AP-1/c-Fos supports SIV and HIV-1 latency in CD4 T cells infected in vivo

Persistent HIV-1 reservoirs of infected CD4 T cells are a major barrier to HIV-1 cure, although the mechanisms by which they are established and maintained in vivo remain poorly characterized. To elucidate host cell gene expression patterns that govern virus gene expression, we analyzed viral RNA+ (vRNA) CD4 T cells of untreated simian immunodeficiency virus (SIV)-infected macaques by single-cell RNA sequencing. A subset of vRNA+ cells distinguished by spliced and high total vRNA (7-10% of reads) expressed diminished FOS, a component of the Activator protein 1 (AP-1) transcription factor, relative to vRNA-low and -negative cells. Conversely, FOS and JUN, another AP-1 component, were upregulated in HIV DNA+ infected cells compared to uninfected cells from people with HIV-1 on suppressive therapy. Inhibiting c-Fos in latently infected primary cells augmented reactivatable HIV-1 infection. These findings implicate AP-1 in latency establishment and maintenance and as a potential therapeutic target to limit HIV-1 reservoirs.

Timing of initiation of anti-retroviral therapy predicts post-treatment control of SIV replication

One approach to 'functional cure' of HIV infection is to induce durable control of HIV replication after the interruption of antiretroviral therapy (ART). However, the major factors that determine the viral 'setpoint' level after treatment interruption are not well understood. Here we combine data on ART interruption following SIV infection for 124 total animals from 10 independent studies across 3 institutional cohorts to understand the dynamics and predictors of post-treatment viral control. We find that the timing of treatment initiation is an important determinant of both the peak and early setpoint viral levels after treatment interruption. During the first 3 weeks of infection, every day of delay in treatment initiation is associated with a 0.22 log10 copies/ml decrease in post-rebound peak and setpoint viral levels. However, delay in initiation of ART beyond 3 weeks of infection is associated with higher post-rebound setpoint viral levels. For animals treated beyond 3 weeks post-infection, viral load at ART initiation was the primary predictor of post-rebound setpoint viral levels. Potential alternative predictors of post-rebound setpoint viral loads including cell-associated DNA or RNA, time from treatment interruption to rebound, and pre-interruption CD8+ T cell responses were also examined in the studies where these data were available. This analysis suggests that optimal timing of treatment initiation may be an important determinant of post-treatment control of HIV.

Long-acting lenacapavir protects macaques against intravenous challenge with simian-tropic HIV

BACKGROUND

Long-acting subcutaneous lenacapavir (LEN), a first-in-class HIV capsid inhibitor approved by the US FDA for the treatment of multidrug-resistant HIV-1 with twice yearly dosing, is under investigation for HIV-1 pre-exposure prophylaxis (PrEP). We previously derived a simian-tropic HIV-1 clone (stHIV-A19) that encodes an HIV-1 capsid and replicates to high titres in pigtail macaques (PTM), resulting in a nonhuman primate model well-suited for evaluating LEN PrEP in vivo.

METHODS

Lenacapavir potency against stHIV-A19 in PTM peripheral blood mononuclear cells in vitro was determined and subcutaneous LEN pharmacokinetics were evaluated in naïve PTMs in vivo. To evaluate the protective efficacy of LEN PrEP, naïve PTMs received either a single subcutaneous injection of LEN (25 mg/kg, N = 3) or vehicle (N = 4) 30 days before a high-dose intravenous challenge with stHIV-A19, or 7 daily subcutaneous injections of a 3-drug control PrEP regimen starting 3 days before stHIV-A19 challenge (N = 3).

FINDINGS

In vitro, LEN showed potent antiviral activity against stHIV-A19, comparable to its potency against HIV-1. In vivo, subcutaneous LEN displayed sustained plasma drug exposures in PTMs. Following stHIV-A19 challenge, while all vehicle control animals became productively infected, all LEN and 3-drug control PrEP animals were protected from infection.

INTERPRETATION

These findings highlight the utility of the stHIV-A19/PTM model and support the clinical development of long-acting LEN for PrEP in humans.

FUNDING

Gilead Sciences as part of a Cooperative Research and Development Agreement between Gilead Sciences and Frederick National Lab; federal funds from the National Cancer Institute, National Institutes of Health, under Contract No. 75N91019D00024/HHSN261201500003I; NIH grant R01AI078788.

Correction: Tracking the Luminal Exposure and Lymphatic Drainage Pathways of Intravaginal and Intrarectal Inocula Used in Nonhuman Primate Models of HIV Transmission

[This corrects the article DOI: 10.1371/journal.pone.0092830.].

Interleukin-10 contributes to reservoir establishment and persistence in SIV-infected macaques treated with antiretroviral therapy

Interleukin-10 (IL-10) is an immunosuppressive cytokine that signals through STAT3 to regulate T follicular helper (Tfh) cell differentiation and germinal center formation. In SIV-infected macaques, levels of IL-10 in plasma and lymph nodes (LNs) were induced by infection and not normalized with antiretroviral therapy (ART). During chronic infection, plasma IL-10 and transcriptomic signatures of IL-10 signaling were correlated with the cell-associated SIV-DNA content within LN CD4+ memory subsets, including Tfh cells, and predicted the frequency of CD4+ Tfh cells and their cell-associated SIV-DNA content during ART, respectively. In ART-treated rhesus macaques, cells harboring SIV-DNA by DNAscope were preferentially found in the LN B cell follicle in proximity to IL-10. Finally, we demonstrated that the in vivo neutralization of soluble IL-10 in ART-treated, SIV-infected macaques reduced B cell follicle maintenance and, by extension, LN memory CD4+ T cells, including Tfh cells and those expressing PD-1 and CTLA-4. Thus, these data support a role for IL-10 in maintaining a pool of target cells in lymphoid tissue that serve as a niche for viral persistence. Targeting IL-10 signaling to impair CD4+ T cell survival and improve antiviral immune responses may represent a novel approach to limit viral persistence in ART-suppressed people living with HIV.

Transient viral replication during analytical treatment interruptions in SIV infected macaques can alter the rebound-competent viral reservoir

Analytical treatment interruptions (ATIs) of antiretroviral therapy (ART) play a central role in evaluating the efficacy of HIV-1 treatment strategies targeting virus that persists despite ART. However, it remains unclear if ATIs alter the rebound-competent viral reservoir (RCVR), the virus population that persists during ART and from which viral recrudescence originates after ART discontinuation. To assess the impact of ATIs on the RCVR, we used a barcode sequence tagged SIV to track individual viral lineages through a series of ATIs in Rhesus macaques. We demonstrate that transient replication of individual rebounding lineages during an ATI can lead to their enrichment in the RCVR, increasing their probability of reactivating again after treatment discontinuation. These data establish that the RCVR can be altered by uncontrolled replication during ATI.

