Alternative Entry Mechanism for Pathogenic Retroviruses

描述（由申请人提供）：与嗜生态逆转录病毒感染相关的小鼠白血病发病机制的关键步骤是由MCF原病毒的邻近整合引起的myc或其他原癌基因的调节改变（12,30,57）。MCF病毒是刺激性嗜生态病毒的基因组与源自缺陷内源性原病毒的宿主序列重组的产物（27- 29,79）。在对人293细胞病毒受体功能的研究中，我们观察到暴露于MCF247病毒的细胞获得的原病毒/细胞比暴露于密切相关的非致病性逆转录病毒的细胞多20-40倍（85）。我们的研究确定了两种可能的解释：1)MCF247 env gp的受体结合亲和力明显低于其他病毒（Ka >100 nM），因此在受体下调阻断额外感染之前，需要显着提高env gp的表达水平。2) MCF247病毒特别适合通过一种不依赖于直接结合细胞受体的替代机制进入某些细胞。通过这种途径，感染是通过MCF247病毒env gp与位于靶细胞表面膜上与其受体复合物中的亲生态病毒的env gp结合而触发的（85）。这种被称为“反式激活”的机制不受MCF247病毒env gp表达的下调，因此其活性与先前感染（和原病毒插入）的数量无关。我们提出，受体和反式激活依赖机制的联合使用使MCF247病毒比其他在正在进行的亲生态病毒复制过程中产生的重组病毒具有选择性优势，这可能部分解释了为什么MCF247病毒具有高度的致白血病性。此外，我们观察到反式激活是FeLV-T感染的唯一机制(8)，FeLV-T是一种重组猫逆转录病毒，与T细胞中大量前病毒积累和凋亡引起的深度免疫缺陷相关（25,66）。FeLV-T和MCF247病毒的行为表明，重组病毒和激发病毒之间的合作相互作用可能有助于感染宿主中准物种的含量和性质。迄今为止，我们对反式激活的研究主要局限于FeLV-T和MCF247病毒的体外实验。我们的具体目标是：

DESCRIPTION (provided by applicant): A key step in the pathogenesis of leukemia associated with ecotropic retrovirus infection in mice is altered regulation of myc or other proto-oncogenes caused by adjacent integration of an MCF provirus (12, 30, 57). MCF viruses are the product of recombination between the genome of the inciting ecotropic virus and host sequences derived from defective endogenous proviruses (27-29, 79). In studies of virus receptor function in human 293 cells, we observed that cells exposed to MCF247 virus acquired 20-40-fold more proviruses/cell than cells exposed to closely related retroviruses that are not pathogenic (85). Our studies have identified two possible explanations for this observation: 1) The receptor binding affinity of the MCF247 env gp is significantly lower than other viruses (Ka >100 nM) and therefore a substantially higher level of env gp expression is required before additional infection is blocked by receptor down regulation. 2) MCF247 viruses are uniquely suited to gain entry into certain cells through an alternative mechanism that is not dependent on direct binding to a cellular receptor. By this route, infection is triggered by binding of the MCF247 virus env gp to the env gp of an ecotropic virus located in a complex with its receptor on the surface membrane of the target cell (85). This mechanism, termed "trans-activation" is not down regulated by expression of the MCF247 virus env gp and therefore its activity is independent of the number of prior infections (and provirus insertions). We propose the combined use of both receptor and trans-activation-dependent mechanisms provides the MCF247 virus with a selective advantage over other recombinant viruses that are created during ongoing ecotropic virus replication and may explain, in part, why MCF247 virus is highly leukemogenic. In addition, we observe that trans-activation is the sole mechanism of infection by FeLV-T (8), a recombinant feline retrovirus associated with profound immunodeficiency caused by massive provirus accumulation and apoptosis in T cells (25, 66). The behavior of FeLV-T and MCF247 viruses suggests that cooperative interactions between recombinant and inciting viruses may contribute to the content and properties of the quasi-species in an infected host. To date, our studies of trans-activation have been largely limited to FeLV-T and MCF247 viruses in vitro. Our specific aims are to: 1. Determine if susceptibility to trans-activation is a common property of pathogenic MCF viruses. 2. Develop an in vitro system to investigate the mechanism of trans-activation. 3. Assess the contribution of trans-activation to the pathogenesis of retrovirus-induced leukemia in mice.