdV-A12 and HAdV-C5 oncoproteins in virus-mediated cell transformation, as summarized here, is relevant for adenovirus research and offers broader insights into viral transformation and oncogenesis.

Structural Determinants within the Adenovirus Early Region 1A Protein Spacer Region Necessary for Tumorigenesis

It has long been established that group A human adenoviruses (HAdV-A12, -A18, and -A31) can cause tumors in newborn rodents, with tumorigenicity related to the presence of a unique spacer region located between conserved regions 2 and 3 within the HAdV-A12 early region 1A (E1A) protein. Group B adenoviruses are weakly oncogenic, whereas most of the remaining human adenoviruses are nononcogenic. In an attempt to understand better the relationship between the structure of the AdE1A spacer region and oncogenicity of HAdVs, the structures of synthetic peptides identical or very similar to the adenovirus 12 E1A spacer region were determined and found to be α-helical using nuclear magnetic resonance (NMR) spectroscopy. This contrasts significantly with some previous suggestions that this region is unstructured. Using available predictive algorithms, the structures of spacer regions from other E1As were also examined, and the extent of the predicted α-helix was found to correlate reasonably well with the tumorigenicity of the respective virus. We suggest that this may represent an as-yet-unknown binding site for a partner protein required for tumorigenesis.IMPORTANCE This research analyzed small peptides equivalent to a region within the human adenovirus early region 1A protein that confers, in part, tumor-inducing properties to various degrees on several viral strains in rats and mice. The oncogenic spacer region is α-helical, which contrasts with previous suggestions that this region is unstructured. The helix is characterized by a stretch of amino acids rich in alanine residues that are organized into a hydrophobic, or "water-hating," surface that is considered to form a major site of interaction with cellular protein targets that mediate tumor formation. The extent of α-helix in E1A from other adenovirus species can be correlated to a limited degree to the tumorigenicity of that virus. Some serotypes show significant differences in predicted structural propensity, suggesting that the amino acid type and physicochemical properties are also of importance.

Divergent Evolution of E1A CR3 in Human Adenovirus Species D

Adenovirus E1A is the first viral protein expressed during infection. E1A controls critical aspects of downstream viral gene expression and cell cycle deregulation, and its function is thought to be highly conserved among adenoviruses. Various bioinformatics analyses of E1A from 38 human adenoviruses of species D (HAdV-D), including likelihood clade model partitioning, provided highly significant evidence of divergence of HAdV-Ds into two distinct groups for the conserved region 3 (CR3), present only in the E1A 13S isoform. This variance within E1A 13S of HAdV-Ds was not found in any other human adenovirus (HAdV) species. By protein sequence and structural analysis, the zinc finger motif of E1A CR3, previously shown as critical for transcriptional activation, showed the greatest differences. Subsequent codon usage bias analysis revealed substantial divergence in E1A 13S between the two groups of HAdV-Ds, suggesting that these two sub-groups of HAdV-D evolved under different cellular conditions. Hence, HAdV-D E1A embodies a previously unappreciated evolutionary divergence among HAdVs.

MHC/HLA Class I Loss in Cancer Cells

In this chapter I describe Tumour Immune Escape mechanisms associated with MHC/HLA class I loss in human and experimental tumours. Different altered HLA class-I phenotypes can be observed that are produced by different molecular mechanisms. Experimental and histological evidences are summarized indicating that at the early stages of tumour development there is an enormous variety of tumour clones with different MHC class I expression patterns. This phase is followed by a strong T cell mediated immune-selection of MHC/HLA class-I negative tumour cells in the primary tumour lesion. This transition period results in a formation of a tumour composed only of HLA-class I negative cells. An updated description of this process observed in a large variety of human tumors is included. In the second section I focus on MHC/HLA class I alterations observed in mouse and human metastases, and describe the generation of different tumor cell clones with altered MHC class I phenotypes, which could be similar or different from the original tumor clone. The biological and immunological relevance of these observations is discussed. Finally, the interesting phenomenon of metastatic dormancy is analyzed in association with a particular MHC class I negative tumor phenotype.

Scientific contributions toward successful cancer immunotherapy in The Netherlands

This historical overview shows that immunologists and clinicians from The Netherlands have contributed in a major way to better insights in the nature of cancer immunity. This work involved elucidation of the nature of cancer-associated antigens in autologous and allogeneic settings in addition to understanding of the cellular basis of natural immune responses against cancers and of important immune evasion mechanisms. Insight into such basic immunological mechanisms has contributed to the development of innovating therapies.