Antibody-mediated depletion of viral reservoirs is limited in SIV-infected macaques treated early with antiretroviral therapy

The effectiveness of virus-specific strategies, including administered HIV-specific mAbs, to target cells that persistently harbor latent, rebound-competent HIV genomes during combination antiretroviral therapy (cART) has been limited by inefficient induction of viral protein expression. To examine antibody-mediated viral reservoir targeting without a need for viral induction, we used an anti-CD4 mAb to deplete both infected and uninfected CD4+ T cells. Ten rhesus macaques infected with barcoded SIVmac239M received cART for 93 weeks starting 4 days after infection. During cART, 5 animals received 5 to 6 anti-CD4 antibody administrations and CD4+ T cell populations were then allowed 1 year on cART to recover. Despite profound CD4+ T cell depletion in blood and lymph nodes, time to viral rebound following cART cessation was not significantly delayed in anti-CD4-treated animals compared with controls. Viral reactivation rates, determined based on rebounding SIVmac239M clonotype proportions, also were not significantly different in CD4-depleted animals. Notably, antibody-mediated depletion was limited in rectal tissue and negligible in lymphoid follicles. These results suggest that, even if robust viral reactivation can be achieved, antibody-mediated viral reservoir depletion may be limited in key tissue sites.

The Landscape of Persistent Viral Genomes in ART-Treated SIV, SHIV, and HIV-2 Infections

Evaluation of HIV cure strategies is complicated by defective proviruses that persist in ART-treated patients but are irrelevant to cure. Non-human primates (NHP) are essential for testing cure strategies. However, the persisting proviral landscape in ART-treated NHPs is uncharacterized. Here, we describe viral genomes persisting in ART-treated, simian immunodeficiency virus (SIV)-infected NHPs, simian-human immunodeficiency virus (SHIV)-infected NHPs, and humans infected with HIV-2, an SIV-related virus. The landscapes of persisting SIV, SHIV, and HIV-2 genomes are also dominated by defective sequences. However, there was a significantly higher fraction of intact SIV proviral genomes compared to ART-treated HIV-1 or HIV-2 infected humans. Compared to humans with HIV-1, SIV-infected NHPs had more hypermutated genomes, a relative paucity of clonal SIV sequences, and a lower frequency of deleted genomes. Finally, we report an assay for measuring intact SIV genomes which may have value in cure research.

Preferential Small Intestine Homing and Persistence of CD8 T Cells in Rhesus Macaques Achieved by Molecularly Engineered Expression of CCR9 and Reduced Ex Vivo Manipulation

Adoptive cell transfer (ACT) is a powerful experimental approach to directly study T-cell-mediated immunity in vivo In the rhesus macaque AIDS virus model, infusing simian immunodeficiency virus (SIV)-infected animals with CD8 T cells engineered to express anti-SIV T-cell receptor specificities enables direct experimentation to better understand antiviral T-cell immunity in vivo Limiting factors in ACT experiments include suboptimal trafficking to, and poor persistence in, the secondary lymphoid tissues targeted by AIDS viruses. Previously, we redirected CD8 T cells to B-cell follicles by ectopic expression of the CXCR5 homing protein. Here, we modify peripheral blood mononuclear cell (PBMC)-derived CD8 T cells to express the CCR9 chemokine receptor, which induces preferential homing of the engineered cells to the small intestine, a site of intense early AIDS virus replication and pathology in rhesus macaques. Additionally, we increase in vivo persistence and overall systemic distribution of infused CD8 T cells, especially in secondary lymphoid tissues, by minimizing ex vivo culture/manipulation, thereby avoiding the loss of CD28+/CD95+ central memory T cells by differentiation in culture. These proof-of-principle results establish the feasibility of preferentially localizing PBMC-derived CD8 T cells to the small intestine and enables the direct experimental ACT-based assessment of the potential role of the quality and timing of effective antiviral CD8 T-cell responses to inhibit viral infection and subsequent replication in small intestine CD4 T cells. More broadly, these results support the engineered expression of homing proteins to direct CD8 T cells to target tissues as a means for both experimental and potential therapeutic advances in T-cell immunotherapies, including cancer.IMPORTANCEAdoptive cell transfer (ACT) of T cells engineered with antigen-specific effector properties can deliver targeted immune responses against malignancies and infectious diseases. Current T-cell-based therapeutic ACT relies on circulatory distribution to deliver engineered T cells to their targets, an approach which has proven effective for some leukemias but provided only limited efficacy against solid tumors. Here, engineered expression of the CCR9 homing receptor redirected CD8 T cells to the small intestine in rhesus macaque ACT experiments. Targeted homing of engineered T-cell immunotherapies holds promise to increase the effectiveness of adoptively transferred cells in both experimental and clinical settings.

Clonal expansion of SIV-infected cells in macaques on antiretroviral therapy is similar to that of HIV-infected cells in humans

Clonal expansion of HIV infected cells plays an important role in the formation and persistence of the reservoir that allows the virus to persist, in DNA form, despite effective antiretroviral therapy. We used integration site analysis to ask if there is a similar clonal expansion of SIV infected cells in macaques. We show that the distribution of HIV and SIV integration sites in vitro is similar and that both viruses preferentially integrate in many of the same genes. We obtained approximately 8000 integration sites from blood samples taken from SIV-infected macaques prior to the initiation of ART, and from blood, spleen, and lymph node samples taken at necropsy. Seven clones were identified in the pre-ART samples; one persisted for a year on ART. An additional 100 clones were found only in on-ART samples; a number of these clones were found in more than one tissue. The timing and extent of clonal expansion of SIV-infected cells in macaques and HIV-infected cells in humans is quite similar. This suggests that SIV-infected macaques represent a useful model of the clonal expansion of HIV infected cells in humans that can be used to evaluate strategies intended to control or eradicate the viral reservoir.

