Restriction of HIV-1 replication in monocytes is abolished by Vpx of SIVsmmPBj

Gene editing of SAMHD1 in macrophage-like cells reveals complex relationships between SAMHD1 phospho-regulation, HIV-1 restriction, and cellular dNTP levels

IMPORTANCE

We introduce BLaER1 cells as an alternative myeloid cell model in combination with CRISPR/Cas9-mediated gene editing to study the influence of sterile α motif and HD domain-containing protein 1 (SAMHD1) T592 phosphorylation on anti-viral restriction and the control of cellular dNTP levels in an endogenous, physiologically relevant context. A proper understanding of the mechanism of the anti-viral function of SAMHD1 will provide attractive strategies aiming at selectively manipulating SAMHD1 without affecting other cellular functions. Even more, our toolkit may inspire further genetic analysis and investigation of restriction factors inhibiting retroviruses and their cellular function and regulation, leading to a deeper understanding of intrinsic anti-viral immunity.

Gene editing of SAMHD1 in macrophage-like cells reveals complex relationships between SAMHD1 phospho-regulation, HIV-1 restriction and cellular dNTP levels

Sterile α motif (SAM) and HD domain-containing protein 1 (SAMHD1) is a dNTP triphosphate triphosphohydrolase (dNTPase) and a potent restriction factor for immunodeficiency virus 1 (HIV-1), active in myeloid and resting CD4+ T cells. The anti-viral activity of SAMHD1 is regulated by dephosphorylation of the residue T592. However, the impact of T592 phosphorylation on dNTPase activity is still under debate. Whether additional cellular functions of SAMHD1 impact anti-viral restriction is not completely understood. We report BLaER1 cells as a novel human macrophage HIV-1 infection model combined with CRISPR/Cas9 knock-in (KI) introducing specific mutations into the SAMHD1 locus to study mutations in a physiological context. Transdifferentiated BLaER1 cells harbor active dephosphorylated SAMHD1 that blocks HIV-1 reporter virus infection. As expected, homozygous T592E mutation, but not T592A, relieved a block to HIV-1 reverse transcription. Co-delivery of VLP-Vpx to SAMHD1 T592E KI mutant cells did not further enhance HIV-1 infection indicating the absence of an additional SAMHD1-mediated antiviral activity independent of T592 de-phosphorylation. T592E KI cells retained dNTP levels similar to WT cells indicating uncoupling of anti-viral and dNTPase activity of SAMHD1. The integrity of the catalytic site in SAMHD1 was critical for anti-viral activity, yet poor correlation of HIV-1 restriction and global cellular dNTP levels was observed in cells harboring catalytic core mutations. Together, we emphasize the complexity of the relationship between HIV-1 restriction, SAMHD1 enzymatic function and T592 phospho-regulation and provide novel tools for investigation in an endogenous and physiological context.

Does Sertraline Affect Hypothalamic Food Intake Peptides in the Rat Experimental Model of Chronic Mild Stress-Induced Depression?

Depression is a chronic, recurrent and life-threatening disease affecting approximately 15% of the world population. Depression is responsible for neuropathologies like decreased neurogenesis and increased dendritic atrophy. Antidepressant treatments increase hippocampal neurogenesis and neurotrophic factor expression. Based on this information, it was aimed to investigate effect of sertraline on depression in rats with chronic mild stress (CMS) model and to determine how it affects cell proliferation and hypothalamic peptide levels in hypothalamus. 56 adult male Wistar albino; control, depression(D), depression + sertraline, sertraline were divided into groups. Various stressors were applied to D for 30 days. Open field test (OFT) and forced swimming test (FST) were conducted to check whether the animals were depressed. On the 16th day osmotic minipump was placed subcutaneously and sertraline (10 mg/kg/day) was administered for 15 days. Behavior tests were done. Hypothalamic peptide gene expression levels were analyzed by quantitative RT-PCR. Statistical evaluations were made using ANOVA. It caused a decrease in the percentage of movement in the D and control groups in the OFT, an increase in the immobility time in the D group in the FST, and an increase in the swimming behavior in the DS group. Animals did not show any anxiological behavior based on the elevated plus maze test results. CMS caused a decrease in GLUT2 and NPY gene expression in the hypothalamus of animals, an increase in POMC and FGFR2, and an increase in IGFIR and GLUT2 gene expression in the DS group. Sertraline has been shown to ameliorate the effects of CMS-induced depression. Sertraline is thought to have a positive regulatory effect on both the formation of neural precursor cells and the survival of newly formed neurons in the hypothalamus. Newly formed neurons in the hypothalamus express food intake-related NPY, POMC, GLUT2 neurons, and thus hypothalamic tanycytes may play a key role in the control of energy metabolism.

