Human immunodeficiency virus type 1 genomic RNA sequences in the female genital tract and blood: compartmentalization and intrapatient recombination

Investigation of human immunodeficiency virus type 1 (HIV-1) in the genital tract of women is crucial to the development of vaccines and therapies. Previous analyses of HIV-1 in various anatomic sites have documented compartmentalization, with viral sequences from each location that were distinct yet phylogenetically related. Full-length RNA genomes derived from different compartments in the same individual, however, have not yet been studied. Furthermore, although there is evidence that intrapatient recombination may occur frequently, recombinants comprising viruses from different sites within one individual have rarely been documented. We compared full-length HIV-1 RNA sequences in the plasma and female genital tract, focusing on a woman with high HIV-1 RNA loads in each compartment who had been infected heterosexually and then transmitted HIV-1 by the same route. We cloned and sequenced 10 full-length HIV-1 RNA genomes from her genital tract and 10 from her plasma. We also compared viral genomes from the genital tract and plasma of four additional heterosexually infected women, sequencing 164 env and gag clones obtained from the two sites. Four of five women, including the one whose complete viral sequences were determined, displayed compartmentalized HIV-1 genomes. Analyses of full-length, compartmentalized sequences made it possible to document complex intrapatient HIV-1 recombinants that were composed of alternating viral sequences characteristic of each site. These findings demonstrate that the genital tract and blood harbor genetically distinct populations of replicating HIV-1 and provide evidence that recombination between strains from the two compartments contributes to rapid evolution of viral sequence variation in infected individuals.     

大豆花叶病毒半夏分离物侵染性克隆构建及鉴定

大豆花叶病毒(Soybean mosaic virus,SMV)作为半夏(Pinellia ternata (Thunb.) Breit.)主要病毒病害之一,已对其产量和品质造成严重影响。构建病毒侵染性克隆是反向遗传学研究病毒基因功能、病毒与宿主相互作用的有力工具,为明确SMV侵染半夏的分子机制,开展SMV全长cDNA侵染性克隆的构建特别重要。因此文中利用Gibson体外重组系统对大豆花叶病毒山西半夏分离物(SMV-SXBX)侵染性克隆进行组装,通过农杆菌浸润法接种健康半夏;进一步通过机械传代、逆转录-聚合酶链式反应(RT-PCR)证实SMV-SXBX侵染性克隆3′末端含有poly(A)尾56 nt时具有稳定侵染性。该方法便捷、高效,且避免了SMV侵染性克隆在大肠杆菌中的不稳定问题。SMV全长侵染性cDNA克隆的构建,为进一步研究SMV复制和发病的分子机制奠定了基础。     

Chimeric Sindbis-Ross River viruses to study interactions between alphavirus nonstructural and structural regions.

Sindbis virus and Ross River virus are alphaviruses whose nonstructural proteins share 64% identity and whose structural proteins share 48% identity. Starting from full-length cDNA clones of both viruses, we have generated two reciprocal Sindbis-Ross River chimeric viruses in which the structural and nonstructural regions have been exchanged. These chimeric viruses replicate readily in several cell lines. Both chimeras grow more poorly than do the parental viruses, with the chimera containing Sindbis virus nonstructural proteins and Ross River virus structural proteins growing considerably better in both mosquito and Vero cell lines than the reciprocal chimera does. The reduction in replicative capacity in comparison with the parental viruses appears to result at least in part from a reduction in RNA synthesis, which suggests that the structural proteins or sequence elements within the structural region interact with the nonstructural proteins or sequence elements within the nonstructural region, that these interactions are required for efficient RNA replication, and that these interactions are suboptimal in the chimeras. The chimeras are able to infect mice, but their growth is attenuated. Western equine encephalitis virus, a virus widely distributed throughout the Americas, has been previously shown to have arisen by natural recombination between two distinct alphaviruses, but other naturally occurring recombinant alphaviruses have not been found. The present results suggest that most nonstructural/structural chimeras that might arise by natural recombination will be viable but that interactions between different regions of the genome, some of which were previously known but some of which remain unknown, limit the ability of such recombinants to become established.     

Heterosexual Transmission of Subtype C HIV-1 Selects Consensus-Like Variants without Increased Replicative Capacity or Interferon-α Resistance

Heterosexual transmission of HIV-1 is characterized by a genetic bottleneck that selects a single viral variant, the transmitted/founder (TF), during most transmission events. To assess viral characteristics influencing HIV-1 transmission, we sequenced 167 near full-length viral genomes and generated 40 infectious molecular clones (IMC) including TF variants and multiple non-transmitted (NT) HIV-1 subtype C variants from six linked heterosexual transmission pairs near the time of transmission. Consensus-like genomes sensitive to donor antibodies were selected for during transmission in these six transmission pairs. However, TF variants did not demonstrate increased viral fitness in terms of particle infectivity or viral replicative capacity in activated peripheral blood mononuclear cells (PBMC) and monocyte-derived dendritic cells (MDDC). In addition, resistance of the TF variant to the antiviral effects of interferon-α (IFN-α) was not significantly different from that of non-transmitted variants from the same transmission pair. Thus neither in vitro viral replicative capacity nor IFN-α resistance discriminated the transmission potential of viruses in the quasispecies of these chronically infected individuals. However, our findings support the hypothesis that within-host evolution of HIV-1 in response to adaptive immune responses reduces viral transmission potential.     