TLR7 agonist administration to SIV-infected macaques receiving early initiated cART does not induce plasma viremia

Reduction/elimination of HIV-1 reservoirs that persist despite combination antiretroviral therapy (cART) will likely require induction of viral expression by residual infected cells and enhanced clearance of these cells. TLR7 agonists have potential to mediate these activities. We evaluated immunologic and virologic effects of repeated doses of the TLR7 agonist GS-9620 in SIV-infected rhesus macaques receiving cART, which was initiated at 13 days after infection and was continued for 75 weeks prior to GS-9620 administration. During cART, GS-9620 induced transient upregulation of IFN-stimulated genes in blood and tissues, increases in plasma cytokines, and changes in immune cell population activation and phenotypes but did not result in measurable increases in plasma viremia or viral RNA-to-viral DNA ratio in PBMCs or tissues nor decreases in viral DNA in PBMC or tissues. SIV-specific CD8+ T cell responses, negligible prior to GS-9620 treatment, were not measurably boosted by treatment; a second course of GS-9620 administration overlapping with later cART discontinuation was associated with increased CD8+ T cell responses during viral recrudescence. These results confirm and extend evidence for GS-9620-mediated enhancement of antiviral immune responses in SIV-infected macaques but suggest that GS-9620-mediated viral induction may depend critically on the timing of initiation and duration of cART and resulting characteristics of viral reservoirs.

Defining early SIV replication and dissemination dynamics following vaginal transmission

Understanding HIV transmission is critical to guide the development of prophylactic interventions to prevent infection. We used a nonhuman primate (NHP) model with a synthetic swarm of sequence-tagged variants of SIVmac239 ("SIVmac239X") and scheduled necropsy during primary infection (days 3 to 14 after challenge) to study viral dynamics and host responses to the establishment and dissemination of infection following vaginal challenge. We demonstrate that local replication was initiated at multiple sites within the female genital tract (FGT), with each site having multiple viral variants. Local replication and spread in the FGT preceded lymphatic dissemination. Innate viral restriction factors were observed but appeared to follow viral replication and were ineffective at blocking initial viral establishment and dissemination. However, major delays were observed in time to dissemination in animals and among different viral variants within the same animal. It will be important to assess how phenotypic differences affect early viral dynamics.

Short Communication: Ultrasensitive Immunoassay for Assessing Residual Simian-Tropic HIV in Nonhuman Primate Models of AIDS

Persistence of replication-competent viral reservoirs during infection remains a barrier to HIV cure, despite the ability of combination antiretroviral therapy (cART) to effectively suppress viral replication. Simian-tropic HIV (stHIV) is a minimally chimeric HIV-1 that is comprised of 94% HIV-1 sequence, contains HIV-1 drug and immunologic targets, and is capable of replicating to high levels and causing authentic HIV-like pathogenesis leading to clinical AIDS in pigtail macaques. Suppression of stHIV replication by cART provides a model for study of viral reservoirs and HIV-specific intervention strategies targeting them. Accurate measurement of reservoir size is crucial for evaluating the effect of any such intervention strategies. Although there are a variety of assays that allow for indirect monitoring of viral reservoir size ex vivo, they each quantify a different aspect of viral reservoirs, and are characterized by conceptual and/or technical limitations. Measurement of viral protein in ex vivo cell culture assays captures the immunologically relevant viral-antigen producing component of the reservoir. This study demonstrates the utility of an ultrasensitive digital HIV Gag p24 immunoassay, which enabled earlier, and more sensitive detection of viral protein in culture supernatants from stimulated CD4+ T cells from stHIV-infected pigtail macaques receiving cART compared with conventional enzyme-linked immunosorbent assay. Protein measurements were highly correlated with cell-free stHIV RNA, as measured by quantitative reverse transcription polymerase chain reaction. This ultrasensitive p24 assay can be used to complement other reservoir measurement tools to assess ongoing replication and reactivation of infectious virus from reservoirs in stHIV-infected pigtail macaques.

The SIV Envelope Glycoprotein, Viral Tropism, and Pathogenesis: Novel Insights from Nonhuman Primate Models of AIDS

BACKGROUND

Cellular tropism of human immunodeficiency virus (HIV-1) is closely linked to interactions between the viral envelope glycoprotein (Env) with CD4 and chemokine receptor family members, CCR5 and CXCR4. This interaction plays a key role in determining anatomic sites that are infected in vivo and the cascade of early and late events that result in chronic immune activation, immunosuppression and ultimately, AIDS. CD4+ T cells are critical to adaptive immune responses, and their early and rapid infection in gut lamina propria and secondary lymphoid tissues in susceptible hosts likely contributes to viral persistence and progression to disease. CD4+ macrophages are also infected, although their role in HIV-1 pathogenesis is more controversial.

METHODS

Pathogenic infection by simian immunodeficiency viruses (SIV) in Asian macaques as models of HIV-1 infection has enabled the impact of cellular tropism on pathogenesis to be directly probed. This review will highlight examples in which experimental interventions during SIV infection or the introduction of viral mutations have altered cellular tropism and, subsequently, pathogenesis.

RESULTS

Alterations to the interaction of Env and its cellular receptors has been shown to result in changes to CD4 dependence, coreceptor specificity, and viral tropism for gut CD4+ T cells and macrophages.

CONCLUSION

Collectively, these findings have yielded novel insights into the critical role of the viral Env and tropism as a driver of pathogenesis and host control and have helped to identify new areas for targeted interventions in therapy and prevention of HIV-1 infection.

Ultrasensitive Immunoassay for Simian Immunodeficiency Virus p27CA

Although effective for suppressing viral replication, combination antiretroviral treatment (cART) does not represent definitive therapy for HIV infection due to persistence of replication-competent viral reservoirs. The advent of effective cART regimens for simian immunodeficiency virus (SIV)-infected nonhuman primates (NHP) has enabled the development of relevant models for studying viral reservoirs and intervention strategies targeting them. Viral reservoir measurements are crucial for such studies but are problematic. Quantitative polymerase chain reaction (PCR) assays overestimate the size of the replication competent viral reservoir, as not all detected viral genomes are intact. Quantitative viral outgrowth assays measure replication competence, but they suffer from limited precision and dynamic range, and require large numbers of cells. Ex vivo virus induction assays to detect cells harboring inducible virus represent an experimental middle ground, but detection of inducible viral RNA in such assays does not necessarily indicate production of virions, while detection of more immunologically relevant viral proteins, including p27^CA, by conventional enzyme-linked immunosorbent assays (ELISA) lacks sensitivity. An ultrasensitive digital SIV Gag p27 assay was developed, which is 100-fold more sensitive than a conventional ELISA. In ex vivo virus induction assays, the quantification of SIV Gag p27 produced by stimulated CD4+ T cells from rhesus macaques receiving cART enabled earlier and more sensitive detection than conventional ELISA-based approaches and was highly correlated with SIV RNA, as measured by quantitative reverse transcription PCR. This ultrasensitive p27 assay provides a new tool to assess ongoing replication and reactivation of infectious virus from reservoirs in SIV-infected NHP.

