Genetic Structure Of Murine Retroviruses

多向性MuLV是由近交系小鼠基因组中存在的生态型MuLV与内源性包膜序列重组而成的。这些病毒表现出感染宿主范围的改变，并利用与生态型MuLV所利用的受体不同的细胞表面受体。在一些情况下，多向MULV直接与发病机制有关，包括诱导增殖性、免疫性和神经系统疾病。感染后产生的变异会导致逆转录病毒的混合感染。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。在小鼠中同时接种多嗜性和生态性MuLV后，我们观察到了对多嗜性病毒感染性传播的深刻影响，同时伴随着在单独接种其中任何一种病毒后不能观察到的非常迅速的神经疾病的诱导。混合感染导致神经系统疾病的一个常见影响是多嗜性病毒在中枢神经系统(CNS)周围组织中的传播大大增强。这一现象是由生态型病毒颗粒内多嗜性病毒基因组的伪分型所介导的。最初在CNS中检测到的多嗜性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非假型的多嗜性病毒粒子进行的。这种快速传播几乎与神经系统症状的出现相吻合。作为生态型MuLV和许多不同的多向性MuLV的混合物接种的小鼠中，神经疾病是明显的，这表明神经致病性可能是多变性MuLV的一般特性。此外，我们的研究表明，中枢神经系统的侵袭需要一个外周复制的阈值，而多方病毒通过多方受体结合蛋白与中枢神经系统细胞上的受体相互作用而传播可能是诱导神经病理的必要条件。我们研究的另一个方面涉及精确识别参与重组的小鼠体内的内源性逆转录病毒，从而产生重组的多嗜性病毒。不同的亲生病毒与不同的内源性前病毒特异性重组产生重组子。确定参与重组的准确序列对于了解促进这一过程的内源性病毒的特征是至关重要的。由于内源性病毒非常相似，以及病毒在复制过程中进化的速度很快，这种鉴定一直难以捉摸。我们已经分离了大部分可能导致重组病毒的内源性前病毒，并发现几乎所有的内源性前病毒都可以通过其env基因的序列异质性来区分。以最小化复制周期的方式检查来自这些小鼠的多嗜性病毒的序列，第一次准确地识别出引起重组病毒的前病毒。此外，系统发育比较发现了一组新的内源性多嗜性前病毒。这一组表现出的特征表明，病毒是迄今在文献中描述的多嗜性前病毒的前体。我们发现，这些前病毒参与重组以产生多向性MuLV，尽管它们的许多后代被报道为有缺陷的。

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.