HIV-2 Vpx neutralizes host restriction factor SAMHD1 to promote viral pathogenesis

SAMHD1, a human host factor found in myeloid cells which restricts HIV-1 replication. It depletes the dNTPs pool for viral cDNA syntheses, thus preventing the viral replication in the cells. The viral accessory protein, Vpx, exists only in SIVmac/HIV-2 particles. Vpx in SIVmac can induce proteosomal degradation of SAMHD1, which then leads to a decrease in the cytoplasmic dNTP pool. The protein-protein interaction between Vpx and SAMHD1 and its consequences are still unclear. Methods: In this study, we cloned, for the first time, Vpx gene from a HIV-2 infected patient and found up to 30% sequence variation compared to known HIV-2 strains. We then analyzed the role of SAMHD1 protein expression in transfected THP-1 and U937 cells by transfecting with the Vpx gene derived from SIVmac, HIV-2 from the NIH sample as well as HIV-2 from a Saudi patient. We found that Vpx gene expression led to reduced levels of intracellular SAMHD1. When the supernatants of the transfected cell lines were examined for secreted cytokines, chemokines and growth factors, Vpx expression seemed to be suppressive of pro-inflammatory response, and skewed the immune response towards an anti-inflammatory response. These results suggest that Vpx can act at two levels: clearance of intracellular restriction factor and suppression of cytokine storm: both aimed at long-term latency and host-pathogen stand-off, suggesting that Vpx is likely to be a potential therapeutic target.

Alarmin S100A9 restricts retroviral infection by limiting reverse transcription in human dendritic cells

Dendritic cells (DC) subsets, like Langerhans cells (LC), are immune cells involved in pathogen sensing. They express specific antimicrobial cellular factors that are able to restrict infection and limit further pathogen transmission. Here, we identify the alarmin S100A9 as a novel intracellular antiretroviral factor expressed in human monocyte-derived and skin-derived LC. The intracellular expression of S100A9 is decreased upon LC maturation and inversely correlates with enhanced susceptibility to HIV-1 infection of LC. Furthermore, silencing of S100A9 in primary human LC relieves HIV-1 restriction while ectopic expression of S100A9 in various cell lines promotes intrinsic resistance to both HIV-1 and MLV infection by acting on reverse transcription. Mechanistically, the intracellular expression of S100A9 alters viral capsid uncoating and reverse transcription. S100A9 also shows potent inhibitory effect against HIV-1 and MMLV reverse transcriptase (RTase) activity in vitro in a divalent cation-dependent manner. Our findings uncover an unexpected intracellular function of the human alarmin S100A9 in regulating antiretroviral immunity in Langerhans cells.