Frequent intrapatient recombination between human immunodeficiency virus type 1 R5 and X4 envelopes: implications for coreceptor switch

Emergence of human immunodeficiency virus type 1 (HIV-1) populations that switch or broaden coreceptor usage from CCR5 to CXCR4 is intimately coupled to CD4+ cell depletion and disease progression toward AIDS. To better understand the molecular mechanisms involved in the coreceptor switch, we determined the nucleotide sequences of 253 V1 to V3 env clones from 27 sequential HIV-1 subtype B isolates from four patients with virus populations that switch coreceptor usage. Coreceptor usage of clones from dualtropic R5X4 isolates was characterized experimentally. Sequence analysis revealed that 9% of the clones from CXCR4-using isolates had originated by recombination events between R5 and X4 viruses. The majority (73%) of the recombinants used CXCR4. Furthermore, coreceptor usage of the recombinants was determined by a small region of the envelope, including V3. This is the first report demonstrating that intrapatient recombination between viruses with distinct coreceptor usage occurs frequently. It has been proposed that X4 viruses are more easily suppressed by the immune system than R5 viruses. We hypothesize that recombination between circulating R5 viruses and X4 viruses can result in chimeric viruses with the potential to both evade the immune system and infect CXCR4-expressing cells. The broadening in cell tropism of the viral population to include CXCR4-expressing cells would gradually impair the immune system and eventually allow the X4 population to expand. In conclusion, intrapatient recombination between viruses with distinct coreceptor usage may contribute to the emergence of X4 viruses in later stages of infection.     

Productive Homologous and Non-homologous Recombination of Hepatitis C Virus in Cell Culture

Genetic recombination is an important mechanism for increasing diversity of RNA viruses, and constitutes a viral escape mechanism to host immune responses and to treatment with antiviral compounds. Although rare, epidemiologically important hepatitis C virus (HCV) recombinants have been reported. In addition, recombination is an important regulatory mechanism of cytopathogenicity for the related pestiviruses. Here we describe recombination of HCV RNA in cell culture leading to production of infectious virus. Initially, hepatoma cells were co-transfected with a replicating JFH1ΔE1E2 genome (genotype 2a) lacking functional envelope genes and strain J6 (2a), which has functional envelope genes but does not replicate in culture. After an initial decrease in the number of HCV positive cells, infection spread after 13–36 days. Sequencing of recovered viruses revealed non-homologous recombinants with J6 sequence from the 5′ end to the NS2–NS3 region followed by JFH1 sequence from Core to the 3′ end. These recombinants carried duplicated sequence of up to 2400 nucleotides. HCV replication was not required for recombination, as recombinants were observed in most experiments even when two replication incompetent genomes were co-transfected. Reverse genetic studies verified the viability of representative recombinants. After serial passage, subsequent recombination events reducing or eliminating the duplicated region were observed for some but not all recombinants. Furthermore, we found that inter-genotypic recombination could occur, but at a lower frequency than intra-genotypic recombination. Productive recombination of attenuated HCV genomes depended on expression of all HCV proteins and tolerated duplicated sequence. In general, no strong site specificity was observed. Non-homologous recombination was observed in most cases, while few homologous events were identified. A better understanding of HCV recombination could help identification of natural recombinants and thereby lead to improved therapy. Our findings suggest mechanisms for occurrence of recombinants observed in patients.     

Intratypic recombination of polioviruses: evidence for multiple crossing-over sites on the viral genome.

Intratypic recombinant polioviruses were isolated from cells that were coinfected with two temperature-sensitive (ts) mutants of poliovirus type 1, ts035Gr and ts247. After phenotypic characterization of these recombinants, their proteins were studied by polyacrylamide gel electrophoresis, and their genomes were analyzed by RNase T1 fingerprinting and partial nucleotide sequencing. Segregation of specific phenotypic and biochemical characteristics inherited from the parental viruses demonstrated that crossing-over could occur in at least four distinct regions of the genome. Possible mechanisms for recombination are discussed.     

