Expression of the adenovirus E1A oncogene during cell transformation is sufficient to induce susceptibility to lysis by host inflammatory cells

Therapeutic targeting of the RB1 pathway in retinoblastoma with the oncolytic adenovirus VCN-01

Retinoblastoma is a pediatric solid tumor of the retina activated upon homozygous inactivation of the tumor suppressor RB1 VCN-01 is an oncolytic adenovirus designed to replicate selectively in tumor cells with high abundance of free E2F-1, a consequence of a dysfunctional RB1 pathway. Thus, we reasoned that VCN-01 could provide targeted therapeutic activity against even chemoresistant retinoblastoma. In vitro, VCN-01 effectively killed patient-derived retinoblastoma models. In mice, intravitreous administration of VCN-01 in retinoblastoma xenografts induced tumor necrosis, improved ocular survival compared with standard-of-care chemotherapy, and prevented micrometastatic dissemination into the brain. In juvenile immunocompetent rabbits, VCN-01 did not replicate in retinas, induced minor local side effects, and only leaked slightly and for a short time into the blood. Initial phase 1 data in patients showed the feasibility of the administration of intravitreous VCN-01 and resulted in antitumor activity in retinoblastoma vitreous seeds and evidence of viral replication markers in tumor cells. The treatment caused local vitreous inflammation but no systemic complications. Thus, oncolytic adenoviruses targeting RB1 might provide a tumor-selective and chemotherapy-independent treatment option for retinoblastoma.

PIDD-dependent activation of caspase-2-mediated mitochondrial injury in E1A-induced cellular sensitivity to macrophage nitric oxide-induced apoptosis

Expression of the adenovirus E1A oncogene sensitizes tumor cells to innate immune rejection by apoptosis induced by macrophage-produced tumor necrosis factor (TNF)-α and nitric oxide (NO). E1A sensitizes cells to TNF-α and NO through two distinct mechanisms, by repressing NF-κB-dependent antiapoptotic responses and enhancing caspase-2 activation and mitochondrial injury, respectively. The mechanisms through which E1A enhances caspase-2 activation in response to NO were unknown. Here, we report that E1A-induced sensitization to NO-induced apoptosis is dependent on expression of PIDD (p53-inducible protein with a death domain) and enhancement of primary immunodeficiency diseases (PIDD) processing for formation of the PIDDosome, the core component of the caspase-2 activation complex. NO-induced apoptosis in E1A-expressing cells did not require expression Bak or Bax, indicating that NO-induced caspase-2-mediated mitochondrial injury does not proceed through the activities of typical, proapoptotic Bcl-2 family members that induce mitochondrial cytochrome C release. These results define a PIDD-dependent pathway, through which E1A enhances casapse-2-mediated mitochondrial injury, resulting in increased sensitivity of mammalian cells to macrophage-induced, NO-mediated apoptosis.

Oncolytic virus and PD-1/PD-L1 blockade combination therapy

Oncolytic viruses are lytic for many types of cancers but are attenuated or replication-defective in normal tissues. Aside from tumor lysis, oncolytic viruses can induce host immune responses against cancer cells and may thus be viewed as a form of immunotherapy. Although recent successes with checkpoint inhibitors have shown that enhancing antitumor immunity can be effective, the dynamic nature of the immunosuppressive tumor microenvironment presents significant hurdles to the broader application of these therapies. Targeting one immune-suppressive pathway may not be sufficient to eliminate tumors. Here we focus on the development of the combination of oncolytic virotherapy with checkpoint inhibitors designed to target the programmed cell death protein 1 and programmed cell death ligand 1 signaling axis. We also discuss future directions for the clinical application of this novel combination therapy.

Adenovirus serotype 5 E1A expressing tumor cells elicit a tumor-specific CD8+ T cell response independent of NKG2D

The expression of the Adenovirus serotype 2 or serotype 5 (Ad2/5) E1A gene in tumor cells upregulates ligands that are recognized by the NKG2D activating receptor, which is expressed on NK cells and T cells, and reduces their tumorigenicity, a process dependent on NK cells and T cells. In some model systems, the forced overexpression of NKG2D ligands on tumor cells induced antigen-specific CD8+ T cells that mediated anti-tumor immunity. We wanted to determine if the interaction of NKG2D ligands on tumor cells that express E1A with NKG2D on immune cells contributed to the ability of E1A to induce a CD8+ T cell anti-tumor response or reduce tumorigenicity. To address these questions, we used the MCA-205 tumor cell line or MCA-205 cells that expressed Ad5 E1A (MCA-205-E1A cells), a fusion protein of E1A and ovalbumin (MCA-205-E1A-OVA) or OVA (MCA-205-OVA). We found that the expression of E1A or E1A–OVA, but not OVA, upregulated the expression of the NKG2D ligand RAE-1 on the surface of MCA-205 cells. Additionally, MCA-205-E1A cells and MCA-205-E1A-OVA cells were more sensitive to NK cell lysis than MCA-205 or MCA-205-OVA cells in WT B6 mice, but not NKG2D deficient B6 mice. Next, we adoptively transferred WT or NKG2D deficient OT-1 T cells (CD8 T cells that recognize OVA residues 257–264) into WT B6 mice or B6 mice that were deficient in NKG2D respectively and measured the expansion of OT-1 cells following immunization with MCA-205-E1A-OVA or MCA-205-OVA cells. We found that the expansion of OT-1 cells following immunization of either OVA-expressing MCA-205 cell lines was not affected by the presence or absence of NKG2D in B6 mice. Finally, we found that the capacity of E1A to reduce the tumorigenicity of MCA-205 cells was not impaired in B6-NKG2D deficient mice as compared to WT B6 mice. Our results suggest that the ability of E1A to reduce the tumorigenicity of MCA-205 cells, or induce an antigen-specific CD8+ T cell response, is independent of the interaction of NKG2D ligands with the NKG2D receptor.

