SMRT sequencing revealed the diversity and characteristics of defective interfering RNAs in influenza A (H7N9) virus infection.

Influenza defective interfering (DI) particles are replication-incompetent viruses carrying large internal deletion in the genome. The loss of essential genetic information causes abortive viral replication, which can be rescued by co-infection with a helper virus that possesses an intact genome. Despite reports of DI particles present in seasonal influenza A H1N1 infections, their existence in human infections by the avian influenza A viruses, such as H7N9, has not been studied. Here we report the ubiquitous presence of DI-RNAs in nasopharyngeal aspirates of H7N9-infected patients. Single Molecule Real Time (SMRT) sequencing was first applied and long-read sequencing analysis showed that a variety of H7N9 DI-RNA species were present in the patient samples and human bronchial epithelial cells. In several abundantly expressed DI-RNA species, long overlapping sequences have been identified around at the breakpoint region and the other side of deleted region. Influenza DI-RNA is known as a defective viral RNA with single large internal deletion. Beneficial to the long-read property of SMRT sequencing, double and triple internal deletions were identified in half of the DI-RNA species. In addition, we examined the expression of DI-RNAs in mice infected with sublethal dose of H7N9 virus at different time points. Interestingly, DI-RNAs were abundantly expressed as early as day 2 post-infection. Taken together, we reveal the diversity and characteristics of DI-RNAs found in H7N9-infected patients, cells and animals. Further investigations on this overwhelming generation of DI-RNA may provide important insights into the understanding of H7N9 viral replication and pathogenesis.

A quantitative approach for measuring the reservoir of latent HIV-1 proviruses.

A stable latent reservoir for HIV-1 in resting CD4+ T cells is the principal barrier to a cure1-3. Curative strategies that target the reservoir are being tested4,5 and require accurate, scalable reservoir assays. The reservoir was defined with quantitative viral outgrowth assays for cells that release infectious virus after one round of T cell activation1. However, these quantitative outgrowth assays and newer assays for cells that produce viral RNA after activation6 may underestimate the reservoir size because one round of activation does not induce all proviruses7. Many studies rely on simple assays based on polymerase chain reaction to detect proviral DNA regardless of transcriptional status, but the clinical relevance of these assays is unclear, as the vast majority of proviruses are defective7-9. Here we describe a more accurate method of measuring the HIV-1 reservoir that separately quantifies intact and defective proviruses. We show that the dynamics of cells that carry intact and defective proviruses are different in vitro and in vivo. These findings have implications for targeting the intact proviruses that are a barrier to curing HIV infection.

HIV-1 proviral landscapes distinguish posttreatment controllers from noncontrollers.

HIV posttreatment controllers (PTCs) represent a natural model of sustained HIV remission, but they are rare and little is known about their viral reservoir. We obtained 1,450 proviral sequences after near-full-length amplification for 10 PTCs and 16 posttreatment noncontrollers (NCs). Before treatment interruption, the median intact and total reservoir size in PTCs was 7-fold lower than in NCs, but the proportion of intact, defective, and total clonally expanded proviral genomes was not significantly different between the 2 groups. Quantification of total but not intact proviral genome copies predicted sustained HIV remission as 81% of NCs, but none of the PTCs had a total proviral genome greater than 4 copies per million peripheral blood mononuclear cells (PBMCs). The results highlight the restricted intact and defective HIV reservoir in PTCs and suggest that total proviral genome burden could act as the first biomarker for identifying PTCs. Total and defective but not intact proviral copy numbers correlated with levels of cell-associated HIV RNA, activated NK cell percentages, and both HIV-specific CD4+ and CD8+ responses. These results support the concept that defective HIV genomes can lead to viral antigen production and interact with both the innate and adaptive immune systems.

Antiviral resistance due to deletion in the neuraminidase gene and defective interfering-like viral polymerase basic 2 RNA of influenza A virus subtype H3N2.