HLA-C downregulation by HIV-1 adapts to host HLA genotype

HIV-1 can downregulate HLA-C on infected cells, using the viral protein Vpu, and the magnitude of this downregulation varies widely between primary HIV-1 variants. The selection pressures that result in viral downregulation of HLA-C in some individuals, but preservation of surface HLA-C in others are not clear. To better understand viral immune evasion targeting HLA-C, we have characterized HLA-C downregulation by a range of primary HIV-1 viruses. 128 replication competent viral isolates from 19 individuals with effective anti-retroviral therapy, show that a substantial minority of individuals harbor latent reservoir virus which strongly downregulates HLA-C. Untreated infections display no change in HLA-C downregulation during the first 6 months of infection, but variation between viral quasispecies can be detected in chronic infection. Vpu molecules cloned from plasma of 195 treatment naïve individuals in chronic infection demonstrate that downregulation of HLA-C adapts to host HLA genotype. HLA-C alleles differ in the pressure they exert for downregulation, and individuals with higher levels of HLA-C expression favor greater viral downregulation of HLA-C. Studies of primary and mutant molecules identify 5 residues in the transmembrane region of Vpu, and 4 residues in the transmembrane domain of HLA-C, which determine interactions between Vpu and HLA. The observed adaptation of Vpu-mediated downregulation to host genotype indicates that HLA-C alleles differ in likelihood of mediating a CTL response that is subverted by viral downregulation, and that preservation of HLA-C expression is favored in the absence of these responses. Finding that latent reservoir viruses can downregulate HLA-C could have implications for HIV-1 cure therapy approaches in some individuals.

Gammaherpesvirus infection and malignant disease in rhesus macaques experimentally infected with SIV or SHIV

Human gammaherpesviruses are associated with malignancies in HIV infected individuals; in macaques used in non-human primate models of HIV infection, gammaherpesvirus infections also occur. Limited data on prevalence and tumorigenicity of macaque gammaherpesviruses, mostly cross-sectional analyses of small series, are available. We comprehensively examine all three-rhesus macaque gammaherpesviruses -Rhesus rhadinovirus (RRV), Rhesus Lymphocryptovirus (RLCV) and Retroperitoneal Fibromatosis Herpesvirus (RFHV) in macaques experimentally infected with Simian Immunodeficiency Virus or Simian Human Immunodeficiency Virus (SIV/SHIV) in studies spanning 15 years at the AIDS and Cancer Virus Program of the Frederick National Laboratory for Cancer Research. We evaluated 18 animals with malignancies (16 lymphomas, one fibrosarcoma and one carcinoma) and 32 controls. We developed real time quantitative PCR assays for each gammaherpesvirus DNA viral load (VL) in malignant and non-tumor tissues; we also characterized the tumors using immunohistochemistry and in situ hybridization. Furthermore, we retrospectively quantified gammaherpesvirus DNA VL and SIV/SHIV RNA VL in longitudinally-collected PBMCs and plasma, respectively. One or more gammaherpesviruses were detected in 17 tumors; generally, one was predominant, and the relevant DNA VL in the tumor was very high compared to surrounding tissues. RLCV was predominant in tumors resembling diffuse large B cell lymphomas; in a Burkitt-like lymphoma, RRV was predominant; and in the fibrosarcoma, RFHV was predominant. Median RRV and RLCV PBMC DNA VL were significantly higher in cases than controls; SIV/SHIV VL and RLCV VL were independently associated with cancer. Local regressions showed that longitudinal VL patterns in cases and controls, from SIV infection to necropsy, differed for each gammaherpesvirus: while RFHV VL increased only slightly in all animals, RLCV and RRV VL increased significantly and continued to increase steeply in cases; in controls, VL flattened. In conclusion, the data suggest that gammaherpesviruses may play a significant role in tumorogenesis in macaques infected with immunodeficiency viruses.

Elevated HLA-A expression impairs HIV control through inhibition of NKG2A-expressing cells

The highly polymorphic human leukocyte antigen (HLA) locus encodes cell surface proteins that are critical for immunity. HLA-A expression levels vary in an allele-dependent manner, diversifying allele-specific effects beyond peptide-binding preference. Analysis of 9763 HIV-infected individuals from 21 cohorts shows that higher HLA-A levels confer poorer control of HIV. Elevated HLA-A expression provides enhanced levels of an HLA-A–derived signal peptide that specifically binds and determines expression levels of HLA-E, the ligand for the inhibitory NKG2A natural killer (NK) cell receptor. HLA-B haplotypes that favor NKG2A-mediated NK cell licensing (i.e., education) exacerbate the deleterious effect of high HLA-A on HIV control, consistent with NKG2A-mediated inhibition impairing NK cell clearance of HIV-infected targets. Therapeutic blockade of HLA-E:NKG2A interaction may yield benefit in HIV disease.

A single gp120 residue can affect HIV-1 tropism in macaques

Species-dependent variation in proteins that aid or limit virus replication determines the ability of lentiviruses to jump between host species. Identifying and overcoming these differences facilitates the development of animal models for HIV-1, including models based on chimeric SIVs that express HIV-1 envelope (Env) glycoproteins, (SHIVs) and simian-tropic HIV-1 (stHIV) strains. Here, we demonstrate that the inherently poor ability of most HIV-1 Env proteins to use macaque CD4 as a receptor is improved during adaptation by virus passage in macaques. We identify a single amino acid, A281, in HIV-1 Env that consistently changes during adaptation in macaques and affects the ability of HIV-1 Env to use macaque CD4. Importantly, mutations at A281 do not markedly affect HIV-1 Env neutralization properties. Our findings should facilitate the design of HIV-1 Env proteins for use in non-human primate models and thus expedite the development of clinically relevant reagents for testing interventions against HIV-1.

Understanding the interactions between viruses and their hosts allows manipulation of primate lentiviruses and the generation of better animal models for HIV/AIDS. Species-dependent differences in cellular proteins that play key roles in virus replication, such as the primary HIV-1 receptor CD4, can limit virus tropism. Our data reveal how adaptation in macaques improves the ability of HIV-1 envelope glycoproteins to use macaque CD4. Moreover, we identify a single amino acid in the HIV-1 envelope glycoprotein CD4 binding site that improves macaque CD4 use by most HIV-1 envelope proteins tested and allows viruses expressing these proteins to replicate efficiently in macaque cells without compromising their sensitivity to various antibodies. These findings should facilitate the development and preclinical evaluation of HIV-1 Env directed prophylactic and therapeutic interventions.

