Selective transmission of R5 HIV-1 over X4 HIV-1 at the dendritic cell-T cell infectious synapse is determined by the T cell activation state

Schlafen 12 restricts HIV-1 latency reversal by a codon-usage dependent post-transcriptional block in CD4+ T cells

Latency is a major barrier towards virus elimination in HIV-1-infected individuals. Yet, the mechanisms that contribute to the maintenance of HIV-1 latency are incompletely understood. Here we describe the Schlafen 12 protein (SLFN12) as an HIV-1 restriction factor that establishes a post-transcriptional block in HIV-1-infected cells and thereby inhibits HIV-1 replication and virus reactivation from latently infected cells. The inhibitory activity is dependent on the HIV-1 codon usage and on the SLFN12 RNase active sites. Within HIV-1-infected individuals, SLFN12 expression in PBMCs correlated with HIV-1 plasma viral loads and proviral loads suggesting a link with the general activation of the immune system. Using an RNA FISH-Flow HIV-1 reactivation assay, we demonstrate that SLFN12 expression is enriched in infected cells positive for HIV-1 transcripts but negative for HIV-1 proteins. Thus, codon-usage dependent translation inhibition of HIV-1 proteins participates in HIV-1 latency and can restrict the amount of virus release after latency reversal.

SARS-CoV-2 and HIV: Impact on Pulmonary Epithelial Cells

The SARS-CoV-2 pandemic provides a natural opportunity for the collision of coronavirus disease-2019 (COVID-19) with chronic infections, which place numerous individuals at high risk of severe COVID-19. Infection with Human Immunodeficiency Virus (HIV), a global epidemic, remains a major public health concern. Whether prior HIV+ status exacerbates COVID-19 warrants investigation. Herein, we characterized the impact of SARS-CoV-2 in human bronchial epithelial cells (HBECs) previously exposed to HIV. We optimized the air-liquid interface (ALI) cell culture technique to allow for challenges with HIV at the basolateral cell surface and SARS-CoV-2 spike protein on the apical surface, followed by genetic analyses for cellular stress/toxicity and innate/adaptive immune responses. Our results suggest that the IL-10 pathway was consistently activated in HBECs treated with spike, HIV, or a combination. Recombinant spike protein elicited COVID-19 cytokine storms while HIV activated different signaling pathways. HIV-treated HBECs could no longer activate NF-kB, pro-inflammatory TRAF-6 ubiquitination nor RIP1 signaling. Combinations of HIV and SARS-CoV-2 spike increased gene expression for activation of endoplasmic reticulum-phagosome pathway and downregulated non-canonical NF-kB pathways that are key in functional regulatory T cells and RNA Polymerase II transcription. Our in vitro studies suggest that prior HIV infection may not exacerbate COVID-19. Further in vivo studies are warranted to advance this field.

Mice with humanized immune system as novel models to study HIV-associated pulmonary hypertension

People living with HIV and who receive antiretroviral therapy have a significantly improved lifespan, compared to the early days without therapy. Unfortunately, persisting viral replication in the lungs sustains chronic inflammation, which may cause pulmonary vascular dysfunction and ultimate life-threatening Pulmonary Hypertension (PH). The mechanisms involved in the progression of HIV and PH remain unclear. The study of HIV-PH is limited due to the lack of tractable animal models that recapitulate infection and pathobiological aspects of PH. On one hand, mice with humanized immune systems (hu-mice) are highly relevant to HIV research but their suitability for HIV-PH research deserves investigation. On another hand, the Hypoxia-Sugen is a well-established model for experimental PH that combines hypoxia with the VEGF antagonist SU5416. To test the suitability of hu-mice, we combined HIV with either SU5416 or hypoxia. Using right heart catheterization, we found that combining HIV+SU5416 exacerbated PH. HIV infection increases human pro-inflammatory cytokines in the lungs, compared to uninfected mice. Histopathological examinations showed pulmonary vascular inflammation with arterial muscularization in HIV-PH. We also found an increase in endothelial-monocyte activating polypeptide II (EMAP II) when combining HIV+SU5416. Therefore, combinations of HIV with SU5416 or hypoxia recapitulate PH in hu-mice, creating well-suited models for infectious mechanistic pulmonary vascular research in small animals.