BACKGROUND AND OBJECTIVE: Antiviral treatment of influenza virus infections can lead to drug resistance of virus. This study investigates a selection of mutations in the full genome of H3N2 influenza A virus isolated from a patient in treatment with oseltamivir. STUDY DESIGN: Respiratory samples from a patient were collected before, during, and after antiviral treatment. Whole genome sequencing of the influenza virus by next generation sequencing, and low-frequency-variant analysis was performed. Neuraminidase-inhibition tests were performed with oseltamivir and zanamivir, and viruses were propagated in sial-transferase gene transfected Madin-Darby Canine Kidney cells. RESULTS: A deletion at amino acid position 245-248 in the neuraminidase gene occurred after initiation of treatment with oseltamivir. The deleted virus had highly reduced inhibition against oseltamivir but was sensitive to zanamivir. Nine days after discontinuation of oseltamivir treatment the deleted H3N2 virus was still present in the patient. After three passages of the deleted virus in cell culture, the deletion was retained. Several variant mutations appeared in the other genes of the H3N2 virus, where most striking were two major out-of-frame deletions in the polymerase basic 2 (PB2) gene, indicating defective interfering-like viral RNA. CONCLUSIONS: The viruses harboring the 245-248 deletion in the neuraminidase gene were still present after discontinuation of oseltamivir treatment and passages in cell cultures, indicating a potential risk for transmission of the deleted virus. Full genome deep sequencing was useful to reveal variant mutations that might be selected due to antiviral treatment, and defective interfering-like viral PB2 RNA in the respiratory samples was detected.

Live Attenuated Influenza Vaccine contains Substantial and Unexpected Amounts of Defective Viral Genomic RNA.

The live attenuated influenza vaccine FluMist® was withdrawn in the USA by the Centers for Disease Control and Prevention after its failure to provide adequate protective immunity during 2013-2016. The vaccine uses attenuated core type A and type B viruses, reconfigured each year to express the two major surface antigens of the currently circulating viruses. Here Fluenz™ Tetra, the European version of this vaccine, was examined directly for defective-interfering (DI) viral RNAs. DI RNAs are deleted versions of the infectious virus genome, and have powerful biological properties including attenuation of infection, reduction of infectious virus yield, and stimulation of some immune responses. Reverse transcription polymerase chain reaction followed by cloning and sequencing showed that Fluenz™ vaccine contains unexpected and substantial amounts of DI RNA arising from both its influenza A and influenza B components, with 87 different DI RNA sequences identified. Flu A DI RNAs from segment 3 replaced the majority of the genomic full-length segment 3, thus compromising its infectivity. DI RNAs arise during vaccine production and non-infectious DI virus replaces infectious virus pro rata so that fewer doses of the vaccine can be made. Instead the vaccine carries a large amount of non-infectious but biologically active DI virus. The presence of DI RNAs could significantly reduce the multiplication in the respiratory tract of the vaccine leading to reduced immunizing efficacy and could also stimulate the host antiviral responses, further depressing vaccine multiplication. The role of DI viruses in the performance of this and other vaccines requires further investigation.

Unexpected complexity in the interference activity of a cloned influenza defective interfering RNA.

BACKGROUND: Defective interfering (DI) viruses are natural antivirals made by nearly all viruses. They have a highly deleted genome (thus being non-infectious) and interfere with the replication of genetically related infectious viruses. We have produced the first potential therapeutic DI virus for the clinic by cloning an influenza A DI RNA (1/244) which was derived naturally from genome segment 1. This is highly effective in vivo, and has unexpectedly broad-spectrum activity with two different modes of action: inhibiting influenza A viruses through RNA interference, and all other (interferon-sensitive) respiratory viruses through stimulating interferon type I. RESULTS: We have investigated the RNA inhibitory mechanism(s) of DI 1/244 RNA. Ablation of initiation codons does not diminish interference showing that no protein product is required for protection. Further analysis indicated that 1/244 DI RNA interferes by replacing the cognate full-length segment 1 RNA in progeny virions, while interfering with the expression of genome segment 1, its cognate RNA, and genome RNAs 2 and 3, but not genome RNA 6, a representative of the non-polymerase genes. CONCLUSIONS: Our data contradict the dogma that a DI RNA only interferes with expression from its cognate full-length segment. There is reciprocity as cloned segment 2 and 3 DI RNAs inhibited expression of RNAs from a segment 1 target. These data demonstrate an unexpected complexity in the mechanism of interference by this cloned therapeutic DI RNA.

Droplet digital PCR: A novel method for detection of influenza virus defective interfering particles.

