Complete suppression of viral gene expression is associated with the onset and progression of lymphoid malignancy: observations in Bovine Leukemia Virus-infected sheep
© Merimi et al; licensee BioMed Central Ltd. 2007
Received: 07 March 2007
Accepted: 23 July 2007
Published: 23 July 2007
During malignant progression, tumor cells need to acquire novel characteristics that lead to uncontrolled growth and reduced immunogenicity. In the Bovine Leukemia Virus-induced ovine leukemia model, silencing of viral gene expression has been proposed as a mechanism leading to immune evasion. However, whether proviral expression in tumors is completely suppressed in vivo was not conclusively demonstrated. Therefore, we studied viral expression in two selected experimentally-infected sheep, the virus or the disease of which had features that made it possible to distinguish tumor cells from their nontransformed counterparts.
In the first animal, we observed the emergence of a genetically modified provirus simultaneously with leukemia onset. We found a Tax-mutated (TaxK303) replication-deficient provirus in the malignant B-cell clone while functional provirus (TaxE303) had been consistently monitored over the 17-month aleukemic period. In the second case, both non-transformed and transformed BLV-infected cells were present at the same time, but at distinct sites. While there was potentially-active provirus in the non-leukemic blood B-cell population, as demonstrated by ex-vivo culture and injection into naïve sheep, virus expression was completely suppressed in the malignant B-cells isolated from the lymphoid tumors despite the absence of genetic alterations in the proviral genome. These observations suggest that silencing of viral genes, including the oncoprotein Tax, is associated with tumor onset.
Our findings suggest that silencing is critical for tumor progression and identify two distinct mechanisms-genetic and epigenetic-involved in the complete suppression of virus and Tax expression. We demonstrate that, in contrast to systems that require sustained oncogene expression, the major viral transforming protein Tax can be turned-off without reversing the transformed phenotype. We propose that suppression of viral gene expression is a contributory factor in the impairment of immune surveillance and the uncontrolled proliferation of the BLV-infected tumor cell.
It is widely accepted that the majority of cancers if not all result from a combination of multiple cellular events leading to malignancy after a prolonged period of clinical latency. Alterations in the cell itself however may not be sufficient to drive full transformation and evidence has emerged that the immune system is playing a critical role in the control of cancer progression. Although the propensity of tumor cells to evade immune attack is well documented [1–3], there is little direct experimental evidence suggesting a correlation between immune evasion through virus- or oncogene-silencing and the onset of overt leukemia.
Sheep are particularly interesting as a large animal model for studying certain aspects of cancer biology. Compared to murine tumor models, information gained from large animal outbred populations such as sheep can be expected to be more informative about human malignancies . Furthermore, sheep develop B-cell leukemia and lymphoma after experimental transmission of BLV, a virus belonging to the deltaretrovirus family, which encompasses HTLV-1 and -2 and STLVs [5–7]. Finally, in contrast to most rodent leukemia models in which a short mean latency precedes the aggressive acute phase, the ovine BLV-associated leukemia effectively recreates the temporal events that occur during the initiation and progression of chronic leukemia such as ATL and B-CLL in human.
In the model of BLV-induced leukemia and lymphoid tumors, viral infection and tumor progression can be monitored over time following injection with either naked proviral DNA or virus-producing cells [8, 9]. BLV-infected sheep consistently develop tumors after a 6-month to 4-year period of latency. The pre-leukemic phase of infection includes the expansion of infected surface immunoglobulin M-positive (sIgM+) B-cells with proviral insertion at multiple sites, whereas a unique integration site represents the molecular signature of the malignant B-cell clone found in each individual after the onset of overt leukemia/lymphoma. Unlike simple retroviruses, which induce tumors by expressing viral products or by proviral insertional mutagenesis, complex oncoretroviruses such as HTLV-1 and BLV induce tumors using mechanisms which involve Tax, the viral transactivator. Tax deregulates signal transduction pathways, acts through the transcriptional modification of host genes and interactions with cellular proteins which create a cellular environment favoring aneuploidy and DNA damage [10–13]. Although Tax is an essential contributor to the oncogenic potential of both viruses, there is compelling evidence that expression of Tax is not sufficient for transformation. Furthermore, the presence of deletions and mutations in tumor-associated proviral sequences, including tax, suggests that neither virus nor Tax expression are required for the maintenance of the transformed phenotype [8, 14, 15].
