Estimating the in-vivo HIV template switching and recombination rate

Identification of a novel first-generation HIV-1 circulating recombinant form (CRF152_DG) among people living with HIV in Karachi, Pakistan

The objective of this study was to characterize a novel circulating recombinant form of human immunodeficiency virus type 1 (HIV-1) among people living with HIV in Karachi, Pakistan. We conducted near-full-length genome (NFLG) sequencing on eight samples exhibiting D/G recombination signals in the pol gene region. We successfully obtained NFLG sequences (790-9,614; with reference to the HXB2 genome) from four of the eight samples and then conducted phylogenetic and recombination analyses on them. The four NFLG sequences from our study and one DG unique recombinant form previously identified in the United Kingdom (GenBank accession: MF109700) formed a distinct monophyletic cluster with an Shimodaira-Hasegawa approximate likelihood ratio test node support value of 100%. Bootscan analyses of the five NFLG sequences of DG recombinants showed that all five NFLGs shared the same unique mosaic pattern of recombination breakpoints between D and G clades, with two D fragments in the pol and vif regions inserted into a G backbone. Subregion phylogenetic analyses confirmed these sequences to be a novel circulating recombinant form (CRF) composed of subtypes D and G. The DG recombinant sequences were eventually designated as CRF152_DG by the Los Alamos HIV Sequence Database staff.

IMPORTANCE

In Pakistan, the genetic diversity of human immunodeficiency virus type 1 (HIV-1) is becoming increasingly complex, compared to the early years of the epidemic that started after the detection of the first cases of HIV-1 in 1987 in Karachi. Based on the available molecular studies, two dominant HIV-1 clades, sub-subtype A1 and CRF02_AG, have been found to co-circulate with other clades, namely B, C, D, G, CRF01_AE, CRF35_A1D, and CRF56_cpx, in various urban areas of Pakistan. Several novel recombinant forms have also been detected. This first report of CRF152_DG highlights the complex nature of the HIV epidemic in Pakistan and emphasizes the importance of continual molecular surveillance (ideally based on whole-genome sequences) of HIV.

Reverse transcription of plasma-derived HIV-1 RNA generates multiple artifacts through tRNA(Lys-3)-priming

In vitro reverse transcription of full-length HIV-1 RNA extracted from the blood plasma of people living with HIV-1 remains challenging. Here, we describe the initiation of reverse transcription of plasma-derived viral RNA in the absence of an exogenous primer. Real-time PCR and Sanger sequencing were applied to identify the source and to monitor the outcome of this reaction. Results demonstrated that during purification of viral RNA from plasma, tRNA(Lys-3) is co-extracted in a complex with the viral RNA. In the presence of a reverse transcription enzyme, this tRNA(Lys-3) can induce reverse transcription, a reaction that is not confined to transcription of the 5' end of the viral RNA. A range of cDNA products is generated, most of them indicative for the occurrence of in vitro strand transfer events that involve translocation of cDNA from the 5' end to random positions on the viral RNA. This process results in the formation of cDNAs with large internal deletions. However, near full-length cDNA and cDNA with sequence patterns resembling multiple spliced HIV-1 RNA were also detected. Despite its potential to introduce significant bias in the interpretation of results across various applications, tRNA(Lys-3)-driven reverse transcription has been overlooked thus far. A more in-depth study of this tRNA-driven in vitro reaction may provide new insight into the complex process of in vivo HIV-1 replication.IMPORTANCEThe use of silica-based extraction methods for purifying HIV-1 RNA from viral particles is a common practice, but it involves co-extraction of human tRNA(Lys-3) due to the strong interactions between these molecules. This co-extraction becomes particularly significant when the extracted RNA is used in reverse transcription reactions, as the tRNA(Lys-3) then serves as a primer. Reverse transcription from tRNA(Lys-3) is not confined to cDNA synthesis of the 5' end of the viral RNA but extends across various regions of the viral genome through in vitro strand transfer events. Co-extraction of tRNA(Lys-3) has been overlooked thus far, despite its potential to introduce bias in downstream, reverse transcription-related applications. The observed events in the tRNA(Lys-3)-induced in vitro reverse transcription resemble in vivo replication processes. Therefore, these reactions may offer a unique model to better understand the replication dynamics of HIV-1.