Impact of the HIV-1 env genetic context outside HR1-HR2 on resistance to the fusion inhibitor enfuvirtide and viral infectivity in clinical isolates

Resistance mutations to the HIV-1 fusion inhibitor enfuvirtide emerge mainly within the drug's target region, HR1, and compensatory mutations have been described within HR2. The surrounding envelope (env) genetic context might also contribute to resistance, although to what extent and through which determinants remains elusive. To quantify the direct role of the env context in resistance to enfuvirtide and in viral infectivity, we compared enfuvirtide susceptibility and infectivity of recombinant viral pairs harboring the HR1-HR2 region or the full Env ectodomain of longitudinal env clones from 5 heavily treated patients failing enfuvirtide therapy. Prior to enfuvirtide treatment onset, no env carried known resistance mutations and full Env viruses were on average less susceptible than HR1-HR2 recombinants. All escape clones carried at least one of G36D, V38A, N42D and/or N43D/S in HR1, and accordingly, resistance increased 11- to 2800-fold relative to baseline. Resistance of full Env recombinant viruses was similar to resistance of their HR1-HR2 counterpart, indicating that HR1 and HR2 are the main contributors to resistance. Strictly X4 viruses were more resistant than strictly R5 viruses, while dual-tropic Envs featured similar resistance levels irrespective of the coreceptor expressed by the cell line used. Full Env recombinants from all patients gained infectivity under prolonged drug pressure; for HR1-HR2 viruses, infectivity remained steady for 3/5 patients, while for 2/5 patients, gains in infectivity paralleled those of the corresponding full Env recombinants, indicating that the env genetic context accounts mainly for infectivity adjustments. Phylogenetic analyses revealed that quasispecies selection is a step-wise process where selection of enfuvirtide resistance is a dominant factor early during therapy, while increased infectivity is the prominent driver under prolonged therapy.     

Linearization of baculovirus DNA enhances the recovery of recombinant virus expression vectors.

Engineered derivatives of Autographa californica multiple nucleocapsid nuclear polyhedrosis virus (AcMNPV) possessing a unique restriction site provide a source of viral DNA that can be linearized by digestion with a specific endonuclease. Circular or linearized DNA from two such viruses were compared in terms of their infectivity and recombinogenic activities. The linear forms were 15- to 150-fold less infectious than the corresponding circular forms, when transfected into Spodoptera frugiperda cells using the calcium phosphate method. Linear viral DNA was, however, proficient at recombination on co-transfection with an appropriate transfer vector. Up to 30% of the progeny viruses were recombinant, a 10-fold higher fraction of recombinants than was obtained from co-transfections with circular AcMNPV DNA. The isolation of a recombinant baculovirus expression vector from any of the AcMNPV transfer vectors currently in use can thus be facilitated by linearization of the viral DNA at the appropriate location.     

Recombination every day: abundant recombination in a virus during a single multi-cellular host infection

Viral recombination can dramatically impact evolution and epidemiology. In viruses, the recombination rate depends on the frequency of genetic exchange between different viral genomes within an infected host cell and on the frequency at which such co-infections occur. While the recombination rate has been recently evaluated in experimentally co-infected cell cultures for several viruses, direct quantification at the most biologically significant level, that of a host infection, is still lacking. This study fills this gap using the cauliflower mosaic virus as a model. We distributed four neutral markers along the viral genome, and co-inoculated host plants with marker-containing and wild-type viruses. The frequency of recombinant genomes was evaluated 21 d post-inoculation. On average, over 50% of viral genomes recovered after a single host infection were recombinants, clearly indicating that recombination is very frequent in this virus. Estimates of the recombination rate show that all regions of the genome are equally affected by this process. Assuming that ten viral replication cycles occurred during our experiment—based on data on the timing of coat protein detection—the per base and replication cycle recombination rate was on the order of 2 × 10⁻⁵ to 4 × 10⁻⁵. This first determination of a virus recombination rate during a single multi-cellular host infection indicates that recombination is very frequent in the everyday life of this virus.     

Recombination in Enteroviruses Is a Biphasic Replicative Process Involving the Generation of Greater-than Genome Length ‘Imprecise’ Intermediates

Recombination in enteroviruses provides an evolutionary mechanism for acquiring extensive regions of novel sequence, is suggested to have a role in genotype diversity and is known to have been key to the emergence of novel neuropathogenic variants of poliovirus. Despite the importance of this evolutionary mechanism, the recombination process remains relatively poorly understood. We investigated heterologous recombination using a novel reverse genetic approach that resulted in the isolation of intermediate chimeric intertypic polioviruses bearing genomes with extensive duplicated sequences at the recombination junction. Serial passage of viruses exhibiting such imprecise junctions yielded progeny with increased fitness which had lost the duplicated sequences. Mutations or inhibitors that changed polymerase fidelity or the coalescence of replication complexes markedly altered the yield of recombinants (but did not influence non-replicative recombination) indicating both that the process is replicative and that it may be possible to enhance or reduce recombination-mediated viral evolution if required. We propose that extant recombinants result from a biphasic process in which an initial recombination event is followed by a process of resolution, deleting extraneous sequences and optimizing viral fitness. This process has implications for our wider understanding of ‘evolution by duplication’ in the positive-strand RNA viruses.     