Changing faces in virology: the dutch shift from oncogenic to oncolytic viruses

Viruses have two opposing faces. On the one hand, they can cause harm and disease. A virus may manifest directly as a contagious disease with a clinical pathology of varying significance. A viral infection can also have delayed consequences, and in rare cases may cause cellular transformation and cancer. On the other hand, viruses may provide hope: hope for an efficacious treatment of serious disease. Examples of the latter are the use of viruses as a vaccine, as transfer vector for therapeutic genes in a gene therapy setting, or, more directly, as therapeutic anticancer agent in an oncolytic-virus therapy setting. Already there is evidence for antitumor activity of oncolytic viruses. The antitumor efficacy seems linked to their capacity to induce a tumor-directed immune response. Here, we will provide an overview on the development of oncolytic viruses and their clinical evaluation from the Dutch perspective.

Blocking oncogenic RAS enhances tumour cell surface MHC class I expression but does not alter susceptibility to cytotoxic lymphocytes

Mutations in the RAS family of oncogenes are highly prevalent in human cancer and, amongst its manifold effects, oncogenic RAS impairs the expression of components of the antigen presentation pathway. This allows evasion of cytotoxic T lymphocytes (CTL). CTL and natural killer (NK) cells are reciprocally regulated by MHC class I molecules and any gain in CTL recognition obtained by therapeutic inactivation of oncogenic RAS may be offset by reduced NK cell activation. We have investigated the consequences of targeted inactivation of oncogenic RAS on the recognition by both CTL and NK cells. Inactivation of oncogenic RAS, either by genetic deletion or inactivation with an inducible intracellular domain antibody (iDAb), increased MHC class I expression in human colorectal cell lines. The common RAS mutations, at codons 12, 13 and 61, all inhibited antigen presentation. Although MHC class I modulates the activity of both CTL and NK cells, the enhanced MHC class I expression resulting from inactivation of mutant KRAS did not significantly affect the in vitro recognition of these cell lines by either class of cytotoxic lymphocyte. These results show that oncogenic RAS and its downstream signalling pathways modulate the antigen presentation pathway and that this inhibition is reversible. However, the magnitude of these effects was not sufficient to alter the in vitro recognition of tumour cell lines by either CTL or NK cells.

Tumorigenic adenovirus 12 cells evade NK cell lysis by reducing the expression of NKG2D ligands

Activation of natural killer (NK) cells depends on a balance between signals received from activation and inhibitory ligands expressed on the surface of target cells. Tumorigenic human adenovirus 12 (Ad12) transformed cells express low levels of the NK cell inhibitory ligand MHC I, but do not exhibit increased sensitivity to NK cell lysis compared to their non-tumorigenic counterparts. Analysis of the expression of activation ligands that bind to the NKG2D receptor revealed that RAE1β and H60 were reduced on the surface of Ad12 mouse cells as well as at the level of transcription. In accord with these results, RAE1 localization to the synapse and sensitivity to NK cell cytotoxicity were also diminished. The reduced transcription of the rat NKG2D ligands, RAEt1L and RRTL, in tumorigenic rat cells compared to non-tumorigenic counterparts implies that both mouse and rat cell lines share a common mechanism of NKG2D ligand activation subverted by Ad12.

Adenovirus E1A interacts directly with, and regulates the level of expression of, the immunoproteasome component MECL1

Proteasomes represent the major non-lysosomal mechanism responsible for the degradation of proteins. Following interferon γ treatment 3 proteasome subunits are replaced producing immunoproteasomes. Adenovirus E1A interacts with components of the 20S and 26S proteasome and can affect presentation of peptides. In light of these observations we investigated the relationship of AdE1A to the immunoproteasome. AdE1A interacts with the immunoproteasome subunit, MECL1. In contrast, AdE1A binds poorly to the proteasome β2 subunit which is replaced by MECL1 in the conversion of proteasomes to immunoproteasomes. Binding sites on E1A for MECL1 correspond to the N-terminal region and conserved region 3. Furthermore, AdE1A causes down-regulation of MECL1 expression, as well as LMP2 and LMP7, induced by interferon γ treatment during Ad infections or following transient transfection. Consistent with previous reports AdE1A reduced IFNγ-stimulated STAT1 phosphorylation which appeared to be responsible for its ability to reduce expression of immunoproteasome subunits.

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.

Gene therapy in The Netherlands: highlights from the Low Countries

Gene therapy is an active research area in The Netherlands and Dutch scientists involved in fundamental and clinical gene therapy research significantly contribute to the progresses made in this field. This ranges from the establishment of the 293, 911 and PER.C6 cell lines, which are used worldwide for the production of replication‐defective adenoviral vectors, to the development of targeted viral vectors and T lymphocytes as well as of non‐viral vectors. Several milestones have been achieved in Dutch clinical gene therapy trials, including the first treatment worldwide of patients with adenosine deaminase deficiency with genetically corrected hematopoietic stem cells in collaboration with French and British scientists. Until now, about 230 patients with various diseases have been treated with viral and non‐viral gene therapy in this country. Ongoing and upcoming Dutch clinical trials focus on the translation of new developments in gene therapy research, including the restoration of genetic defects other than SCID, and the use of oncolytic adenoviruses and targeted T cells for the treatment of cancer. The growing commercial interest in Dutch clinical gene therapy is reflected by the involvement of two Dutch companies in ongoing trials as well as the participation of Dutch clinical centres in large phase III international multicenter immuno‐gene therapy trials on prostate cancer sponsored by an American company. Translational gene therapy research in The Netherlands is boosted at a governmental level by the Dutch Ministry of Health via a dedicated funding programme. This paper presents an overview on milestones in Dutch basic gene therapy research as well as on past, present and future clinical gene therapy trials in The Netherlands. Copyright © 2007 John Wiley & Sons, Ltd.