BLV and HTLV-1 infection are both characterized by low or undetectable viral expression in vivo but cells isolated from an infected individual during the pre-malignant phase spontaneously express viral proteins in vitro [16, 17]. However, in B-cell tumors isolated from BLV-infected sheep and cell lines that were derived from these tumors, we previously observed the presence of a silent provirus [8, 15, 18]. We raised the hypothesis that silencing of viral genes might be a strategy to circumvent effective immune attack. Because in BLV-infected sheep from earlier studies, the malignant cells were not easily distinguishable from their non-transformed infected counterparts, we studied viral expression in two selected BLV-infected individuals the virus or the disease of which had features that made it possible to separate tumor cells from non malignant cells. We found a correlation between the complete suppression of provirus expression and tumor onset, providing experimental evidence that virus and Tax silencing are critical if not mandatory for progression to overt malignancy.
Sheep S2531: a case illustrating tumor-associated virus silencing by a genetic mechanism
Expression vectors for Tax2531 were then constructed by exchanging the wild-type tax sequence in pSGTax with the PCR-amplified tax DNA from either pre-leukemic (position 8149 = G) or leukemic (position 8149 = A) S2531 samples respectively. HeLa cells were co-transfected with each pSGTax2531 construct together with the pLTRLuc reporter plasmid containing the firefly luciferase gene under the control of the BLV promoter as previously described . Luciferase activities examined 42 hours post-transfection of pSGTax2531 constructs from samples 17-months post-inoculation were not significantly different from background levels generated by the control vector pSGc, confirming the transactivation-deficient phenotype associated with the genetic change observed in the tumor-derived proviral tax. As expected, constructs expressing tax sequences isolated from earlier samples, before the onset of leukemia, were consistently positive (Fig. 1A,B). Furthermore, two naïve sheep injected with the cloned S2531 proviral DNA isolated from leukemic cells failed to seroconvert and BLV-specific PCR was consistently negative, conclusively demonstrating that the tumor-associated S2531 provirus was non functional (data not shown). Thus, in S2531, while functional provirus had been consistently monitored over the 17-month aleukemic period, we exclusively found the transactivation-deficient provirus in both the peripheral lymphoid tumors and the blood isolated after progression to the acute leukemic phase. Finally, we examined whether the silent replication-deficient provirus might have been present as a minor form in the inoculum used to infect S2531. Therefore, we subcloned the PCR-amplified tax products obtained with DNA extracted from S19 PBMCs in the pCRScript®-SK(+) vector system (Stratagene) and sequenced multiple tax clones. Among a total of twenty sequenced clones we found two clones the sequence of which corresponded to the mutated tax (TaxK303), suggesting that besides wild-type replication-competent provirus (TaxE303) a minor population of replication-deficient provirus was present in the cells that served to infect S2531 (data not shown). Although it remains to be understood how and where a transactivation-deficient provirus was able to persist in S2531 before eventually giving rise to a transformed B-cell, our data show that while functional provirus was the major replicative form present over the pre-malignant stage, a transactivation-deficient provirus was selected after progression to acute leukemia. This in vivo follow-up strongly suggests that switching off Tax and virus expression is associated with the onset of full-blown malignancy.
