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A stable latent reservoir for HIV-1 in resting CD4 + T cells is the principal barrier to a cure 1 – 3 . Curative strategies that target the reservoir are being tested 4 , 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 activation 1 . However, these quantitative outgrowth assays and newer assays for cells that produce viral RNA after activation 6 may underestimate the reservoir size because one round of activation does not induce all proviruses 7 . 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 defective 7 – 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. An assay to measure the latent HIV-1 reservoir that separately quantifies intact and defective proviruses will facilitate evaluation of HIV-1 cure strategies by measuring the provirues that pose a barrier to cure.

Human immunodeficiency virus 1 (HIV-1) reservoir cells persist lifelong despite antiretroviral treatment 1 , 2 but may be vulnerable to host immune responses that could be exploited in strategies to cure HIV-1. Here we used a single-cell, next-generation sequencing approach for the direct ex vivo phenotypic profiling of individual HIV-1-infected memory CD4 + T cells from peripheral blood and lymph nodes of people living with HIV-1 and receiving antiretroviral treatment for approximately 10 years. We demonstrate that in peripheral blood, cells harbouring genome-intact proviruses and large clones of virally infected cells frequently express ensemble signatures of surface markers conferring increased resistance to immune-mediated killing by cytotoxic T and natural killer cells, paired with elevated levels of expression of immune checkpoint markers likely to limit proviral gene transcription; this phenotypic profile might reduce HIV-1 reservoir cell exposure to and killing by cellular host immune responses. Viral reservoir cells harbouring intact HIV-1 from lymph nodes exhibited a phenotypic signature primarily characterized by upregulation of surface markers promoting cell survival, including CD44, CD28, CD127 and the IL-21 receptor. Together, these results suggest compartmentalized phenotypic signatures of immune selection in HIV-1 reservoir cells, implying that only small subsets of infected cells with optimal adaptation to their anatomical immune microenvironment are able to survive during long-term antiretroviral treatment. The identification of phenotypic markers distinguishing viral reservoir cells may inform future approaches for strategies to cure and eradicate HIV-1. A proteogenomic profiling analysis of single cells from the blood and lymph nodes of individuals living with HIV-1 reveals that CD4 + memory T cells harbouring intact provirus show signatures associated with resistance to immune-mediated killing and cell survival.

After initiating antiretroviral therapy (ART), a rapid decline in HIV viral load is followed by a long period of undetectable viremia. Viral outgrowth assay suggests the reservoir continues to decline slowly. Here, we use full-length sequencing to longitudinally study the proviral landscape of four subjects on ART to investigate the selective pressures influencing the dynamics of the treatment-resistant HIV reservoir. We find intact and defective proviruses that contain genetic elements favoring efficient protein expression decrease over time. Moreover, proviruses that lack these genetic elements, yet contain strong donor splice sequences, increase relatively to other defective proviruses, especially among clones. Our work suggests that HIV expression occurs to a significant extent during ART and results in HIV clearance, but this is obscured by the expansion of proviral clones. Paradoxically, clonal expansion may also be enhanced by HIV expression that leads to splicing between HIV donor splice sites and downstream human exons. How HIV reservoirs are shaped over time on antiviral therapy is poorly understood. Here, the authors analyze the dynamics of the HIV reservoir by longitudinal proviral sequencing revealing that HIV reservoir expression can contribute to its clearance and paradoxically even to its persistence.

The Staphylococcus epidermidis CRISPR-Cas system can prevent lytic infection but tolerate lysogenization by temperate phage through a transcription-dependent DNA targeting mechanism. Target discrimination by bacterial immune systems Temperate phages that integrate into the bacterial genome can carry genes that confer a fitness advantage. However, it has been unclear how this potential beneficial interaction is balanced against host defence by CRISPR-Cas immune systems, which defend bacteria against phage infection using Cas nucleases and small RNA guides that provide sequence specificity for cleavage of target sites in the phage genome. Here Luciano Marraffini and colleagues show that the Staphylococcus epidermidis CRISPR-Cas system can prevent lytic phage infection but tolerate lysogenization by temperate phage through a transcription-dependent DNA targeting mechanism. This work expands the repertoire of CRISPR-based immune functions to include a facility for conditional tolerance of foreign elements. A fundamental feature of immune systems is the ability to distinguish pathogenic from self and commensal elements, and to attack the former but tolerate the latter 1 . Prokaryotic CRISPR-Cas immune systems defend against phage infection by using Cas nucleases and small RNA guides that specify one or more target sites for cleavage of the viral genome 2 , 3 . Temperate phages include viruses that can integrate into the bacterial chromosome, and they can carry genes that provide a fitness advantage to the lysogenic host 4 , 5 . However, CRISPR-Cas targeting that relies strictly on DNA sequence recognition provides indiscriminate immunity both to lytic and lysogenic infection by temperate phages 6 —compromising the genetic stability of these potentially beneficial elements altogether. Here we show that the Staphylococcus epidermidis CRISPR-Cas system can prevent lytic infection but tolerate lysogenization by temperate phages. Conditional tolerance is achieved through transcription-dependent DNA targeting, and ensures that targeting is resumed upon induction of the prophage lytic cycle. Our results provide evidence for the functional divergence of CRISPR-Cas systems and highlight the importance of targeting mechanism diversity. In addition, they extend the concept of ‘tolerance to non-self’ to the prokaryotic branch of adaptive immunity.