HIV-1 Vpu Mediates HLA-C Downregulation

Many pathogens evade cytotoxic T lymphocytes (CTLs) by downregulating HLA molecules on infected cells, but the loss of HLA can trigger NK cell-mediated lysis. HIV-1 is thought to subvert CTLs while preserving NK cell inhibition by Nef-mediated downregulation of HLA-A and -B but not HLA-C molecules. We find that HLA-C is downregulated by most primary HIV-1 clones, including transmitted founder viruses, in contrast to the laboratory-adapted NL4-3 virus. HLA-C reduction is mediated by viral Vpu and reduces the ability of HLA-C restricted CTLs to suppress viral replication in CD4+ cells in vitro. HLA-A/B are unaffected by Vpu, and primary HIV-1 clones vary in their ability to downregulate HLA-C, possibly in response to whether CTLs or NK cells dominate immune pressure through HLA-C. HIV-2 also suppresses HLA-C expression through distinct mechanisms, underscoring the immune pressure HLA-C exerts on HIV. This viral immune evasion casts new light on the roles of CTLs and NK cells in immune responses against HIV.

Genetically-barcoded SIV facilitates enumeration of rebound variants and estimation of reactivation rates in nonhuman primates following interruption of suppressive antiretroviral therapy

HIV and SIV infection dynamics are commonly investigated by measuring plasma viral loads. However, this total viral load value represents the sum of many individual infection events, which are difficult to independently track using conventional sequencing approaches. To overcome this challenge, we generated a genetically tagged virus stock (SIVmac239M) with a 34-base genetic barcode inserted between the vpx and vpr accessory genes of the infectious molecular clone SIVmac239. Next-generation sequencing of the virus stock identified at least 9,336 individual barcodes, or clonotypes, with an average genetic distance of 7 bases between any two barcodes. In vitro infection of rhesus CD4+ T cells and in vivo infection of rhesus macaques revealed levels of viral replication of SIVmac239M comparable to parental SIVmac239. After intravenous inoculation of 2.2x10⁵ infectious units of SIVmac239M, an average of 1,247 barcodes were identified during acute infection in 26 infected rhesus macaques. Of the barcodes identified in the stock, at least 85.6% actively replicated in at least one animal, and on average each barcode was found in 5 monkeys. Four infected animals were treated with combination antiretroviral therapy (cART) for 82 days starting on day 6 post-infection (study 1). Plasma viremia was reduced from >10⁶ to <15 vRNA copies/mL by the time treatment was interrupted. Virus rapidly rebounded following treatment interruption and between 87 and 136 distinct clonotypes were detected in plasma at peak rebound viremia. This study confirmed that SIVmac239M viremia could be successfully curtailed with cART, and that upon cART discontinuation, rebounding viral variants could be identified and quantified. An additional 6 animals infected with SIVmac239M were treated with cART beginning on day 4 post-infection for 305, 374, or 482 days (study 2). Upon treatment interruption, between 4 and 8 distinct viral clonotypes were detected in each animal at peak rebound viremia. The relative proportions of the rebounding viral clonotypes, spanning a range of 5 logs, were largely preserved over time for each animal. The viral growth rate during recrudescence and the relative abundance of each rebounding clonotype were used to estimate the average frequency of reactivation per animal. Using these parameters, reactivation frequencies were calculated and ranged from 0.33–0.70 events per day, likely representing reactivation from long-lived latently infected cells. The use of SIVmac239M therefore provides a powerful tool to investigate SIV latency and the frequency of viral reactivation after treatment interruption.

Elucidation of HIV dynamics is essential for a thorough understanding of viral transmission, therapeutic interventions, pathogenesis, and immune evasion. The complex dynamics of reservoir establishment and viral recrudescence upon therapy removal present the primary obstacles to developing a functional cure. We sought to develop a virus model system for use in nonhuman primates that allows for the genetic discrimination of nearly 10,000 otherwise isogenic clones. This “synthetic swarm” adds a genetic component to viral dynamics where individual viral lineages can be tracked and monitored during infection. Here we utilized this model to identify the dynamics of viral reservoir establishment and rebound. We found that after 300 or more days of therapy, between 4 and 8 distinct viral lineages could be detected upon therapeutic intervention. Using the relative proportion of each distinct genetic barcoded virus and the overall viral load curve, we could estimate the time and rate of reactivation from latency. On average, we found 1 reactivation event every 2 days with reactivation of the first rebounding variant within days of therapeutic interruption. This virus model will be useful for testing various approaches to reduce the latent viral reservoir and to molecularly track viral dynamics in all stages of infection.

Derivation and Characterization of a CD4-Independent, Non-CD4-Tropic Simian Immunodeficiency Virus

CD4 tropism is conserved among all primate lentiviruses and likely contributes to viral pathogenesis by targeting cells that are critical for adaptive antiviral immune responses. Although CD4-independent variants of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) have been described that can utilize the coreceptor CCR5 or CXCR4 in the absence of CD4, these viruses typically retain their CD4 binding sites and still can interact with CD4. We describe the derivation of a novel CD4-independent variant of pathogenic SIVmac239, termed iMac239, that was used to derive an infectious R5-tropic SIV lacking a CD4 binding site. Of the seven mutations that differentiate iMac239 from wild-type SIVmac239, a single change (D178G) in the V1/V2 region was sufficient to confer CD4 independence in cell-cell fusion assays, although other mutations were required for replication competence. Like other CD4-independent viruses, iMac239 was highly neutralization sensitive, although mutations were identified that could confer CD4-independent infection without increasing its neutralization sensitivity. Strikingly, iMac239 retained the ability to replicate in cell lines and primary cells even when its CD4 binding site had been ablated by deletion of a highly conserved aspartic acid at position 385, which, for HIV-1, plays a critical role in CD4 binding. iMac239, with and without the D385 deletion, exhibited an expanded host range in primary rhesus peripheral blood mononuclear cells that included CCR5(+) CD8(+) T cells. As the first non-CD4-tropic SIV, iMac239-ΔD385 will afford the opportunity to directly assess the in vivo role of CD4 targeting on pathogenesis and host immune responses.