Sheep S267: a case illustrating tumor-associated virus silencing by an epigenetic mechanism
Characterization of PBMC- and lymphoma-derived B-cells isolated from sheep S267
Cells isolated from:
cytokine-independent growth/capacity to derive cell lines
in vivo infectious potential
Using the BLV-associated ovine model of leukemia and based on the observations in two experimental sheep, we provide evidence for the role of virus and oncogene silencing as an important step in the onset of lymphoid malignancy. In the first animal, S2531, we identified a correlation between the genetic modification of the proviral structure and the emergence of leukemia. We found a Tax-mutated (TaxK303) replication-deficient provirus integrated into the genome of the malignant B-cell clone while recombinant functional provirus (TaxE303) had been consistently monitored over the aleukemic period. Although sequencing of individual tax clones identified the presence of a replication-deficient proviral form in the inoculum, our data provide no clues as to how this provirus might persist in the infected host. It will be important to sort out from our future studies whether the TaxK303 defective provirus found at the time of leukemia development in S2531 was already present in the pre-tumoral clone early after infection. A study is ongoing to answer this question, based on a BLV-specific inverse PCR technique for the detection of tumor-specific integration sites developed by Moules et al. . Using this method, BLV-positive pre-malignant clones are detectable as early as two weeks after virus exposure. Whatever the mechanism responsible for this genetic modification, our observations suggest that switching off expression of Tax, the essential contributor to the oncogenic potential of BLV, is linked with the onset of acute leukemia. We propose that in this particular case, the mechanism by which the immune system destroys developing malignancies is evaded by the malignant cell by reducing its intrinsic immunogenicity, possibly through recombination-mediated virus silencing. In the second case, S267, both non-transformed and transformed BLV-infected cells were present at the same time, but at clearly distinct sites. While there was potentially-active provirus in the non-leukemic blood B-cell population, as demonstrated by ex-vivo culture and injection into naïve recipients, virus expression was completely suppressed in the malignant B-cells isolated from the lymphoid tumors despite the absence of genetic alterations in the proviral genome. This independent observation reinforces our previous conclusion and suggests that besides genetic alterations, epigenetic mechanisms might be involved in tumor-associated silencing. Altogether, our findings strongly support the hypothesis that switching-off viral gene expression, including Tax, the essential contributor to the oncogenic potential of BLV, is critical, if not mandatory, for progression to overt malignancy.
Sheep infected by BLV mount a strong immune response to viral antigens. Active killing of infected cells might play a decisive role in limiting BLV gene expression, but seems unable to prevent – or perhaps paradoxically favors – the development of a malignant clone harboring a silent provirus. It is tempting to assign our observations to the failure of the immune system to eliminate the infected cell given the absence of proper expression of immunogenic proteins, in this case Tax. Tax is the major target of CTLs in HTLV-associated disease , and we found significant levels of Tax-specific CTLs in BLV-infected sheep (Van den Broeke, unpublished results). The lack of immunogenicity of naturally occurring tumors is often understood in terms of a suboptimal condition in the tumor microenvironment to generate protective immunity, regulatory T-cell activity, dendritic cell dysfunction, production of suppressive factors such as IL-10, or changes in the pattern of antigen expression [1, 3, 26], but so far there was no example of complete suppression of tumor antigen expression, especially if this antigen is the major transforming protein.
The demonstration in S2531 of a link between the interruption of the long clinical latency and the complete suppression of viral expression suggests that silencing is a late event in the multi-step process leading to the uncontrolled growth of a transformed B-cell clone and the onset of the fatal acute stage of the disease. Early after infection, cells that do not express viral proteins might have a survival advantage because they escape CTLs, but such cells will not outgrow the cells that express virus because of the absence of functional Tax protein capable of transactivating the host cell pathways responsible for enhanced B-cell proliferation. However, if virus silencing occurs when the cell has undergone sufficient events to reach a point of no return, impairment of immune surveillance might allow the uncontrolled proliferation of this fully-transformed B-cell clone. Whatever the mechanism – genetic or epigenetic – it is critical for achieving complete silencing of all viral genes. Cellular changes that have occurred during the process of leukemogenesis are such that even the Tax oncoprotein can be turned off without reversing the transformed phenotype. Loss of Tax and virus expression has been extensively documented in HTLV-1-associated disease and both genetic and epigenetic silencing mechanisms have been described [13, 27, 28]. This study in sheep contributes to the further understanding of tumor-associated silencing. In particular, the analysis of sequential samples of the same individual from pre-tumoral to overt leukemia and the documentation of the timing of the Tax expression reduction are unique. Our findings are in strong contrast with observations in other viral-associated malignancies including HPV-, EBV-, and HBV-associated cancers, as well as tumors mediated by simple oncornaviruses that all require sustained oncogene or transforming gene expression. This observation also raises a major concern for the application of effective anti-tumor immunotherapy. CTLs to the oncogenic protein might be effective when elicited during the chronic pre-leukemic stage, but would be irrelevant for eliminating malignant cells that do not longer express the initially-immunogenic target antigen after tumor progression.