Interference with SAMHD1 Restores Late Gene Expression of Modified Vaccinia Virus Ankara in Human Dendritic Cells and Abrogates Type I Interferon Expression

Attenuated poxviruses like modified vaccinia virus Ankara (MVA) are promising vectors for vaccines against infectious diseases and cancer. However, host innate immune responses interfere with the viral life cycle and also influence the immunogenicity of vaccine vectors. Sterile alpha motif (SAM) domain and histidine-aspartate (HD) domain-containing protein 1 (SAMHD1) is a phosphohydrolase and reduces cellular deoxynucleoside triphosphate (dNTP) concentrations, which impairs poxviral DNA replication in human dendritic cells (DCs). Human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus (SIV) encode an accessory protein called viral protein X (Vpx) that promotes proteasomal degradation of SAMHD1, leading to a rapid increase in cellular dNTP concentrations. To study the function of SAMHD1 during MVA infection of human DCs, the SIV vpx gene was introduced into the MVA genome (resulting in recombinant MVA-vpx). Infection of human DCs with MVA-vpx led to SAMHD1 protein degradation and enabled MVA-vpx to replicate its DNA genome and to express genes controlled by late promoters. Late gene expression by MVA-vpx might improve its vaccine vector properties; however, type I interferon expression was unexpectedly blocked by Vpx-expressing MVA. MVA-vpx can be used as a tool to study poxvirus-host interactions and vector safety.IMPORTANCE SAMHD1 is a phosphohydrolase and reduces cellular dNTP concentrations, which impairs poxviral DNA replication. The simian SIV accessory protein Vpx promotes degradation of SAMHD1, leading to increased cellular dNTP concentrations. Vpx addition enables poxviral DNA replication in human dendritic cells (DCs), as well as the expression of viral late proteins, which is normally blocked. SAMHD1 function during modified vaccinia virus Ankara (MVA) infection of human DCs was studied with recombinant MVA-vpx expressing Vpx. Infection of human DCs with MVA-vpx decreased SAMHD1 protein amounts, enabling MVA DNA replication and expression of late viral genes. Unexpectedly, type I interferon expression was blocked after MVA-vpx infection. MVA-vpx might be a good tool to study SAMHD1 depletion during poxviral infections and to provide insights into poxvirus-host interactions.

Human SAMHD1 restricts the xenotransplantation relevant porcine endogenous retrovirus (PERV) in non-dividing cells

The release of porcine endogenous retrovirus (PERV) particles from pig cells is a potential risk factor during xenotransplantation by way of productively infecting the human transplant recipient. Potential countermeasures against PERV replication are restriction factors that block retroviral replication. SAMHD1 is a triphosphohydrolase that depletes the cellular pool of dNTPs in non-cycling cells starving retroviral reverse transcription. We investigated the antiviral activity of human SAMHD1 against PERV and found that SAMHD1 potently restricts its reverse transcription in human monocytes, monocyte-derived dendritic cells (MDDC), or macrophages (MDM) and in monocytic THP-1 cells. Degradation of SAMHD1 by SIVmac Vpx or CRISPR/Cas9 knock-out of SAMHD1 allowed for PERV reverse transcription. Addition of deoxynucleosides alleviated the SAMHD1-mediated restriction suggesting that SAMHD1-mediated degradation of dNTPs restricts PERV replication in these human immune cells. In conclusion, our findings highlight SAMHD1 as a potential barrier to PERV transmission from pig transplants to human recipients during xenotransplantation.

Tropism, intracerebral distribution, and transduction efficiency of HIV- and SIV-based lentiviral vectors after injection into the mouse brain: a qualitative and quantitative in vivo study