Characterization of HIV-1 CRF02_AG/A3/G unique recombinant forms identified among children in Larkana, Pakistan

Co-circulation of different human immunodeficiency virus type 1 HIV-1 subtypes among infected populations can lead to the generation of new recombinants. In Pakistan, subtype A1 and CRF02_AG are the dominant strains circulating among key populations. The high prevalence of new HIV infections among the key populations highlights the possibility of recombination between the dominant strains, which can lead to the generation of new recombinants. Here, we identified a recombinant cluster composed of CRF02_AG, sub-subtype A3, and subtype G among HIV-infected children in Larkana. For the study, 10 retrospectively collected samples, with recombination signals in the pol gene, were used to perform a near full-length genome NFLG sequencing. Of the 10 samples, NFLG was successfully sequenced from seven samples. Phylogenetic analysis of the seven NFLGs showed that all recombinants formed a distinct monophyletic cluster and were distinct from known HIV-1 circulating recombinant forms CRFs. Recombination analyses showed that all seven NFLGs shared a similar recombinant structure consisting of CRF02_AG, sub-subtype A3, and subtype G, with a sub-subtype A3 fragment inserted into pol and vif regions spanning from (HXB2: 4218-5518), and a subtype G fragment inserted into vpu, rev, tat and env regions spanning from (HXB2: 5957-8250) of the CRF02_AG backbone. The identification of unique recombinant forms may indicate the presence and transmission of several co-circulating lineages in Larkana, giving rise to newer CRFs. This study also highlights the importance of continuous molecular surveillance to fully understand HIV-1 genetic diversity in Pakistan, particularly in Larkana, which is the epicenter of HIV outbreaks.

Revisiting the recombinant history of HIV-1 group M with dynamic network community detection

Recombination is a major mechanism through which HIV type 1 (HIV-1) maintains genetic diversity and interferes with viral eradication efforts. There is growing evidence demonstrating a recombinant origin of primate lentiviruses including HIV-1 group M (HIV-1/M). Inferring the extent of recombination across the entire HIV-1/M genome is of great importance as it provides deeper insights into the origin, dynamics, and evolution of the global pandemic. Here we propose an alternative method that can reconstruct the extent of genome-wide recombination in HIV-1, uncover reticulate patterns, and serve as a framework for HIV-1 classification. Our method provides an alternative approach for understanding the roles of virus recombination in the early evolutionary history of zoonosis for other emerging viruses.

The prevailing abundance of full-length HIV type 1 (HIV-1) genome sequences provides an opportunity to revisit the standard model of HIV-1 group M (HIV-1/M) diversity that clusters genomes into largely nonrecombinant subtypes, which is not consistent with recent evidence of deep recombinant histories for simian immunodeficiency virus (SIV) and other HIV-1 groups. Here we develop an unsupervised nonparametric clustering approach, which does not rely on predefined nonrecombinant genomes, by adapting a community detection method developed for dynamic social network analysis. We show that this method (dynamic stochastic block model [DSBM]) attains a significantly lower mean error rate in detecting recombinant breakpoints in simulated data (quasibinomial generalized linear model (GLM), P<8×10−8), compared to other reference-free recombination detection programs (genetic algorithm for recombination detection [GARD], recombination detection program 4 [RDP4], and RDP5). When this method was applied to a representative sample of n = 525 actual HIV-1 genomes, we determined k = 29 as the optimal number of DSBM clusters and used change-point detection to estimate that at least 95% of these genomes are recombinant. Further, we identified both known and undocumented recombination hotspots in the HIV-1 genome and evidence of intersubtype recombination in HIV-1 subtype reference genomes. We propose that clusters generated by DSBM can provide an informative framework for HIV-1 classification.