Animals and animal samples
All sheep were housed at the Centre de Recherches Vétérinaires et Agrochimiques (Brussels, Belgium). Experimental procedures were approved by the Comité d'Ethique Médicale de la Faculté de Médecine ULB and were conducted in accordance with national and institutional guidelines for animal care and use. S2531 was inoculated intradermally with 107 PBMCs isolated from a BLV-infected animal (S19) described earlier . S267 was injected with naked proviral DNA of an infectious BLV variant (pBLVX3C) , isogenic to the full-length wild-type 344 provirus used for in vivo infection of sheep [9, 20–23]. Blood was collected in EDTA-containing tubes and PBMCs were isolated using standard Ficoll-Hypaque separation. S267 lymphoid tumors were collected at necropsy, minced through a nylon mesh cell strainer (Becton-Dickinson) to obtain single-cell suspensions. Sheep used for injection with S267-derived cell populations were inoculated with 2 × 107 BL267, L267, or CL267 respectively. Anti-p24 antibody titers and viral load were determined as previously described .
PBMCs and single cell suspensions isolated from BLV-infected sheep were cultured at a concentration of 106 cells/ml in OPTIMEM medium (Invitrogen) supplemented with 10% FCS, 1 mM sodium pyruvate, 2 mM glutamine, non-essential amino acids and 100 μg/ml kanamycin as previously described .
Southern blot, PCR, RT-PCR and sequence analysis
Genomic DNA was prepared and analyzed by Southern blot and PCR analysis as previously described . The nylon-bound Sac I or EcoRI-digested genomic DNAs were hybridized with a 32P-labeled BLV full-length proviral DNA probe (Fig. 2A). Primers for PCR were as follow (nucleotide positions according to Sagata : Tax1 [7321–7340]: 5'-GATGCCTGGTGCCCCCTCTG-3', Tax2 [7604–7623]: 5'-ACCGTCGCTAGAGGCCGAGG-3', U3 [8599–8618]:5'-GCCAGACGCCCTTGGAGCGC-3'. Tax1-Tax2 and Tax1-U3 were paired together for proviral DNA detection and sequencing respectively. For RT-PCR experiments, total RNA was extracted using the Tripure reagent according to the manufacturer's protocol (Roche). 1 μg of RNA was reverse transcribed and amplified using the Titan RT-PCR system according to the protocol supplied by the manufacturer (Roche). Primers EnvA [4766–4787]: 5'-TCCTGGCTACTAACCCCCCCGT-3', and Tax2 were used for the detection of the 2.1 kb doubly-spliced tax/rex mRNA as previously described , generating a fragment of 482 bp (Fig. 2A). For provirus sequencing, amplification of selected regions was performed using the Pfu proofreading DNA polymerase (Stratagene) and the purified products were sequenced using the Thermosequenase radiolabeled terminator cycle sequencing method (GE Healthcare Biosciences).