Interleukin-27 promotes autophagy in human serum-induced primary macrophages via an mTOR- and LC3-independent pathway

Interleukin-27 (IL-27) is a cytokine that suppresses human immunodeficiency virus (HIV)-1 infection in macrophages and is considered as an immunotherapeutic reagent for infectious diseases. It is reported that IL-27 suppresses autophagy in Mycobacterium tuberculosis-infected macrophages; however, a role for IL-27 on autophagy induction has been less studied. In this study, we investigated the impact of IL-27 in both autophagy induction and HIV-1 infection in macrophages. Primary human monocytes were differentiated into macrophages using human AB serum (huAB) alone, macrophage-colony stimulating factor (M-CSF) alone, or a combination of IL-27 with huAB or M-CSF. Electron microscopy and immunofluorescence staining demonstrated that a 20-fold increase in autophagosome formation was only detected in IL-27 + huAB-induced macrophages. Western blot analysis indicated that the autophagosome induction was not linked to either dephosphorylation of the mammalian target of rapamycin (mTOR) or lipidation of microtubule-associated protein 1A/1B-light chain 3 (LC3), an autophagosomal marker, implying that IL-27 can induce autophagy through a novel non-canonical pathway. Here we show for the first time that IL-27 induces autophagy during monocyte-to-macrophage differentiation in a subtype-dependent manner.

Perspectives on Non-BLT Humanized Mouse Models for Studying HIV Pathogenesis and Therapy

A variety of humanized mice, which are reconstituted only with human hematopoietic stem cells (HSC) or with fetal thymus and HSCs, have been developed and widely utilized as in vivo animal models of HIV-1 infection. The models represent some aspects of HIV-mediated pathogenesis in humans and are useful for the evaluation of therapeutic regimens. However, there are several limitations in these models, including their incomplete immune responses and poor distribution of human cells to the secondary lymphoid tissues. These limitations are common in many humanized mouse models and are critical issues that need to be addressed. As distinct defects exist in each model, we need to be cautious about the experimental design and interpretation of the outcomes obtained using humanized mice. Considering this point, we mainly characterize the current conventional humanized mouse reconstituted only with HSCs and describe past achievements in this area, as well as the potential contributions of the humanized mouse models for the study of HIV pathogenesis and therapy. We also discuss the use of various technologies to solve the current problems. Humanized mice will contribute not only to the pre-clinical evaluation of anti-HIV regimens, but also to a deeper understanding of basic aspects of HIV biology.

Substantial induction of non-apoptotic CD4 T-cell death during the early phase of HIV-1 infection in a humanized mouse model

Several mechanisms underline induction of CD4 T-cell death by human immunodeficiency virus (HIV) infection. For a long time, apoptosis was considered central to cell death involved in the depletion of CD4 T cells during HIV infection. However, which types of cell death are induced during the early phase of HIV infection in vivo remains unclear. In this study, CD4 T-cell death induced in early HIV infection was characterized using humanized mice challenged with CCR5-tropic (R5) or CXCR4-tropic (X4) HIV-1. Results showed that CD4 T-cell death was induced in the spleen 3 days post-challenge with both R5 and X4 HIV-1. Although cell death without caspase-1 and caspase-3/7 activation was preferentially observed, caspase-1⁺ pyroptosis was also significantly induced within the memory subpopulation by R5 or X4 HIV-1 and the naïve subpopulation by X4 HIV-1. In contrast, caspase-3/7⁺ apoptosis was not enhanced by either R5 or X4 HIV-1. Furthermore, phosphorylated mixed lineage kinase domain-like protein⁺ necroptosis was induced by only X4 HIV-1. These findings indicate that various types of non-apoptotic CD4 T-cell death, such as pyroptosis and necroptosis, are induced during the early phase of HIV infection in vivo.