Possible role of arginase-1 in concomitant tumor immunity

The expression of Adenovirus serotype 2 or serotype 5 (Ad2/5) E1A in tumor cells reduces their tumorigenicity in vivo by enhancing the NK cell mediated and T cell mediated anti-tumor immune response, an activity that correlates with the ability of E1A to bind p300. We determined if E1A could be used as a molecular adjuvant to enhance antigen-specific T cell responses to a model tumor antigen, ovalbumin (OVA). To achieve this goal, we stably expressed a fusion protein of E1A and OVA (MCA-205-E1A-OVA), OVA (MCA-205-OVA) or a mutant version of E1A unable to bind p300 and OVA (E1A-Δp300-OVA) in the B6-derived, highly tumorigenic MCA-205 tumor cell line. MCA-205-E1A-OVA tumor cells were over 10,000 fold less tumorigenic than MCA-205-OVA, MCA-205-E1A-Δp300-OVA, or MCA-205 in B6 mice. However, immunization of B6 mice with live MCA-205-OVA, MCA-205-E1A-Δp300-OVA and MCA-E1A-OVA tumor cells induced nearly equivalent OVA-specific CD4 T cells and CD8 CTL responses. Further studies revealed that mice with primary, enlarging MCA-205-OVA or MCA-205-E1A-Δp300-OVA tumors on one flank exhibited OVA-specific anti-tumor T cell responses that rejected a tumorigenic dose of MCA-205-OVA cells on the contralateral flank (concomitant tumor immunity). Next we found that tumor associated macrophages (TAMs) in progressive MCA-205-OVA tumors, but not MCA-205-E1A-OVA tumors that expressed high levels of arginase-1, which is known to have local immunosuppressive activities. In summary, immunization of mice with MCA-205 cells expressing OVA, E1A-Δp300-OVA or E1A-OVA induced equivalent OVA-specific CD4 and CD8 anti-tumor responses. TAMs found in MCA-205-OVA, but not MCA-205-E1A-OVA, tumors expressed high levels of arginase-1. We hypothesize that the production of arginase-1 by TAMs in MCA-205-OVA or MCA-205-E1A-Δp300-OVA tumor cells leads to an ineffective anti-tumor immune response in the tumor microenvironment, but does not result in inhibition of a systemic anti-tumor immunity.

Quantification of high-capacity helper-dependent adenoviral vector genomes in vitro and in vivo, using quantitative TaqMan real-time polymerase chain reaction

First-generation adenoviral (Ad) and high-capacity adenoviral (HC-Ad) vectors are efficient delivery vehicles for transferring therapeutic transgenes in vivo into tissues/organs. The initial successes reported with adenoviral vectors in preclinical trials have been limited by immune-related adverse side effects. This has been, in part, attributed to the use of poorly characterized preparations of adenoviral vectors and also to the untoward immune adverse side effects elicited when high doses of these vectors were used. HC-Ads have several advantages over Ads, including the lack of viral coding sequences, which after infection and uncoating, makes them invisible to the host's immune system. Another advantage is their large cloning capacity (up to approximately 35 kb). However, accurate characterization of HC-Ad vectors, and of contaminating replication-competent adenovirus (RCA) or helper virus, is necessary before these preparations can be used safely in clinical trials. Consequently, the development of accurate, simple, and reproducible methods to standardize and validate adenoviral preparations for the presence of contaminant genomes is required. By using a molecular method that allows accurate, reproducible, and simultaneous determination of HC-Ad, contaminating helper virus, and RCA genome copy numbers based on real-time quantitative PCR, we demonstrate accurate detection of these three genomic entities, within CsCl-purified vector stocks, total DNA isolated from cells transduced in vitro, and from brain tissue infected in vivo. This approach will allow accurate assessment of the levels and biodistribution of HC-Ad and improve the safety and efficacy of clinical trials.

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.

Macrophages kill human papillomavirus type 16 E6-expressing tumor cells by tumor necrosis factor alpha- and nitric oxide-dependent mechanisms

The expression of adenovirus serotype 2 or 5 (Ad2/5) E1A sensitizes cells to killing by NK cells and activated macrophages, a property that correlates with the ability of E1A to bind the transcriptional coadaptor proteins p300-CBP. The E6 oncoproteins derived from the high-risk human papillomaviruses (HPV) interact with p300 and can complement mutant forms of E1A that cannot interact with p300 to induce cellular immortalization. Therefore, we determined if HPV type 16 (HPV16) E6 could sensitize cells to killing by macrophages and NK cells. HPV16 E6 expression sensitized human (H4 and C33A) and murine (MCA-102) cell lines to lysis by macrophages but not by NK cells. The lysis of cells that expressed E6 by macrophages was p53 independent but dependent on the production of tumor necrosis factor alpha (TNF-alpha) or nitric oxide (NO) by macrophages. Unlike cytolysis assays with macrophages, E6 expression did not significantly sensitize cells to lysis by the direct addition of NO or TNF-alpha. Like E1A, E6 has been reported to sensitize cells to lysis by TNF-alpha by inhibiting the TNF-alpha-induced activation of NF-kappaB. We found that E1A, but not E6, blocked the TNF-alpha-induced activation of NF-kappaB, an activity that correlated with E1A-p300 binding. In summary, Ad5 E1A and HPV16 E6 sensitized cells to lysis by macrophages. Unlike E1A, E6 did not block the ability of TNF-alpha to activate NF-kappaB or sensitize cells to lysis by NK cells, TNF-alpha, or NO. Thus, there appears to be a spectrum of common and unique biological activities that result as a consequence of the interaction of E6 or E1A with p300-CBP.