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.

Enhancement of growth and survival and alterations in Bcl-family proteins in beta-thalassemic erythroid progenitors by novel short-chain fatty acid derivatives

Accelerated apoptosis of erythroid progenitors is a characteristic of beta-thalassemia which presents a significant barrier to definitive therapeutic approaches utilizing induction of endogenous fetal globin gene expression. gamma-globin gene expression may not be inducible in, or may not be able to rescue, erythroid cells in which programmed cell death is initiated early in erythroblast development. In this report, short-chain fatty acid derivatives (SCFADs) which induce fetal globin gene expression were tested for their ability to promote proliferation and survival of erythroid progenitors cultured from beta-thalassemic subjects, and of cytokine-dependent erythroid cell lines. Certain SCFADs promoted thalassemic Bfu-e growth and cytokine-independent growth and survival of erythroid cell lines. A 40-80% increase in erythroid Bfu-e colony number was observed in cultures established with any of five mitogenic SCFADs, compared to control or butyrate-treated cultures from the same subjects. Immunoblot analysis demonstrated that these same SCFADs also regulated the expression of specific protein inhibitors of apoptosis. Anti-apoptotic ratios of the proteins Bcl-xL/Bcl-xS in thalassemic Bfu-e were increased by 30-120% with exposure to the SCFDs, compared to the ratios in the same cells cultured under control conditions. Similar anti-apoptotic increases in Mcl-1L/Mcl-1S ratios were induced by the SCFADs. These findings suggest that select fetal globin-inducing SCFADs which enhance proliferation of beta-thalassemia progenitors may enhance survival of these progenitors by altering levels of Bcl-family protein members. This combination of effects should enhance erythroid cell survival in the beta-thalassemia syndromes, allowing fetal globin gene expression to be induced more effectively than currently available, growth-suppressing, fetal globin-inducing agents, such as the butyrates or chemotherapeutic agents.

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.

Blockage of CRM1-dependent nuclear export of the adenovirus type 5 early region 1B 55-kDa protein augments oncogenic transformation of primary rat cells

The 55-kDa gene product from subgroup C adenovirus type 5 (Ad5) early region 1 (E1B-55kDa) plays a central role in the oncogenic transformation of primary rodent cells primarily by inactivating transcriptional and presumably other functional properties of the tumor suppressor protein p53. We have previously shown that Ad5 E1B-55kDa possesses a leucine-rich nuclear export signal (NES), which confers rapid nucleocytoplasmic shuttling via the CRM1-dependent export pathway. In this study we report that an export-deficient mutant of the viral protein (E1B-NES) substantially enhances focus formation of primary baby rat kidney cells in combination with Ad E1A. Transformed rat cells stably expressing the E1B-NES protein exhibited increased tumorigenicity and accelerated tumor growth in nude mice compared to transformants containing the wild-type E1B product. This 'gain of function' correlated with enhanced inhibition of p53 transactivation in transient reporter assays and the accumulation of the mutant protein and p53 in several dot-like subnuclear aggregates. Interestingly, these structures also contained a large fraction of cellular promyelocytic leukemia protein (PML), a known regulator of p53. These data indicate that E1B-NES promotes oncogenic transformation by combinatorial mechanisms that involve modulation of p53 in the context of PML nuclear bodies. In sum, these results extend our previous observation that inhibition of PML activities by E1B-55kDa is required for efficient focus formation and provide further support for the view that blocking p53 transcriptional functions is the principal mechanism by which the Ad protein contributes to complete cell transformation in conjunction with Ad E1A.