Combination antiretroviral therapy controls but does not cure HIV-1 infection because a small fraction of cells harbor latent viruses that can produce rebound viremia when therapy is interrupted. The circulating latent virus reservoir has been documented by a variety of methods, most prominently by viral outgrowth assays (VOAs) in which CD4⁺ T cells are activated to produce virus in vitro, or more recently by amplifying proviral near full-length (NFL) sequences from DNA. Analysis of samples obtained in clinical studies in which individuals underwent analytical treatment interruption (ATI), showed little if any overlap between circulating latent viruses obtained from outgrowth cultures and rebound viruses from plasma. To determine whether intact proviruses amplified from DNA are more closely related to rebound viruses than those obtained from VOAs, we assayed 12 individuals who underwent ATI after infusion of a combination of two monoclonal anti–HIV-1 antibodies. A total of 435 intact proviruses obtained by NFL sequencing were compared with 650 latent viruses from VOAs and 246 plasma rebound viruses. Although, intact NFL and outgrowth culture sequences showed similar levels of stability and diversity with 39% overlap, the size of the reservoir estimated from NFL sequencing was larger than and did not correlate with VOAs. Finally, intact proviruses documented by NFL sequencing showed no sequence overlap with rebound viruses; however, they appear to contribute to recombinant viruses found in plasma during rebound.

Chromosomal integration of genome-intact HIV-1 sequences into the host genome creates a reservoir of virally infected cells that persists throughout life, necessitating indefinite antiretroviral suppression therapy. During effective antiviral treatment, the majority of these proviruses remain transcriptionally silent, but mechanisms responsible for viral latency are insufficiently clear. Here, we used matched integration site and proviral sequencing (MIP-Seq), an experimental approach involving multiple displacement amplification of individual proviral species, followed by near-full-length HIV-1 next-generation sequencing and corresponding chromosomal integration site analysis to selectively map the chromosomal positions of intact and defective proviruses in 3 HIV-1-infected individuals undergoing long-term antiretroviral therapy. Simultaneously, chromatin accessibility and gene expression in autologous CD4+ T cells were analyzed by assays for transposase-accessible chromatin using sequencing (ATAC-Seq) and RNA-Seq. We observed that in comparison to proviruses with defective sequences, intact HIV-1 proviruses were enriched for non-genic chromosomal positions and more frequently showed an opposite orientation relative to host genes. In addition, intact HIV-1 proviruses were preferentially integrated in either relative proximity to or increased distance from active transcriptional start sites and to accessible chromatin regions. These studies strongly suggest selection of intact proviruses with features of deeper viral latency during prolonged antiretroviral therapy, and may be informative for targeting the genome-intact viral reservoir.

The Intact Proviral DNA Assay (IPDA) is widely used to quantify genome-intact HIV proviruses in people living with HIV, but viral sequence diversity has been observed to cause assay failures due to primer/probe mismatches. Adapted for subtype C, IPDA-BC is a modified version of the IPDA validated on South African HIV-1 subtype C. India is also impacted by subtype C, but IPDA performance within-subtype across geographical regions is not well studied. We analyzed Indian (IN) and South African (ZA) subtype C sequences in silico, hypothesizing that IPDA-BC may underperform with IN viruses. Primer/probe binding was predicted using three increasingly stringent nucleotide mismatch criteria, whose sensitivity and specificity were evaluated against experimental IPDA outcomes. Phylogenetic analyses confirmed that IN and ZA subtype C sequences form distinct clusters with significant compartmentalization (p < 0.003). Across criteria, up to 6–10% decreases in primer/probe binding were observed in IN versus ZA, with the env forward primer being the most affected. These criteria showed low sensitivity (18–53%) and variable specificity (67–100%) in predicting experimental outcomes. In conclusion, even within subtype, HIV-1 variation across geographical regions may impact IPDA performance, underscoring the need for improved predictive models to guide assay design for global HIV cure research.

