Vpx rescue of HIV-1 from the antiviral state in mature dendritic cells is independent of the intracellular deoxynucleotide concentration

CRL4-DCAF1 Ubiquitin Ligase Dependent Functions of HIV Viral Protein R and Viral Protein X

The Human Immunodeficiency Virus (HIV) encodes several proteins that contort the host cell environment to promote viral replication and spread. This is often accomplished through the hijacking of cellular ubiquitin ligases. These reprogrammed complexes initiate or enhance the ubiquitination of cellular proteins that may otherwise act to restrain viral replication. Ubiquitination of target proteins may alter protein function or initiate proteasome-dependent destruction. HIV Viral Protein R (Vpr) and the related HIV-2 Viral Protein X (Vpx), engage the CRL4-DCAF1 ubiquitin ligase complex to target numerous cellular proteins. In this review we describe the CRL4-DCAF1 ubiquitin ligase complex and its interactions with HIV Vpr and Vpx. We additionally summarize the cellular proteins targeted by this association as well as the observed or hypothesized impact on HIV.

Enhanced infection efficiency and cytotoxicity mediated by vpx-containing lentivirus in chimeric antigen receptor macrophage (CAR-M)

Genetically modified macrophage infusion has been proven to be a novel treatment for cancer. One of the most important processes in macrophage-based therapy is the efficient transfer of genes. HIV-1-derived lentiviruses were widely used as delivery vectors in chimeric antigen receptor T and NK cell construction. While macrophages are relatively refractory to this lentiviral vector transduction as a result of the myeloid-specific restriction factor SAMHD1, which inhibited the virion cycle through exhausting the dNTPs pool and degradating RNAs. An efficient macrophage transduction strategy has been developed via packaging the HIV-2 accessory protein Vpx into the virion. Vpx counteracts SAMHD1 through CRL4 (DCAF1) E3 ubiquitin ligase mediated SAMHD1 degradation, yet the influence by the introduction of Vpx on macrophage has not been fully evaluated. Here, we constructed the chimeric lentiviral vector HIV-1-Vpx and systematically analyzed the infection efficiency of this vector in time-dependent manner. Our results showed that the simplified chimeric virus exhibited dramatically enhanced infection in human macrophages compared to normal lentivirus. Moreover, transcriptome sequencing was performed to evaluate the cellular status after chimeric virus infection. The sequencing results indicated that Vpx introduction promoted macrophage remodeling towards a proinflammatory phenotype, without affecting classic M1/M2 cell surface markers. Our results suggest that the Vpx-containing lentivirus could be used as an ideal tool for the generation of genetically engineered macrophages with high gene transfer efficiency and poised proinflammatory gene sets, especially for solid tumor treatment.

Sulforaphane Reduces SAMHD1 Phosphorylation To Protect Macrophages from HIV-1 Infection

The cellular protein SAMHD1 is important for DNA repair, suppressing LINE elements, controlling deoxynucleoside triphosphate (dNTP) concentrations, maintaining HIV-1 latency, and preventing excessive type I interferon responses. SAMHD1 is also a potent inhibitor of HIV-1 and other significant viral pathogens. Infection restriction is due in part to the deoxynucleoside triphosphatase (dNTPase) activity of SAMHD1 but is also mediated through a dNTPase-independent mechanism that has been described but not explored. The phosphorylation of SAMHD1 at threonine 592 (T592) controls many of its functions. Retroviral restriction, irrespective of dNTPase activity, is linked to unphosphorylated T592. Sulforaphane (SFN), an isothiocyanate, protects macrophages from HIV infection by mobilizing the transcription factor and antioxidant response regulator Nrf2. Here, we show that SFN and other clinically relevant Nrf2 mobilizers reduce SAMHD1 T592 phosphorylation to protect macrophages from HIV-1. We further show that SFN, through Nrf2, triggers the upregulation of the cell cycle control protein p21 in human monocyte-derived macrophages to contribute to SAMHD1 activation. We additionally present data that support another, potentially redox-dependent mechanism employed by SFN to contribute to SAMHD1 activation through reduced phosphorylation. This work establishes the use of exogenous Nrf2 mobilizers as a novel way to study virus restriction by SAMHD1 and highlights the Nrf2 pathway as a potential target for the therapeutic control of SAMHD1 cellular and antiviral functions. IMPORTANCE Here, we show, for the first time, that the treatment of macrophages with Nrf2 mobilizers, known activators of antioxidant responses, increases the fraction of SAMHD1 without a regulatory phosphate at position 592. We demonstrate that this decreases infection of macrophages by HIV-1. Phosphorylated SAMHD1 is important for DNA repair, the suppression of LINE elements, the maintenance of HIV-1 in a latent state, and the prevention of excessive type I interferon responses, while unphosphorylated SAMHD1 blocks HIV infection. SAMHD1 impacts many viruses and is involved in various cancers, so knowledge of how it works and how it is regulated has broad implications for the development of therapeutics. Redox-modulating therapeutics are already in clinical use or under investigation for the treatment of many conditions. Thus, understanding the impact of redox modifiers on controlling SAMHD1 phosphorylation is important for many areas of research in microbiology and beyond.

Post-translational modifications inducing proteasomal degradation to counter HIV-1 infection

Post-translational modifications (PTMs) are integral to regulating a wide variety of cellular processes in eukaryotic cells, such as regulation of protein stability, alteration of celluar location, protein activity modulation, and regulation of protein interactions. HIV-1, like other eukaryotic viruses, and its infected host exploit the proteasomal degradation system for their respective proliferation and survival, using various PTMs, including but not limited to ubiquitination, SUMOylation, NEDDylation, interferon-stimulated gene (ISG)ylation. Essentially all viral proteins within the virions -- and in the HIV-1-infected cells -- interact with their cellular counterparts for this degradation, utilizing ubiquitin (Ub), and the Ub-like (Ubl) modifiers less frequently, to eliminate the involved proteins throughout the virus life cycle, from the entry step to release of the assembled virus particles. Such interplay is pivotal for, on the one hand, the cell to restrict proliferation of the infecting virus, and on the other, for molecular counteraction by the virus to overcome this cellular protein-imposed restriction. Recent reports indicate that not only viral/cellular proteins but also viral/viral protein interactions play vital roles in regulating viral protein stability. We hence give an overview of the molecular processes of PTMs involved in proteasomal degradation of the viral and cellular proteins, and the viral/viral and viral/cellular protein interplay in restriction and competition for HIV-1 vs. host cell survival. Insights in this realm could open new avenues for developing therapeutics against HIV-1 via targeting specific steps of the proteasome degradation pathway during the HIV-1 life cycle.