IMPORTANCE

CD4 tropism is an invariant feature of primate lentiviruses and likely plays a key role in pathogenesis by focusing viral infection onto cells that mediate adaptive immune responses and in protecting virions attached to cells from neutralizing antibodies. Although CD4-independent viruses are well described for HIV and SIV, these viruses characteristically retain their CD4 binding site and can engage CD4 if available. We derived a novel CD4-independent, CCR5-tropic variant of the pathogenic molecular clone SIVmac239, termed iMac239. The genetic determinants of iMac239's CD4 independence provide new insights into mechanisms that underlie this phenotype. This virus remained replication competent even after its CD4 binding site had been ablated by mutagenesis. As the first truly non-CD4-tropic SIV, lacking the capacity to interact with CD4, iMac239 will provide the unique opportunity to evaluate SIV pathogenesis and host immune responses in the absence of the immunomodulatory effects of CD4(+) T cell targeting and infection.

Short Communication: Comparative Evaluation of Coformulated Injectable Combination Antiretroviral Therapy Regimens in Simian Immunodeficiency Virus-Infected Rhesus Macaques

The use of nonhuman primate (NHP) models to study persistent residual virus and viral eradication strategies in combination antiretroviral therapy (cART)-treated individuals requires regimens that effectively suppress SIV replication to clinically relevant levels in macaques. We developed and evaluated two novel cART regimens in SIVmac239-infected rhesus macaques: (1) a "triple regimen" containing the nucleo(s/t)ide reverse transcriptase inhibitors emtricitabine (FTC) and tenofovir disoproxil fumarate [TDF, prodrug of tenofovir (TFV, PMPA)] with the integrase strand transfer inhibitor dolutegravir (DTG) (n = 3), or (2) a "quad regimen" containing the same three drugs plus the protease inhibitor darunavir (DRV) (n = 3), with each regimen coformulated for convenient administration by a single daily subcutaneous injection. Plasma drug concentrations were consistent across animals within the triple and quad regimen-treated groups, although DTG levels were lower in the quad regimen animals. Time to achieve plasma viral loads stably <30 viral RNA copies/ml ranged from 12 to 20 weeks of treatment between animals, and viral loads <30 viral RNA copies/ml plasma were maintained through 40 weeks of follow-up on cART. Notably, although we show virologic suppression and development of viral resistance in a separate cohort of SIV-infected animals treated with oral DRV monotherapy, the addition of DRV in the quad regimen did not confer an apparent virologic benefit during early treatment, hence the quad regimen-treated animals were switched to the triple regimen after 4 weeks. This coformulated triple cART regimen can be safely, conveniently, and sustainably administered to durably suppress SIV replication to clinically relevant levels in rhesus macaques.

Experimental colitis in SIV-uninfected rhesus macaques recapitulates important features of pathogenic SIV infection

Mucosal damage to the gastrointestinal (GI) tract with resulting microbial translocation is hypothesized to significantly contribute to the heightened and persistent chronic inflammation and immune activation characteristic to HIV infection. Here we employ a non-human primate model of chemically induced colitis in SIV-uninfected rhesus macaques that we developed using dextran sulfate sodium (DSS), to directly test this hypothesis. DSS treatment results in GI barrier damage with associated microbial translocation, inflammation and immune activation. The progression and severity of colitis are longitudinally monitored by a magnetic resonance imaging approach. DSS treatment of SIV-infected African green monkeys, a natural host species for SIV that does not manifest GI tract damage or chronic immune activation during infection, results in colitis with elevated levels of plasma SIV RNA, sCD14, LPS, CRP and mucosal CD4+ T-cell loss. Together these results support the hypothesis that GI tract damage leading to local and systemic microbial translocation, and associated immune activation, are important determinants of AIDS pathogenesis.

HIV-1 infection in humans and SIV infection in rhesus macaques are associated with mucosal damage to the gastrointestinal tract, microbial translocation and chronic immune activation. Here the authors develop a non-human primate DSS colitis model that recapitulates these aspects of the disease in uninfected rhesus macaques.

Selection of unadapted, pathogenic SHIVs encoding newly transmitted HIV-1 envelope proteins

Infection of macaques with chimeric viruses based on SIVMAC but expressing the HIV-1 envelope (Env) glycoproteins (SHIVs) remains the most powerful model for evaluating prevention and therapeutic strategies against AIDS. Unfortunately, only a few SHIVs are currently available. Furthermore, their generation has required extensive adaptation of the HIV-1 Env sequences in macaques so they may not accurately represent HIV-1 Env proteins circulating in humans, potentially limiting their translational utility. We developed a strategy for generating large numbers of SHIV constructs expressing Env proteins from newly transmitted HIV-1 strains. By inoculating macaques with cocktails of multiple SHIV variants, we selected SHIVs that can replicate and cause AIDS-like disease in immunologically intact rhesus macaques without requiring animal-to-animal passage. One of these SHIVs could be transmitted mucosally. We demonstrate the utility of the SHIVs generated by this method for evaluating neutralizing antibody administration as a protection against mucosal SHIV challenge.

Generation and characterization of a SIVmac239 clone corrected at four suboptimal nucleotides

Background

SIVmac239 is a commonly used virus in non-human primate models of HIV transmission and pathogenesis. Previous studies identified four suboptimal nucleotides in the SIVmac239 genome, which putatively inhibit its replicative capacity. Since all four suboptimal changes revert to the optimal nucleotide consensus sequence during viral replication in vitro and in vivo, we sought to eliminate the variability of generating these mutations de novo and increase the overall consistency of viral replication by introducing the optimal nucleotides directly to the infectious molecular clone.

Results

Using site directed mutagenesis of the full-length/nef-open SIVmac239 clone, we reverted all four nucleotides to the consensus/optimal base to generate SIVmac239Opt and subsequently tested its infectivity and replicative capacity in vitro and in vivo. In primary and cell line cultures, we observed that the optimized virus displayed consistent modest but not statistically significant increases in replicative kinetics compared to wild type. In vivo, SIVmac239Opt replicated to high peak titers with an average of 1.2 × 10⁸ viral RNA copies/ml at day 12 following intrarectal challenge, reaching set-point viremia of 1.2 × 10⁶ viral RNA copies/ml by day 28. Although the peak and set point viremia means were not statistically different from the original “wild type” SIVmac239, viral load variation at set point was greater for SIVmac239WT compared to SIVmac239Opt (p = 0.0015) demonstrating a greater consistency of the optimized virus. Synonymous mutations were added to the integrase gene of SIVmac239Opt to generate a molecular tag consisting of ten genetically distinguishable viral variants referred to as SIVmac239OptX (Del Prete et al., J Virol. doi:10.1128/JVI.01026-14, 2014). Replication dynamics in vitro of these optimized clones were not statistically different from the parental clones. Interestingly, the consistently observed rapid reversion of the primer binding site suboptimal nucleotide is not due to viral RT error but is changed post-integration of a mismatched base via host proofreading mechanisms.