Viruses are important ecological, biogeochemical, and evolutionary drivers in every environment. Upon infection, they often cause the lysis of the host cell. However, some viruses exhibit alternative life cycles, such as chronic infections without cell lysis. The nature and the impact of chronic infections in prokaryotic host organisms remains largely unknown. Here, we characterize a novel haloarchaeal virus, Haloferax volcanii pleomorphic virus 1 (HFPV-1), which is currently the only virus infecting the model haloarchaeon Haloferax volcanii DS2, and demonstrate that HFPV-1 and H. volcanii are a great model system to study virus–host interactions in archaea. HFPV-1 is a pleomorphic virus that causes a chronic infection with continuous release of virus particles, but host and virus coexist without cell lysis or the appearance of resistant cells. Despite an only minor impact of the infection on host growth, we uncovered an extensive remodeling of the transcriptional program of the host (up to 1,049 differentially expressed genes). These changes are highlighted by a down-regulation of two endogenous provirus regions in the host genome, and we show that HFPV-1 infection is strongly influenced by a cross-talk between HFPV-1 and one of the proviruses mediated by a superinfection-like exclusion mechanism. Furthermore, HFPV-1 has a surprisingly wide host range among haloarchaea, and purified virus DNA can cause an infection after transformation into the host, making HFPV-1 a candidate for being developed into a genetic tool for a range of so far inaccessible haloarchaea.

BackgroundAntiretroviral therapy (ART) halts HIV-1 replication, decreasing viremia to below the detection limit of clinical assays. However, some individuals experience persistent nonsuppressible viremia (NSV) originating from CD4+ T cell clones carrying infectious proviruses. Defective proviruses represent over 90% of all proviruses persisting during ART and can express viral genes, but whether they can cause NSV and complicate ART management is unknown.MethodsWe undertook an in-depth characterization of proviruses causing NSV in 4 study participants with optimal adherence and no drug resistance. We investigated the impact of the observed defects on 5'-leader RNA properties, virus infectivity, and gene expression. Integration-site specific assays were used to track these proviruses over time and among cell subsets.ResultsClones carrying proviruses with 5'-leader defects can cause persistent NSV up to approximately 103 copies/mL. These proviruses had small, often identical deletions or point mutations involving the major splicing donor (MSD) site and showed partially reduced RNA dimerization and nucleocapsid binding. Nevertheless, they were inducible and produced noninfectious virions containing viral RNA, but lacking envelope.ConclusionThese findings show that proviruses with 5'-leader defects in CD4+ T cell clones can give rise to NSV, affecting clinical care. Sequencing of the 5'-leader can help in understanding failure to completely suppress viremia.FundingOffice of the NIH Director and National Institute of Dental and Craniofacial Research, NIH; Howard Hughes Medical Institute; Johns Hopkins University Center for AIDS Research; National Institute for Allergy and Infectious Diseases (NIAID), NIH, to the PAVE, BEAT-HIV, and DARE Martin Delaney collaboratories.

To combat hostile viruses, bacteria and archaea have evolved a unique antiviral defense system composed of clustered regularly interspaced short palindromic repeats (CRISPRs), together with CRISPR-associated genes (Cas). The CRISPR/Cas9 system develops an adaptive immune resistance to foreign plasmids and viruses by creating site-specific DNA double-stranded breaks (DSBs). Here we adapt the CRISPR/Cas9 system to human cells for intracellular defense against foreign DNA and viruses. Using HIV-1 infection as a model, our results demonstrate that the CRISPR/Cas9 system disrupts latently integrated viral genome and provides long-term adaptive defense against new viral infection, expression and replication in human cells. We show that engineered human-induced pluripotent stem cells stably expressing HIV-targeted CRISPR/Cas9 can be efficiently differentiated into HIV reservoir cell types and maintain their resistance to HIV-1 challenge. These results unveil the potential of the CRISPR/Cas9 system as a new therapeutic strategy against viral infections. The CRISPR/Cas9 system can be used for genome editing. Here, Liao et al . show that the system can be adapted to inhibit HIV expression and replication, excise the integrated HIV genome and provide long-term protection against new infections in human cells, including pluripotent stem cells.