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

An in vivo functional genetic screen reveals a role for the TRK-T3 oncogene in tumor progression

Over the past decades, much has been learnt about the genes that contribute to oncogenic transformation of primary cells in vitro. However, much less is known about the genes that contribute to the later stages of tumor progression, in which cells of ever increasing malignancy arise through clonal selection in vivo. To search for genes that confer a tumor progression phenotype in vivo, we have used a functional genetic approach. We used adenovirus-transformed mouse embryo fibroblasts, which are tumorigenic in immunodeficient nude mice, but not in immunocompetent mice, due to strong cytotoxic T-cell-mediated immune rejection. We infected these cells in vitro with several high-complexity retroviral cDNA expression libraries and selected rare variants that formed tumors in immunocompetent mice. Using this approach, we identify here the TRK-T3 oncogene as a tumor progression gene. TRK-T3 does not inhibit T-cell reactivity towards the tumor cells. Instead, we find that cells expressing TRK-T3 enhances in vivo growth rate, most likely by stimulating anchorage-independent proliferation in growth factor-limiting conditions. Our data indicate that cDNA expression libraries can be used to identify tumor progression genes in vivo that cannot be readily identified using in vitro cell culture systems.

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.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Adenovirus E1A, not human papillomavirus E7, sensitizes tumor cells to lysis by macrophages through nitric oxide- and TNF-alpha-dependent mechanisms despite up-regulation of 70-kDa heat shock protein

Expression of adenovirus (Ad) serotype 2 or 5 (Ad2/5) E1A or human papillomavirus (HPV)16 E7 reportedly sensitizes cells to lysis by macrophages. Macrophages possess several mechanisms to kill tumor cells including TNF-alpha, NO, reactive oxygen intermediates (ROI), and Fas ligand (FasL). E1A sensitizes cells to apoptosis by TNF-alpha, and macrophages kill E1A-expressing cells, in part through the elaboration of TNF-alpha. However, E1A also up-regulates the expression of 70-kDa heat shock protein, a protein that inhibits killing by TNF-alpha and NO, thereby protecting cells from lysis by macrophages. Unlike E1A, E7 does not sensitize cells to killing by TNF-alpha, and the effector mechanism(s) used by macrophages to kill E7-expressing cells remain undefined. The purpose of this study was to further define the capacity of and the effector mechanisms used by macrophages to kill tumor cells that express Ad5 E1A or HPV16 E7. We found that Ad5 E1A, but not HPV16 E7, sensitized tumor cells to lysis by macrophages. Using macrophages derived from mice unable to make TNF-alpha, NO, ROI, or FasL, we determined that macrophages used NO, and to a lesser extent TNF-alpha, but not FasL or ROI, to kill E1A-expressing cells. Through the use of S-nitroso-N-acetylpenicillamine, which releases NO upon exposure to an aqueous environment, E1A was shown to directly sensitize tumor cells to NO-induced death. E1A sensitized tumor cells to lysis by macrophages despite up-regulating the expression of 70-kDa heat shock protein. In summary, E1A, but not E7, sensitized tumor cells to lysis by macrophages. Macrophages killed E1A-expressing cells through NO- and TNF-alpha-dependent mechanisms.

Subversion of B lymphocyte signaling by infectious agents

Infectious agents and their hosts interact in a complex manner, involving not only superficially apparent mechanisms, but also the signaling machinery that governs host cells responses. Thus, signaling events, surface molecule expression, and transcriptional control may be affected in various cell types, with profound consequences for the function of individual cells and organ systems. Studies of the biochemistry of cell signaling and cell invasion by infectious agents have begun to detail the interplay between elements of infectious organisms and the host at the molecular level. Consequently, the resulting interferences with lymphocyte signaling may disturb the function of the immune system. In B cells, alterations of immune receptor signaling has implications for human diseases. By affecting the mechanisms of the host's immune defense, this may not only lead to inadequate elimination of an infectious agent, but also to autoimmunity or neoplasia.

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.

Differential expression of tapasin and immunoproteasome subunits in adenovirus type 5- versus type 12-transformed cells

Adenovirus type 12 (Ad12)-transformed baby rat kidney (BRK) cells are oncogenic in syngeneic immunocompetent rats in contrast to adenovirus type 5 (Ad5)-transformed BRK cells, which are not oncogenic in these animals. A significant factor contributing to the difference in oncogenicity may be the low levels of major histocompatibility complex (MHC) class I membrane expression in Ad12-transformed BRK cells as compared with those in Ad5-transformed BRK cells, which presumably results in escape from killing by cytotoxic T lymphocytes. Here we show that, in addition to the decreased levels of expression of the MHC class I heavy chain and the peptide transporter Tap-2, the expression levels of the chaperone Tapasin and the immunoproteasome components MECL-1, PA28-alpha, and PA28-beta also are much lower in Ad12- than in Ad5-transformed BRK cells. The low expression levels of these proteins may contribute to the escape from killing by cytotoxic T lymphocytes, because the generation of optimal peptides and loading of these peptides on MHC class I require these components. Increased levels of phosphorylated signal transducer and activator of transcription-1 protein and expression of IFN regulatory factor-7 were found in Ad5- versus Ad12-transformed BRK cells. Therefore, the critical alteration leading to the plethora of differences may be an interferon (-related) effect.