Conclusions

Overall, our results demonstrate that SIVmac239Opt is a functional alternative to parental SIVmac239 with marginally faster replication dynamics and with increased replication uniformity providing a more consistent and reproducible infection model in nonhuman primates.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0175-3) contains supplementary material, which is available to authorized users.

Relative expression levels of the HLA class-I proteins in normal and HIV-infected cells

The expression level of HLA class-I proteins is known to influence pathological outcomes: pathogens downregulate HLA to evade host immune responses, host inflammatory reactions upregulate HLA, and differences among people with regard to the steady-state expression levels of HLA associate with disease susceptibility. Yet precise quantification of relative expression levels of the various HLA loci is difficult because of the tremendous polymorphism of HLA. We report relative expression levels of HLA-A, HLA-B, HLA-C, and HLA-E proteins for the specific haplotype A*02:01, B*44:02, C*05:01, which were characterized using two independent methods based on flow cytometry and mass spectrometry. PBLs from normal donors showed that HLA-A and HLA-B proteins are expressed at similar levels, which are 13-18 times higher than HLA-C by flow cytometry and 4-5 times higher than HLA-C by mass spectrometry; these differences may reflect variation in the conformation or location of proteins detected. HLA-E was detected at a level 25 times lower than that of HLA-C by mass spectrometry. Primary CD4(+) T cells infected with HIV in vitro were also studied because HIV downregulates selective HLA types. HLA-A and HLA-B were reduced on HIV-infected cells by a magnitude that varied between cells in an infected culture. Averaging all infected cells from an individual showed HLA-A to be 1-3 times higher and HLA-B to be 2-5 times higher than HLA-C by flow cytometry. These results quantify substantial differences in expression levels of the proteins from different HLA loci, which are very likely physiologically significant on both uninfected and HIV-infected cells.

Potent restriction of HIV-1 and SIVmac239 replication by African green monkey TRIM5α

Background

The TRIM5α protein is a principal restriction factor that contributes to an HIV-1 replication block in rhesus macaque CD4⁺ T cells by preventing reverse transcription. HIV-1 restriction is induced in human CD4⁺ T cells by expression of rhesus TRIM5α as well as those of other old world monkeys. While TRIM5α restriction has been extensively studied in single-round infection assays, fewer studies have examined restriction after extended viral replication.

Results

To examine TRIM5α restriction of replication, we studied the ability of TRIM5α proteins from African green monkey (AgmTRIM5α) and gorilla (gorTRIM5α) to restrict HIV-1 and SIV_mac239 replication. These xenogeneic TRIM5α genes were transduced into human Jurkat-CCR5 cells (JR5), which were then exposed to HIV-1 or SIV_mac239. In our single-round infection assays, AgmTRIM5α showed a relatively modest 4- to 10-fold restriction of HIV-1 and SIV_mac239, while gorTRIM5α produced a 2- and 3-fold restriction of HIV-1 and SIV_mac239, respectively, consistent with the majority of previously published single-round studies. To assess the impact of these modest effects on infection, we tested restriction in replication systems initiated with either cell-free or cell-to-cell challenges. AgmTRIM5α powerfully restricted both HIV-1 and SIV_mac239 replication 14 days after cell-free infection, with a ≥ 3-log effect. Moreover, expression of AgmTRIM5α restricted HIV-1 and SIV_mac239 replication by 2-logs when co-cultured with infected JR5 cells for 12 days. In contrast, neither expression of gorTRIM5α nor rhesus TRIM5α induced significant resistance when co-cultured with infected cells. Follow up experiments showed that the observed differences between replication and infection were not due to assembly defects as xenogeneic TRIM5α expression had no effect on either virion production or specific infectivity.

Conclusions

Our results indicate that AgmTRIM5α has a much greater effect on extended replication than on any single infection event, suggesting that AgmTRIM5α restriction acts cumulatively, building up over many rounds of replication. Furthermore, AgmTRIM5α was able to potently restrict both HIV-1 and SIV replication in a cell-to-cell infection challenge. Thus, AgmTRIM5α is unique among the TRIM5α species tested to date, being able to restrict even at the high multiplicities of infection presented by mixed culture with nonrestrictive infected cells.

Maturation of the HIV-1 core by a non-diffusional phase transition

The formation of the HIV-1 core is the final step in the viral maturation pathway, resulting in the formation of infectious virus. Most current models for HIV-1 core formation suggest that, upon proteolytic cleavage from the immature Gag, capsid (CA) dissociates into the viral interior before reforming into the core. Here we present evidence for an alternate view of core formation by taking advantage of our serendipitous observation of large membrane-enclosed structures in HIV-1 supernatants from infected cells. Cryo-electron tomographic studies show that these structures, which contain ordered arrays of what is likely the membrane-associated matrix protein, contain multiple cores that can be captured at different stages of maturation. Our studies suggest that HIV maturation involves a non-diffusional phase transition in which the detaching layer of the cleaved CA lattice is gradually converted into a roll that ultimately forms the surface of the mature conical core.

Current models of HIV maturation involve the diffusion of the cleaved capsid protein into the viral core. Here, Frank et al. use cryo-electron tomography to characterize HIV assembly intermediates, and propose a novel maturation mechanism involving a non-diffusional phase transition.

HIV-1-induced AIDS in monkeys

Primate lentiviruses exhibit narrow host tropism, reducing the occurrence of zoonoses but also impairing the development of optimal animal models of AIDS. To delineate the factors limiting cross-species HIV-1 transmission, we passaged a modified HIV-1 in pigtailed macaques that were transiently depleted of CD8(+) cells during acute infection. During adaptation over four passages in macaques, HIV-1 acquired the ability to antagonize the macaque restriction factor tetherin, replicated at progressively higher levels, and ultimately caused marked CD4(+) T cell depletion and AIDS-defining conditions. Transient treatment with an antibody to CD8 during acute HIV-1 infection caused rapid progression to AIDS, whereas untreated animals exhibited an elite controller phenotype. Thus, an adapted HIV-1 can cause AIDS in macaques, and stark differences in outcome can be determined by immunological perturbations during early infection.