Constructs and luciferase assays
DNA extracted from PBMCs isolated from S2531 at different times post-infection was amplified using primers Tax1/U3. Eco RI-restricted products were inserted into pSGTax  for exchange with the wild-type sequence. Each pSGTax2531 construct was used in HeLa co-transfection with pLTR-Luc, and luciferase activities were measured as described . pSGTax contains the wild-type tax downstream of the CMV promoter; pLTR-Luc expresses the firefly luciferase under the control of the BLV-LTR promoter.
Proviral DNA from S2531 leukemic cells was cloned by insertion of EcoRI-restricted genomic DNA into the Lambda Dash® II vector (Stratagene) according to the manufacturer and used to evaluate the infectious potential in sheep.
Adult T-cell Leukemia
B-cell Chronic Lymphocytic Leukemia
Bovine Leukemia Virus
Human Papilloma Virus
Human T-lymphotropic Virus-1
Moloney Murine Leukemia Virus
Peripheral Blood Mononuclear Cells
Simian T-lymphotropic Virus
White Blood Cell.
This work was supported by the Fonds National de la Recherche Scientifique (F.N.R.S.), the Medic Foundation, the International Brachet Foundation, the Fondation Bekales, les Amis de l'Institut Bordet (Y.C.), and Télévie Grants to M.M. and M.S.
We thank Jean-Marie Londes for skilful help with the animals.
- Kim R, Emi M, Tanabe K, Arihiro K: Tumor-driven evolution of immunosuppressive networks during malignant progression. Cancer Res. 2006, 66: 5527-5536. 10.1158/0008-5472.CAN-05-4128.View ArticlePubMedGoogle Scholar
- Marincola FM, Jaffee EM, Hicklin DJ, Ferrone S: Escape of human solid tumors from T-cell recognition: molecular mechanisms and functional significance. Adv Immunol. 2000, 74: 181-273.View ArticlePubMedGoogle Scholar
- Pinzon-Charry A, Maxwell T, Lopez JA: Dendritic cell dysfunction in cancer: a mechanism for immunosuppression. Immunol Cell Biol. 2005, 83: 451-461. 10.1111/j.1440-1711.2005.01371.x.View ArticlePubMedGoogle Scholar
- Hein WR, Griebel PJ: A road less travelled: large animal models in immunological research. Nat Rev Immunol. 2003, 3: 79-84. 10.1038/nri977.View ArticlePubMedGoogle Scholar
- Burny A, Dequiedt,F.,Droogmans,L.,Grimonpont,C.,Kerkhofs,P.,Mammerickx,M.,Portetelle,D.,Van den Broeke,A.,and Kettman,R.: Bovine Leukemia Virus: biology and mode of transformation. In: Viruses and Cancer Minson, A C , Neil, J C and McRae, M A (eds), Cambridge University Press, Cambridge. 1994, 313-334.Google Scholar
- Willems L, Burny A, Collete D, Dangoisse O, Dequiedt F, Gatot JS, Kerkhofs P, Lefebvre L, Merezak C, Peremans T, Portetelle D, Twizere JC, Kettmann R: Genetic determinants of bovine leukemia virus pathogenesis. AIDS Res Hum Retroviruses. 2000, 16: 1787-1795. 10.1089/08892220050193326.View ArticlePubMedGoogle Scholar
- Gillet N, Florins A, Boxus M, Burteau C, Nigro A, Vandermeers F, Balon H, Bouzar AB, Defoiche J, Burny A, Reichert M, Kettmann R, Willems L: Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human. Retrovirology. 2007, 4: 18-10.1186/1742-4690-4-18.PubMed CentralView ArticlePubMedGoogle Scholar