HIV-1-Specific Immunodominant T-Cell Responses Drive the Dynamics of HIV-1 Recombination Following Superinfection

A series of HIV-1 CRF01_AE/CRF07_BC recombinants were previously found to have emerged gradually in a superinfected patient (patient LNA819). However, the extent to which T-cell responses influenced the development of these recombinants after superinfection is unclear. In this study, we undertook a recombination structure analysis of the gag, pol, and nef genes from longitudinal samples of patient LNA819. A total of 9 pol and 5 nef CRF01_AE/CRF07_BC recombinants were detected. The quasispecies makeup and the composition of the pol and nef gene recombinants changed continuously, suggestive of continuous evolution in vivo. T-cell responses targeting peptides of the primary strain and the recombination regions were screened. The results showed that Pol-LY10, Pol-RY9, and Nef-GL9 were the immunodominant epitopes. Pol-LY10 overlapped with the recombination breakpoints in multiple recombinants. For the LY10 epitope, escape from T-cell responses was mediated by both recombination with a CRF07_BC insertion carrying the T467E/T472V variants and T467N/T472V mutations originating in the CRF01_AE strain. In pol recombinants R8 and R9, the recombination breakpoints were located ~23 amino acids upstream of the RY9 epitope. The appearance of new recombination breakpoints harboring a CRF07_BC insertion carrying a R984K variant was associated with escape from RY9-specific T-cell responses. Although the Nef-GL9 epitope was located either within or 10~11 amino acids downstream of the recombination breakpoints, no variant of this epitope was observed in the nef recombinants. Instead, a F85V mutation originating in the CRF01_AE strain was the main immune escape mechanism. Understanding the cellular immune pressure on recombination is critical for monitoring the new circulating recombinant forms of HIV and designing epitope-based vaccines. Vaccines targeting antigens that are less likely to escape immune pressure by recombination and/or mutation are likely to be of benefit to patients with HIV-1.

OUP accepted manuscript

Background

Methods

Results

Conclusions

Modeling HIV multiple infection

Multiple infection of target cells by human immunodeficiency virus (HIV) may lead to viral escape from host immune responses and drug resistance to antiretroviral therapy, bringing more challenges to the control of infection. The mechanisms underlying HIV multiple infection and their relative contributions are not fully understood. In this paper, we develop and analyze a mathematical model that includes sequential cell-free virus infection (i.e.one virus is transmitted each time in a sequential infection of target cells by virus) and cell-to-cell transmission (i.e.multiple viral genomes are transmitted simultaneously from infected to uninfected cells). By comparing model prediction with the distribution data of proviral genomes in HIV-infected spleen cells, we find that multiple infection can be well explained when the two modes of viral transmission are both included. Numerical simulation using the parameter estimates from data fitting shows that the majority of T cell infections are attributed to cell-to-cell transmission and this transmission mode also accounts for more than half of cell's multiple infections. These results suggest that cell-to-cell transmission plays a critical role in forming HIV multiple infection and thus has important implications for HIV evolution and pathogenesis.

Genetically barcoded SIV reveals the emergence of escape mutations in multiple viral lineages during immune escape

The rapidity of replication coupled with a high mutation rate enables HIV to evade selective pressures imposed by host immune responses. Investigating the ability of HIV to escape different selection forces has generally relied on population-level measures, such as the time to detectable escape mutations in plasma and the rate these mutations subsequently take over the virus population. Here we employed a barcoded synthetic swarm of simian immunodeficiency virus (SIV) in rhesus macaques to investigate the generation and selection of escape mutations within individual viral lineages at the Mamu-A*01-restricted Tat-SL8 epitope. We observed the persistence of more than 1,000 different barcode lineages following selection after acquiring escape mutations. Furthermore, the increased resolution into the virus population afforded by barcode analysis revealed changes in the population structure of the viral quasispecies as it adapted to immune pressure. The high frequency of emergence of escape mutations in parallel viral lineages at the Tat-SL8 epitope highlights the challenge posed by viral escape for the development of T cell-based vaccines. Importantly, the level of viral replication required for generating escape mutations in individual lineages can be directly estimated using the barcoded virus, thereby identifying the level of efficacy required for a successful vaccine to limit escape. Overall, assessing the survival of barcoded viral lineages during selection provides a direct and quantitative measure of the stringency of the underlying genetic bottleneck, making it possible to predict the ability of the virus to escape selective forces induced by host immune responses as well as during therapeutic interventions.