High-fidelity PCR amplification of infectious copies of the complete simian virus 40 genome from plasmids and virus-infected cell lysates

We describe here a long-polymerase chain reaction (PCR) method that can be used to amplify complete simian virus 40 (SV40) DNA with high fidelity, and we show that authentic, viable virus can be produced from molecular clones of the PCR-amplified viral DNAs. A commercial long-PCR kit that employed a combination of Taq and GB-D polymerases was used, together with a pair of overlapping primers that recognized a unique EcoRI site in the SV40 genome. Efficient amplification required linearization of the circular SV40 genomic DNAs with EcoRI. Entire SV40 genomes were successfully PCR-amplified from an SV40 plasmid and from two different SV40-infected cell lysates and were cloned into pUC-19. Three separate segments of the cloned viral genomes were DNA sequenced, and no nucleotide changes relative to the parental virus were detected, suggesting that the viral DNAs had been amplified with high fidelity. Each PCR clone was infectious, and no differences were detected in the growth characteristics of viruses derived from these clones as compared to the original viral strain. The procedure we utilized shortens and simplifies the molecular cloning of small double-stranded DNA viruses and will be useful for viral diagnostic tests and for recovery of virus from clinical samples. The results of these experiments have broad implications, as the methodology is applicable to many systems.     

Full-length infectious clone of a low passage dengue virus serotype 2 from Brazil

Full-length dengue virus (DENV) cDNA clones are an invaluable tool for many studies, including those on the development of attenuated or chimeric vaccines and on host-virus interactions. Furthermore, the importance of low passage DENV infectious clones should be highlighted, as these may harbour critical and unique strain-specific viral components from field-circulating isolates. The successful construction of a functional Brazilian low passage DENV serotype 2 full-length clone through homologous recombination reported here supports the use of a strategy that has been shown to be highly useful by our group for the development of flavivirus infectious clones and replicons.     

High-frequency RNA recombination of murine coronaviruses.

The RNA genome of coronaviruses consists of a single species of nonsegmented RNA. In this communication, we demonstrate that the RNA genomes of different strains of murine coronaviruses recombine during mixed infection at a very high frequency. Susceptible cells were coinfected with a temperature-sensitive mutant of one strain of mouse hepatitis virus (MHV) and a wild-type virus of a different strain. Of 21 randomly isolated viruses released from the coinfected cells at the nonpermissive temperature, 2 were recombinants which differed in the site of recombination. After three serial passages of the original virus pool derived from the mixed infection, the majority of the progeny viruses were recombinants. These recombinant viruses represented at least five different recombination sites between the two parental MHV strains. Such a high-frequency recombination between nonsegmented RNA genomes of MHV suggests that segmented RNA intermediates might be generated during MHV replication. We propose that the RNA replication of MHV proceeds in a discontinuous and nonprocessive manner, thus generating free segmented RNA intermediates, which could be used in RNA recombination via a copy-choice mechanism.     

Dynamics of mutation and recombination in a replicating population of complementing, defective viral genomes

In a previous study, we documented that serial passage of a biological clone of foot-and-mouth disease virus (FMDV) at high multiplicity of infection (moi) in cell culture resulted in viral populations dominated by defective genomes that included internal in-frame deletions, affecting the L and capsid-coding regions, and were infectious by complementation. In the present study, analyses of the defective genomes present in individual viral plaques, and of consensus nucleotide sequences determined for the entire genomes of sequential samples, have revealed a continuous dynamics of mutation and recombination. At some points of high genetic instability, multiple minority genomes with different internal deletions co-existed in the population. At later passages, a new defective RNA arose and displaced a related, previously dominant RNA. Nucleotide sequences of the different genomic forms found in sequential isolates have revealed an accumulation of mutations at an average rate of 0.12 substitutions per genome per passage. At the regions around the deletion sites, substantial, minor or no nucleotide sequence identity is found, suggesting relaxed sequence requirements for the occurrence of internal deletions. Competition experiments indicate a selective advantage of late phase defective genomes over their precursor forms. The defective genome-based FMDV retained an expansion of host cell tropism, undergone by the standard virus at a previous stage of the same evolutionary lineage. Thus, despite a complex dynamics of mutation and recombination, and phases of high genetic instability, a biologically relevant phenotypic trait was stably maintained after the evolutionary transition towards a primitive genome segmentation. The results extend the concept of a complex spectrum of mutant genomes to a complex spectrum of defective genomes in some evolutionary transitions of RNA viruses.