HIV X4 Variants Increase Arachidonate 5-Lipoxygenase in the Pulmonary Microenvironment and are associated with Pulmonary Arterial Hypertension

Pulmonary Arterial Hypertension (PAH) is overrepresented in People Living with Human Immunodeficiency Virus (PLWH). HIV protein gp120 plays a key role in the pathogenesis of HIV-PAH. Genetic changes in HIV gp120 determine viral interactions with chemokine receptors; specifically, HIV-X4 viruses interact with CXCR4 while HIV-R5 interact with CCR5 co-receptors. Herein, we leveraged banked samples from patients enrolled in the NIH Lung HIV studies and used bioinformatic analyses to investigate whether signature sequences in HIV-gp120 that predict tropism also predict PAH. Further biological assays were conducted in pulmonary endothelial cells in vitro and in HIV-transgenic rats. We found that significantly more persons living with HIV-PAH harbor HIV-X4 variants. Multiple HIV models showed that recombinant gp120-X4 as well as infectious HIV-X4 remarkably increase arachidonate 5-lipoxygenase (ALOX5) expression. ALOX5 is essential for the production of leukotrienes; we confirmed that leukotriene levels are increased in bronchoalveolar lavage fluid of HIV-infected patients. This is the first report associating HIV-gp120 genotype to a pulmonary disease phenotype, as we uncovered X4 viruses as potential agents in the pathophysiology of HIV-PAH. Altogether, our results allude to the supplementation of antiretroviral therapy with ALOX5 antagonists to rescue patients with HIV-X4 variants from fatal PAH.

Syphilis infection does not affect immunodeficiency progression in HIV-infected men who have sex with men in China

Syphilis and human immunodeficiency virus (HIV) co-infection is expected to play a role in HIV-1-related immunodeficiency progression; however, studies involving syphilis/HIV co-infection have not been conclusive. We investigated the factors associated with co-infection of syphilis and HIV and to assess the effect of syphilis on HIV progression in the context of HIV-1 diversity in an observational cohort of 246 newly-diagnosed HIV-infected but antiretroviral therapy-naive men who have sex with men enrolled in Guangzhou, China between 2008 and 2012. CD4+ cell counts of all the participants were measured from the time of diagnosis until 2015 with an average of 32 ± 18 months. Logistic analysis indicated that patients with syphilis/HIV co-infection were more likely to be older with an adjusted odds ratio (AOR) of 2.48 (95% CI: 1.28–4.80) for those aged between 31 and 40 years and 3.20 (1.11–9.22) for those aged ≥40 years as compared to 16–30  year-olds. The AOR of patients infected with HIV-1 CRF07_BC as compared to CRF01_AE was 2.14 (95% CI: 1.01–4.53). Co-infection of syphilis and HIV was associated with lower baseline CD4+ cell count (0.45, 95% CI: 0.22–0.94), but was not associated with HIV disease progression (HR: 1.03; 95% CI, 0.86–1.23) based on Kaplan–Meier analysis. Our results provide new evidence about the interaction between syphilis and HIV and indicate differential rates of immunodeficiency progression as a function of HIV-1 genetic diversity.

Diverse effects of interferon alpha on the establishment and reversal of HIV latency

HIV latency is the major barrier to a cure for people living with HIV (PLWH) on antiretroviral therapy (ART) because the virus persists in long-lived non-proliferating and proliferating latently infected CD4⁺ T cells. Latently infected CD4⁺ T cells do not express viral proteins and are therefore not visible to immune mediated clearance. Therefore, identifying interventions that can reverse latency and also enhance immune mediated clearance is of high interest. Interferons (IFNs) have multiple immune enhancing effects and can inhibit HIV replication in activated CD4⁺ T cells. However, the effects of IFNs on the establishment and reversal of HIV latency is not understood. Using an in vitro model of latency, we demonstrated that plasmacytoid dendritic cells (pDC) inhibit the establishment of HIV latency through secretion of type I IFNα, IFNβ and IFNω but not IFNε or type III IFNλ1 and IFNλ3. However, once latency was established, IFNα but no other IFNs were able to efficiently reverse latency in both an in vitro model of latency and CD4⁺ T cells collected from PLWH on suppressive ART. Binding of IFNα to its receptor expressed on primary CD4⁺ T cells did not induce activation of the canonical or non-canonical NFκB pathway but did induce phosphorylation of STAT1, 3 and 5 proteins. STAT5 has been previously demonstrated to bind to the HIV long terminal repeat and activate HIV transcription. We demonstrate diverse effects of interferons on HIV latency with type I IFNα; inhibiting the establishment of latency but also reversing HIV latency once latency is established.