Expression of an E1A/E7 chimeric protein sensitizes tumor cells to killing by activated macrophages but not NK cells

Adenovirus (Ad) E1A and human papillomavirus (HPV) E7 express homologous conserved regions (CRs) that mediate their shared biological functions. Despite their similarities, the expression of E1A sensitizes tumor cells to killing by NK cells and macrophages but the expression of E7 does not, a factor that may contribute to the dissimilar oncogenicities of Ad and HPV. This study was undertaken to define molecular differences between E1A and E7 that are responsible for the ability of E1A and the inability of E7 to sensitize cells to killing by NK cells and macrophages. Genetic mapping studies using human fibrosarcoma cells (H4) that stably expressed mutant forms of E1A showed that only those forms of E1A that interacted with the transcriptional coadaptor protein p300 sensitized cells to killing by NK cells and macrophages. E7 lacks the N-terminal p300-binding region present in E1A. Therefore, a chimeric E1A/E7 gene was constructed that included the N terminus and the CR1 (p300-binding) domain of E1A fused to CR2 and the C-terminal sequences of E7. The E1A/E7 protein interacted with p300 and pRb and immortalized primary mouse embryo fibroblasts (MEF). The expression of E1A/E7 sensitized H4 and MEF cells to killing by activated macrophages but not to killing by NK cells. Therefore, N-terminal differences between E1A and E7 that map to the E1A-p300 binding region accounted for differences in their abilities to sensitize cells to killing by macrophages. However, regions in addition to the E1A-p300 binding region are required to sensitize cells to killing by NK cells.

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.

Adenovirus E3-6.7K maintains calcium homeostasis and prevents apoptosis and arachidonic acid release

E3-6.7K is a small and hydrophobic membrane glycoprotein encoded by the E3 region of subgroup C adenovirus. Recently, E3-6.7K has been shown to be required for the downregulation of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptors by the adenovirus E3/10.4K and E3/14.5K complex of proteins. We demonstrate here that E3-6.7K has additional protective roles, independent of other virus proteins. In transfected Jurkat T-cell lymphoma cells, E3-6.7K was found to maintain endoplasmic reticulum-Ca(2+) homeostasis and inhibit the induction of apoptosis by thapsigargin. The presence of E3-6.7K also lead to a reduction in the TNF-induced release of arachidonic acid from transfected U937 human histiocytic lymphoma cells. In addition, E3-6.7K protected cells against apoptosis induced through Fas, TNF receptor, and TRAIL receptors. Therefore, E3-6.7K confers a wide range of protective effects against both Ca(2+) flux-induced and death receptor-mediated apoptosis.

MHC class I molecules on adenovirus E1A-expressing tumor cells inhibit NK cell killing but not NK cell-mediated tumor rejection

Expression of adenovirus E1A gene products in tumor cells enhances NK cell lysis in vitro and NK-mediated rejection in vivo, despite increasing class I molecules on tumor cells. It is unclear why the increased expression of MHC class I molecules does not appear to confer resistance to killing by NK cells. One possibility is the unique capacity of E1A to sensitize cells to multiple NK cell killing mechanisms including perforin/granzyme, Fas ligand, tumor necrosis factor-alpha and TRAIL. To examine this issue, MCA-102-E1A tumor cells (H-2(b)) that express E1A and are NK sensitive were transfected with H-2D(d), the ligand for the NK inhibitory receptor, Ly49A. Expression of H-2D(d) molecules by MCA-102-E1A cells protected them from lysis by a Ly49A(+) NK cell clone and Ly49A(+) NK cells isolated from C57BL/6 nude mice. In contrast, NK cell-mediated rejection of MCA-102-E1A tumor cells was not inhibited by the expression of H-2D(d) molecules, nor was killing by polyclonal populations of NK cells isolated from C57BL/6-nude mice. H-2D(d) interacts with several inhibitory Ly49 receptors that are non-clonally expressed on NK cells in C57BL/6 mice: Ly49A (20% of NK cells), Ly49G2 (54% of NK cells) and Ly49C/I (47% of NK cells). Our data indicate that while E1A sensitizes cells to NK cell killing, it does not interfere with signal transduction by inhibitory NK receptors. Therefore, a small population of NK cells that do not express Ly49A, Ly49G2 or Ly49C/I inhibitory receptors are likely responsible for the rejection of MCA-102-E1A-D(d) tumor cells in vivo.

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.

Synergy between the herpes simplex virus tk/ganciclovir prodrug suicide system and the topoisomerase I inhibitor topotecan

An established principle of antineoplastic chemotherapy is that multidrug regimens are generally superior to single-agent therapy. This prompted us to elucidate whether the topoisomerase inhibitor topotecan (TPT) could enhance the efficacy of the herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) system for the treatment of cancer. We assessed the interaction between these two treatments in murine MC38 and human HT-29 colon carcinoma cell lines that were genetically modified to constitutively express HSV-tk, sensitizing them to GCV. Synergistic cell killing was observed in a clonogenic assay over most of the cytotoxic dose range by the median-effect principle of Chou and Talalay (Adv. Enzyme Regul. 1984; 22:27-55). Subcutaneous tumor models, using the same cell lines in C57BL/6 and athymic nude mice, respectively, demonstrated that the combination of GCV and TPT resulted in statistically significant enhanced survival relative to single-agent treatment. In addition, nude mice bearing HT-29 tumor xenografts were treated with an Ad5 E1b Mr 55,000 attenuated replication-competent adenovirus expressing HSV-tk (Ad.TK(RC)) either alone or in combination with GCV and/or TPT. These experiments demonstrated that Ad.TK(RC) followed by GCV and TPT was more efficacious than any other treatment tested. Our results suggest that for antineoplastic therapy, molecular chemotherapy based on the HSV-tk/GCV system combined with traditional chemotherapy is a logical and practical future direction to pursue. Suicide gene therapy is the approach whereby genetically altering a cell makes it susceptible to an otherwise relatively nontoxic prodrug. By this approach it is possible to achieve relatively high concentrations of the toxic metabolites in the transduced cells while maintaining low systemic levels of the active drug. The most often used metabolic suicide gene transfer system is the HSV-tk/GCV paradigm, which is currently being used in cancer therapy or as a safety modality. The low response rate observed in the early clinical HSV-tk cancer trials may be due to failure in achieving adequate transduction efficiency and/or prodrug concentration within the tumor. The combination of such suicide gene prodrug systems with adjunctive drugs resulting in synergistic cytotoxicity might improve the clinical utility of this approach.