Defective interfering (DI) particles are viruses that carry one or more large, internal deletions in the viral genome. These deletions occur commonly in RNA viruses due to polymerase error and yield incomplete genomes that typically lack essential coding regions. The presence of DI particles in a virus population can have a major impact on the efficiency of viral growth and is an important variable to consider in interpreting experimental results. Herein, we sought to develop a robust methodology for the quantification of DI particles within influenza A virus stocks. We took advantage of reverse transcription followed by droplet digital PCR (RT ddPCR), a highly sensitive and precise technology for determination of template concentrations without the use of a standard curve. Results were compared to those generated using standard RT qPCR. Both assays relied on the use of primers binding to terminal regions conserved in DI gene segments described to date, and internal primers targeting regions typically missing from DI particles. As it has been reported previously, we observed a lower coefficient of variation among technical replicates for ddPCR compared to qPCR. Results furthermore established RT ddPCR as a sensitive and quantitative method for detecting DI gene segments within influenza A virus stocks.

Development of a Model System for Tick-Borne Flavivirus Persistence in HEK 293T Cells.

UNLABELLED: We devised a model system to study persistent infection by the tick-borne flavivirus Langat virus (LGTV) in 293T cells. Infection with a molecularly cloned LGTV strain produced an acute lytic crisis that left few surviving cells. The culture was repopulated by cells that were ~90% positive for LGTV E protein, thus initiating a persistent infection that was maintained for at least 35 weeks without additional lytic crises. Staining of cells for viral proteins and ultrastructural analysis revealed only minor differences from the acute phase of infection. Infectious LGTV decreased markedly over the study period, but the number of viral genomes remained relatively constant, suggesting the development of defective interfering particles (DIPs). Viral genome changes were investigated by RNA deep sequencing. At the initiation of persistent infection, levels of DIPs were below the limit of detection at a coverage depth of 11,288-fold, implying that DIPs are not required for initiation of persistence. However, after 15 passages, DIPs constituted approximately 34% of the total LGTV population (coverage of 1,293-fold). Furthermore, at this point, one specific DIP population predominated in which nucleotides 1058 to 2881 had been deleted. This defective genome specified an intact polyprotein that coded for a truncated fusion protein containing 28 N-terminal residues of E and 134 C-terminal residues of NS1. Such a fusion protein has not previously been described, and a possible function in persistent infection is uncertain. DIPs are not required for the initiation of persistent LGTV infection but may play a role in the maintenance of viral persistence. IMPORTANCE: Tick-borne flaviviruses are significant infectious agents that cause serious disease and death in humans worldwide. Infections are characterized by severe neurological symptoms, such as meningitis and encephalitis. A high percentage of people who get infected and recuperate from the acute phase of infection continue to suffer from chronic debilitating neurological sequelae, most likely as a result of nervous tissue damage, viral persistence, or both. However, little is known about mechanisms of viral persistence. Therefore, we undertook studies to investigate the persistence of Langat virus, a member of the tick-borne flavivirus group, in a mammalian cell line. Using next-generation sequencing, we determined that defective viral genomes do not play a role in the initiation of persistence, but their occurrence seems to be nonstochastic and could play a role in the maintenance of viral persistence via the expression of a novel envelope-NS1 fusion protein.

Defective DNAs of beet curly top virus from long-term survivor sugar beet plants.

Long-term surviving sugar beet plants were investigated after beet curly top virus infection to characterize defective (D) viral DNAs as potential symptom attenuators. Twenty or 14 months after inoculation, 20 D-DNAs were cloned and sequenced. In contrast to known D-DNAs, they exhibited a large range of sizes. Deletions were present in most open reading frames except ORF C4, which encodes a pathogenicity factor. Direct repeats and inverted sequences were observed. Interestingly, the bidirectional terminator of transcription was retained in all D-DNAs. A model is presented to explain the deletion sites and sizes with reference to the viral minichromosome structure, and symptom attenuation by D-DNAs is discussed in relation to RNA interference.

Analysis of complete genomes of isolates of the Wheat dwarf virus from new geographical locations and descriptions of their defective forms.