Identification of Host Trafficking Genes Required for HIV-1 Virological Synapse Formation in Dendritic Cells

The lentivirus human immunodeficiency virus (HIV) targets and destroys CD4⁺ T cells, leaving the host vulnerable to life-threatening opportunistic infections associated with AIDS. Dendritic cells (DCs) form a virological synapse (VS) with CD4⁺ T cells, enabling the efficient transfer of virus between the two cells. We have identified cellular factors that are critical in the induction of the VS. We show that ADP-ribosylation factor 1 (ARF1), bridging integrator 1 (BIN1), and Rab GTPases RAB7L1 and RAB8A are important regulators of HIV-1 trafficking to the VS and therefore the infection of CD4⁺ T cells. We found these cellular factors were essential for endosomal protein trafficking and formation of the VS and that depletion of target proteins prevented virus trafficking to the plasma membrane by retaining virus in intracellular vesicles. Identification of key regulators in HIV-1 trans-infection between DC and CD4⁺ T cells has the potential for the development of targeted therapy to reduce trans-infection of HIV-1 in vivo.

Dendritic cells (DCs) are one of the earliest targets of HIV-1 infection acting as a “Trojan horse,” concealing the virus from the innate immune system and delivering it to T cells via virological synapses (VS). To explicate how the virus is trafficked through the cell to the VS and evades degradation, a high-throughput small interfering RNA screen targeting membrane trafficking proteins was performed in monocyte-derived DCs. We identified several proteins including BIN-1 and RAB7L1 that share common roles in transport from endosomal compartments. Depletion of target proteins resulted in an accumulation of virus in intracellular compartments and significantly reduced viral trans-infection via the VS. By targeting endocytic trafficking and retromer recycling to the plasma membrane, we were able to reduce the virus’s ability to accumulate at budding microdomains and the VS. Thus, we identify key genes involved in a pathway within DCs that is exploited by HIV-1 to traffic to the VS.

IMPORTANCE The lentivirus human immunodeficiency virus (HIV) targets and destroys CD4⁺ T cells, leaving the host vulnerable to life-threatening opportunistic infections associated with AIDS. Dendritic cells (DCs) form a virological synapse (VS) with CD4⁺ T cells, enabling the efficient transfer of virus between the two cells. We have identified cellular factors that are critical in the induction of the VS. We show that ADP-ribosylation factor 1 (ARF1), bridging integrator 1 (BIN1), and Rab GTPases RAB7L1 and RAB8A are important regulators of HIV-1 trafficking to the VS and therefore the infection of CD4⁺ T cells. We found these cellular factors were essential for endosomal protein trafficking and formation of the VS and that depletion of target proteins prevented virus trafficking to the plasma membrane by retaining virus in intracellular vesicles. Identification of key regulators in HIV-1 trans-infection between DC and CD4⁺ T cells has the potential for the development of targeted therapy to reduce trans-infection of HIV-1 in vivo.

Tissue memory CD4+ T cells expressing IL-7 receptor-alpha (CD127) preferentially support latent HIV-1 infection

The primary reservoir for HIV is within memory CD4+ T cells residing within tissues, yet the features that make some of these cells more susceptible than others to infection by HIV is not well understood. Recent studies demonstrated that CCR5-tropic HIV-1 efficiently enters tissue-derived memory CD4+ T cells expressing CD127, the alpha chain of the IL7 receptor, but rarely completes the replication cycle. We now demonstrate that the inability of HIV to replicate in these CD127-expressing cells is not due to post-entry restriction by SAMHD1. Rather, relative to other memory T cell subsets, these cells are highly prone to undergoing latent infection with HIV, as revealed by the high levels of integrated HIV DNA in these cells. Host gene expression profiling revealed that CD127-expressing memory CD4+ T cells are phenotypically distinct from other tissue memory CD4+ T cells, and are defined by a quiescent state with diminished NFκB, NFAT, and Ox40 signaling. However, latently-infected CD127+ cells harbored unspliced HIV transcripts and stimulation of these cells with anti-CD3/CD28 reversed latency. These findings identify a novel subset of memory CD4+ T cells found in tissue and not in blood that are preferentially targeted for latent infection by HIV, and may serve as an important reservoir to target for HIV eradication efforts.

TLR4-Mediated Pathway Triggers Interferon-Independent G0 Arrest and Antiviral SAMHD1 Activity in Macrophages

Macrophages exist predominantly in two distinct states, G0 and a G1-like state that is accompanied by phosphorylation of SAMHD1 at T592. Here, we demonstrate that Toll-like receptor 4 (TLR4) activation can potently induce G0 arrest and SAMHD1 antiretroviral activity by an interferon (IFN)-independent pathway. This pathway requires TLR4 engagement with TRIF, but not involvement of TBK1 or IRF3. Exclusive Myd88 activators are unable to trigger G0 arrest or SAMHD1 dephosphorylation, demonstrating this arrest is also Myd88/nuclear factor κB (NF-κB) independent. The G0 arrest is accompanied by p21 upregulation and CDK1 depletion, consistent with the observed SAMHD1 dephosphorylation at T592. Furthermore, we show by SAMHD1 knockdown that the TLR4-activated pathway potently blocks HIV-1 infection in macrophages specifically via SAMHD1. Together, these data demonstrate that macrophages can mobilize an intrinsic cell arrest and anti-viral state by activating TLR4 prior to IFN secretion, thereby highlighting the importance of cell-cycle regulation as a response to pathogen-associated danger signals in macrophages.

Involvement of SAMHD1 in dNTP homeostasis and the maintenance of genomic integrity and oncotherapy (Review)

Sterile alpha motif and histidine/aspartic acid domain‑containing protein 1 (SAMHD1), the only deoxynucleotide triphosphate (dNTP) hydrolase in eukaryotes, plays a crucial role in regulating the dynamic balance and ratio of cellular dNTP pools. Furthermore, SAMHD1 has been reported to be involved in the pathological process of several diseases. Homozygous SAMHD1 mutations have been identified in immune system disorders, such as autoimmune disease Aicardi‑Goutières syndrome (AGS), whose primary pathogenesis is associated with the abnormal accumulation and disproportion of dNTPs. SAMHD1 is also considered to be an intrinsic virus‑restriction factor by suppressing the viral infection process, including reverse transcription, replication, packaging and transmission. In addition, SAMHD1 has been shown to promote genome integrity during homologous recombination following DNA damage, thus being considered a promising candidate for oncotherapy applications. The present review summarizes the molecular mechanisms of SAMHD1 regarding the regulation of dNTP homeostasis and DNA damage response. Additionally, its potential effects on tumorigenesis and oncotherapy are reported.