Single genome sequencing of near full-length HIV-1 RNA using a limiting dilution approach

Sequencing very long stretches of the HIV-1 genome can advance studies on virus evolution and in vivo recombination but remains technically challenging. We developed an efficient procedure to sequence near full-length HIV-1 RNA using a two-amplicon approach. The whole genome was successfully amplified for 107 (88%) of 121 plasma samples including samples from patients infected with HIV-1 subtype A1, B, C, D, F1, G, H, CRF01_AE and CRF02_AG. For the 17 samples with a viral load below 1000 c/ml and the 104 samples with a viral load above 1000 c/ml, the amplification efficiency was respectively 53% and 94%. The sensitivity of the method was further evaluated using limiting dilution of RNA extracted from a plasma pool containing an equimolar mixture of three HIV-1 subtypes (B, C and CRF02_AG) and diluted before and after cDNA generation. Both RNA and cDNA dilution showed comparable sensitivity and equal accuracy in reflecting the subtype distribution of the plasma pool. One single event of in vitro recombination was detected amongst the 41 sequences obtained after cDNA dilution but no indications for in vitro recombination were found after RNA dilution. In conclusion, a two-amplicon strategy and limiting dilution of viral RNA followed by reverse transcription, nested PCR and Sanger sequencing, allows near full genome sequencing of individual HIV-1 RNA molecules. This method will be a valuable tool in the study of virus evolution and recombination.

Retroviral nucleocapsid proteins and DNA strand transfers

An infectious retroviral particle contains 1000-1500 molecules of the nucleocapsid protein (NC) that cover the diploid RNA genome. NC is a small zinc finger protein that possesses nucleic acid chaperone activity that enables NC to rearrange DNA and RNA molecules into the most thermodynamically stable structures usually those containing the maximum number of base pairs. Thanks to the chaperone activity, NC plays an essential role in reverse transcription of the retroviral genome by facilitating the strand transfer reactions of this process. In addition, these reactions are involved in recombination events that can generate multiple drug resistance mutations in the presence of anti-HIV-1 drugs. The strand transfer reactions rely on base pairing of folded DNA/RNA structures. The molecular mechanisms responsible for NC-mediated strand transfer reactions are presented and discussed in this review. Antiretroviral strategies targeting the NC-mediated strand transfer events are also discussed.

Dynamics of HIV-1 coinfection in different susceptible target cell populations during cell-free infection

HIV-1 mutations rapidly accumulate through genetic recombination events, which require the infection of a single cell by two virions (coinfection). Accumulation of mutations in the viral population may lead to immune escape and high-level drug resistance. The existence of cell subpopulations characterized by different susceptibility to HIV-1 infection has been proposed as an important parameter driving coinfection (Dang et al., 2004). While the mechanism and the quantification of HIV-1 coinfection have been recently investigated by mathematical models, the detailed dynamics of this process during cell-free infection remains elusive. In this study, we constructed ordinary differential equations considering the heterogeneity of target cell populations during cell-free infection in cell culture, and reproduced the cell culture experimental data. Our mathematical analyses showed that the presence of two differently susceptible target cell subpopulations could explain our experimental datasets, while increasing the number of subpopulations did not improve the fitting. In addition, we quantitatively demonstrated that cells infected by multiple viruses mainly accumulated from one cell subpopulation under cell-free infection conditions. In particular, the frequency of infection events in the more susceptible subpopulation was 6.11-higher than that from the other subpopulation, and 98.3% of coinfected cells emerged from the more susceptible subpopulation. Our mathematical-experimental approach is able to extract such a quantitative information, and can be easily applied to other virus infections.

Tracking HIV-1 recombination to resolve its contribution to HIV-1 evolution in natural infection

Recombination in HIV-1 is well documented, but its importance in the low-diversity setting of within-host diversification is less understood. Here we develop a novel computational tool (RAPR (Recombination Analysis PRogram)) to enable a detailed view of in vivo viral recombination during early infection, and we apply it to near-full-length HIV-1 genome sequences from longitudinal samples. Recombinant genomes rapidly replace transmitted/founder (T/F) lineages, with a median half-time of 27 days, increasing the genetic complexity of the viral population. We identify recombination hot and cold spots that differ from those observed in inter-subtype recombinants. Furthermore, RAPR analysis of longitudinal samples from an individual with well-characterized neutralizing antibody responses shows that recombination helps carry forward resistance-conferring mutations in the diversifying quasispecies. These findings provide insight into molecular mechanisms by which viral recombination contributes to HIV-1 persistence and immunopathogenesis and have implications for studies of HIV transmission and evolution in vivo.