E1A oncogene-induced cellular sensitization to immune-mediated apoptosis is independent of p53 and resistant to blockade by E1B 19 kDa protein

E1A oncogene expression sensitizes mammalian cells to apoptosis triggered by cytolytic lymphocytes (CL) [16]. Most studies suggest that E1A-induced apoptosis involves a p53-dependent cellular pathway that is blocked by the E1B 19 kDa gene product. In this study, the roles of p53 and E1B 19 kDa were tested for E1A sensitization to CL-induced apoptosis in contrast with apoptosis triggered by TNF alpha or chemical injuries. E1A sensitization to immune-mediated (CL- or TNF-induced) apoptosis was independent of p53 expression and was resistant to blockade by E1B 19 kDa protein in mouse and hamster cells. In contrast, the p53 requirement for chemically induced apoptosis of E1A-sensitized cells varied with the agent used to treat cells. Apoptosis induced by diverse chemical agents (hygromycin, beauvericin, etoposide, H(2)O(2)) was blocked by E1B 19 kDa expression. Therefore, both the p53-dependence and the E1B 19 kDa blockade of E1A-induced cellular sensitization to apoptotic injury depend on the type of proapoptotic injury tested. These data suggest that the mechanisms by which E1A sensitizes tumor cells to immune-mediated apoptosis and to rejection by immunocompetent animals do not require cellular expression of wild-type p53 and can function independently of the Bcl-2-like, antiapoptotic mechanisms of E1B 19 kDa.

Adenoviral vectors capable of replication improve the efficacy of HSVtk/GCV suicide gene therapy of cancer

A major obstacle to the success of gene therapy strategies that directly target cancer cells is the poor vector distribution within solid tumors. To address this problem, we developed an E1b 55 kDa attenuated, replication-competent adenovirus (Ad.TKRC) which expresses the herpes simplex-1 thymidine kinase (HSVtk) gene to sensitize tumors to ganciclovir (GCV). Efficacy of this combined strategy was tested in nude mice with subcutaneous human A375 melanoma and ME180 cervical carcinomas. Intratumoral injection of a replication-defective adenoviral vector expressing HSVtk (Ad.TK) followed by GCV treatment resulted in doubling of the survival time of mice bearing A375 tumors and 20% long-term survival of mice with ME180 tumors. Treatment of tumors with Ad.TKRC without GCV resulted in a similar antitumor effect, confirming that the replicating vector has an oncolytic effect. When GCV was initiated 3 days after Ad.TKRC injection, survival of mice with each tumor type was greatly prolonged, with 60% of animals with ME180 tumors surviving for over 160 days. These results confirm that both the oncolysis caused by a replicating virus and suicide/prodrug gene therapy with HSVtk/GCV have potent antitumor effects. When combined, these two approaches are complementary resulting in a significantly improved treatment outcome.

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

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

Role of p300-family proteins in E1A oncogene induction of cytolytic susceptibility and tumor cell rejection

The mechanism by which the adenoviral (Ad) E1A oncogene induces cellular susceptibility to lysis by killer lymphocytes involves interactions between its first exon and different second-exon accessory regions. Mutational analysis showed that two first-exon regions--one in the N terminus and one in the conserved region 1 (CR1) domain--are necessary for this activity. E1A complex formation with cellular p300 protein through these first-exon-encoded regions correlated with induction of the cytolytic susceptible phenotype but was only effective in the context of E1A second-exon expression. An E1A first-exon deletion that prevented p300 binding eliminated both oncoprotein-induced cytolytic susceptibility and rejection of transfected sarcoma cells by immunocompetent animals. These results suggest that the E1A oncogene induces cytolytic susceptibility and tumor rejection by interactions with cellular proteins of the p300 family that affect transcription of genes involved in the cellular response to injury inflicted by host killer cells.

Adenovirus E1A proteins regulate phosphoenolpyruvate carboxykinase gene transcription through multiple mechanisms

Recently, Kalvakolanu et al. (Kalvakolanu, D. V. R., Liu, J., Hanson, R. W., Harter, M. L., and Sen, G. C. (1992) J. Biol. Chem. 267, 2530-2536) showed that E1A inhibited the basal and cAMP-stimulated transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK). This inhibition was mediated by the conserved region 1 (CR1) domain of E1A, which has been shown by other laboratories to bind to the cellular transcriptional adaptor proteins, p300 and cAMP response element binding protein (CREB)-binding protein (CBP). The PEPCK gene promoter contains a functional cAMP-response element, through which CREB and, therefore, CBP modulate transcription, and a consensus p300 DNA binding sequence is also present in a distal protein binding site of the promoter. We hypothesized that E1A might inhibit PEPCK gene transcription by binding to p300 and/or CBP. Surprisingly, we found that E1A consistently stimulated basal transcription from the PEPCK promoter in transfection assays in adenovirus (Ad)-infected HepG2 hepatoma cells or E1A-expressing, stably transfected 3T3 fibroblasts and nuclear run-on assays in Ad-infected H4IIE hepatoma cells. E1A also enhanced the stimulation of PEPCK gene transcription by Bt2cAMP. In transfection assays, wild type Ad5 expressing both 243R and 289R forms of E1A or a mutant virus expressing the 289R form alone stimulated transcription from the PEPCK promoter by approximately 5-fold 20 h postinfection. However, no stimulation was observed in cells infected with a virus expressing either the 243R protein alone or a 289R protein from which conserved region 3 (CR3) was mutated. Mutation or deletion of CR1 of E1A had no significant effect on transcription from the PEPCK promoter. Mutations within conserved region 2 (CR2) of E1A that inhibit the binding of E1A to the retinoblastoma gene product (pRb) further enhanced the stimulation of transcription from the PEPCK promoter by 2 3-fold compared with wild type E1A. These findings suggested that the normal function of pRb is to stimulate PEPCK gene transcription, and that this process is inhibited by the binding of E1A to pRb. This hypothesis was confirmed by overexpressing pRb in HepG2 cells, which stimulated transcription from the PEPCK promoter. Our findings indicate that Ad E1A regulates PEPCK gene transcription through a stimulatory mechanism involving CR3, and by attenuating a stimulatory effect of pRb through CR2.