HIV-2/SIV Vpx targets a novel functional domain of STING to selectively inhibit cGAS-STING-mediated NF-κB signalling

Innate immunity is the first line of host defence against pathogens. Suppression of innate immune responses is essential for the survival of all viruses. However, the interplay between innate immunity and HIV/SIV is only poorly characterized. We have discovered Vpx as a novel inhibitor of innate immune activation that associates with STING signalosomes and interferes with the nuclear translocation of NF-κB and the induction of innate immune genes. This new function of Vpx could be separated from its role in mediating degradation of the antiviral factor SAMHD1, and is conserved among diverse HIV-2/SIV Vpx. Vpx selectively suppressed cGAS-STING-mediated nuclear factor-κB signalling. Furthermore, Vpx and Vpr had complementary activities against cGAS-STING activity. Since SIV_MAC lacking both Vpx and Vpr was less pathogenic than SIV deficient for Vpr or Vpx alone, suppression of innate immunity by HIV/SIV is probably a key pathogenic determinant, making it a promising target for intervention.

Cyclophilin A protects HIV-1 from restriction by human TRIM5α

The HIV-1 capsid (CA) protein lattice encases viral genomic RNA and regulates steps essential to target cell invasion¹. Cyclophilin A (CypA) has interacted with the CA of lentiviruses related to HIV-1 for millions of years^2–7. Disruption of the CA-CypA interaction decreases HIV-1 infectivity in human cells^8–12, but stimulates infectivity in non-human primate cells^13–15. Genetic and biochemical data suggest that CypA protects HIV-1 from a CA-specific restriction factor in human cells^16–20. Discovery of the CA-specific restriction factor tripartite-containing motif 5α (TRIM5α)²¹, and of multiple, independently-derived, TRIM5-CypA fusion genes^{4,5,15,22–26}, pointed to human TRIM5α as the CypA-sensitive restriction factor. However, HIV-1 restriction by human TRIM5α in tumor cell lines is minimal²¹, and inhibition of such activity by CypA has not been detected²⁷. Here, exploiting reverse genetic tools optimized for primary human blood cells, we demonstrate that disruption of the CA-CypA interaction renders HIV-1 susceptible to potent restriction by human TRIM5α, with the block occurring before reverse transcription. Endogenous TRIM5α associated with virion cores as they entered the cytoplasm, but only when the CA-CypA interaction was disrupted. These experiments resolve the long-standing mystery of the role of CypA in HIV-1 replication by demonstrating that this ubiquitous cellular protein shields HIV-1 from previously inapparent restriction by human TRIM5α.

Role of the Ubiquitin Proteasome System (UPS) in the HIV-1 Life Cycle

Given that the ubiquitin proteasome system (UPS) is the major protein degradation process in the regulation of a wide variety of cellular processes in eukaryotic cells, including alteration of cellular location, modulation of protein activity, and regulation of protein interaction, it is reasonable to suggest that the infecting HIV-1 and the invaded hosts exploit the UPS in a contest for survival and proliferation. However, to date, regulation of the HIV-1 life cycle has been mainly explained by the stage-specific expression of HIV-1 viral genes, not by elimination processes of the synthesized proteins after completion of their duties in the infected cells, which is also quintessential for understanding the molecular processes of the virus life cycle and thereby HIV-1 pathogenesis. In fact, several previous publications have indicated that the UPS plays a critical role in the regulation of the proteasomal degradation of viral and cellular counterparts at every step of the HIV-1 life cycle, from the virus entry to release of the assembled virus particles, which is integral for the regulation of survival and proliferation of the infecting HIV-1 and to replication restriction of the invading virus in the host. However, it is unknown whether and how these individual events taking place at different stages of the HIV-1 life cycle are orchestrated as an overall strategy to overcome the restrictions conferred by the host cells. Thus, in this review, we overview the interplay between HIV-1 viral and cellular proteins for restrictions/competitions for proliferation of the virus in the infected cell, which could open a new avenue for the development of therapeutics against HIV-1 via targeting a specific step of the proteasome degradation pathway during the HIV-1 life cycle.

The ability of SAMHD1 to block HIV-1 but not SIV requires expression of MxB

SAMHD1 is a human restriction factor known to prevent infection of macrophages, resting CD4+ T cells, and dendritic cells by HIV-1. To test the contribution of MxB to the ability of SAMHD1 to block HIV-1 infection, we created human THP-1 cell lines that were knocked out for expression of MxB, SAMHD1, or both. Interestingly, MxB depletion renders SAMHD1 ineffective against HIV-1 but not SIVmac. We observed similar results in human primary macrophages that were knockdown for the expression of MxB. To understand how MxB assists SAMHD1 restriction of HIV-1, we examined direct interaction between SAMHD1 and MxB in pull-down experiments. In addition, we investigated several properties of SAMHD1 in the absence of MxB expression, including subcellular localization, phosphorylation of the SAMHD1 residue T592, and dNTPs levels. These experiments showed that SAMHD1 restriction of HIV-1 requires expression of MxB.

HUSH, a Link Between Intrinsic Immunity and HIV Latency

A prominent obstacle to HIV eradication in seropositive individuals is the viral persistence in latent reservoir cells, which constitute an HIV sanctuary out of reach of highly active antiretroviral therapies. Thus, the study of molecular mechanisms governing latency is a very active field that aims at providing solutions to face the reservoirs issue. Since the past 15 years, another major field in HIV biology focused on the discovery and study of restriction factors that shape intrinsic immunity, while engaging in a molecular battle against HIV. Some of these restrictions factors act at early stages of the virus life cycle, alike SAMHD1 antagonized by the viral protein Vpx, while others are late actors. Until recently, no such factor was identified in the nucleus and found active at the level of provirus expression, a crucial step where latency may take place. Today, two studies highlight Human Silencing Hub (HUSH) as a potential restriction factor that controls viral expression and is antagonized by Vpx. This Review discusses HUSH restriction in the light of the actual knowledge of intrinsic immunity and HIV latency.

Intron-containing RNA from the HIV-1 provirus activates type I interferon and inflammatory cytokines

HIV-1-infected people who take drugs that suppress viremia to undetectable levels are protected from developing AIDS. Nonetheless, HIV-1 establishes proviruses in long-lived CD4⁺ memory T cells, and perhaps other cell types, that preclude elimination of the virus even after years of continuous antiviral therapy. Here we show that the HIV-1 provirus activates innate immune signaling in isolated dendritic cells, macrophages, and CD4⁺ T cells. Immune activation requires transcription from the HIV-1 provirus and expression of CRM1-dependent, Rev-dependent, RRE-containing, unspliced HIV-1 RNA. If rev is provided in trans, all HIV-1 coding sequences are dispensable for activation except those cis-acting sequences required for replication or splicing. Our results indicate that the complex, post-transcriptional regulation intrinsic to HIV-1 RNA is detected by the innate immune system as a danger signal, and that drugs which disrupt HIV-1 transcription or HIV-1 RNA metabolism would add qualitative benefit to current antiviral drug regimens.