Down-regulation of HLA-A expression correlates with a better prognosis in colorectal cancer patients

To evaluate the prognostic impact of human leukocyte antigen class I (HLA-I) expression on immune surveillance in colorectal cancer, we studied 88 curatively resected tumors for HLA-A and HLA-B/C expression and correlated these data to clinical and histopathological parameters. HLA-A was normal (all tumor cells had HLA expression) in 32%, reduced (HLA-negative and -positive tumor cells coexisted) in 56%, or absent (no tumor cells expressed HLA) in 12% of evaluable cases. HLA-B/C was normal in 47%, reduced in 47%, and absent in 7% of the cases. Considering both markers, total HLA-I expression was normal in 27%, reduced in 63%, absent in 7%, and could not be evaluated in 3% of the cases due to absent HLA-A expression in tumor and normal cells. Down-regulation of HLA-A expression significantly correlated with a lower tumor stage (p = 0.005), mucinous tumors (p = 0.05), a lower incidence of recurrences (p = 0.03), and a longer disease-free survival (p = 0.02). Down-regulation of HLA-B/C expression correlated with a lower tumor stage (p < 0.001) and a longer disease-free survival (p = 0.04). In multivariate analysis, HLA-A down-regulation was the only prognostic factor correlated with a longer disease-free survival (p = 0.02). Six tumors were negative for HLA-A and -B/C and did not recur during follow-up. Therefore, we analyzed microsatellite instability (MSI) in these cases. Three of these six tumors indeed showed down-regulation of MLH-1, MSH-2, or MSH-6, indicating a MSI-high phenotype. Beta-2-microglobulin protein expression was lost in five of six of the HLA-I-negative cases, but frame shift mutations in three repetitive sequences in beta2-microglobulin were absent. In contrast, loss of MLH-1, MSH-2, and MSH-6-protein expression was only observed in two of nine matched controls with reduced or normal HLA-A and -B/C expression. Our data showed that HLA-I was down-regulated in 72% of colorectal cancers and provided independent prognostic information for a longer disease-free survival. The better prognosis may be caused by elimination of HLA-negative cells by natural killer cells or by an attenuated tumor aggressiveness, as is seen in tumors with a MSI-high phenotype.

Antigen degradation or presentation by MHC class I molecules via classical and non-classical pathways

Major histocompatibility complex (MHC) class I molecules usually present endogenous peptides at the cell surface. This is the result of a cascade of events involving various dedicated proteins like the peptide transporter associated with antigen processing (TAP) and the ER chaperone tapasin. However, alternative ways for class I peptide loading exist which may be highly relevant in a process called cross-priming. Both pathways are described here in detail. One major difference between these pathways is that the proteases involved in the generation of peptides are different. How proteases and peptidases influence peptide generation and degradation will be discussed. These processes determine the amount of peptides available for TAP translocation and class I binding and ultimately the immune response.

Injection of v-mos-transformed and irradiated macrophages leads to longlasting specific acceptance of MHC-allogeneic heart grafts and specific prolongation of skin graft survival in mice

In vitro studies have revealed that the v-mos-transformed clone mos2 (mos2) of the murine macrophage cell line P388D1 (D1) (H-2d) is capable of inducing a state of specific unresponsiveness in MHC-allogeneic unprimed T cells. Here, we present data on the in vivo relevance of these findings. Male C57bl/6 mice (H-2b) were injected i.p. 6 times with 10(7) of the following irradiated cell types: D1, mos3, mos2, DBA/2-(H-2d) or C3H-(H-2k) spleen macrophages. DBA/2 and C3H skin or heart grafts were performed 10 days after the last injection. The normal rejection time for allogeneic skin was 7.5 days and for allogeneic hearts, was 12.8 days. After injection of D1 or mos3, DBA/2 skin grafts were rejected after 4.5 and 6.5 days, respectively, and the hearts, after 15.4 and 18.6 days, respectively. Third-partly C3H grafts were rejected normally (7.0 days). In contrast, injection of mos2 led to prolongation of DBA skin graft survival to 12.3 days. DBA/2 hearts were accepted for more than 160 days as revealed by heart beating. Again, C3H grafts were rejected normally (11.0 days). DBA/2 skin grafts on day 102 after heart grafting survived for 30 days, indicating hyporesponsiveness against these grafts. These results confirmed the in vitro findings. The mos2 cells obviously induced a state of specific unresponsiveness in otherwise unmanipulated recipients. However, the duration of this unresponsiveness induced by the injection of irradiated cells was dependent on the organ type.