Recombination in the rabies virus and other lyssaviruses

Recombination is a common event in RNA viruses; however, in the rabies virus there have been only a few reports of isolated recombination events. Comprehensive analysis found traces of recent recombination events within Arctic, Arctic-like and Africa 1b rabies virus groups, as well as recombination between distinct lyssaviruses. Recombination breakpoints were not linked to gene boundaries and could be detected all over the genome. However, there was no evidence that recombination is an important factor in the genetic variability of the rabies virus. It is therefore likely that recombination in the rabies virus is limited by ecological factors (e.g., rare co-circulation of distinguishable lineages and a narrow window for productive coinfection in most carnivore hosts), rather than molecular barriers (e.g., incompatibility of genome fragments).

Are RNA Viruses Candidate Agents for the Next Global Pandemic? A Review

Pathogenic RNA viruses are potentially the most important group involved in zoonotic disease transmission, and they represent a challenge for global disease control. Their biological diversity and rapid adaptive rates have proved to be difficult to overcome and to anticipate by modern medical technology. Also, the anthropogenic change of natural ecosystems and the continuous population growth are driving increased rates of interspecies contacts and the interchange of pathogens that can develop into global pandemics. The combination of molecular, epidemiological, and ecological knowledge of RNA viruses is therefore essential towards the proper control of these emergent pathogens. This review outlines, throughout different levels of complexity, the problems posed by RNA viral diseases, covering some of the molecular mechanisms allowing them to adapt to new host species-and to novel pharmaceutical developments-up to the known ecological processes involved in zoonotic transmission.

Mechanisms and Factors That Drive Extensive Human Immunodeficiency Virus Type-1 Hypervariability: An Overview

The extensive hypervariability of human immunodeficiency virus type-1 (HIV-1) populations represents a major barrier against the success of currently available antiretroviral therapy. Moreover, it is still the most important obstacle that faces the development of an effective preventive vaccine against this infectious virus. Indeed, several factors can drive such hypervariability within and between HIV-1 patients. These factors include: first, the very low fidelity nature of HIV-1 reverse transcriptase; second, the extremely high HIV-1 replication rate; and third, the high genomic recombination rate that the virus has. All these factors together with the APOBEC3 proteins family and the immune and antiviral drugs pressures drive the extensive hypervariability of HIV-1 populations. Studying these factors and the mechanisms that drive such hypervariability will provide valuable insights that may guide the development of effective therapeutic and preventive strategies against HIV-1 infection in the near future. To this end, in this review, we summarized recent advances in this area of HIV-1 research.

A step forward understanding HIV-1 diversity

Human immunodeficiency virus (HIV) populations are characterized by extensive genetic diversity. Antigenic diversification is essential for escape from immune selection and therapy, and remains one of the major obstacles for the development of an efficient vaccine strategy. Even if intensive efforts have been made for understanding the molecular mechanisms responsible for genetic diversity in HIV, conclusive data in vivo is still lacking. Recent works have addressed this issue, focusing on the identification of the sources of genetic diversity during in vivo infections and on the estimate of the pervasiveness of genetic recombination during replication in vivo. Surprisingly, it appears that despite the error-prone nature of the viral polymerase, the bulk of mutations found in patients are indeed due to the effect of a cellular restriction factor. This factor tends to hypermutate the viral genome abolishing viral infectivity. When hypermutation is incomplete, the virus retains infectivity and converts the effect of the cellular factor to its advantage by exploiting it to generate genetic diversity that is beneficial for viral propagation. This view contrasts the long-standing dogma that viral diversity is due to the intrinsic error-prone nature of the viral replication cycle. Besides hypermutations and mutations, recombination is also a pervasive source of genetic diversity. The estimate of the frequency at which this process takes place in vivo has remained elusive, despite extensive efforts in this sense. Now, using single genome amplification, and starting from publically available datasets, it has been obtained a confirmation of the estimates previously made using tissue culture studies. These recent findings are presented here and their implications for the development of future researches are discussed.