During HIV infection, antiviral therapy can suppress viraemia to undetectable levels and hinder the progression towards AIDS; however the HIV-1 provirus can remain in long-lived CD4⁺ memory T cells. Here the authors show that intronic RNA from the HIV-1 provirus can induce type I interferon and inflammatory cytokine production.

Incomplete Suppression of HIV-1 by SAMHD1 Permits Efficient Macrophage Infection

Background:

Sterile alpha motif and histidine/aspartic acid domain-containing protein (SAMHD1) is a dNTP triphosphorylase that reduces cellular dNTP levels in non-dividing cells, such as macrophages. Since dNTPs are required for reverse transcription, HIV-2 and most SIVs encode a Vpx protein that promotes proteasomal degradation of SAMHD1. It is unclear how HIV-1, which does not appear to harbor a SAMHD1 escape mechanism, is able to infect macrophages in the face of SAMHD1 restriction.

Methods:

To assess whether HIV-1 had a mechanism to negate SAMHD1 activity, we compared SAMHD1 and dNTP levels in macrophages infected by HIV-1 and SIV. We examined whether macrophages infected by HIV-1 still harbored antiviral levels of SAMHD1 by assessing their susceptibility to superinfection by vpx-deleted SIV. Finally, to assess whether HIV-1 reverse transcriptase (RT) has adapted to a low dNTP environment, we evaluated SAMHD1 sensitivity of chimeric HIV-1 and SIV variants in which the RT regions were functionally exchanged.

Results:

Here, we demonstrate that HIV-1 efficiently infects macrophages without modulating SAMHD1 activity or cellular dNTP levels, and that macrophages permissive to HIV-1 infection remained refractory to superinfection by vpx-deleted SIV. Furthermore, through the use of chimeric HIV/SIV, we demonstrate that the differential sensitivity of HIV-1 and SIV to SAMHD1 restriction is not dictated by RT.

Conclusions:

Our study reveals fundamental differences between HIV-1 and SIV in the strategy used to evade restriction by SAMHD1 and suggests a degree of resistance of HIV-1 to the antiviral environment created by SAMHD1. Understanding how these cellular restrictions antagonize viral replication will be important for the design of novel antiviral strategies.

Primate immunodeficiency virus proteins Vpx and Vpr counteract transcriptional repression of proviruses by the HUSH complex

Host factors that silence provirus transcription in CD4⁺ memory T cells help HIV-1 escape eradication by the host immune system and by antiviral drugs¹. These same factors, though, must be overcome for HIV-1 to propagate. Here we show that Vpx and Vpr encoded by diverse primate immunodeficiency viruses activate provirus transcription. Vpx and Vpr are adaptor proteins for the DCAF1-CUL4A/B E3 ubiquitin ligase that degrade SAMHD1 and increase reverse transcription^2–4. Nonetheless, Vpx and Vpr have effects on reporter gene expression that are not explained by SAMHD1 degradation^5–8. A screen for factors that mimic these effects identified the Human Silencing Hub (HUSH) complex, FAM208A (TASOR/RAP140), MPHOSPH8 (MPP8), PPHLN1 (PERIPHILIN), and MORC2^9–13. Vpx associated with the HUSH complex and decreased steady-state level of these proteins in a DCAF1/CUL4A/B/proteasome-dependent manner^14,15. Replication kinetics of HIV-1 and SIV_MAC was accelerated to a similar extent by vpx or FAM208A knockdown. Finally, vpx increased steady-state levels of LINE-1 ORF1p, as previously described for FAM208A disruption¹¹. These results demonstrate that the HUSH complex represses primate immunodeficiency virus transcription, and that, to counteract this restriction, viral Vpx or Vpr proteins degrade the HUSH complex.

IL-15 regulates susceptibility of CD4+ T cells to HIV infection

HIV integrates into the host genome to create a persistent viral reservoir. Stimulation of CD4⁺ memory T lymphocytes with common γc-chain cytokines renders these cells more susceptible to HIV infection, making them a key component of the reservoir itself. IL-15 is up-regulated during primary HIV infection, a time when the HIV reservoir established. Therefore, we investigated the molecular and cellular impact of IL-15 on CD4⁺ T-cell infection. We found that IL-15 stimulation induces SAM domain and HD domain-containing protein 1 (SAMHD1) phosphorylation due to cell cycle entry, relieving an early block to infection. Perturbation of the pathways downstream of IL-15 receptor (IL-15R) indicated that SAMHD1 phosphorylation after IL-15 stimulation is JAK dependent. Treating CD4⁺ T cells with Ruxolitinib, an inhibitor of JAK1 and JAK2, effectively blocked IL-15-induced SAMHD1 phosphorylation and protected CD4⁺ T cells from HIV infection. Using high-resolution single-cell immune profiling using mass cytometry by TOF (CyTOF), we found that IL-15 stimulation altered the composition of CD4⁺ T-cell memory populations by increasing proliferation of memory CD4⁺ T cells, including CD4⁺ T memory stem cells (T_SCM). IL-15-stimulated CD4⁺ T_SCM, harboring phosphorylated SAMHD1, were preferentially infected. We propose that IL-15 plays a pivotal role in creating a self-renewing, persistent HIV reservoir by facilitating infection of CD4⁺ T cells with stem cell-like properties. Time-limited interventions with JAK1 inhibitors, such as Ruxolitinib, should prevent the inactivation of the endogenous restriction factor SAMHD1 and protect this long-lived CD4⁺ T-memory cell population from HIV infection.

Control of HIV infection by IFN-α: implications for latency and a cure

Viral infections, including HIV, trigger the production of type I interferons (IFNs), which in turn, activate a signalling cascade that ultimately culminates with the expression of anti-viral proteins. Mounting evidence suggests that type I IFNs, in particular IFN-α, play a pivotal role in limiting acute HIV infection. Highly active anti-retroviral treatment reduces viral load and increases life expectancy in HIV positive patients; however, it fails to fully eliminate latent HIV reservoirs. To revisit HIV as a curable disease, this article reviews a body of literature that highlights type I IFNs as mediators in the control of HIV infection, with particular focus on the anti-HIV restriction factors induced and/or activated by IFN-α. In addition, we discuss the relevance of type I IFN treatment in the context of HIV latency reversal, novel therapeutic intervention strategies and the potential for full HIV clearance.