Lentiviruses are suitable to transfer potential therapeutic genes into non-replicating cells such as neurons, but systematic in vivo studies on transduction of neural cells within the complete brain are missing. We analysed the distribution of transduced cells with respect to brain structure, virus tropism, numbers of transduced neurons per brain, and influence of the Vpx or Vpr accessory proteins after injection of vectors based on SIVsmmPBj, HIV-2, and HIV-1 lentiviruses into the right striatum of the mouse brain. Transduced cells were found ipsilaterally around the injection canal, in corpus striatum and along corpus callosum, irrespective of the vector type. All vectors except HIV-2SEW transduced also single cells in the olfactory bulb, hippocampus, and cerebellum. Vector HIV-2SEW was the most neuron specific. However, vectors PBjSEW and HIV-1SEW transduced more neurons per brain (means 41,299 and 32,309) than HIV-2SEW (16,102). In the presence of Vpx/Vpr proteins, HIV-2SEW(Vpx) and HIV-1SEW(Vpr) showed higher overall transduction efficiencies (30,696 and 27,947 neurons per brain) than PBjSEW(Vpx) (6636). The distances of transduced cells from the injection canal did not differ among the viruses but correlated positively with the numbers of transduced neurons. The presence of Vpx/Vpr did not increase the numbers of transduced neurons. Parental virus type and the vector equipment seem to influence cellular tropism and transduction efficiency. Thus, precision of injection and choice of virus pseudotype are not sufficient when targeted lentiviral vector transduction of a defined brain cell population is required.

TLR7/8 agonist induces a post-entry SAMHD1-independent block to HIV-1 infection of monocytes

Background

Monocytes, the primary myeloid cell-type in peripheral blood, are resistant to HIV-1 infection as a result of the lentiviral restriction factor SAMHD1. Toll-like receptors recognize microbial pathogen components, inducing the expression of antiviral host proteins and proinflammatory cytokines. TLR agonists that mimic microbial ligands have been found to have activity against HIV-1 in macrophages. The induction of restriction factors in monocytes by TLR agonist activation has not been well studied. To analyze restriction factor induction by TLR activation in monocytes, we used the imidazoquinoline TLR7/8 agonist R848 and infected with HIV-1 reporter virus that contained packaged viral accessory protein Vpx, which allows the virus to escape SAMHD1-mediated restriction. 

Results

R848 prevented the replication of Vpx-containing HIV-1 and HIV-2 in peripheral blood mononuclear cells and monocytes. The block was post-entry but prior to reverse transcription of the viral genomic RNA. The restriction was associated with destabilization of the genomic RNA molecules of the in-coming virus particle. R848 treatment of activated T cells did not protect them from infection but treated monocytes produced high levels of proinflammatory cytokines, including type-I IFN that protected bystander activated T cells from infection.

Conclusion

The activation of TLR7/8 induces two independent restrictions to HIV-1 replication in monocytes: a cell-intrinsic block that acts post-entry to prevent reverse transcription; and a cell-extrinsic block, in which monocytes produce high levels of proinflammatory cytokines (primarily type-I IFN) that protects bystander monocytes and T lymphocytes. The cell-intrinsic block may result from the induction of a novel restriction factor, which can be termed Lv5 and acts by destabilizing the in-coming viral genomic RNA, either by the induction of a host ribonuclease or by disrupting the viral capsid. TLR agonists are being developed for therapeutic use to diminish the size of the latent provirus reservoir in HIV-1 infected individuals. Such drugs may both induce latent provirus expression and restrict virus replication during treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0316-3) contains supplementary material, which is available to authorized users.

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

BACKGROUND

SIVMAC/HIV-2 Vpx recruits the CUL4A-DCAF1 E3 ubiquitin ligase complex to degrade the deoxynucleotide hydrolase SAMHD1. This increases the concentration of deoxynucleotides available for reverse transcription in myeloid cells and resting T cells. Accordingly, transduction of these cells by SIVMAC requires Vpx. Virus-like particles containing SIVMAC Vpx (Vpx-VLPs) also increase the efficiency of HIV-1 transduction in these cells, and rescue transduction by HIV-1, but not SIVMAC, in mature monocyte-derived dendritic cells (MDDCs). Differences in Vpx mechanism noted at that time, along with recent data suggesting that SAMHD1 gains additional restriction capabilities in the presence of type I IFN prompted further examination of the role of Vpx and SAMHD1 in HIV-1 transduction of mature MDDCs.