Oncogenicity of human papillomavirus- or adenovirus-transformed cells correlates with resistance to lysis by natural killer cells

The reasons for the dissimilar oncogenicities of human adenoviruses and human papillomaviruses (HPV) in humans are unknown but may relate to differences in the capacities of the E1A and E7 proteins to target cells for rejection by the host natural killer (NK) cell response. As one test of this hypothesis, we compared the abilities of E1A- and E7-expressing human fibroblastic or keratinocyte-derived human cells to be selectively killed by either unstimulated or interferon (IFN)-activated NK cells. Cells expressing the E1A oncoprotein were selectively killed by unstimulated NK cells, while the same parental cells but expressing the HPV type 16 (HPV-16) or HPV-18 E7 oncoprotein were resistant to NK cell lysis. The ability of IFN-activated NK cells to selectively kill virally transformed cells depends on IFN's ability to induce resistance to NK cell lysis in normal (i.e., non-viral oncogene-expressing) but not virally transformed cells. E1A blocked IFN's induction of cytolytic resistance, resulting in the selective lysis of adenovirus-transformed cells by IFN-activated NK cells. The extent of IFN-induced NK cell killing of E1A-expressing cells was proportional to the level of E1A expression and correlated with the ability of E1A to block IFN-stimulated gene expression in target cells. In contrast, E7 blocked neither IFN-stimulated gene expression nor IFN's induction of cytolytic resistance, thereby precluding the selective lysis of HPV-transformed cells by IFN-activated NK cells. In conclusion, E1A expression marks cells for destruction by the host NK cell response, whereas the E7 oncoprotein lacks this activity.

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.

Effect of temperature on the expression of major histocompatibility complex class-I antigens

In the present study we investigated the effect of temperature on MHC class-I gene expression in BALB/C 3T3 cells incubated for 5 days at 34 degrees C, 37 degrees C and 39 degrees C. FACS analysis revealed no significant difference in the cell surface expression of any of the 3 major class-I antigens at 34 degrees C and 37 degrees C. Strikingly, however, when the level of the respective mRNA was determined, only that of the H-2K was comparable at both temperatures, whereas the levels of the H-2D and H-2L mRNA were profoundly higher at 37 degrees C. These data appear to reflect a differential temperature-related transcriptional control of the different class-I genes or a different temperature effect on the stability of their mRNA. The absence of a parallel increase in surface expression of the corresponding H-2D and H-2L antigens may result from some translational or post-translational limiting factors. At 39 degrees C, however, these limiting factors seem to be overcome since the surface expression of all the 3 antigens was remarkably increased although the level of their encoding mRNA was rather lower than in 37 degrees C. This stimulatory effect might be ascribed to heat shock proteins which are known to arise in cells at heat or other stress conditions. They participate in assembly and disassembly of various protein complexes and in transport of certain proteins across intracellular membranes. Such proteins may have arisen in our cells at 39 degrees C and facilitated the intracellular assembly of the class-I molecules and their transport to the cell surface. The possible implication of such heat shock proteins in the anti-tumor effect of hyperthermia is discussed.

E1A oncogene induction of cytolytic susceptibility eliminates sarcoma cell tumorigenicity

The manner in which oncogenes influence tumorigenicity beyond their ability to immortalize cells is uncertain. We tested the hypothesis that, in addition to subverting cellular growth controls, oncogenes can actively determine tumor-inducing capacity by affecting neoplastic cell susceptibility to destruction by the host cellular immune response. The adenovirus type 5 E1A oncogene, which induces susceptibility to lysis by natural killer cells and encodes epitopes recognized by cytotoxic T lymphocytes, was transfected into highly tumorigenic sarcoma cells. E1A expression in these sarcoma cells eliminated their tumorigenicity in recipients with natural killer cell activity that was competent to lyse these E1A-positive targets. Thymus-dependent responses were not required for tumor rejection. These results indicate that oncogene-regulated cellular pathways that affect neoplastic cell susceptibility to natural killer cell lytic mechanisms may influence tumor development in the immunocompetent host.

The in vivo clearance of Ha-ras transformants by natural killer cells

The experiments in this study were designed to test the hypothesis that natural killer (NK) cells play a role in host surveillance against early neoplastic changes in the malignant process. C3H 10T1/2 mouse fibroblasts were transfected with a pSV2-neo plasmid vector which contains EJ, the mutated c-Ha-ras, regulated by its own promoter. Control cells were transfected with pSV2-neo alone and did not contain the ras gene. Oncogene-transfected cells were compared with control cells for lung colony formation following tail vein injection into C3H mice. Intravenous injection of ras-transfected 10T1/2 cells induced marked lung colony formation in vivo, whereas C3H 10T1/2 parental lines or 10T1/2 cells transfected with pSV2-neo alone induced no lung colonies in C3H mice. The colonising potential of ras transfectants could be decreased by augmentation of NK activity by injection of polyinosinic cytidylic acid and increased by depletion of NK effectors with anti-asialo GM1. Experiments with beige mice demonstrated that the mortality of syngeneic, NK-deficient C3H-bg/bg mice injected with ras tranfectants was significantly greater than similarly treated NK-normal C3H(-)+/bg littermate controls. The results support the view that NK cells are capable in vivo of recognizing early defined stages in the neoplastic process initiated by oncogenes.