Comparative Analysis of Immune Cells Reveals a Conserved Regulatory Lexicon

Most well-characterized enhancers are deeply conserved. In contrast, genome-wide comparative studies of steady-state systems showed that only a small fraction of active enhancers are conserved. To better understand conservation of enhancer activity, we used a comparative genomics approach that integrates temporal expression and epigenetic profiles in an innate immune system. We found that gene expression programs diverge among mildly induced genes, while being highly conserved for strongly induced genes. The fraction of conserved enhancers varies greatly across gene expression programs, with induced genes and early-response genes, in particular, being regulated by a higher fraction of conserved enhancers. Clustering of conserved accessible DNA sequences within enhancers resulted in over 60 sequence motifs including motifs for known factors, as well as many with unknown function. We further show that the number of instances of these motifs is a strong predictor of the responsiveness of a gene to pathogen detection.

DNA damage induced by topoisomerase inhibitors activates SAMHD1 and blocks HIV-1 infection of macrophages

We report that DNA damage induced by topoisomerase inhibitors, including etoposide (ETO), results in a potent block to HIV-1 infection in human monocyte-derived macrophages (MDM). SAMHD1 suppresses viral reverse transcription (RT) through depletion of cellular dNTPs but is naturally switched off by phosphorylation in a subpopulation of MDM found in a G1-like state. We report that SAMHD1 was activated by dephosphorylation following ETO treatment, along with loss of expression of MCM2 and CDK1, and reduction in dNTP levels. Suppression of infection occurred after completion of viral DNA synthesis, at the step of 2LTR circle and provirus formation. The ETO-induced block was completely rescued by depletion of SAMHD1 in MDM Concordantly, infection by HIV-2 and SIVsm encoding the SAMHD1 antagonist Vpx was insensitive to ETO treatment. The mechanism of DNA damage-induced blockade of HIV-1 infection involved activation of p53, p21, decrease in CDK1 expression, and SAMHD1 dephosphorylation. Therefore, topoisomerase inhibitors regulate SAMHD1 and HIV permissivity at a post-RT step, revealing a mechanism by which the HIV-1 reservoir may be limited by chemotherapeutic drugs.

p21 Restricts HIV-1 in Monocyte-Derived Dendritic Cells through the Reduction of Deoxynucleoside Triphosphate Biosynthesis and Regulation of SAMHD1 Antiviral Activity

HIV-1 infection of noncycling cells, such as dendritic cells (DCs), is impaired due to limited availability of deoxynucleoside triphosphates (dNTPs), which are needed for HIV-1 reverse transcription. The levels of dNTPs are tightly regulated during the cell cycle and depend on the balance between dNTP biosynthesis and degradation. SAMHD1 potently blocks HIV-1 replication in DCs, although the underlying mechanism is still unclear. SAMHD1 has been reported to be able to degrade dNTPs and viral nucleic acids, which may both hamper HIV-1 reverse transcription. The relative contribution of these activities may differ in cycling and noncycling cells. Here, we show that inhibition of HIV-1 replication in monocyte-derived DCs (MDDCs) is associated with an increased expression of p21cip1/waf, a cell cycle regulator that is involved in the differentiation and maturation of DCs. Induction of p21 in MDDCs decreases the pool of dNTPs and increases the antiviral active isoform of SAMHD1. Although both processes are complementary in inhibiting HIV-1 replication, the antiviral activity of SAMHD1 in our primary cell model appears to be, at least partially, independent of its dNTPase activity. The reduction in the pool of dNTPs in MDDCs appears rather mostly due to a p21-mediated suppression of several enzymes involved in dNTP synthesis (i.e., RNR2, TYMS, and TK-1). These results are important to better understand the interplay between HIV-1 and DCs and may inform the design of new therapeutic approaches to decrease viral dissemination and improve immune responses against HIV-1.IMPORTANCE DCs play a key role in the induction of immune responses against HIV. However, HIV has evolved ways to exploit these cells, facilitating immune evasion and virus dissemination. We have found that the expression of p21, a cyclin-dependent kinase inhibitor involved in cell cycle regulation and monocyte differentiation and maturation, potentially can contribute to the inhibition of HIV-1 replication in monocyte-derived DCs through multiple mechanisms. p21 decreased the size of the intracellular dNTP pool. In parallel, p21 prevented SAMHD1 phosphorylation and promoted SAMHD1 dNTPase-independent antiviral activity. Thus, induction of p21 resulted in conditions that allowed the effective inhibition of HIV-1 replication through complementary mechanisms. Overall, p21 appears to be a key regulator of HIV infection in myeloid cells.

The poly-proline tail of SIVmac Vpx provides gain of function for resistance to a cryptic proteasome-dependent degradation pathway

The lentiviral accessory protein Vpx is critical for viral infection of myeloid cells and acts by hijacking CRL4(DCAF1) E3 ubiquitin ligase to induce the degradation of the host restriction factor SAMHD1. It has been observed that the sequences from HIV-2 and SIVsmm/SIVmac Vpx contain a poly-proline tail which is distinct from other SIV Vpx proteins. However, the role of this region in Vpx function is controversial. Herein, we found proteasome-dependent degradation of a Vpx mutant lacking the poly-proline tail in the nucleus in a CRL4(DCAF1) E3 ligase-independent fashion. Unlike wild-type Vpx, the poly-proline tail mutant Vpx is partly defective in enhancing viral infection in macrophages. Our findings suggest that during Vpx evolution, Vpx of the HIV-2/SIVsm/SIVmac lineage is targeted by a CRL4(DCAF1) E3 ligase-independent ubiquitination pathway, and have gained this interesting region, allowing them to maintain nuclear accumulation as part of their adaptation to host cell regulation.

Roles of SAMHD1 in antiviral defense, autoimmunity and cancer

The enzyme, sterile α motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) diminishes infection of human immunodeficiency virus type 1 (HIV-1) by hydrolyzing intracellular deoxynucleotide triphosphates (dNTPs) in myeloid cells and resting CD4+ T cells. This dNTP degradation reduces the dNTP concentration to a level insufficient for viral cDNA synthesis, thereby inhibiting retroviral replication. This antiviral enzymatic activity can be inhibited by viral protein X (Vpx). The HIV-2/SIV Vpx causes degradation of SAMHD1, thus interfering with the SAMHD1-mediated restriction of retroviral replication. Recently, SAMHD1 has been suggested to restrict HIV-1 infection by directly digesting genomic HIV-1 RNA through a still controversial RNase activity. Here, we summarize the current knowledge about structure, antiviral mechanisms, intracellular localization, interferon-regulated expression of SAMHD1. We also describe SAMHD1-deficient animal models and an antiviral drug on the basis of disrupting proteasomal degradation of SAMHD1. In addition, the possible roles of SAMHD1 in regulating innate immune sensing, Aicardi-Goutières syndrome and cancer are discussed in this review.