Identification of CCAAT displacement protein (CDP/cut) as a locus-specific repressor of major histocompatibility complex gene expression in human tumor cells

Human major histocompatibility (MHC) class I antigen expression is important in controlling the metastatic growth of malignant tumors. Locus-specific down-regulation of MHC class I gene expression is frequently observed in human tumors, leading to decreased susceptibility to cytotoxic T-cell-mediated lysis. The mechanism of this down-regulation is incompletely understood. Here, we describe the identification of human CCAAT displacement protein (CDP/cut) as a locus-specific repressor of HLA-B and C gene expression. Transient and stable transfections in HeLa and K562 cells demonstrated the presence of a repressor element 650 base pairs upstream of the first exon of HLA-B7. A specific binding complex with the HLA-B7 and Cw2 repressor elements was demonstrated by EMSA. Formation of the EMSA complex was inhibited specifically with polyclonal antiserum to human CDP/cut, demonstrating that CDP/cut binds the HLA-B7 repressor element. The corresponding region of the HLA-A2 promoter neither repressed HLA-A2 gene expression nor bound CDP/cut. Overexpression of CDP/cut in cell lines deficient in CDP/cut resulted in a nearly 4-fold repression of reporter constructs containing the HLA-B7 repressor element but not the corresponding region of the HLA-A2 promoter. Repression of HLA-B and C gene expression by CDP/cut does not involve displacement of NF-Y, nor is CDP/cut associated with the histone deacetylase HDAC1 when bound to the HLA-B7 repressor element. To our knowledge, these results identify CDP/cut as the first example of a locus-specific repressor of MHC class I gene transcription in human tumor cells.

cDNA micro array identification of a gene differentially expressed in adenovirus type 5- versus type 12-transformed cells

Proteins encoded by non-oncogenic adenovirus type 5 and oncogenic adenovirus type 12 differentially affect expression of a number of cellular genes. We have used cDNA micro array analysis to identify a cellular gene that is expressed in Ad12- but not in Ad5-transformed cells. This cellular gene was found to be the gene encoding follistatin-related protein, a TGF-beta inducible gene. Consistently, a constitutive factor binding to Smad binding elements was found in adenovirus type 12-transformed cells.

Dissimilar immunogenicities of human papillomavirus E7 and adenovirus E1A proteins influence primary tumor development

Although human papillomaviruses (HPV) and adenoviruses (Ad) both transform cells by expressing functionally related oncogenes (Ad-E1A/E1B; HPV-E7/E6), only HPV are oncogenic in humans. Prior studies have shown that HPV-transformed cells are resistant to NK cell lysis and E7- and E6-specific CTL are inefficiently generated in women with HPV-induced cervical cancer. Therefore, we postulated that the dissimilar oncogenicities of Ad and HPV may be caused by a protective NK and T cell response that is triggered by transformed cells expressing E1A, but not by E7. To test this hypothesis, mice that were either immunologically intact, lacked T cells, or lacked both NK and T cells were challenged with Ad serotype 5 (Ad5)-E1A- or HPV16-E7-transfected tumor cells. E7-expressing tumor cells were resistant to NK cell lysis in vitro and failed to elicit a measurable anti-tumor NK or T cell response in vivo. The concomitant expression of E6 did not change this phenotype. In contrast, E1A-expressing tumor cells were sensitive to NK lysis in vitro and triggered a protective NK and T cell immune response in vivo. These data suggest differences in the capacities of E1A or E7 oncoproteins to trigger protective anti-tumor immune responses may contribute to the dissimilar oncogenicities of Ad and HPV in humans.

Immune selection and genetic sequence variation in core and envelope regions of hepatitis C virus

How Hepatitis C Virus (HCV) causes persistent infection is unknown. One hypothesis is that HCV evades the host immune response through mutation in immune epitopes. We have investigated mutations in the HCV genome to see if they cluster within immune epitopes; and we have studied the effect of antibody deficiency on mutation rates. We studied patients with chronic hepatitis C, 3 with antibody deficiency and 3 with normal immunity. Regions of the core and envelope genes of HCV, encoding cytotoxic (CTL), and B cell epitopes were sequenced at 2 time points, 2 years apart. The diversity of quasispecies increased with time. The HCV genetic mutation rate was higher than previously predicted. The cryptic nucleotide mutation rate in core was similar to that observed in envelope, suggesting that the error rate of the HCV RNA polymerase is similar in both regions. In contrast, the coding mutation rate was decreased in core and increased in envelope. No genetic mutation was seen in any of the core CTL epitopes despite detectable cellular responses. All patients had mutations within a previously described envelope CTL epitope but did not exhibit immune responses to either index or mutated peptides. There was no difference in mutation rates in any cellular or humoral epitopes between patients with antibody deficiency and normal immunity. Thus we have found no evidence that mutations were selected by T-lymphocytes or antibodies. These findings implicate alternative virus-host interactions in the selection of HCV mutations.

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.

