DNA methylation of retroviral promoters and enhancers localized in the provirus 5′ long terminal repeat (LTR) is considered to be a mechanism of transcriptional suppression that allows retroviruses to evade host immune responses and antiretroviral drugs. However, the role of DNA methylation in the control of HIV-1 latency has never been unambiguously demonstrated, in contrast to the apparent importance of transcriptional interference and chromatin structure, and has never been studied in HIV-1-infected patients. Here, we show in an in vitro model of reactivable latency and in a latent reservoir of HIV-1-infected patients that CpG methylation of the HIV-1 5′ LTR is an additional epigenetic restriction mechanism, which controls resistance of latent HIV-1 to reactivation signals and thus determines the stability of the HIV-1 latency. CpG methylation acts as a late event during establishment of HIV-1 latency and is not required for the initial provirus silencing. Indeed, the latent reservoir of some aviremic patients contained high proportions of the non-methylated 5′ LTR. The latency controlled solely by transcriptional interference and by chromatin-dependent mechanisms in the absence of significant promoter DNA methylation tends to be leaky and easily reactivable. In the latent reservoir of HIV-1-infected individuals without detectable plasma viremia, we found HIV-1 promoters and enhancers to be hypermethylated and resistant to reactivation, as opposed to the hypomethylated 5′ LTR in viremic patients. However, even dense methylation of the HIV-1 5′LTR did not confer complete resistance to reactivation of latent HIV-1 with some histone deacetylase inhibitors, protein kinase C agonists, TNF-α, and their combinations with 5-aza-2deoxycytidine: the densely methylated HIV-1 promoter was most efficiently reactivated in virtual absence of T cell activation by suberoylanilide hydroxamic acid. Tight but incomplete control of HIV-1 latency by CpG methylation might have important implications for strategies aimed at eradicating HIV-1 infection.

Hepatitis C virus (HCV) entry is dependent on coreceptor complex formation between the tetraspanin superfamily member CD81 and the tight junction protein claudin-1 (CLDN1) on the host cell membrane. The receptor tyrosine kinase EGFR acts as a cofactor for HCV entry by promoting CD81-CLDN1 complex formation via unknown mechanisms. We identify the GTPase HRas, activated downstream of EGFR signaling, as a key host signal transducer for EGFR-mediated HCV entry. Proteomic analysis revealed that HRas associates with tetraspanin CD81, CLDN1, and the previously unrecognized HCV entry cofactors integrin β1 and Ras-related protein Rap2B in hepatocyte membranes. HRas signaling is required for lateral membrane diffusion of CD81, which enables tetraspanin receptor complex assembly. HRas was also found to be relevant for entry of other viruses, including influenza. Our data demonstrate that viruses exploit HRas signaling for cellular entry by compartmentalization of entry factors and receptor trafficking.

Chronic hepatitis B virus (HBV) infection is a major cause of chronic liver disease and cancer worldwide. The mechanisms of viral genome sensing and the evasion of innate immune responses by HBV infection are still poorly understood. Recently, the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) was identified as a DNA sensor. In this study, we investigated the functional role of cGAS in sensing HBV infection and elucidate the mechanisms of viral evasion. We performed functional studies including loss-of-function and gain-of-function experiments combined with cGAS effector gene expression profiling in an infectious cell culture model, primary human hepatocytes, and HBV-infected human liver chimeric mice. Here, we show that cGAS is expressed in the human liver, primary human hepatocytes, and human liver chimeric mice. While naked relaxed-circular HBV DNA is sensed in a cGAS-dependent manner in hepatoma cell lines and primary human hepatocytes, host cell recognition of viral nucleic acids is abolished during HBV infection, suggesting escape from sensing, likely during packaging of the genome into the viral capsid. While the hepatocyte cGAS pathway is functionally active, as shown by reduction of viral covalently closed circular DNA levels in gain-of-function studies, HBV infection suppressed cGAS expression and function in cell culture models and humanized mice. Conclusion: HBV exploits multiple strategies to evade sensing and antiviral activity of cGAS and its effector pathways.