Antiretroviral therapy (ART) cannot cure HIV or eliminate infection from long-lived and proliferating latently infected CD4⁺ T cells. Plasmacytoid dendritic cells (pDC) are major producers of interferons (IFNs), which have multiple effects on viral replication and immunity including suppression of viral expression that could favor HIV latency. Van Der Sluis et al. show that type I IFNs inhibit the establishment of HIV latency, however, once established, latency can be reversed by IFNα but not by other type I or type III IFNs. These observations demonstrate that pDC through type I IFNs are important in HIV latency and can potentially be manipulated to eliminate latent infection.

Combination Immune Checkpoint Blockade to Reverse HIV Latency

In people living with HIV on antiretroviral therapy, HIV latency is the major barrier to a cure. HIV persists preferentially in CD4⁺ T cells expressing multiple immune checkpoint (IC) molecules, including programmed death (PD)-1, T cell Ig and mucin domain-containing protein 3 (TIM-3), lymphocyte associated gene 3 (LAG-3), and T cell immunoreceptor with Ig and ITIM domains (TIGIT). We aimed to determine whether these and other IC molecules have a functional role in maintaining HIV latency and whether blocking IC molecules with Abs reverses HIV latency. Using an in vitro model that establishes latency in both nonproliferating and proliferating human CD4⁺ T cells, we show that proliferating cells express multiple IC molecules at high levels. Latent infection was enriched in proliferating cells expressing PD-1. In contrast, nonproliferating cells expressed IC molecules at significantly lower levels, but latent infection was enriched in cells expressing PD-1, TIM-3, CTL-associated protein 4 (CTLA-4), or B and T lymphocyte attenuator (BTLA). In the presence of an additional T cell-activating stimulus, staphylococcal enterotoxin B, Abs to CTLA-4 and PD-1 reversed HIV latency in proliferating and nonproliferating CD4⁺ T cells, respectively. In the absence of staphylococcal enterotoxin B, only the combination of Abs to PD-1, CTLA-4, TIM-3, and TIGIT reversed latency. The potency of latency reversal was significantly higher following combination IC blockade compared with other latency-reversing agents, including vorinostat and bryostatin. Combination IC blockade should be further explored as a strategy to reverse HIV latency.

Dendritic Cells, the Double Agent in the War Against HIV-1

Human Immunodeficiency Virus (HIV) infects cells from the immune system and has thus developed tools to circumvent the host immunity and use it in its advance. Dendritic cells (DCs) are the first immune cells to encounter the HIV, and being the main antigen (Ag) presenting cells, they link the innate and the adaptive immune responses. While DCs work to promote an efficient immune response and halt the infection, HIV-1 has ways to take advantage of their role and uses DCs to gain faster and more efficient access to CD4⁺ T cells. Due to their ability to activate a specific immune response, DCs are promising candidates to achieve the functional cure of HIV-1 infection, but knowing the molecular partakers that determine the relationship between virus and cell is the key for the rational and successful design of a DC-based therapy. In this review, we summarize the current state of knowledge on how both DC subsets (myeloid and plasmacytoid DCs) act in presence of HIV-1, and focus on different pathways that the virus can take after binding to DC. First, we explore the consequences of HIV-1 recognition by each receptor on DCs, including CD4 and DC-SIGN. Second, we look at cellular mechanisms that prevent productive infection and weapons that turn cellular defense into a Trojan horse that hides the virus all the way to T cell. Finally, we discuss the possible outcomes of DC-T cell contact.