- Van den Broeke A, Bagnis C, Ciesiolka M, Cleuter Y, Gelderblom H, Kerkhofs P, Griebel P, Mannoni P, Burny A: In vivo rescue of a silent tax-deficient bovine leukemia virus from a tumor-derived ovine B-cell line by recombination with a retrovirally transduced wild-type tax gene. J Virol. 1999, 73: 1054-1065.PubMed CentralPubMedGoogle Scholar
- Willems L, Kettmann R, Dequiedt F, Portetelle D, Voneche V, Cornil I, Kerkhofs P, Burny A, Mammerickx M: In vivo infection of sheep by bovine leukemia virus mutants. J Virol. 1993, 67: 4078-4085.PubMed CentralPubMedGoogle Scholar
- Klener P, Szynal M, Cleuter Y, Merimi M, Duvillier H, Lallemand F, Bagnis C, Griebel P, Sotiriou C, Burny A, Martiat P, Van Den BA: Insights into gene expression changes impacting B-cell transformation: cross-species microarray analysis of bovine leukemia virus tax-responsive genes in ovine B cells. J Virol. 2006, 80: 1922-1938. 10.1128/JVI.80.4.1922-1938.2006.PubMed CentralView ArticlePubMedGoogle Scholar
- Ng PW, Iha H, Iwanaga Y, Bittner M, Chen Y, Jiang Y, Gooden G, Trent JM, Meltzer P, Jeang KT, Zeichner SL: Genome-wide expression changes induced by HTLV-1 Tax: evidence for MLK-3 mixed lineage kinase involvement in Tax-mediated NF-kappaB activation. Oncogene. 2001, 20: 4484-4496. 10.1038/sj.onc.1204513.View ArticlePubMedGoogle Scholar
- Szynal M, Cleuter Y, Beskorwayne T, Bagnis C, Van LC, Kerkhofs P, Burny A, Martiat P, Griebel P, Van Den BA: Disruption of B-cell homeostatic control mediated by the BLV-Tax oncoprotein: association with the upregulation of Bcl-2 and signaling through NF-kappaB. Oncogene. 2003, 22: 4531-4542. 10.1038/sj.onc.1206546.View ArticlePubMedGoogle Scholar
- Matsuoka M, Jeang KT: Human T-cell leukaemia virus type 1 (HTLV-1) infectivity and cellular transformation. Nat Rev Cancer. 2007, 7: 270-280. 10.1038/nrc2111.View ArticlePubMedGoogle Scholar
- Takeda S, Maeda M, Morikawa S, Taniguchi Y, Yasunaga J, Nosaka K, Tanaka Y, Matsuoka M: Genetic and epigenetic inactivation of tax gene in adult T-cell leukemia cells. Int J Cancer. 2004, 109: 559-567. 10.1002/ijc.20007.View ArticlePubMedGoogle Scholar
- Van den Broeke A, Cleuter Y, Chen G, Portetelle D, Mammerickx M, Zagury D, Fouchard M, Coulombel L, Kettmann R, Burny A: Even transcriptionally competent proviruses are silent in bovine leukemia virus-induced sheep tumor cells. Proc Natl Acad Sci U S A. 1988, 85: 9263-9267. 10.1073/pnas.85.23.9263.PubMed CentralView ArticlePubMedGoogle Scholar
- Hanon E, Asquith RE, Taylor GP, Tanaka Y, Weber JN, Bangham CR: High frequency of viral protein expression in human T cell lymphotropic virus type 1-infected peripheral blood mononuclear cells. AIDS Res Hum Retroviruses. 2000, 16: 1711-1715. 10.1089/08892220050193191.View ArticlePubMedGoogle Scholar
- Powers MA, Radke K: Activation of bovine leukemia virus transcription in lymphocytes from infected sheep: rapid transition through early to late gene expression. J Virol. 1992, 66: 4769-4777.PubMed CentralPubMedGoogle Scholar
- Van den Broeke A: Isolation and culture of B lymphoblastoid cell lines from Bovine Leukemia Virus-induced tumors. In:"Immunology methods manual", In vitro experimental immunology in sheep, Yvan Lefkovits (ed), Academic Press. 1997, 2127-2132.Google Scholar