Recently, the importance of the Geminiviruses infecting cereal crops has been appreciated, and they are now being studied in detail. Barley and wheat strains of Wheat dwarf virus are recorded in most European countries. Information on complete sequences of isolates from the United Kingdom, Spain, and Austria are reported here for the first time. Analysis revealed that their sequences are very stable. Recombination between strains was recorded only for the barley strain. We identified several defective forms of the barley strain from barley and wheat, which do not influence symptom expression. Sequences of barley isolates infecting wheat were obtained that did not differ from the isolates from barley. Based on specific features of the SIR of the barley strains, it is suggested that they are assigned to one of the two proposed new clusters, A1 or A2.

Biological and genomic analysis of a PBSX-like defective phage induced from Bacillus pumilus AB94180.

Defective prophages, which are found in the genomes of many bacteria, are unable to complete a viral replication cycle and propagate in their hosts as healthy prophages. They package random DNA fragments derived from various sites of the host chromosome instead of their own genomes. In this study, we characterized a defective phage, PBP180, which was induced from Bacillus pumilus AB94180 by treatment with mitomycin C. Electron microscopy showed that the PBP180 particle has a head with a hexagonal outline of ~40 nm in diameter and a long tail. The DNA packaged in the PBP180 head consists of 8-kb DNA fragments from random portions of the host chromosome. The head and tail proteins of the PBP180 particle consist of four major proteins of approximately 49, 33, 16 and 14 kDa. The protein profile of PBP180 is different from that of PBSX, a well-known defective phage induced from Bacillus subtilis 168. A killing activity test against two susceptible strains each of B. subtilis and B. pumilus showed that the defective particles of PBP180 killed three strains other than its own host, B. pumilus AB94180, differing from the host-killing ranges of the defective phages PBSX, PBSZ (induced from B. subtilis W23), and PBSX4 (induced from B. pumilus AB94044). The genome of the PBP180 prophage, which is integrated in the B. pumilus AB94180 chromosome, is 28,205 bp in length, with 40 predicted open reading frames (ORFs). Further genomic comparison of prophages PBP180, PBSX, PBSZ and other PBSX-like prophage elements in B. pumilus strains revealed that their overall architectures are similar, but significant low homology exists in ORF29-ORF38, which presumably encode tail fiber proteins involved in recognition and killing of susceptible strains.

A naturally occurring defective DNA satellite associated with a monopartite begomovirus: evidence for recombination between alphasatellite and betasatellite.

Monopartite begomoviruses and their associated satellites form unique disease complexes that have emerged as a serious threat to agriculture worldwide. It is well known that frequent recombination contributes to the diversification and evolution of geminiviruses. In this study, we identified a novel defective satellite molecule (RecSat) in association with Tobacco leaf curl Yunnan virus (TbLCYNV) in a naturally infected tobacco plant. Sequence analysis showed that Recsat comprises 754 nucleotides in size and is a chimera involving alphasatellite and betasatellite sequences, containing both betasatellite-conserved region and alphasatellite stem-loop structure. Recombination analysis revealed that RecSat has arisen from three independent recombination events likely involving Tomato yellow leaf curl China betasatellite, Ageratum yellow vein China betasatellite and Tobacco curly shoot alphasatellite. Co-inoculation of RecSat with TbLCYNV induced symptoms indistinguishable from those induced by TbLCYNV alone in Nicotiana benthamiana. Southern blot hybridization showed that RecSat could be trans-replicated stably in N. benthamiana plants by TbLCYNV, and impaired the accumulation of helper virus and co-inoculated alphasatellite. Our results provide the first evidence for recombination between two distinct types of satellites among geminivirus complex and highlight recombination as a driving force for geminivirus evolution.

Quantitative RT-PCR for titration of replication-defective recombinant Semliki Forest virus.