Vpx overcomes a SAMHD1-independent block to HIV reverse transcription that is specific to resting CD4 T cells

Early after entry into monocytes, macrophages, dendritic cells, and resting CD4 T cells, HIV encounters a block, limiting reverse transcription (RT) of the incoming viral RNA genome. In this context, dNTP triphosphohydrolase SAM domain and HD domain-containing protein 1 (SAMHD1) has been identified as a restriction factor, lowering the concentration of dNTP substrates to limit RT. The accessory lentiviral protein X (Vpx) proteins from the major simian immunodeficiency virus of rhesus macaque, sooty mangabey, and HIV-2 (SIVsmm/SIVmac/HIV-2) lineage packaged into virions target SAMHD1 for proteasomal degradation, increase intracellular dNTP pools, and facilitate HIV cDNA synthesis. We find that virion-packaged Vpx proteins from a second SIV lineage, SIV of red-capped mangabeys or mandrills (SIVrcm/mnd-2), increased HIV infection in resting CD4 T cells, but not in macrophages, and, unexpectedly, acted in the absence of SAMHD1 degradation, dNTP pool elevation, or changes in SAMHD1 phosphorylation. Vpx rcm/mnd-2 virion incorporation resulted in a dramatic increase of HIV-1 RT intermediates and viral cDNA in infected resting CD4 T cells. These analyses also revealed a barrier limiting HIV-1 infection of resting CD4 T cells at the level of nuclear import. Single amino acid changes in the SAMHD1-degrading Vpx mac239 allowed it to enhance early postentry steps in a Vpx rcm/mnd-2-like fashion. Moreover, Vpx enhanced HIV-1 infection of SAMHD1-deficient resting CD4 T cells of a patient with Aicardi-Goutières syndrome. These results indicate that Vpx, in addition to SAMHD1, overcomes a previously unappreciated restriction for lentiviruses at the level of RT that acts independently of dNTP concentrations and is specific to resting CD4 T cells.

Ebola Virus Glycoprotein with Increased Infectivity Dominated the 2013-2016 Epidemic

The magnitude of the 2013-2016 Ebola virus disease (EVD) epidemic enabled an unprecedented number of viral mutations to occur over successive human-to-human transmission events, increasing the probability that adaptation to the human host occurred during the outbreak. We investigated one nonsynonymous mutation, Ebola virus (EBOV) glycoprotein (GP) mutant A82V, for its effect on viral infectivity. This mutation, located at the NPC1-binding site on EBOV GP, occurred early in the 2013-2016 outbreak and rose to high frequency. We found that GP-A82V had heightened ability to infect primate cells, including human dendritic cells. The increased infectivity was restricted to cells that have primate-specific NPC1 sequences at the EBOV interface, suggesting that this mutation was indeed an adaptation to the human host. GP-A82V was associated with increased mortality, consistent with the hypothesis that the heightened intrinsic infectivity of GP-A82V contributed to disease severity during the EVD epidemic.

Restriction by SAMHD1 Limits cGAS/STING-Dependent Innate and Adaptive Immune Responses to HIV-1

SAMHD1 is a restriction factor for HIV-1 infection. SAMHD1 mutations cause the autoinflammatory Aicardi-Goutières syndrome that is characterized by chronic type I interferon (IFN) secretion. We show that the spontaneous IFN response in SAMHD1-deficient cells and mice requires the cGAS/STING cytosolic DNA-sensing pathway. We provide genetic evidence that cell-autonomous control of lentivirus infection in myeloid cells by SAMHD1 limits virus-induced production of IFNs and the induction of co-stimulatory markers. This program of myeloid cell activation required reverse transcription, cGAS and STING, and signaling through the IFN receptor. Furthermore, SAMHD1 reduced the induction of virus-specific cytotoxic T cells in vivo. Therefore, virus restriction by SAMHD1 limits the magnitude of IFN and T cell responses. This demonstrates a competition between cell-autonomous virus control and subsequent innate and adaptive immune responses, a concept with important implications for the treatment of infection.

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Spontaneous IFN production in SAMHD1-deficient cells requires cGAS and STING

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During HIV-1 infection, SAMHD1 limits activation of myeloid cells

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cGAS and STING detect HIV-1 infection in SAMHD1-deficient cells and induce IFN

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SAMHD1 prevents virus-specific CD8 T cell responses in vivo

TGFβ Induces a SAMHD1-Independent Post-Entry Restriction to HIV-1 Infection of Human Epithelial Langerhans Cells

Sterile alpha motif (SAM) and histidine-aspartic (HD) domains protein 1 (SAMHD1) was previously identified as a critical post-entry restriction factor to HIV-1 infection in myeloid dendritic cells. Here we show that SAMHD1 is also expressed in epidermis-isolated Langerhans cells (LC), but degradation of SAMHD1 does not rescue HIV-1 or vesicular stomatitis virus G-pseudotyped lentivectors infection in LC. Strikingly, using Langerhans cells model systems (mutz-3-derived LC, monocyte-derived LC [MDLC], and freshly isolated epidermal LC), we characterize previously unreported post-entry restriction activity to HIV-1 in these cells, which acts at HIV-1 reverse transcription, but remains independent of restriction factors SAMHD1 and myxovirus resistance 2 (MX2). We demonstrate that transforming growth factor-β signaling confers this potent HIV-1 restriction in MDLC during their differentiation and blocking of mothers against decapentaplegic homolog 2 (SMAD2) signaling in MDLC restores cells' infectivity. Interestingly, maturation of MDLC with a toll-like receptor 2 agonist or transforming growth factor-α significantly increases cells' susceptibility to HIV-1 infection, which may explain why HIV-1 acquisition is increased during coinfection with sexually transmitted infections. In conclusion, we report a SAMHD1-independent post-entry restriction in MDLC and LC isolated from epidermis, which inhibits HIV-1 replication. A better understanding of HIV-1 restriction and propagation from LC to CD4(+) T cells may help in the development of new microbicides or vaccines to curb HIV-1 infection at its earliest stages during mucosal transmission.