Differential induction of cytolytic susceptibility by E1A, myc, and ras oncogenes in immortalized cells

The E1A oncogene of adenovirus serotypes 2 and 5 induces susceptibility to the cytolytic effects of natural killer lymphocytes and activated macrophages when expressed in infected and transformed mammalian cells (cytolysis-susceptible phenotype). E1A and the oncogenes v-myc, long-terminal-repeat-promoted c-myc, and activated c-ras share the ability to immortalize transfected low-passage rodent cells. The cytolytic phenotypes of well-characterized rodent cell lines immortalized by these three oncogenes were defined. In contrast to target cells expressing the intact E1A gene, myc- and ras-expressing, immortalized primary transfectants were resistant to lysis by both types of killer cell populations. The same patterns of susceptibility (E1A) and resistance (myc and ras) to cytolysis were observed in oncogene-transfected continuous rat (REF52) and mouse (NIH 3T3) cell lines, indicating that differences in the cytolytic phenotypes associated with expression of these oncogenes are not due to cell selection during immortalization. The results suggest that the E1A oncogene may possess a functional domain that is different from those of other oncogenes, such as myc and ras, and that the activity linked to this postulated domain is dissociable from the process of immortalization.

Bovine naturally cytolytic cell activation against bovine herpes virus type 1-infected cells does not require late viral glycoproteins

Freshly isolated or overnight cultured bovine peripheral blood mononuclear (PBM) cells lysed bovine herpes virus 1 (BHV-1)-infected allogeneic and xenogeneic target cells but not non-infected target cells. To determine if late viral genes contribute to target cell lysis, phosphonoacetic acid (PAA), an inhibitor of DNA polymerase activity, was used to block DNA replication that is required for expression of late viral proteins. Both adherent and non-adherent (NA) cell populations mediated lysis against PAA-treated BHV-1-infected target cells in both 4- and 20-hr assays, indicating recognition and killing occurred in the absence of expression of late BHV-1 glycoproteins. Thus recognition of BHV-1 by bovine natural cytolytic effector cells does not require recognition of late BHV-1 glycoproteins for killing virally infected target cells.

Use of anti-HLA antibodies to mask major histocompatibility complex gene products on tumor cells can enhance susceptibility of these cells to lysis by natural killer cells

The role of major histocompatibility gene products (i.e., HLA molecules) in rendering tumor cells resistant to natural killer (NK) cell-mediated lysis was investigated by using mouse monoclonal antibodies to bind and mask HLA or non-HLA gene products on the cell membrane of human allogeneic tumor targets. Enhanced lysis of resistant lymphoid and certain other solid tumor cell lines was observed only when monoclonals used reacted to class I and II HLA molecules but not non-HLA molecules on tumor targets. Enhanced lysis was not due to antibody dependent cellular cytotoxicity or due to an effect of antibody on NK effectors. Of importance, normal autologous and allogeneic human lymphocytes could not be lysed by NK cells despite blast transformation with mitogens or masking of HLA membrane determinants on blasts with monoclonal antibodies. Enhanced lysis, in the presence of antibody to HLA antigens, was not due to increased NK cell binding to tumor targets, but a consequence of enhanced postbinding lysis. Studies using granules obtained from NK cells indicated that masking of HLA antigens did not enhance the susceptibility of tumor targets to cytolysins. Such observations would suggest that HLA antigens on tumor targets inhibit the triggering of effector cells (and release of cytolysins) after recognition and binding of NK cells to target cells.

The relationship between MHC antigen expression and metastasis

From the studies summarized here a complex picture of the role played by MHC products in determining tumorigenicity and metastasis is emerging. In order to be able to understand this relationship better, it is necessary to consider several factors. 1. Each tumor system or neoplastic tissue is unique, and its behavior reflects the influence of cell-specific characteristics, as well as its ability to modulate other cells and tissues--including cells belonging to the immune system--and also to be modulated by other cells and soluble factors. 2. Since metastasis formation is a multistep process in which only small subpopulations of tumor cells with complex and defined phenotypes are able to colonize secondary tissues, elimination of even one single phenotypic component of this structured process can easily reverse the metastatic capacity of the cells. Acquisition of metastatic ability, on the other hand, would be a more difficult task, since any new characteristic expressed by the cells or induced experimentally, such as gene transfection or results of IFN treatment, must be expressed in a temporal manner and in concert with other cellular characteristics. Therefore, an experimental protocol measuring a specific element in determining metastasis can easily produce conflicting results, depending on the type of cells and genetic background of the host studied. 3. The level of specific MHC products on tumor cells is one among many other cell characteristics that may determine the metastatic potential of cells. Moreover, each of the class 1 MHC products, and the relationship among them, including other than the classical K, L, or D products (Brickell et al., 1983), should be regarded as independent entities, with possible different regulatory roles in cell-cell recognition, in a general sense, which may be involved in determining invasiveness and homing as well as recognition by the immune system. 4. Both specific T-cell and nonspecific natural mediated immunity (which is much less understood) are involved in the selection of the metastatic cell population. 5. Immunogenicity of tumors is not necessarily determined by high levels of MHC antigen expression; it is also dependent on the level of TSA. Thus, immunoselection mediated by T lymphocytes during metastasis formation could be directed against both MHC and TSA antigens. Therefore, low expression of MHC antigens by metastatic cells as a result of immunoselection is not always observed.(ABSTRACT TRUNCATED AT 400 WORDS)

The E1a gene of adenovirus type 2 reduces the metastatic potential of ras-transformed rat embryo cells

We have previously demonstrated that second-passage rat embryo cells transformed by the ras oncogene alone are both tumorigenic and highly metastatic when injected into nude mice. In contrast, rat embryo cells cotransformed with the ras oncogene and the adenovirus type 2 (Ad2) E1a gene are tumorigenic but either fail to metastasize or exhibit a very low metastatic potential. In this report, we demonstrate that transfection of the Ad2 E1a gene into four independent ras-transformed rat embryo cell lines results in a dramatic reduction in metastatic potential relative to that of the parental cell line. Transfection of cDNAs for the 12S and 13S E1a transcripts showed that the 12S cDNA was highly effective in reducing the metastatic potential of ras-transformed cell lines, while the 13S cDNA showed an effect in only one of the two cell lines tested. This effect is specific to the Ad2 E1a gene, since ras-transformed cell lines expressing the Ad12 E1a gene maintained their high metastatic potential. We hypothesize that the Ad2 E1a gene may regulate the expression of one or more cellular genes that contribute to the metastatic phenotype.

