Genetic Structure Of Murine Retroviruses

多嗜性MuLV是通过亲嗜性MuLV与近交系小鼠基因组中存在的内源性包膜序列重组而形成的。病毒表现出改变的感染宿主范围，并利用与亲嗜性MuLV所利用的受体不同的细胞表面受体。在一些情况下，多变性MuLV直接参与发病机制，包括诱导增殖性、免疫性和神经系统疾病。感染后变异体的产生导致混合逆转录病毒感染。该项目的一个方面涉及研究逆转录病毒在混合感染中的相互作用。在小鼠中共接种嗜多性和嗜亲性MuLV后，我们观察到对嗜多性病毒的感染性传播的显著影响，同时非常快速地诱导神经系统疾病，在单独接种任一病毒后未观察到。导致神经系统疾病的混合感染的常见效应是多变性病毒在中枢神经系统（CNS）外周组织中的传播大大增强。这种现象是由嗜亲性病毒颗粒内的多嗜性病毒基因组的假型化介导的。最初在CNS中检测到的多变性MuLV也是假型的，然而随后病毒在CNS中的快速传播似乎是通过未假型化的多变性病毒体进行的。这种快速传播几乎与神经系统症状的发作同时发生。神经系统疾病在作为嗜亲性MuLV与许多不同的嗜多性MuLV的混合物接种的小鼠中是明显的，这表明神经致病性可能是嗜多性MuLV的一般性质。此外，我们的研究表明，外周复制的阈值是必需的侵入中枢神经系统，和传播的多变性病毒通过相互作用的多变性受体结合蛋白与受体的中枢神经系统细胞可能是一个必要的诱导神经病理。我们研究的另一个方面涉及小鼠内源性逆转录病毒的精确鉴定，这些逆转录病毒参与重组产生重组多变病毒。不同的亲嗜性病毒特异性地与不同的内源性前病毒重组以产生重组体。确定参与重组的精确序列对于理解促进这一过程的内源性病毒的特征至关重要。由于内源性病毒非常相似以及病毒在复制过程中的快速进化速率，这种鉴定一直是难以捉摸的。我们已经分离了NFS/N小鼠中可能产生重组病毒的大多数内源性前病毒，并发现几乎所有前病毒都可以通过其env基因的序列异质性相互区分。以最小化复制周期数的方式对来自这些小鼠的多变病毒的序列进行检查，首次精确鉴定了产生重组病毒的前病毒。此外，系统发育的比较已经确定了一组新的内源性多变前病毒。这一组表现出的特征表明病毒是迄今为止在文献中描述的多变前病毒的祖先。我们已经发现，这些前病毒参与重组，以产生多嗜性MuLV，即使他们的许多后代已被报道是有缺陷的。

Polytropic MuLVs are formed by recombination of ecotropic MuLVs with endogenous envelope sequences present in the genomes of inbred mouse strains. The viruses exhibit an altered infectious host range and utilize a cell surface receptor distinct from the receptor utilized by ecotropic MuLVs. In several instances polytropic MuLVs have been directly implicated in pathogenesis, including the induction of proliferative, immunological, and neurological disorders. The generation of variants after infection results in a mixed retrovirus infection. One aspect of the project involves studies of the interactions of retroviruses in mixed infections. Upon co-inoculation of polytropic and ecotropic MuLVs in mice we have observed profound effects on the infectious spread of the polytropic virus, concomitant with a very rapid induction of neurological disease not observed after inoculation with either virus alone. A common effect of mixed infections resulting in neurological disease is a greatly enhanced spread of the polytropic virus in tissues peripheral to the central nervous system (CNS). This phenomenon is mediated by pseudotyping of polytropic viral genomes within ecotropic virus particles. Polytropic MuLVs initially detected in the CNS are also pseudotyped, however a subsequent rapid spread of the virus in the CNS appears to proceed by polytropic virions that are not pseudotyped. This rapid spread is nearly coincident with the onset of neurological symptoms. Neurological disease is evident in mice inoculated as a mixture of ecotropic MuLVs with a number of different polytropic MuLVs, suggesting that neuropathogenicity may be a general property of polytropic MuLVs. In addition, our studies suggest that a threshold of peripheral replication is required for invasion of the CNS, and that spread of the polytropic virus through interaction of the polytropic receptor-binding protein with receptors on CNS cells may be a requirement for the induction of neuropathology. Another aspect of our studies involves the precise identification of endogenous retroviruses in mice that participate in recombination giving rise to recombinant polytropic viruses. Different ecotropic viruses specifically recombine with different endogenous proviruses to give rise to the recombinants. Determination of the precise sequences that participate in recombination is essential to understanding characteristics of the endogenous viruses that facilitate this process. Such identification has been elusive because of the very close similarity of the endogenous viruses as well as the rapid rate of evolution of the viruses during replication. We have isolated most of the endogenous proviruses in NFS/N mice that could potentially give rise to the recombinant viruses and found that nearly all could be distinguished from one another by sequence heterogeneity in their env genes. Examination of the sequences of polytropic viruses derived from these mice in a manner that minimized the number of replication cycles has, for the first time, precisely identified proviruses that give rise to the recombinant viruses. Furthermore, phylogenetic comparisons have identified a new group of endogenous polytropic proviruses. This group exhibits characteristics indicative of viruses that were progenitors to the polytropic proviruses that have thus far been described in the literature. We have found that these proviruses participate in recombination to generate polytropic MuLVs, even though many of their descendants have been reported to be defective.