The Frequency of Cytidine Editing of Viral DNA Is Differentially Influenced by Vpx and Nucleosides during HIV-1 or SIVMAC Infection of Dendritic Cells

Two cellular factors are currently known to modulate lentiviral infection specifically in myeloid cells: SAMHD1 and APOBEC3A (A3A). SAMHD1 is a deoxynucleoside triphosphohydrolase that interferes with viral infection mostly by limiting the intracellular concentrations of dNTPs, while A3A is a cytidine deaminase that has been described to edit incoming vDNA. The restrictive phenotype of myeloid cells can be alleviated through the direct degradation of SAMHD1 by the HIV-2/SIV_SM Vpx protein or else, at least in the case of HIV-1, by the exogenous supplementation of nucleosides that artificially overcome the catabolic activity of SAMHD1 on dNTPs. Here, we have used Vpx and dNs to explore the relationship existing between vDNA cytidine deamination and SAMHD1 during HIV-1 or SIV_MAC infection of primary dendritic cells. Our results reveal an interesting inverse correlation between conditions that promote efficient infection of DCs and the extent of vDNA editing that may reflect the different susceptibility of vDNA to cytoplasmic effectors during the infection of myeloid cells.

Optimization of human dendritic cell sample preparation for mass spectrometry-based proteomic studies

Dendritic cells (DCs) are specialized leukocytes that orchestrate the adaptive immune response. Mass spectrometry (MS)-based proteomic study of these cells presents technical challenges, especially when the DCs are human in origin due to the paucity of available biological material. Here, to maximize MS coverage of the global human DC proteome, different cell disruption methods, lysis conditions, protein precipitation, and protein pellet solubilization and denaturation methods were compared. Mechanical disruption of DC cell pellets under cryogenic conditions, coupled with the use of RIPA (radioimmunoprecipitation assay) buffer, was shown to be the method of choice based on total protein extraction and on the solubilization and identification of nuclear proteins. Precipitation by acetone was found to be more efficient than that by 10% trichloroacetic acid (TCA)/acetone, allowing in excess of 28% more protein identifications. Although being an effective strategy to eliminate the detergent residue, the acetone wash step caused a loss of protein identifications. However, this potential drawback was overcome by adding 1% sodium deoxycholate into the dissolution buffer, which enhanced both solubility of the precipitated proteins and digestion efficiency. This in turn resulted in 6 to 11% more distinct peptides and 14 to 19% more total proteins identified than using 0.5M triethylammonium bicarbonate alone, with the greatest increase (34%) for hydrophobic proteins.

Lv4 Is a Capsid-Specific Antiviral Activity in Human Blood Cells That Restricts Viruses of the SIVMAC/SIVSM/HIV-2 Lineage Prior to Integration

HIV-2 and SIV_MAC are AIDS-causing, zoonotic lentiviruses that jumped to humans and rhesus macaques, respectively, from SIV_SM-bearing sooty mangabey monkeys. Cross-species transmission events such as these sometimes necessitate virus adaptation to species-specific, host restriction factors such as TRIM5. Here, a new human restriction activity is described that blocks viruses of the SIV_SM/SIV_MAC/HIV-2 lineage. Human T, B, and myeloid cell lines, peripheral blood mononuclear cells and dendritic cells were 4 to >100-fold less transducible by VSV G-pseudotyped SIV_MAC, HIV-2, or SIV_SM than by HIV-1. In contrast, transduction of six epithelial cell lines was equivalent to that by HIV-1. Substitution of HIV-1 CA with the SIV_MAC or HIV-2 CA was sufficient to reduce HIV-1 transduction to the level of the respective vectors. Among such CA chimeras there was a general trend such that CAs from epidemic HIV-2 Group A and B isolates were the most infectious on human T cells, CA from a 1° sooty mangabey isolate was the least infectious, and non-epidemic HIV-2 Group D, E, F, and G CAs were in the middle. The CA-specific decrease in infectivity was observed with either HIV-1, HIV-2, ecotropic MLV, or ALV Env pseudotypes, indicating that it was independent of the virus entry pathway. As₂O₃, a drug that suppresses TRIM5-mediated restriction, increased human blood cell transduction by SIV_MAC but not by HIV-1. Nonetheless, elimination of TRIM5 restriction activity did not rescue SIV_MAC transduction. Also, in contrast to TRIM5-mediated restriction, the SIV_MAC CA-specific block occurred after completion of reverse transcription and the formation of 2-LTR circles, but before establishment of the provirus. Transduction efficiency in heterokaryons generated by fusing epithelial cells with T cells resembled that in the T cells, indicative of a dominant-acting SIV_MAC restriction activity in the latter. These results suggest that the nucleus of human blood cells possesses a restriction factor specific for the CA of HIV-2/SIV_MAC/SIV_SM and that cross-species transmission of SIV_SM to human T cells necessitated adaptation of HIV-2 to this putative restriction factor.

HIV-1 and HIV-2, the two lentiviruses that cause AIDS in humans, are members of a family of such viruses that infect African primates. HIV-1 is a zoonosis that was transmitted to humans from chimpanzees. HIV-2 was transmitted to humans from sooty mangabey monkeys. In several documented cases of cross-species transmission of lentiviruses it has been shown that replication of the virus in the new host species necessitated that the virus adapt to species-specific antiviral factors in the host. Here we report that human blood cells possess an antiviral activity that exhibits specificity for viruses of the HIV-2/SIV_MAC/SIV_SM lineage, with restriction being greatest for SIV_SM and the least for epidemic HIV-2. Here we show that this dominant-acting, antiviral activity is specific for the capsid and blocks the virus after it enters the nucleus. The evidence suggests that, in order to jump from sooty mangabey monkeys to humans, the capsid of these viruses changed in order to adapt to this antiviral activity. In keeping with the practice concerning anti-lentiviral activities we propose to call this new antiviral activity Lv4.

The Expression of Functional Vpx during Pathogenic SIVmac Infections of Rhesus Macaques Suppresses SAMHD1 in CD4+ Memory T Cells

For nearly 20 years, the principal biological function of the HIV-2/SIV Vpx gene has been thought to be required for optimal virus replication in myeloid cells. Mechanistically, this Vpx activity was recently reported to involve the degradation of Sterile Alpha Motif and HD domain-containing protein 1 (SAMHD1) in this cell lineage. Here we show that when macaques were inoculated with either the T cell tropic SIVmac239 or the macrophage tropic SIVmac316 carrying a Vpx point mutation that abrogates the recruitment of DCAF1 and the ensuing degradation of endogenous SAMHD1 in cultured CD4⁺ T cells, virus acquisition, progeny virion production in memory CD4⁺ T cells during acute infection, and the maintenance of set-point viremia were greatly attenuated. Revertant viruses emerging in two animals exhibited an augmented replication phenotype in memory CD4⁺ T lymphocytes both in vitro and in vivo, which was associated with reduced levels of endogenous SAMHD1. These results indicate that a critical role of Vpx in vivo is to promote the degradation of SAMHD1 in memory CD4⁺ T lymphocytes, thereby generating high levels of plasma viremia and the induction of immunodeficiency.