CD4- and Time-Dependent Susceptibility of HIV-1-Infected Cells to Antibody-Dependent Cellular Cytotoxicity

HIV-1-specific antibody-dependent cellular cytotoxicity (ADCC) antibodies within HIV-1-positive (HIV-1⁺) individuals predominantly target CD4-induced (CD4i) epitopes on HIV-1 envelope glycoprotein (Env). These CD4i epitopes are usually concealed on the surface of infected cells due to CD4 downregulation by the HIV-1 accessory proteins Nef and Vpu. We hypothesized that early-stage infected cells in the process of downregulating CD4 could be more susceptible to ADCC than late-stage infected cells that have fully downregulated CD4. There was significantly higher binding of antibodies within plasma from HIV-1-infected individuals to early-stage infected cells expressing intermediate levels of CD4 (CD4-intermediate cells) than in late-stage infected cells expressing low levels of CD4 (CD4-low cells). However, we noted that HIV-1-uninfected bystander cells and HIV-1-infected cells, at various stages of downregulating CD4, were all susceptible to NK cell-mediated ADCC. Importantly, we observed that the cytolysis of bystander cells and early infected cells in this culture system was driven by sensitization of target cells by inoculum-derived HIV-1 Env or virions. This phenomenon provided Env to target cells prior to de novo Env expression, resulting in artifactual ADCC measurements. Future studies should take into consideration the inherent caveats of in vitro infection systems and develop improved models to address the potential role for ADCC against cells with nascent HIV-1 infection.IMPORTANCE An increasing body of evidence suggests that ADCC contributes to protection against HIV-1 acquisition and slower HIV-1 disease progression. Targeting cells early during the infection cycle would be most effective in limiting virus production and spread. We hypothesized that there could be a time-dependent susceptibility of HIV-1-infected cells to ADCC in regard to CD4 expression. We observed NK cell-mediated ADCC of HIV-1-infected cells at multiple stages of CD4 downregulation. Importantly, ADCC of early infected cells appeared to be driven by a previously unappreciated problem of soluble Env and virions from the viral inoculum sensitizing uninfected cells to ADCC prior to de novo Env expression. These results have implications for studies examining ADCC against cells with nascent HIV-1 infection.

A CCR5+ memory subset within HIV-1-infected primary resting CD4+ T cells is permissive for replication-competent, latently infected viruses in vitro

OBJECTIVE

Resting CD4⁺ T cells are major reservoirs of latent HIV-1 infection, and may be formed during the early phase of the infection. Although CCR5-tropic (R5) HIV-1 is highly transmissible during the early phase, newly infected individuals have usually been exposed to a mixture of R5 and CXCR4-tropic (X4) viruses, and X4 viral DNA is also detectable in the host. Our aim was to identify which subsets of resting CD4⁺ T cells contribute to forming the latent reservoir in the presence of both X4 and R5 viruses.

RESULTS

Primary resting CD4⁺ naïve T (T_N) cells, CCR5^- memory T (T_M) cells, and CCR5⁺ T_M cells isolated by flow cytometry were infected simultaneously with X4 and R5 HIV-1, which harbored different reporter genes, and were cultured in the resting condition. Flow cytometry at 3 days post-infection demonstrated that X4 HIV-1⁺ cells were present in all three subsets of cells, whereas R5 HIV-1⁺ cells were present preferentially in CCR5⁺ T_M cells, but not in T_N cells. Following CD3/CD28-mediated activation at 3 days post-infection, numbers of R5 HIV-1⁺ cells and X4 HIV-1⁺ cells increased significantly only in the CCR5⁺ T_M subset, suggesting that it provides a major reservoir of replication-competent, latently infected viruses.