Interaction of p27 with E1A and its effect on CDK kinase activity

The interaction of p27 with adenovirus (Ad) E1A was investigated to study its possible role in cell-cycle regulation and transformation by E1A. In in vitro binding assays, recombinant p27 proteins were shown to bind 12S and 13S E1A products of both Ad12 and Ad5. The amino-terminal region of p27, but not the carboxyl-terminal region, was responsible for the E1A binding. In the Ad12 E1A proteins, the C-terminal region showed relative importance in p27 binding. Phosphorylation of histone H1 or E1A proteins by CDK2 complex was inhibited by p27, but, in contrast, p27 stimulated the phosphorylation of E1A proteins by CDK4. Thus, the interaction of p27 and E1A proteins may modulate the function of E1A in cell-cycle control by regulating E1A phosphorylation.

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.

Downregulation of Waf1, C2, C3, and major histocompatibility complex class I loci within an 18-cM region of chromosome 17 in adenovirus-transformed mouse cells

In this study, the expression of the p53 tumor suppressor gene and the p53-regulated Mdm2 and Waf1 genes was evaluated in adenovirus (Ad)-transformed mouse cells. The expected levels of p53 mRNA and protein and Mdm2 mRNA were detected in all transformed cells. However, the level of Waf1 mRNA was markedly reduced in Ad12-transformed cells and in some Ad5-transformed cells. Waf1 expression was not reduced in untransformed mouse cells infected with Ad12 or Ad5. Expression of the class I major histocompatibility complex (MHC) locus was downregulated in 13 Ad-transformed cell lines (derived from four different strains of mice) that exhibited reduced expression of Waf1. Waf1 is located on mouse chromosome 17 proximal to the MHC class I locus. To determine whether other chromosome 17 genes were downregulated, the cells were examined for expression of other genetic loci. Of those tested, only the C2 and C3 complement loci were expressed in mouse fibroblasts. Expression of C2 (which is within the MHC) and expression of C3 (which is 15 cM distal to the MHC) were downregulated in those transformed cells in which Waf1 and MHC class I were downregulated. The Ad12- and Ad5-transformed cells that expressed low levels of Waf1, MHC class I, C2, and C3 formed tumors in syngeneic adult mice. These data suggest that the downregulation of multiple genes within the 32 Mb of mouse chromosome 17 that includes the Waf1 locus to the C3 locus occurs in Ad mouse-cell transformation and may contribute to the tumorigenicity of transformed cells.

Viruses use stealth technology to escape from the host immune system

In this review, we focus on recent investigations that reveal novel mechanisms by which viruses evade detection and elimination by the host immune system. In particular, we consider the evasion mechanisms of five persistent viruses: herpes simplex virus, human cytomegalovirus, mouse cytomegalovirus, Epstein-Barr virus and adenovirus. Unravelling the strategies used by viruses to survive within the host could identify new targets for antiviral drugs and for improved vaccines. Identification of the mechanisms that underlie these strategies might also reveal new, fundamental features of biology that occur in uninfected cells and are exploited by viruses.

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.

Susceptibility to natural killer cells and down regulation of MHC class I expression in adenovirus 12 transformed cells are regulated by different E1A domains

All human adenoviruses transform rodent cells in vitro, but only cells transformed by serotypes belonging to subgroups A (Ad12) and B (Ad3) are tumorigenic for immunocompetent animals. In these cells, the expression of MHC-class I antigens is repressed and might allow them to escape from recognition by cytotoxic T lymphocytes (CTL) and to develop in tumor. Furthermore, these cell lines appear resistant to lysis by natural killer (NK) cells. To determine the E1A domain(s) responsible for these properties several cell lines were created by transforming baby rat kidney (BRK) cells with a set of plasmids expressing different Ad2/Ad12 hybrid E1A gene products. The MHC class 1 gene expression was inhibited in cells expressing the Ad12 13S mRNA product and in cells transformed with Ad2/Ad12 hybrid E1A gene product harboring the C-terminal part of the conserved region (CR) 3 of Ad12. Susceptibility of these transformed cell lines to NK cells was determined by cytolytic assays. The results obtained suggest that two Ad12 E1A domains are required to induce resistance of the cell lines to NK cells.

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.