RESULTS

When challenged with Vpx-VLPs, SAMHD1 was degraded in MDDCs even after cells had been matured with LPS, though there was no increase in deoxynucleotide levels. Steady-state levels of HIV-1 late reverse transcription products in mature MDDCs were increased to the same extent by either Vpx-VLPs or exogenous nucleosides. In contrast, only Vpx-VLPs increased the levels of 2-LTR circles and proviral DNA in myeloid cells. These results demonstrate that exogenous nucleosides and Vpx-VLPs both increase the levels of HIV-1 cDNA in myeloid cells, but only Vpx-VLPs rescue 2-LTR circles and proviral DNA in myeloid cells with a previously established antiviral state. Finally, since trans-acting Vpx-VLPs provide long-lasting rescue of HIV-1 vector transduction in the face of the antiviral state, and exogenous nucleosides do not, exogenous nucleosides were used to achieve efficient transduction of MDDCs by vectors that stably encode Vprs and Vpxs from a collection of primate lentiviruses. Vpr from SIVDEB or SIVMUS, Vpx from SIVMAC251 or HIV-2, but not SIVRCM, degraded endogenous SAMHD1, increased steady-state levels of HIV-1 cDNA, and rescued HIV-1 from the antiviral state in MDDCs.

CONCLUSION

Inhibition of deoxynucleotide hydrolysis by promoting SAMHD1 degradation is not the only mechanism by which Vpx rescues HIV-1 in MDDCs from the antiviral state. Vpx has an additional effect on HIV-1 transduction of these cells that occurs after completion of reverse transcription and acts independently of deoxynucleotide levels.

Antagonism of SAMHD1 is actively maintained in natural infections of simian immunodeficiency virus

Restriction factors are effectors of the innate immune response to viral pathogens that inhibit viral replication by operating as molecular barriers to steps of the viral life cycle. The restriction factor SAMHD1 blocks lentiviral reverse transcription in myeloid cells and resting CD4+ T cells. Many lineages of lentiviruses, including HIV-2 and other simian immunodeficiency viruses, encode accessory genes that serve to counteract host SAMHD1 restriction by causing degradation of the antiviral factor. The viral accessory protein Vpr is responsible for SAMHD1 degradation in some lineages of lentiviruses, whereas in others the related protein Vpx assumes this task. However, HIV-1 has no SAMHD1 degradation capability, leading to questions about the selective advantage of this activity. We use an evolutionary approach to examine the importance of SAMHD1 antagonism for viral fitness by studying adaptation to host SAMHD1 in natural simian immunodeficiency virus infections of African Green Monkeys. We identified multiple SAMHD1 haplotypes in African Green Monkeys and find that the vpr gene from different strains of Simian Immunodeficiency Virus has adapted to the polymorphisms of the African Green Monkey population in which it is found. Such evidence of viral adaptation to host restriction indicates that SAMHD1 antagonism is actively maintained in natural infections and that this function must be advantageous to viral fitness, despite its absence in HIV-1.

Cellular and Biochemical Mechanisms of the Retroviral Restriction Factor SAMHD1

Replication of HIV-1 and other retroviruses is dependent on numerous host proteins in the cells. Some of the host proteins, however, function as restriction factors to block retroviral infection of target cells. The host protein SAMHD1 has been identified as the first mammalian deoxynucleoside triphosphate triphosphohydrolase (dNTPase), which blocks the infection of HIV-1 and other retroviruses in non-cycling immune cells. SAMHD1 protein is highly expressed in human myeloid-lineage cells and CD4⁺ T-lymphocytes, but its retroviral restriction function is only observed in noncycling cells. Recent studies have revealed biochemical mechanisms of SAMHD1-mediated retroviral restriction. In this review, the latest progress on SAMHD1 research is summarized and the mechanisms by which SAMHD1 mediates retroviral restriction are analyzed. Although the physiological function of SAMHD1 is largely unknown, this review provides perspectives about the role of endogenous SAMHD1 protein in maintaining normal cellular function, such as nucleic acid metabolism and the proliferation of cells.