Virus titration may constitute a drawback in the development and use of replication-defective viral vectors like Semliki Forest virus (SFV). The standardization and validation of a reverse transcription quantitative PCR (qRT-PCR) method for SFV titration is presented here. The qRT-PCR target is located within the nsp1 gene of the non-structural polyprotein SFV region (SFV RNA), which allows the strategy to be used for several different recombinant SFV constructs. Titer determinations were carried out by performing virus titration and infection assays with SFVs containing an RNA coding region for the rabies virus glycoprotein (RVGP) or green fluorescent protein (GFP). Results showed that the standardized qRT-PCR is applicable for different SFV constructs, and showed good reproducibility. To evaluate the correlation between the amount of functional SFV RNA in a virus lot and its infectivity in BHK-21 cell cultures, a temperature mediated titer decrease was performed and successfully quantitated by qRT-PCR. When used for cell infection at the same multiplicity of infection (MOI), the temperature treated SFV-RVGP samples induced the same levels of RVGP expression. Similarly, when different SFV-GFP lots with different virus titers, as accessed by qRT-PCR, were used for cell infection at the same MOI, the cultures showed comparable amounts of fluorescent cells. The data demonstrate a good correlation between the amount of virus used for infection, as measured by its SFV RNA, and the protein synthesis in the cells. In conclusion, the qRT-PCR method developed here is accurate and enables the titration of replication-defective SFV vectors, an essential aid for viral vector development as well as for establishment of production bioprocesses.

Generation of high-titer defective HSV-1 vectors.

Basic Protocol 1 describes the generation of helper virus stocks. Preparation of recombinant amplicon vector particles by transfection of amplicon and superinfection of helper virus into cells, and harvesting of packaged particles, is delineated in Basic Protocol 3. Thorough characterization of each amplicon viral vector stock involves measuring (1) the helper virus plaque-forming units per ml (pfu/ml) on 2-2 cells and (2) the amplicon stock infectious units per ml (iu/ml) on PC12 cells. The Support Protocols detail methods for determining titers of helper virus by plaque assay, and of amplicon stocks by vector assay.

Production of hepatitis B defective particles is dependent on liver status.

Defective hepatitis B virus (dHBV) generated from spliced RNA is detected in the sera of HBV-chronic carriers. Our study was designed to determine whether the proportion of dHBV changed during the course of infection, and to investigate whether dHBV might interfere with HBV replication. To achieve this, HBV wild-type and dHBV levels were determined by Q-PCR in sera from 56 untreated chronic patients and 23 acute patients, in sequential samples from 4 treated-patients and from liver-humanized mice after HBV infection. The proportion of dHBV was higher in patients with severe compared to null/moderate liver disease or with acute infection. Follow-up showed that the proportion of dHBV increased during disease progression. By contrast, a low and stable proportion of dHBV was observed in the humanized-mouse model of HBV infection. Our results highlight a regulation of the proportion of dHBV during liver disease progression that is independent of interference with viral replication.

HIV-1 genome is often defective in PBMCs and rectal tissues after long-term HAART as a result of APOBEC3 editing and correlates with the size of reservoirs.

OBJECTIVES: Precise characterization of viruses present in reservoirs in long-term pretreated patients will be a major issue to consider in the context of viral eradication. We assessed the frequency of defective viruses present in cellular reservoirs. METHODS: Peripheral blood mononuclear cells (PBMCs) and rectal biopsy samples were compared between five patients on successful long-term highly active antiretroviral therapy (HAART) (>7 years without blips) and five untreated patients. Molecular cloning and sequencing of the reverse transcriptase region were used to detect the presence of and quantify in-frame stop codons in HIV quasi-species. The relationship between the size of the reservoir and the frequency of defective genomes was assessed. RESULTS: Defective genomes were systematically detected in all patients on long-term HAART in both compartments (PBMCs and rectal tissues), with a higher level of defective genomes per sample compared with PBMCs of untreated patients. A high level of defective genomes was correlated with a small size of HIV proviral DNA. Regarding the nucleotide context, guanine (G) to adenine (A) substitution at tryptophan positions was responsible for the appearance of 89% of all in-frame stop codons in the context of G-to-A hypermutation, likely reflecting APOBEC3 footprints on the viral genome. CONCLUSIONS: We propose a scenario whereby defective genomes accumulate during HAART treatment, eventually reaching a viral extinction threshold. In the context of viral eradication, measurement of the relative amounts of defective and non-defective viruses (by molecular cloning and ultradeep sequencing) should be used as a new criterion for eradicating HIV.

Production of baculovirus defective interfering particles during serial passage is delayed by removing transposon target sites in fp25k.