STING agonists activate latently infected cells and enhance SIV-specific responses ex vivo in naturally SIV controlled cynomolgus macaques

To achieve a functional cure for HIV, treatment regimens that eradicate latently HIV-infected cells must be established. For this, many groups have attempted to reactivate latently-infected cells to induce cytopathic effects and/or elicit cytotoxic T lymphocyte (CTL)/NK cell-mediated immune responses to kill these cells. We believe that not only the reactivation of latently-infected cells, but also the induction of strong CTL responses, would be required for this. Here, we used typical immune activators that target pattern recognition receptors (PRRs). For our experimental model, we identified eight SIV-infected cynomolgus monkeys that became natural controllers of viremia. Although plasma viral loads were undetectable, we could measure SIV-DNA by qPCR in peripheral blood mononuclear cells (PBMCs). Using these PBMCs, we screened 10 distinct PRR ligands to measure IFN-α and IFN-γ production. Among these, STING ligands, cGAMP and c-di-AMP, and the TLR7/8 agonist R848 markedly increased cytokine levels. Both R848 and STING ligands could reactivate latently-infected cells in both cynomolgus monkeys and human PBMCs in vitro. Furthermore, c-di-AMP increased the frequency of SIV Gag-specific CD8⁺ T cells including polyfunctional CD8⁺ T cells, as compared to that in untreated control or R848-treated cells. Together, STING ligands might be candidates for HIV treatment.

HIV latency can be established in proliferating and nonproliferating resting CD4+ T cells in vitro: implications for latency reversal

OBJECTIVE

To determine whether latency can be established and reversed in both proliferating and nonproliferating CD4+ T cells in the same model in vitro.

METHODS

Activated CD4+ T cells were infected with either a nonreplication competent, luciferase reporter virus or wild-type full-length enhanced green fluorescent protein (EGFP) reporter virus and cultured for 12 days. The cells were then sorted by flow cytometry to obtain two distinct T-cell populations that did not express the T-cell activation markers, CD69, CD25 and human leukocyte antigen (HLA)-DR: CD69CD25HLA-DR small cells (nonblasts) that had not proliferated in vitro following mitogen stimulation and CD69CD25HLA-DR large cells (which we here call transitional blasts) that had proliferated. The cells were then reactivated with latency-reversing agents and either luciferase or EGFP quantified.

RESULTS

Inducible luciferase expression, consistent with latent infection, was observed in nonblasts and transitional blasts following stimulation with either phorbol-myristate-acetate/phytohemagglutinin (3.8 ± 1 and 2.9 ± 0.5 fold above dimethyl sulfoxide, respectively) or romidepsin (2.1 ± 0.6 and 1.8 ± 0.2 fold above dimethyl sulfoxide, respectively). Constitutive expression of luciferase was higher in transitional blasts compared with nonblasts. Using wild-type full-length EGFP reporter virus, inducible virus was observed in nonblasts but not in transitional blasts. No significant difference was observed in the response to latency-reversing agents in either nonblasts or transitional blasts.

CONCLUSION

HIV latency can be established in vitro in resting T cells that have not proliferated (nonblasts) and blasts that have proliferated (transitional blasts). This model could potentially be used to assess new strategies to eliminate latency.

Programmed cell death-1 contributes to the establishment and maintenance of HIV-1 latency

OBJECTIVE

In HIV-infected individuals on antiretroviral therapy (ART), latent HIV is enriched in CD4 T cells expressing immune checkpoint molecules, in particular programmed cell death-1 (PD-1). We therefore assessed the effect of blocking PD-1 on latency, both in vitro and in vivo.

METHODS

HIV latency was established in vitro following coculture of resting CD4+ T cells with myeloid dendritic cells. Expression of PD-1 was quantified by flow cytometry, and latency assessed in sorted PD-1high and PD-1low/-nonproliferating CD4+ memory T cells. The role of PD-1 in the establishment of latency was determined by adding anti-PD-1 (pembrolizumab) to cocultures before and after infection. In addition, a single infusion of anti-PD-1 (nivolumab) was administered to an HIV-infected individual on ART with metastatic melanoma, and cell-associated HIV DNA and RNA, and plasma HIV RNA were quantified.