Restriction of retroviral infection of macrophages

Primate immunodeficiency viruses are highly specialized lentiviruses that have evolved to successfully infect and persist for the lifetime of the host. Despite encountering numerous potent antiviral factors, HIVs and SIVs are successful pathogens due to the acquisition of equally potent countermeasures in the form of accessory genes. The accessory gene Vpx encoded by HIV-2 and a subset of SIVs have a profound effect on the ability of lentiviruses to infect non-dividing cells, such as macrophages. Although most virus replication occurs in activated CD4(+) T cells, myeloid lineage cells are natural targets of infection and play a central role in virus transmission, dissemination, and persistence. However, myeloid lineage cells are poorly sensitive to lentiviral infection due partly to the high-level expression of a host protein that regulates nucleic acid metabolism named SAMHD1. Degradation of SAMHD1 is induced by Vpx to eliminate this intrinsic antiviral factor. Importantly, SAMHD1 has also been implicated as a negative regulator of the innate immune response, so the interplay between SAMHD1 and Vpx is likely to have significant consequences for virus replication, persistence, and immune control.

SAMHD1 restricts HIV-1 infection in dendritic cells (DCs) by dNTP depletion, but its expression in DCs and primary CD4+ T-lymphocytes cannot be upregulated by interferons

Background

SAMHD1 is an HIV-1 restriction factor in non-dividing monocytes, dendritic cells (DCs), macrophages, and resting CD4⁺ T-cells. Acting as a deoxynucleoside triphosphate (dNTP) triphosphohydrolase, SAMHD1 hydrolyzes dNTPs and restricts HIV-1 infection in macrophages and resting CD4⁺ T-cells by decreasing the intracellular dNTP pool. However, the intracellular dNTP pool in DCs and its regulation by SAMHD1 remain unclear. SAMHD1 has been reported as a type I interferon (IFN)-inducible protein, but whether type I IFNs upregulate SAMHD1 expression in primary DCs and CD4⁺ T-lymphocytes is unknown.

Results

Here, we report that SAMHD1 significantly blocked single-cycle and replication-competent HIV-1 infection of DCs by decreasing the intracellular dNTP pool and thereby limiting the accumulation of HIV-1 late reverse transcription products. Type I IFN treatment did not upregulate endogenous SAMHD1 expression in primary DCs or CD4⁺ T-lymphocytes, but did in HEK 293T and HeLa cell lines. When SAMHD1 was over-expressed in these two cell lines to achieve higher levels than that in DCs, no HIV-1 restriction was observed despite partially reducing the intracellular dNTP pool.

Conclusions

Our results suggest that SAMHD1-mediated reduction of the intracellular dNTP pool in DCs is a common mechanism of HIV-1 restriction in myeloid cells. Endogenous expression of SAMHD1 in primary DCs or CD4⁺ T-lymphocytes is not upregulated by type I IFNs.

Restricting HIV the SAMHD1 way: through nucleotide starvation

HIV replication is limited by cellular restriction factors, such as APOBEC and tetherin, which themselves are counteracted by viral proteins. SAMHD1 was recently identified as a novel HIV restriction factor in myeloid cells, and was shown to be blocked by the lentiviral protein Vpx. SAMHD1 limits viral replication through an original mechanism: it hydrolyses intracellular dNTPs in non-cycling cells, thus decreasing the amount of these key substrates, which are required for viral DNA synthesis. In this Progress article, we describe how SAMHD1 regulates the pool of intracellular nucleotides to control HIV replication and the innate immune response.

Neuropeptide receptor ligands as drugs for psychiatric diseases: the end of the beginning?