- Calomme C, Dekoninck A, Nizet S, Adam E, Nguyen TL, Van Den BA, Willems L, Kettmann R, Burny A, Van LC: Overlapping CRE and E box motifs in the enhancer sequences of the bovine leukemia virus 5' long terminal repeat are critical for basal and acetylation-dependent transcriptional activity of the viral promoter: implications for viral latency. J Virol. 2004, 78: 13848-13864. 10.1128/JVI.78.24.13848-13864.2004.PubMed CentralView ArticlePubMedGoogle Scholar
- Rice NR, Stephens RM, Burny A, Gilden RV: The gag and pol genes of bovine leukemia virus: nucleotide sequence and analysis. Virology. 1985, 142: 357-377. 10.1016/0042-6822(85)90344-7.View ArticlePubMedGoogle Scholar
- Rice NR, Stephens RM, Couez D, Deschamps J, Kettmann R, Burny A, Gilden RV: The nucleotide sequence of the env gene and post-env region of bovine leukemia virus. Virology. 1984, 138: 82-93. 10.1016/0042-6822(84)90149-1.View ArticlePubMedGoogle Scholar
- Willems L, Portetelle D, Kerkhofs P, Chen G, Burny A, Mammerickx M, Kettmann R: In vivo transfection of bovine leukemia provirus into sheep. Virology. 1992, 189: 775-777. 10.1016/0042-6822(92)90604-N.View ArticlePubMedGoogle Scholar
- Willems L, Thienpont E, Kerkhofs P, Burny A, Mammerickx M, Kettmann R: Bovine leukemia virus, an animal model for the study of intrastrain variability. J Virol. 1993, 67: 1086-1089.PubMed CentralPubMedGoogle Scholar
- Moules V, Pomier C, Sibon D, Gabet AS, Reichert M, Kerkhofs P, Willems L, Mortreux F, Wattel E: Fate of premalignant clones during the asymptomatic phase preceding lymphoid malignancy. Cancer Res. 2005, 65: 1234-1243. 10.1158/0008-5472.CAN-04-1834.View ArticlePubMedGoogle Scholar
- Bangham CR, Osame M: Cellular immune response to HTLV-1. Oncogene. 2005, 24: 6035-6046. 10.1038/sj.onc.1208970.View ArticlePubMedGoogle Scholar
- Khazaie K, von BH: The impact of CD4+CD25+ Treg on tumor specific CD8+ T cell cytotoxicity and cancer. Semin Cancer Biol. 2006, 16: 124-136. 10.1016/j.semcancer.2005.11.006.View ArticlePubMedGoogle Scholar
- Taniguchi Y, Nosaka K, Yasunaga J, Maeda M, Mueller N, Okayama A, Matsuoka M: Silencing of human T-cell leukemia virus type I gene transcription by epigenetic mechanisms. Retrovirology. 2005, 2: 64-10.1186/1742-4690-2-64.PubMed CentralView ArticlePubMedGoogle Scholar
- Kamoi K, Yamamoto K, Misawa A, Miyake A, Ishida T, Tanaka Y, Mochizuki M, Watanabe T: SUV39H1 interacts with HTLV-1 Tax and abrogates Tax transactivation of HTLV-1 LTR. Retrovirology. 2006, 3: 5-10.1186/1742-4690-3-5.PubMed CentralView ArticlePubMedGoogle Scholar
- Sagata N, Yasunaga T, Tsuzuku-Kawamura J, Ohishi K, Ogawa Y, Ikawa Y: Complete nucleotide sequence of the genome of bovine leukemia virus: its evolutionary relationship to other retroviruses. Proc Natl Acad Sci U S A. 1985, 82: 677-681. 10.1073/pnas.82.3.677.PubMed CentralView ArticlePubMedGoogle Scholar
- Willems L, Heremans H, Chen G, Portetelle D, Billiau A, Burny A, Kettmann R: Cooperation between bovine leukaemia virus transactivator protein and Ha-ras oncogene product in cellular transformation. EMBO J. 1990, 9: 1577-1581.PubMed CentralPubMedGoogle Scholar
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