Merimi Makram, Klener Pavel, Szynal Maud, Cleuter Yvette, Bagnis Claude, Kerkhofs Pierre, Burny Arsène, Martiat Philippe, Van den Broeke Anne
Laboratory of Experimental Hematology, Institut Jules Bordet, Université Libre de Bruxelles, Brussels, Belgium.
Retrovirology. 2007 Jul 23;4:51. doi: 10.1186/1742-4690-4-51.
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.
在恶性进展过程中，肿瘤细胞需要获得导致不受控制生长和免疫原性降低的新特性。在牛白血病病毒诱导的绵羊白血病模型中，病毒基因表达的沉默被认为是导致免疫逃逸的一种机制。然而，肿瘤中前病毒表达在体内是否被完全抑制尚未得到确凿证明。因此，我们研究了两只经实验感染的绵羊中的病毒表达情况，这两只绵羊所感染的病毒或疾病具有能够区分肿瘤细胞与其未转化对应细胞的特征。
在第一只动物中，我们观察到与白血病发病同时出现了一种基因改造的前病毒。我们在恶性B细胞克隆中发现了一个Tax突变（TaxK303）的复制缺陷型前病毒，而在17个月的无白血病期一直监测到功能性前病毒（TaxE303）。在第二个案例中，未转化和转化的BLV感染细胞同时存在，但位于不同部位。虽然通过体外培养和注射到未感染的绵羊中证明非白血病血液B细胞群体中存在潜在活跃的前病毒，但从淋巴肿瘤中分离出的恶性B细胞中的病毒表达被完全抑制，尽管前病毒基因组没有基因改变。这些观察结果表明，包括癌蛋白Tax在内的病毒基因沉默与肿瘤发生有关。
我们的研究结果表明，沉默对于肿瘤进展至关重要，并确定了两种不同的机制——遗传和表观遗传——参与病毒和Tax表达的完全抑制。我们证明，与需要持续癌基因表达的系统不同，主要的病毒转化蛋白Tax可以被关闭而不逆转转化表型。我们提出，病毒基因表达的抑制是免疫监视受损和BLV感染的肿瘤细胞不受控制增殖的一个促成因素。