Decreased tumor surveillance in perforin-deficient mice

Immune surveillance against tumors usually depends on T cell recognition of tumor antigens presented by major histocompatibility complex (MHC) molecules, whereas MHC class I- tumors may be controlled by natural killer (NK) cells. Perforin-dependent cytotoxicity is a major effector function of CD8+ MHC class I-restricted T cells and of NK cells. Here, we used perforin-deficient C57BL/6 (PKO) mice to study involvement of perforin and Fas ligand in tumor surveillance in vivo. We induced tumors in PKO and normal C57BL/6 mice by (a) injection of different syngeneic tumor cell lines of different tissue origin in naive and primed mice; (b) administration of the chemical carcinogens methylcholanthrene (MCA) or 12-O-tetradecanoylphorbol-13-acetate (TPA) plus 7,12-dimethylbenzanthracene (DMBA), or (c) by injection of acutely oncogenic Moloney sarcoma virus. The first set of models analyzes the defense against a tumor load given at once, whereas the last two sets give information on immune defense against tumors at the very moment of their generation. Most of the tumor cell lines tested were eliminated 10-100-fold better by C57BL/6 mice in an unprimed situation; after priming, the differences were more pronounced. Lymphoma cells transfected with Fas were controlled 10-fold better by PKO and C57BL/6 mice when compared to untransfected control cells, indicating some role for FasL in tumor control. MCA-induced tumors arose more rapidly and with a higher incidence in PKO mice compared to C57BL/6 or CD8-deficient mice. DMBA+TPA-induced skin papillomas arose with similar high incidence and comparable kinetics in both mouse strains. C57BL/6 and PKO mice have a similar incidence of Moloney murine sarcoma and leukemia virus-induced sarcomas, but tumors are larger and regression is retarded in PKO mice. Thus, perforin-dependent cytotoxicity is not only a crucial mechanism of both cytotoxic T lymphocyte- and NK-dependent resistance to injected tumor cell lines, but also operates during viral and chemical carcinogenesis in vivo. Experiments addressing the role of Fas-dependent cytotoxicity by studying resistance to tumor cell lines that were stably transfected with Fas neither provided evidence for a major role of Fas nor excluded a minor contribution of Fas in tumor surveillance.

The dual effect of adenovirus type 5 E1A 13S protein on NF-kappaB activation is antagonized by E1B 19K

The genomes of human adenoviruses encode several regulatory proteins, including the two differentially spliced gene products E1A and E1B. Here, we show that the 13S but not the 12S splice variant of E1A of adenovirus type 5 can activate the human transcription factor NF-kappaB in a bimodal fashion. One mode is the activation of NF-kappaB containing the p65 subunit from the cytoplasmic NF-kappaB-IkappaB complex. This activation required reactive oxygen intermediates and the phosphorylation of IkappaBalpha at serines 32 and 36, followed by IkappaBalpha degradation and the nuclear uptake of NF-kappaB. In addition, 13S E1A stimulated the transcriptional activity of the C-terminal 80 amino acids of p65 at a core promoter with either a TATA box or an initiator (INR) element. The C-terminal 80 amino acids of p65 were found to associate with E1A in vitro. The activation of NF-kappaB-dependent reporter gene transcription by E1A was potently suppressed upon coexpression of the E1B 19-kDa protein (19K). E1B 19K prevented both the activation of NF-kappaB and the E1A-mediated transcriptional enhancement of p65. These inhibitory effects were not found for the 55-kDa splice variant of the E1B protein. We suggest that the inductive effect of E1A 13S on the host factor NF-kappaB, whose activation is important for the transcription of various adenovirus genes, must be counteracted by the suppressive effect of E1B 19K so that the adenovirus-infected cell can escape the immune-stimulatory and apoptotic effects of NF-kappaB.

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.

Perforin dependence of natural killer cell-mediated tumor control in vivo

Adaptive immune surveillance by T cells against infections and tumors depends on the presence of antigenic peptides presented by major histocompatibility complex (MHC) molecules. If antigenic tumor-specific peptides or MHC class I molecules are absent, the adaptive T cell immune response fails. Natural killer (NK) cells seem to complement the specific T cells by recognizing target cells lacking MHC class I (e.g. RMA-S). The role of perforin, which is crucially involved in T cell and NK cell-mediated target cell lysis, was evaluated in mice lacking perforin with respect to their capacity to eliminate a syngeneic lymphoid tumor. Here, we show that growth of MHC class I RMA-S tumor cells in unprimed mice was controlled by NK cells through perforin-dependent cytotoxicity.

Dominant-negative p53 can restore tumorigenicity of L-929 cells in immunocompetent mice

To study the effect of a transforming allele of the tumor suppressor p53 upon the anti-tumor immune response, antigenic L-929 cells were transfected with the dominant-negative valine135 mutant of murine p53. Several p53val135-expressing transfectants formed non-regressing tumors in immunocompetent hosts. The growth rates of tumorigenic and non-tumorigenic clones were equivalent in vitro in sublethally irradiated C3H/HeN mice and in nude mice. Tumorigenic and non-tumorigenic p53val135-expressing L-929 clones expressed equivalent levels of cell surface class I major histocompatibility complex (MHC) glycoproteins. Immunization with a tumorigenic Lp53val135 clone protected mice from subsequent challenge and primed MHC class I-restricted cytotoxic T-lymphocytic precursors. Secretion of an immunosuppressive cytokine, transforming growth factor beta-1 and sensitivity to tumor necrosis factor-alpha were equivalent from tumorigenic and non-tumorigenic cell lines. These data suggest that expression of a transforming allele of p53 can allow L-929 cells to escape the host immune system.