Tracking the luminal exposure and lymphatic drainage pathways of intravaginal and intrarectal inocula used in nonhuman primate models of HIV transmission

Over 80% of sexual HIV-1 transmissions originate from a single viral variant, but the underlying basis for this transmission bottleneck remains to be elucidated. Nonhuman primate models of mucosal virus transmission allow opportunities to gain insight into the basis of this mucosal bottleneck. We used simulated inocula consisting of either non-infectious vital dye or contrast dye with non-invasive magnetic resonance imaging (MRI) to visualize mucosal exposure and passive lymphatic drainage patterns following vaginal and rectal exposures in Indian origin rhesus macaques. Results revealed a limited overall distance of dye coverage from the anal verge following 1 ml (n = 8) intrarectally administered, which greatly increased with a 3 ml (n = 8) volume. Intravaginal dye exposure using 2 ml revealed complete coverage of the mucosa of the vagina and ectocervix, however dye was not detectable in the endocervix, uterus, fallopian tubes or ovaries in nuliparous sexually mature rhesus macaques (n = 9). In addition, following submucosal and intranodal injections of vital dye or MRI contrast dye in the rectum (n = 9), or distal and proximal vagina (n = 4), the lymphatic drainage pathways were identified as first the internal then common iliac chain followed by para-aortic lymph nodes. Drainage from the distal descending colon (n = 8) was via the para-colonic lymph nodes followed by the inferior mesenteric and para-aortic lymph nodes. Analysis after vaginal challenge with infectious SIVmac239 followed by euthanasia at day 3 revealed a pattern of viral dissemination consistent with the imaging results. These results provide insights into potential patterns of viral dissemination that can help guide efforts to better elucidate the earliest events of virus transmission and potential intervention strategies.

Comparative characterization of transfection- and infection-derived simian immunodeficiency virus challenge stocks for in vivo nonhuman primate studies

Simian immunodeficiency virus (SIV) stocks for in vivo nonhuman primate models of AIDS are typically generated by transfection of 293T cells with molecularly cloned viral genomes or by expansion in productively infected T cells. Although titers of stocks are determined for infectivity in vitro prior to in vivo inoculation, virus production methods may differentially affect stock features that are not routinely analyzed but may impact in vivo infectivity, mucosal transmissibility, and early infection events. We performed a detailed analysis of nine SIV stocks, comprising five infection-derived SIVmac251 viral swarm stocks and paired infection- and transfected-293T-cell-derived stocks of both SIVmac239 and SIVmac766. Representative stocks were evaluated for (i) virus content, (ii) infectious titer, (iii) sequence diversity and polymorphism frequency by single-genome amplification and 454 pyrosequencing, (iv) virion-associated Env content, and (v) cytokine and chemokine content by 36-plex Luminex analysis. Regardless of production method, all stocks had comparable particle/infectivity ratios, with the transfected-293T stocks possessing the highest overall virus content and infectivity titers despite containing markedly lower levels of virion-associated Env than infection-derived viruses. Transfected-293T stocks also contained fewer and lower levels of cytokines and chemokines than infection-derived stocks, which had elevated levels of multiple analytes, with substantial variability among stocks. Sequencing of the infection-derived SIVmac251 stocks revealed variable levels of viral diversity between stocks, with evidence of stock-specific selection and expansion of unique viral lineages. These analyses suggest that there may be underappreciated features of SIV in vivo challenge stocks with the potential to impact early infection events, which may merit consideration when selecting virus stocks for in vivo studies.

CCR5 blockade is well tolerated and induces changes in the tissue distribution of CCR5+ and CD25+ T cells in healthy, SIV-uninfected rhesus macaques

BACKGROUND

CCR5 is a main co-receptor for HIV, but also homes lymphocytes to sites of inflammation. We hypothesized that inhibition of CCR5 signaling would reduce HIV-associated chronic immune activation.

METHODS

To test this hypothesis, we administered an antagonistic anti-CCR5 monoclonal antibody (HGS101) to five uninfected rhesus macaques (RMs) and monitored lymphocyte dynamics in blood and tissue.

RESULTS

CCR5 blockade resulted in decreased levels of CCR5+ T cells in blood and, at later timepoints, in lymph nodes. Additionally, the levels of CD25+ T cells increased in lymph nodes, but decreased in blood, bone marrow, and rectal mucosa. Finally, a profile of gene expression from HGS101-treated RMs revealed a subtle, but consistent, in vivo signature of CCR5 blockade that suggests a mild immune-modulatory effect.

CONCLUSIONS

Treatment with anti-CCR5 antibody induces changes in the tissue distribution of CCR5+ and CD25+ T cells that may impact on the overall levels of immune activation during HIV and SIV infection.

TCR triggering transcriptionally downregulates CCR5 expression on rhesus macaque CD4(+) T-cells with no measurable effect on susceptibility to SIV infection

Studies using transformed human cell lines suggest that most SIV strains use CCR5 as co-receptor. Our analysis of primary rhesus macaque CD4(+) T-cell clones revealed marked differences in susceptibility to SIV(mac)239 infection. We investigated whether different levels of CCR5 expression account for clonal differences in SIV(mac)239 susceptibility. Macaque CD4(+) T-cells showed significant CCR5 downregulation 1-2days following CD3 mAb stimulation, which gradually recovered at resting state, 7-10days after activation. Exposure of clones to SIV(mac)239 during their CCR5(low) or CCR5(high) expression states revealed differences in SIV susceptibility independent of surface CCR5 levels. Furthermore, a CCR5 antagonist similarly reduced SIV(mac)239 infection of clones during their CCR5(low) or CCR5(high) expression states. Our data suggest a model where i) very low levels of CCR5 are sufficient for efficient SIV infection, ii) CCR5 levels above this threshold do not enhance infection, and iii) low level infection can occur in the absence of CCR5.

AMPK-mediated inhibition of mTOR kinase is circumvented during immediate-early times of human cytomegalovirus infection

Human cytomegalovirus (HCMV) infection increases synthetic rates in infected cells. The resulting increase in energy utilization could potentially increase the AMP:ATP ratio, causing activation of 5'-AMP-activated protein kinase (AMPK). Activated AMPK promotes inhibition of mammalian target of rapamycin (mTOR) kinase, which could be deleterious to the viral infection. Using the AMPK-activating drug 5-amino-4-imidazolecarboxamide ribose (AICAR), we showed that, by 12 h post-HCMV infection, inhibition of mTOR by AMPK is circumvented. However, growth curves showed that progeny virion production is inhibited when AICAR is added, suggesting other inhibitory effects of AICAR or activated AMPK.