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We demonstrate mechanisms by which HIV-1 appears to facilitate its own infection in ex vivo-infected human lymphoid tissue. In this system, HIV-1 readily infects various CD4 ؉ T cells, but productive viral infection was supported predominantly by activated T cells expressing either CD25 or HLA-DR or both (CD25/ HLA-DR) but not other activation markers: There was a strong positive correlation (r ‫؍‬ 0.64, P ‫؍‬ .001) between virus production and the number of CD25 ؉ / HLA-DR IntroductionCD4 ϩ T lymphocytes are the major target for HIV-1 infection, 1 and their loss is the hallmark of HIV-1 disease. [2][3][4] It is well established that the critical event of HIV-1 infection occurs in lymphoid tissue where T lymphocytes expressing CD4 constitute a highly heterogeneous population different in many parameters, in particular, their activation status. [5][6][7][8] Unlike single cell cultures in vitro, the tissue microenvironment provides conditions for both activated and nonactivated cells to be productively infected. 9,10 Nevertheless, CD4 ϩ T-cell activation is thought to be a major factor in facilitating HIV-1 infection of these cells. 11,12 This, and several other observations, have led to the widely accepted hypothesis that tissue activation is a major force, driving HIV-1 disease progression (for review, see Grossman et al 13,14 ). The fraction of activated T lymphocytes and other cell types is increased in HIV-1-infected patients, and we have recently documented distorted activation pattern of lymphocytes in lymph nodes and tonsils from HIV-1-infected patients. 15 However, the patterns of activation determining cell susceptibility to productive HIV-1 infection, the contribution of nonactivated cells to the viral load, and the relationship between activation status and cell loss in lymphoid tissues remain largely unknown, in part because of the lack of an adequate experimental model to address these problems.Here, to reveal mechanisms connecting cell activation and HIV-1 infection, we used ex vivo-infected human lymphoid tissues. These tissues support productive HIV-1 infection ex vivo without exogenous activation 16,17 that is needed to efficiently infect peripheral blood mononuclear cells (PBMCs); such conditions are unlikely to reflect the conditions of cell activation in vivo even remotely. Moreover, in ex vivo tissues, similar to in vivo, 10,18 both activated and nonactivated cells become productively infected, 19 providing an experimental system to address some aspects of cell activation in HIV-1-infected human lymphoid tissue.By comparing matched infected and noninfected lymphoid tissues from individual donors, we demonstrated here that viral load in this system depends on the number of activated target cells, but only of a particular pattern, CD25 ϩ /HLA-DR ϩ . Furthermore, we found that viral infection mobilizes new HIV-1 cell targets by activating uninfected cells to express the very same pattern of markers that is associated with the efficient HIV-1 replication. HIV-1 infection of these cells drives th...

To design strategies to purge latent reservoirs of human immunodeficiency virus type 1 (HIV-1), we investigated mechanisms by which a non-tumor-promoting phorbol ester, prostratin, inhibits infection of CD4 ؉ T lymphocytes and at the same time reactivates virus from latency.

The human butyrophilin (BTN) 3 or CD277 molecules belong to the B7 family members and are expressed in various immune cells such as T and NK cells. Here, we show that CD277 triggering considerably enhances TCR-induced cytokine production and cell proliferation, even when another co-stimulatory molecule, CD28, is engaged. These CD277-induced additive functional effects are in accordance with the detection of early T-cell activation events such as TCR-induced cell signaling being increased upon CD277 engagement. However, we found that CD277 triggering is not involved in CD16-or NKp46-induced NK cell activation. BTN3/CD277 comprises three structurally related members, BTN3A1, BTN3A2 and BTN3A3. CD277 antibodies recognize all isoforms and we describe a differential expression of BTN3 isoforms between T and NK cells that could explain differential CD277 functions between T and NK cells. Our results show that, while T cells express all BTN3/CD277 transcripts, NK cells express mostly BTN3A2, which lacks the B30.2 intracellular domain. Furthermore, NKp30-induced cytokine production is decreased by the specific engagement of BTN3A2, but not by BTN3A1 triggering. Thus, we provide new insights into the CD277 co-stimulatory pathway that may differentially participate in the regulation of various cell-mediated immune responses.

BackgroundHuman immunodeficiency virus type 1 (HIV-1) latency represents the major barrier to virus eradication in infected individuals because cells harboring latent HIV-1 provirus are not affected by current antiretroviral therapy (ART). We previously demonstrated that DNA methylation of HIV-1 long terminal repeat (5’ LTR) restricts HIV-1 reactivation and, together with chromatin conformation, represents an important mechanism of HIV-1 latency maintenance. Here, we explored the new issue of temporal development of DNA methylation in latent HIV-1 5’ LTR.ResultsIn the Jurkat CD4+ T cell model of latency, we showed that the stimulation of host cells contributed to de novo DNA methylation of the latent HIV-1 5’ LTR sequences. Consecutive stimulations of model CD4+ T cell line with TNF-α and PMA or with SAHA contributed to the progressive accumulation of 5’ LTR DNA methylation. Further, we showed that once established, the high DNA methylation level of the latent 5’ LTR in the cell line model was a stable epigenetic mark. Finally, we explored the development of 5’ LTR DNA methylation in the latent reservoir of HIV-1-infected individuals who were treated with ART. We detected low levels of 5’ LTR DNA methylation in the resting CD4+ T cells of the group of patients who were treated for up to 3 years. However, after long-term ART, we observed an accumulation of 5’ LTR DNA methylation in the latent reservoir. Importantly, within the latent reservoir of some long-term-treated individuals, we uncovered populations of proviral molecules with a high density of 5’ LTR CpG methylation.ConclusionsOur data showed the presence of 5’ LTR DNA methylation in the long-term reservoir of HIV-1-infected individuals and implied that the transient stimulation of cells harboring latent proviruses may contribute, at least in part, to the methylation of the HIV-1 promoter.Electronic supplementary materialThe online version of this article (doi:10.1186/s13148-016-0185-6) contains supplementary material, which is available to authorized users.