Influence of Mixed Retrovirus Infections on Leukemia and Neurological disease

小鼠白血病病毒（MuLV）在小鼠中的许多疾病的诱导涉及称为多嗜性MuLV的变体逆转录病毒的参与。 这些包括诱导增殖、免疫和神经系统疾病。 多嗜性MuLV是通过外源性嗜亲性MuLV与存在于近交系小鼠品系基因组中的内源性包膜序列重组而形成的，从而产生利用不同细胞表面受体进行感染的病毒。 亲嗜性MuLV的感染宿主范围仅限于小鼠;然而，用亲嗜性MuLV接种小鼠后产生的重组多效性病毒能够感染许多其他物种以及小鼠。 因此，嗜多性病毒的产生导致具有不同感染特性的病毒的混合逆转录病毒感染。 我们早期的研究强烈表明，亲嗜性和多嗜性MuLV在宿主中的相互作用在促进肿瘤发生中起作用。 最近，我们研究了逆转录病毒在体内混合感染中的相互作用，通过共接种小鼠与嗜多性MuLV分离株和嗜亲性MuLV的混合物。 与用混合物中的单个病毒感染相比，用确定的逆转录病毒混合物感染的小鼠表现出显著改变的病理学。 这些包括一种多变性MuLV诱导增殖性疾病的高度显著延迟，以及另一种多变性分离株导致神经系统疾病突然发展的显著协同效应。 在这两种情况下，共接种小鼠中的嗜多性病毒载量显著增加，而嗜亲性MuLV水平不变。 此外，在共接种小鼠中，嗜亲性MuLV在嗜亲性病毒粒子内几乎完全假型化。 存在许多可能的机制，其可以促进多变性MuLV的深度体内扩增，包括由于嗜亲性病毒粒子内的假型化或共感染细胞中多变性病毒的可能反式激活而增强的病毒传播。为了在一个不太复杂的系统中研究这些问题，我们将这些研究扩展到研究体外细胞系的混合逆转录病毒感染。 我们已经发现，共感染嗜亲性或嗜亲性病毒的多嗜性MuLV导致扩增和假型特征与我们在体内观察到的非常相似。 共感染细胞释放的多变性感染力显著增加，而同向性感染力保持不变。 此外，感染性的增加伴随着嗜亲性病毒粒子内的嗜多性基因组的广泛假型化。 这一观察结果延伸到来自不同组织和不同小鼠的细胞系，表明这是混合感染组分的特征，而不是细胞的特性。 从共感染细胞释放的多变性MuLV感染性的升高可能是由于释放的多变性基因组水平增加所致。 或者，观察到的增加可能反映了嗜亲性或嗜亲性病毒粒子比嗜多性病毒粒子高得多的特异性感染性。 我们已经发现，在具有嗜亲性或嗜亲性MuLV的多变性MuLV混合物中，多变性病毒滴度的至少一些增加可以归因于包装效率的增加以及多变性基因组从共感染细胞中的释放。 对释放不同水平的嗜性病毒的克隆细胞系的分析表明，在这些细胞与嗜性病毒共感染后，这些细胞系中的每一个都可以被诱导释放类似的高水平的嗜性病毒。 这些结果进一步表明，共感染嗜亲性病毒有利于包装和释放的嗜多性基因组，可能反映了一个固有的缺陷的嗜多性包膜。 在这方面，与嗜亲性和嗜亲性病毒不同，重组多嗜性病毒是嵌合病毒，其中病毒的包膜没有与其他结构基因共同进化。 因此，多变性病毒的包膜蛋白在子代病毒的包装和释放中可能不那么有效地起作用。 2013年，我们继续研究释放不同水平的感染性多变病毒的克隆细胞系。 我们已经证实了一个显着增加感染性释放后，与亲嗜性MuLV的重叠感染。 此外，尽管释放的病毒体RNA水平适度增加，表明多变基因组的包装和释放增强，但这种增加不足以解释在重复感染后观察到的感染性的显著增加。 此外，我们发现，在重复感染之前，从克隆细胞系释放的病毒粒子的水平与所有克隆系相似，即使从细胞释放的感染性非常不同。 此外，从每个克隆细胞系收获的病毒在传代到新细胞上时表现出正常的感染性。 这些结果表明，病毒的特异性感染性可能受到产生病毒的细胞的强烈影响，而不是病毒本身的特性。

The induction of many diseases in mice by murine leukemia viruses (MuLVs), involves the participation of variant retroviruses termed polytropic MuLVs. These include the induction of proliferative, immunological and neurological disorders. Polytropic MuLVs are formed by recombination of exogenous ecotropic MuLVs with endogenous envelope sequences present in the genomes of inbred mouse strains resulting in viruses which utilize a distinct cell-surface receptor for infection. The infectious host range of ecotropic MuLVs is limited to mice; however the recombinant polytropic viruses generated after inoculation of mice with ecotropic MuLVs are capable of infecting a number of other species as well as mice. Thus, the generation of polytropic viruses results in a mixed retrovirus infection of viruses with different infectious properties. Our earlier studies strongly suggest that the interactions of ecotropic and polytropic MuLVs in the host play a role in facilitating oncogenesis. More recently we have investigated the interactions of retroviruses in mixed infections in vivo by co-inoculation of mice with mixtures of polytropic MuLV isolates and ecotropic MuLVs. Mice infected with defined mixtures of retroviruses exhibit dramatically altered pathology compared to infection with the individual viruses of the mixture. These included a highly significant delay in the induction of proliferative disease with one polytropic MuLV and a profound synergistic effect resulting in the abrupt development of a neurological disease with another polytropic isolate. In both instances the polytropic virus load in the co-inoculated mice was markedly enhanced while the level of the ecotropic MuLV was unchanged. Furthermore, the polytropic MuLV was nearly completely pseudotyped within ecotropic virions in co-inoculated mice. There are a number of