Differential expression of the ASGM1 antigen on anti-reovirus and alloreactive cytotoxic T lymphocytes (CTL)

Anti-reovirus cytotoxic effectors were found to be: (i) H-2 restricted; (ii) virus specific; (iii) non-lytic (in 4 h) for natural killer (NK)-sensitive YAC-1 cells; and (iv) positive for the Thy-1 and Lyt-2 lymphocyte markers. Thus, anti-reovirus cytotoxic effectors have the functional and phenotype characteristics of cytotoxic T lymphocyte (CTL). A significant fraction of anti-viral CTL, as well as alloreactive CTL, were also found to be positive for the asialo GM1 (ASGM1) cell surface antigen, generally considered to be a NK cell marker. ASGM1 expression on these CTL, as determined by sensitivity to antibody plus complement (C), appeared to be highly variable and dependent on two factors-the nature of the antigenic stimulus (viral vs. alloantigen), and the mouse strain from which the CTL originated. Thus, ASGM1 antigen expression on CTL appears to be regulated and may be under the control of lymphokines, development differentiation signals and/or other strain-dependent genetic factors.

The E1a gene of adenovirus type 2 reduces the metastatic potential of ras-transformed rat embryo cells

We have previously demonstrated that second-passage rat embryo cells transformed by the ras oncogene alone are both tumorigenic and highly metastatic when injected into nude mice. In contrast, rat embryo cells cotransformed with the ras oncogene and the adenovirus type 2 (Ad2) E1a gene are tumorigenic but either fail to metastasize or exhibit a very low metastatic potential. In this report, we demonstrate that transfection of the Ad2 E1a gene into four independent ras-transformed rat embryo cell lines results in a dramatic reduction in metastatic potential relative to that of the parental cell line. Transfection of cDNAs for the 12S and 13S E1a transcripts showed that the 12S cDNA was highly effective in reducing the metastatic potential of ras-transformed cell lines, while the 13S cDNA showed an effect in only one of the two cell lines tested. This effect is specific to the Ad2 E1a gene, since ras-transformed cell lines expressing the Ad12 E1a gene maintained their high metastatic potential. We hypothesize that the Ad2 E1a gene may regulate the expression of one or more cellular genes that contribute to the metastatic phenotype.

Recognition of adenovirus E1A gene products on immortalized cell surfaces by cytotoxic T lymphocytes

The experiments described in this report were designed to examine whether target cells transfected with the adenovirus E1A gene and exhibiting increased susceptibility to lysis by natural killer cells and activated macrophages (J. L. Cook, T. A. Walker, A. M. Lewis, Jr., H. E. Ruley, F. L. Graham, and S. H. Pilder, Proc. Natl. Acad. Sci. USA 83:6965-6969, 1986) also express E1A proteins on their surfaces. MT1A, 12S, and 13S are strain Fischer baby rat kidney (BRK) cell lines immortalized by transfection with plasmids containing only the E1A gene of nononcogenic adenovirus. All of these cell lines were effective in stimulating the generation of cytotoxic T lymphocytes (CTL) in vitro, provided that the cultures were supplemented with an exogenous source of lymphokine and that the responding lymphocytes were from syngeneic Fischer rats previously immunized with a cell line containing the intact E1A gene. HrA2, a Fischer BRK cell line immortalized by transfection with a plasmid containing only exon 1 of the E1A gene, did not generate, nor was it lysed by, E1A-specific CTL. The cytolytic activity of E1A-specific CTL was blocked by antiserum from Fischer rats immunized with purified E1A proteins synthesized in Escherichia coli, supporting the conclusion that an epitope on E1A proteins encoded by the intact E1A gene constitutes part of the CTL target structure on adenovirus-transformed cells. These data suggest that in addition to their functions within host cells, E1A gene products are important immunogenic determinants on the surfaces of adenovirus-transformed cells.

Transformation of NIH 3T3 cells by cotransfection with c-src and nuclear oncogenes

pp60c-src, the cellular homolog of the Rous sarcoma virus transforming protein, does not completely transform cells even when present at high levels, but has been shown to be involved in polyomavirus-induced transformation when activated by polyomavirus middle T (pmt)-antigen binding. Here we show that cotransfection, but not solo transfection, of expression plasmids for c-src and either adenovirus E1A, v-myc, c-myc, or the 5' half of polyomavirus large T (pltN) antigen into NIH 3T3 cells induces anchorage-independent growth, enhanced focus formation, and, for pltN cotransfection, tumorigenicity in adult NFS mice. Enhancement of transformation was not observed with polyomavirus small t (pst) antigen. Cotransfection of c-src with pltN induced modification of pp60c-src that altered its electrophoretic mobility and in vivo phosphorylation state and stimulated its in vitro kinase activity. Similar alterations were not seen after c-src-E1A cotransfection, suggesting that at least two different mechanisms of enhancement are involved.

Regulation of the metastatic phenotype by the E1A gene of adenovirus-2

We have previously demonstrated that rat embryo cells transformed by the ras oncogene alone are both tumorigenic and highly metastatic when injected into nude mice. In contrast, rat embryo cells transformed with the ras oncogene and the adenovirus 2 (Ad2) Ela gene are tumorigenic but either fail to metastasize, or exhibit a very low metastatic potential. Here we demonstrate that transfection of the Ad2 Ela gene into several of the ras transformed rat embryo cell lines results in a dramatic reduction in metastatic potential relative to the parental cell line. Transfection of cDNAs for the 12S and 13S Ela transcripts showed that both gene products are capable of reducing the metastatic potential of the ras transformed cell lines, however the 12S cDNA was more effective. This effect is specific to the Ad2 Ela gene as ras transformed cell lines expressing the Ad12 Ela gene or the human N-myc gene maintained their high metastatic potential. We hypothesize that the Ad2 Ela gene may regulate the expression of one or more cellular genes that contribute to the metastatic phenotype.