The search for novel drugs for treating psychiatric disorders is driven by the growing medical need to improve on the effectiveness and side-effect profile of currently available therapies. Given the wealth of preclinical data supporting the role of neuropeptides in modulating behaviour, pharmaceutical companies have been attempting to target neuropeptide receptors for over two decades. However, clinical studies with synthetic neuropeptide ligands have been unable to confirm the promise predicted by studies in animal models. Here, we analyse preclinical and clinical results for neuropeptide receptor ligands that have been studied in clinical trials for psychiatric diseases, including agents that target the receptors for tachykinins, corticotropin-releasing factor, vasopressin and neurotensin, and suggest new ways to exploit the full potential of these candidate drugs.

Measles virus glycoprotein-pseudotyped lentiviral vectors are highly superior to vesicular stomatitis virus G pseudotypes for genetic modification of monocyte-derived dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells capable of promoting or regulating innate and adaptive immune responses against non-self antigens. To better understand the DC biology or to use them for immune intervention, a tremendous effort has been made to improve gene transfer in these cells. Lentiviral vectors (LVs) have conferred a huge advantage in that they can transduce nondividing cells such as human monocyte-derived DCs (MDDCs) but required high amounts of viral particles and/or accessory proteins such as Vpx or Vpr to achieve sufficient transduction rates. As a consequence, these LVs have been shown to cause dramatic functional modifications, such as the activation or maturation of transduced MDDCs. Taking advantage of new pseudotyped LVs, i.e., with envelope glycoproteins from the measles virus (MV), we demonstrate that MDDCs are transduced very efficiently with these new LVs compared to the classically used vesicular stomatitis virus G-pseudotyped LVs and thus allowed to achieve high transduction rates at relatively low multiplicities of infection. Moreover, in this experimental setting, no activation or maturation markers were upregulated, while MV-LV-transduced cells remained able to mature after an appropriate Toll-like receptor stimulation. We then demonstrate that our MV-pseudotyped LVs use DC-SIGN, CD46, and CD150/SLAM as receptors to transduce MDDCs. Altogether, our results show that MV-pseudotyped LVs provide the most accurate and simple viral method for efficiently transferring genes into MDDCs without affecting their activation and/or maturation status.

SAMHD1-deficient CD14+ cells from individuals with Aicardi-Goutières syndrome are highly susceptible to HIV-1 infection

Myeloid blood cells are largely resistant to infection with human immunodeficiency virus type 1 (HIV-1). Recently, it was reported that Vpx from HIV-2/SIVsm facilitates infection of these cells by counteracting the host restriction factor SAMHD1. Here, we independently confirmed that Vpx interacts with SAMHD1 and targets it for ubiquitin-mediated degradation. We found that Vpx-mediated SAMHD1 degradation rendered primary monocytes highly susceptible to HIV-1 infection; Vpx with a T17A mutation, defective for SAMHD1 binding and degradation, did not show this activity. Several single nucleotide polymorphisms in the SAMHD1 gene have been associated with Aicardi-Goutières syndrome (AGS), a very rare and severe autoimmune disease. Primary peripheral blood mononuclear cells (PBMC) from AGS patients homozygous for a nonsense mutation in SAMHD1 (R164X) lacked endogenous SAMHD1 expression and support HIV-1 replication in the absence of exogenous activation. Our results indicate that within PBMC from AGS patients, CD14+ cells were the subpopulation susceptible to HIV-1 infection, whereas cells from healthy donors did not support infection. The monocytic lineage of the infected SAMHD1 -/- cells, in conjunction with mostly undetectable levels of cytokines, chemokines and type I interferon measured prior to infection, indicate that aberrant cellular activation is not the cause for the observed phenotype. Taken together, we propose that SAMHD1 protects primary CD14+ monocytes from HIV-1 infection confirming SAMHD1 as a potent lentiviral restriction factor.