Accumulation of baculovirus defective interfering particle (DIP) and few polyhedra (FP) mutants is a major limitation to continuous large-scale baculovirus production in insect-cell culture. Although overcoming these mutations would result in a cheaper platform for producing baculovirus biopesticides, little is known regarding the mechanism of FP and DIP formation. This issue was addressed by comparing DIP production of wild-type (WT) Autographa californica multiple nucleopolyhedrovirus (AcMNPV) with that of a recombinant AcMNPV (denoted Ac-FPm) containing a modified fp25k gene with altered transposon insertion sites that prevented transposon-mediated production of the FP phenotype. In addition to a reduction in the incidence of the FP phenotype, DIP formation was delayed on passaging of Ac-FPm compared with WT AcMNPV. Specifically, the yield of DIP DNA in Ac-FPm was significantly lower than in WT AcMNPV up to passage 16, thereby demonstrating that modifying the transposon insertion sites increases the genomic stability of AcMNPV. A critical component of this investigation was the optimization of a systematic method based on the use of pulsed-field gel electrophoresis (PFGE) to characterize extracellular virus DNA. Specifically, PFGE was used to detect defective genomes, determine defective genome sizes and quantify the amount of defective genome within a heterogeneous genome population of passaged virus.

Two types of defective RNAs arising from the tomato black ring virus genome.

Short defective RNAs (D-RNAs) associated with tomato black ring virus (TBRV) were isolated, cloned and sequenced. As a result, two types of D-RNAs associated with different TBRV isolates were identified. Both types were derived from RNA1. The first one contained sequences from the 5' and 3' untranslated regions (UTR) and from the 5' region of a single large open reading frame. The second one included a portion of the coding region for the RNA-dependent RNA polymerase flanked by a short fragment of the 5' UTR and the entire 3' UTR. The possible nature and origin of these RNA species is discussed.

Generation of replication-defective virus-based vaccines that confer full protection in sheep against virulent bluetongue virus challenge.

The reverse genetics technology for bluetongue virus (BTV) has been used in combination with complementing cell lines to recover defective BTV-1 mutants. To generate a potential disabled infectious single cycle (DISC) vaccine strain, we used a reverse genetics system to rescue defective virus strains with large deletions in an essential BTV gene that encodes the VP6 protein (segment S9) of the internal core. Four VP6-deficient BTV-1 mutants were generated by using a complementing cell line that provided the VP6 protein in trans. Characterization of the growth properties of mutant viruses showed that each mutant has the necessary characteristics for a potential vaccine strain: (i) viral protein expression in noncomplementing mammalian cells, (ii) no infectious virus generated in noncomplementing cells, and (iii) efficient replication in the complementing VP6 cell line. Further, a defective BTV-8 strain was made by reassorting the two RNA segments that encode the two outer capsid proteins (VP2 and VP5) of a highly pathogenic BTV-8 with the remaining eight RNA segments of one of the BTV-1 DISC viruses. The protective capabilities of BTV-1 and BTV-8 DISC viruses were assessed in sheep by challenge with specific virulent strains using several assay systems. The data obtained from these studies demonstrated that the DISC viruses are highly protective and could offer a promising alternative to the currently available attenuated and killed virus vaccines and are also compliant as DIVA (differentiating infected from vaccinated animals) vaccines.

Ambivalent effects of defective DNA in beet curly top virus-infected transgenic sugarbeet plants.

Beet curly top virus (BCTV) limits sugarbeet production considerably. Previous studies have shown that infections are associated with the generation of defective DNAs (D-DNA) which may attenuate symptoms. Transgenic sugarbeet lines were established carrying a partial direct repeat construct of D-DNA in order to examine whether they are useful as a means of generating tolerance against BCTV. Thirty four independent transgenic lines were challenged. Viral full-length and D-DNAs were monitored by polymerase chain reaction (PCR) or rolling circle amplification (RCA) and restriction fragment length polymorphism (RFLP). The differential accumulation of both DNA species was compared with symptom severity during the course of infection. RCA/RFLP allowed the discrimination of two D-DNA classes which were either derived from the transgenic construct (D(0)) or had been generated de novo (D(n)). The statistical analysis of the results showed that the presence of D(0)-DNA correlated with increased symptom severity, whereas D(n)-DNAs correlated with attenuated symptoms.