Adenovirus E1A gene induction of susceptibility to lysis by natural killer cells and activated macrophages in infected rodent cells

Rodent cells immortalized by the E1A gene of nononcogenic adenoviruses are susceptible to lysis by natural killer (NK) cells and activated macrophages. This cytolysis-susceptible phenotype may contribute to the rejection of adenovirus-transformed cells by immunocompetent animals. Such increased cytolytic susceptibility has also been observed with infected rodent cells. This infection model provided a means to study the role of E1A gene products in induction of cytolytic susceptibility without cell selection during transformation. Deletion mutations outside of the E1A gene had no effect on adenovirus type 2 (Ad2) or Ad5 induction of cytolytic susceptibility in infected hamster cells, while E1A-minus mutant viruses could not induce this phenotype. E1A mutant viruses that induced expression of either E1A 12S or 13S mRNA in infected cells were competent to induce cytolytic susceptibility. Furthermore, there was a correlation between the accumulation of E1A gene products in Ad5-infected cells and the level of susceptibility of such target cells to lysis by NK cells. The results of coinfection studies indicated that the E1A gene products of highly oncogenic Ad12 could not complement the lack of induction of cytolytic susceptibility by E1A-minus Ad5 virus in infected cells and also could not block induction of this infected-cell phenotype by Ad5. These data suggest that expression of the E1A gene of nononcogenic adenoviruses may cause the elimination of infected cells by the immunologically nonspecific host inflammatory cell response prior to cellular transformation. The lack of induction of this cytolysis-susceptible phenotype by Ad12 E1A may result in an increased persistence of Ad12-infected cells in vivo and may lead to an increased Ad12-transformed cell burden for the host.

Transformation of NIH 3T3 cells by cotransfection with c-src and nuclear oncogenes

pp60c-src, the cellular homolog of the Rous sarcoma virus transforming protein, does not completely transform cells even when present at high levels, but has been shown to be involved in polyomavirus-induced transformation when activated by polyomavirus middle T (pmt)-antigen binding. Here we show that cotransfection, but not solo transfection, of expression plasmids for c-src and either adenovirus E1A, v-myc, c-myc, or the 5' half of polyomavirus large T (pltN) antigen into NIH 3T3 cells induces anchorage-independent growth, enhanced focus formation, and, for pltN cotransfection, tumorigenicity in adult NFS mice. Enhancement of transformation was not observed with polyomavirus small t (pst) antigen. Cotransfection of c-src with pltN induced modification of pp60c-src that altered its electrophoretic mobility and in vivo phosphorylation state and stimulated its in vitro kinase activity. Similar alterations were not seen after c-src-E1A cotransfection, suggesting that at least two different mechanisms of enhancement are involved.

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

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

Immunological surveillance against DNA-virus-transformed cells: correlations between natural killer cell cytolytic competence and tumor susceptibility of athymic rodents

Adenovirus type 2 (Ad2)-transformed hamster and rat cells are susceptible to lysis by natural killer (NK) cells from the host of origin and are nontumorigenic in immunocompetent hamsters and rats, respectively. These NK-cell-susceptible, virus-transformed cells are, however, highly tumorigenic in athymic (nude) mice--animals with intact NK-cell responses. In vitro lysis of these xenogeneic, Ad2-transformed cells by nude-mouse NK cells was found to be defective. In contrast, Ad2-transformed hamster and rat cells were highly susceptible to lysis by nude-rat NK cells. Furthermore, xenogeneic, Ad2-transformed hamster cells were nontumorigenic in nude rats unless the NK-cell responses of the challenged animals were compromised. The results of the nude-rat studies show that thymus-dependent, cytotoxic T-lymphocyte-mediated, host cellular immune responses are not essential for rejection of xenogeneic cells transformed by nononcogenic Ad2. The data suggest instead that immunologically nonspecific host cellular immune responses, such as those mediated by NK cells, are sufficient for rejection of Ad2-transformed cells. These results indicate that biologically important differences exist in the NK-cell-mediated defenses mounted by nude mice and nude rats against transformed cells that may account for the different patterns of tumor induction by various neoplastic cell types in these athymic animals.

Tumorigenicity of hamster and mouse cells transformed by adenovirus types 2 and 5 is not influenced by the level of class I major histocompatibility antigens expressed on the cells

Inbred hamster and mouse cells transformed by the nononcogenic adenovirus (Ad) serotypes, Ad2 and Ad5, are nontumorigenic in syngeneic adult animals, while cells from these species transformed by the highly oncogenic Ad12 are tumorigenic in such rodents. By immunoprecipitation and flow cytometry, cells from four of six Ad2- and Ad5-transformed hamster and mouse lines expressed high levels of cell-surface class I major histocompatibility complex (MHC) antigens, while cells from two of these six lines expressed low levels of cell-surface class I MHC antigens. The levels of class I MHC proteins expressed by cells from these latter two lines were comparable to the levels of cell-surface class I MHC proteins expressed by cells from Ad12-transformed hamster and mouse lines. Moreover, an Ad2-transformed line that had become highly oncogenic after in vivo adaptation showed the same high level of MHC expression as the nononcogenic parent. The amounts of class I mRNA, analyzed by RNA blotting, were, in general, consistent with the levels of class I antigens expressed on the surfaces of these cells. These results indicate that there is no correlation between the tumorigenicity in immunocompetent syngeneic adult rodents of Ad2- and Ad5-transformed hamster and mouse cells and the level of class I MHC antigens expressed on the surfaces of these cells. Thus, the expression of different levels of class I MHC proteins does not seem to explain the differences in the oncogenicity between nononcogenic and highly oncogenic human Ad serotypes.