possible mechanisms which could facilitate the profound in vivo amplification of the polytropic MuLVs including enhanced spread of the virus due to pseudotyping within ecotropic virions or possibly transactivation of the polytropic virus in co-infected cells. To examine these questions in a less complex system we have extended these studies to examine mixed retrovirus infections of an in vitro cell line. We have found that co-infection of polytropic MuLVs with ecotropic or amphotropic viruses results in amplification and pseudotyping characteristics remarkably similar to what we have observed in vivo. The polytropic infectivity released from co-infected cells is markedly increased while the ecotropic infectivity remains unaltered. Further, the increase in infectivity is accompanied by extensive pseudotyping of the polytropic genome within ecotropic virions. This observation extended to cell lines from different tissues and different mice indicating that it was a feature of the components of the mixed infection rather than a property of the cells. The elevation of polytropic MuLV infectivity released from co-infected cells could have resulted from an increase in the level of polytropic genomes released. Alternatively, the observed increase could reflect a much higher specific infectivity of ecotropic or amphotropic virions compared to polytropic virions. We have found in polytropic MuLV mixtures with either ecotropic or amphotropic MuLVs, that at least some of the increase in polytropic virus titer can be attributed to an increase in the efficiency of packaging and release of the polytropic genome from co-infected cells. Analyses of clonal cell lines releasing different levels of polytropic viruses indicated that each of these lines could be induced to release similar high levels of polytropic virus upon co-infection of these cells with an ecotropic virus. These results further suggest that co-infection with an ecotropic virus facilitates the packaging and release of the polytropic genome, possibly reflecting an inherent defectiveness of the pollytropic envelope. In this regard, unlike ecotropic and amphotropic viruses, recombinant polytropic viruses are chimeric viruses in which the envelope of the virus has not co-evolved with the other structural genes. Thus the envelope protein of polytropic viruses may not function as efficiently in packaging and release of progeny viruses. In 2013 we have continued studies with clonal cell lines releasing different levels of infectious polytropic viruses. We have confirmed a remarkable increase in infectivity released upon superinfection with ecotropic MuLVs. Moreover, although the level of released virion RNA is moderately increased, indicating that packaging and release of the polytropic genome is enhanced, the increase is insufficient to account for the marked increase in infectivity observed upon superinfection. Further, we found that the level of virions released from the clonal cell lines before superinfection is similar with all of the clonal lines even though the infectivity released from the cells is very different. Moreover, virus harvested from each of the clonal cell lines exhibit normal infectivity when passaged onto new cells. These results indicate that the specific infectivity of the virus can be strongly influenced by the cell in which it is produced rather than a property of the virus itself.