Lentiviral accessory proteins play important roles in antagonizing host proteins aimed at suppressing HIV-1 replication at a cellular level. The SIV/HIV-2 protein Vpx counteracts SAMHD1, a previously unknown antiviral factor within myeloid blood cells, rendering these cells permissive to primate immunodeficiency viruses. We confirm in this study that Vpx interacts with SAMHD1 leading to ubiquitin-mediated degradation of SAMHD1, and renders CD14 positive monocytes susceptible to HIV-1 infection. We provide new insights into the ability of SAMHD1 to protect monocytic cells from HIV-1 infection by using primary cells from patients with Aicardi-Goutières syndrome (AGS) lacking endogenous SAMHD1 expression. We show that peripheral monocytic cells of AGS patients are highly permissive to HIV-1. Thus, our study demonstrates that SAMHD1 is critical for restriction of HIV-1 infection in monocytes adding SAMHD1 as a novel innate defense factor.

Primate and feline lentiviruses in current intrinsic immunity research: the cat is back

Retroviral restriction factor research is explaining long-standing lentiviral mysteries. Asking why a particular retrovirus cannot complete a critical part of its life cycle in cells of a particular species has been the starting point for numerous discoveries, including heretofore elusive functions of HIV-1 accessory genes. The potential for therapeutic application is substantial. Analyzing the feline immunodeficiency virus (FIV) life cycle has been instrumental and the source of some surprising observations in this field. FIV is restricted in cells of various primates by several restriction factors including APOBEC3 proteins and, uniquely, TRIM proteins from both Old and New World monkeys. In contrast, the feline genome does not encode functional TRIM5alpha or TRIMCyp proteins and HIV-1 is primarily blocked in feline cells by APOBEC3 proteins. These can be overcome by inserting FIV vif or even SIVmac vif into HIV-1. The domestic cat and its lentivirus are positioned to offer strategic research opportunities as the field moves forward.

Limelight on two HIV/SIV accessory proteins in macrophage infection: is Vpx overshadowing Vpr?

HIV viruses encode a set of accessory proteins, which are important determinants of virulence due to their ability to manipulate the host cell physiology for the benefit of the virus. Although these viral proteins are dispensable for viral growth in many in vitro cell culture systems, they influence the efficiency of viral replication in certain cell types. Macrophages are early targets of HIV infection which play a major role in viral dissemination and persistence in the organism. This review focuses on two HIV accessory proteins whose functions might be more specifically related to macrophage infection: Vpr, which is conserved across primate lentiviruses including HIV-1 and HIV-2, and Vpx, a protein genetically related to Vpr, which is unique to HIV-2 and a subset of simian lentiviruses. Recent studies suggest that both Vpr and Vpx exploit the host ubiquitination machinery in order to inactivate specific cellular proteins. We review here why it remains difficult to decipher the role of Vpr in macrophage infection by HIV-1 and how recent data underscore the ability of Vpx to antagonize a restriction factor which counteracts synthesis of viral DNA in monocytic cells.

Interaction of Vpx and apolipoprotein B mRNA-editing catalytic polypeptide 3 family member A (APOBEC3A) correlates with efficient lentivirus infection of monocytes

The accessory protein Vpx is encoded by lentiviruses of the human immunodeficiency virus type 2 (HIV-2) and the simian immunodeficiency SIVsm/SIVmac lineage. It is packaged into virions and is indispensable in early steps of monocyte infection. HIV-1, which does not encode Vpx, is not able to infect human monocytes, but Vpx enables infection with HIV-1. The underlying mechanism is not completely understood. In this work, we focus on Vpx-mediated intracellular postentry events as counteraction of host cell proteins. We found that Vpx binds to apolipoprotein B mRNA-editing catalytic polypeptide 3 family member A (APOBEC3A; A3A), a member of the family of cytidine deaminases, present in monocytes. This interaction led to a reduction of the steady-state protein level of A3A. A single-point mutation in Vpx (H82A) abrogated binding to A3A and single-round infection of monocytes by HIV-1. Taken together, our data indicate that lentiviral Vpx counteracts A3A in human monocytes.