Primate lentiviruses, such as HIV and its SIV simian relative, encode accessory proteins that suppress cellular restriction factors interfering with efficient replication. One of these, designated Vpx, is produced in infected cells by HIV-2 and some SIV strains, which cause endemic infections in African monkeys. The primary function of Vpx has long been thought to facilitate infectivity in dendritic cells and macrophage by degrading the Sterile Alpha Motif and HD domain-containing protein 1 (SAMHD1), which restricts virus replication in these cells. Using SIVmac carrying a mutated Vpx gene with a single amino acid change that prevents it from binding to DCAF1 and subsequently mediating the degradation of SAMHD1, we show that virus infection of CD4⁺ T lymphocytes is markedly compromised both in vitro and in vivo. The SIV Vpx mutant is severely attenuated in establishing new infections in inoculated rhesus monkeys, in producing high levels of virus progeny, in degrading SAMHD1 in memory CD4⁺ T cell in infected animals, and in inducing symptomatic disease. Thus, although once considered to be only critical for optimal replication in macrophage based on earlier studies performed with cultured cells, the SIV Vpx protein is functionally important in vivo for establishing the primary infection in rhesus macaques, sustaining high levels of virus replication in CD4⁺ T lymphocytes, and promoting the onset of symptomatic immunodeficiency.

The susceptibility of primate lentiviruses to nucleosides and Vpx during infection of dendritic cells is regulated by CA

The block toward human immunodeficiency virus type 1 (HIV-1) infection of dendritic cells (DCs) can be relieved by Vpx (viral protein X), which degrades sterile alpha motif-hydroxylase domain 1 (SAMHD1) or by exogenously added deoxynucleosides (dNs), lending support to the hypothesis that SAMHD1 acts by limiting deoxynucleoside triphosphates (dNTPs). This notion has, however, been questioned. We show that while dNs and Vpx increase the infectivity of HIV-1, only the latter restores the infectivity of a simian immunodeficiency virus of macaques variant, SIVMACΔVpx virus. This distinct behavior seems to map to CA, suggesting that species-specific CA interactors modulate infection of DCs.

Mouse knockout models for HIV-1 restriction factors

Infection of cells with human immunodeficiency virus 1 (HIV-1) is controlled by restriction factors, host proteins that counteract a variety of steps in the life cycle of this lentivirus. These include SAMHD1, APOBEC3G and tetherin, which block reverse transcription, hypermutate viral DNA and prevent progeny virus release, respectively. These and other HIV-1 restriction factors are conserved and have clear orthologues in the mouse. This review summarises studies in knockout mice lacking HIV-1 restriction factors. In vivo experiments in such animals have not only validated in vitro data obtained from cultured cells, but have also revealed new findings about the biology of these proteins. Indeed, genetic ablation of HIV-1 restriction factors in the mouse has provided evidence that restriction factors control retroviruses and other viruses in vivo and has led to new insights into the mechanisms by which these proteins counteract infection. For example, in vivo experiments in knockout mice demonstrate that virus control exerted by restriction factors can shape adaptive immune responses. Moreover, the availability of animals lacking restriction factors opens the possibility to study the function of these proteins in other contexts such as autoimmunity and cancer. Further in vivo studies of more recently identified HIV-1 restriction factors in gene targeted mice are, therefore, justified.

Cullin4A and cullin4B are interchangeable for HIV Vpr and Vpx action through the CRL4 ubiquitin ligase complex

Human immunodeficiency virus (HIV) seizes control of cellular cullin-RING E3 ubiquitin ligases (CRLs) to promote viral replication. HIV-1 Vpr and HIV-2/simian immunodeficiency virus (SIV) Vpr and Vpx engage the cullin4 (CUL4)-containing ubiquitin ligase complex (CRL4) to cause polyubiquitination and proteasomal degradation of host proteins, including ones that block infection. HIV-1 Vpr engages CRL4 to trigger the degradation of uracil-N-glycosylase 2 (UNG2). Both HIV-1 Vpr and HIV-2/SIV Vpr tap CRL4 to initiate G2 cell cycle arrest. HIV-2/SIV Vpx secures CRL4 to degrade the antiviral protein SAMHD1. CRL4 includes either cullin4A (CUL4A) or cullin4B (CUL4B) among its components. Whether Vpr or Vpx relies on CUL4A, CUL4B, or both to act through CRL4 is not known. Reported structural, phenotypic, and intracellular distribution differences between the two CUL4 types led us to hypothesize that Vpr and Vpx employ these in a function-specific manner. Here we determined CUL4 requirements for HIV-1 and HIV-2/SIV Vpr-mediated G2 cell cycle arrest, HIV-1 Vpr-mediated UNG2 degradation, and HIV-2 Vpx-mediated SAMHD1 degradation. Surprisingly, CUL4A and CUL4B are exchangeable for CRL4-dependent Vpr and Vpx action, except in primary macrophages, where Vpx relies on both CUL4A and CUL4B for maximal SAMHD1 depletion. This work highlights the need to consider both CUL4 types for Vpr and Vpx functions and also shows that the intracellular distribution of CUL4A and CUL4B can vary by cell type.

IMPORTANCE

The work presented here shows for the first time that HIV Vpr and Vpx do not rely exclusively on CUL4A to cause ubiquitination through the CRL4 ubiquitin ligase complex. Furthermore, our finding that intracellular CUL4 and SAMHD1 distributions can vary with cell type provides the basis for reconciling previous disparate findings regarding the site of SAMHD1 depletion. Finally, our observations with primary immune cells provide insight into the cell biology of CUL4A and CUL4B that will help differentiate the functions of these similar proteins.

New insights into an X-traordinary viral protein

Vpx is a protein encoded by members of the HIV-2/SIVsmm and SIVrcm/SIVmnd-2 lineages of primate lentiviruses, and is packaged into viral particles. Vpx plays a critical role during the early steps of the viral life cycle and has been shown to counteract SAMHD1, a restriction factor in myeloid and resting T cells. However, it is becoming evident that Vpx is a multifunctional protein in that SAMHD1 antagonism is likely not its sole role. This review summarizes the current knowledge on this X-traordinary protein.

Conference highlights of the 16th International Conference on Human Retrovirology: HTLV and related retroviruses, 26-30 June 2013, Montreal, Canada

The 16th International Conference on Human Retrovirology: HTLV and Related Retroviruses was held in Montreal, Québec from June 26th to June 30th, 2013 and was therefore hosted by a Canadian city for the first time. The major topic of the meeting was human T-lymphotropic viruses (HTLVs) and was covered through distinct oral and poster presentation sessions: clinical research, animal models, immunology, molecular and cellular biology, human endogenous and emerging exogenous retroviruses and virology. In this review, highlights of the meeting are provided by different experts for each of these research areas.