RESULTS

HIV latency was significantly enriched in PD-1high compared with PD-1low/- nonproliferating, CD4 memory T cells. Sorting for an additional immune checkpoint molecule, T-cell immunoglobulin domain and mucin domain-3, in combination with PD-1, further enriched for latency. Blocking PD-1 prior to HIV infection, in vitro, resulted in a modest but significant decrease in latently infected cells in all donors (n = 6). The administration of anti-PD-1 to an HIV-infected individual on ART resulted in a significant increase in cell-associated HIV RNA in CD4 T cells, without significant changes in HIV DNA or plasma HIV RNA, consistent with reversal of HIV latency.

CONCLUSION

PD-1 contributes to the establishment and maintenance of HIV latency and should be explored as a target, in combination with other immune checkpoint molecules, to reverse latency.

The Pathway To Establishing HIV Latency Is Critical to How Latency Is Maintained and Reversed

HIV infection requires lifelong antiretroviral therapy because of the persistence of latently infected CD4⁺ T cells. The induction of virus expression from latently infected cells occurs following T cell receptor (TCR) activation, but not all latently infected cells respond to TCR stimulation. We compared two models of latently infected cells using an enhanced green fluorescent protein (EGFP) reporter virus to infect CCL19-treated resting CD4⁺ (rCD4⁺) T cells (preactivation latency) or activated CD4⁺ T cells that returned to a resting state (postactivation latency). We isolated latently infected cells by sorting for EGFP-negative (EGFP^-) cells after infection. These cells were cultured with antivirals and stimulated with anti-CD3/anti-CD28, mitogens, and latency-reversing agents (LRAs) and cocultured with monocytes and anti-CD3. Spontaneous EGFP expression was more frequent in postactivation than in preactivation latency. Stimulation of latently infected cells with monocytes/anti-CD3 resulted in an increase in EGFP expression compared to that for unstimulated controls using the preactivation latency model but led to a reduction in EGFP expression in the postactivation latency model. The reduced EGFP expression was not associated with reductions in the levels of viral DNA or T cell proliferation but depended on direct contact between monocytes and T cells. Monocytes added to the postactivation latency model during the establishment of latency reduced spontaneous virus expression, suggesting that monocyte-T cell interactions at an early time point postinfection can maintain HIV latency. This direct comparison of pre- and postactivation latency suggests that effective strategies needed to reverse latency will depend on how latency is established.IMPORTANCE One strategy being evaluated to eliminate latently infected cells that persist in HIV-infected individuals on antiretroviral therapy (ART) is to activate HIV expression or production with the goal of inducing virus-mediated cytolysis or immune-mediated clearance of infected cells. The gold standard for the activation of latent virus is T cell receptor stimulation with anti-CD3/anti-CD28. However, this stimulus activates only a small proportion of latently infected cells. We show clear differences in the responses of latently infected cells to activating stimuli based on how latent infection is established, an observation that may potentially explain the persistence of noninduced intact proviruses in HIV-infected individuals on ART.

eIF4A2 is a host factor required for efficient HIV-1 replication

Host factors are required for efficient HIV-1 replication. To identify these factors, genome-wide RNA interference screening was performed using a human T cell line. In the present study, we assessed whether eukaryotic translation initiation factor 4A isoform 2 (eIF4A2), a DEAD-box protein identified in our screen, is necessary for efficient HIV-1 replication. Exploiting MT4C5 cells depleted of eIF4A2 by stable expression of eIF4A2-specific short-hairpin RNA (shRNA) using a lentiviral system, we found that depletion of eIF4A2 markedly inhibited the infection of a replication-competent reporter HIV-1. eIF4A2 depletion reduced the efficiency of viral cDNA synthesis with virion entry into target cells being unaffected. Depletion of eIF4A2 also inhibited HIV-1 spreading infection in a knockdown level-dependent manner. These results suggest that HIV-1 requires eIF4A2 for optimal replication in human T cells.

HIV LTR-Driven Antisense RNA by Itself Has Regulatory Function and May Curtail Virus Reactivation From Latency

Latently infected T lymphocytes are an important barrier toward eliminating a persistent HIV infection. Here we describe an HIV-based recombinant fluorescent-lentivirus referred to as “rfl-HIV” that enables to analyze sense and antisense transcription by means of fluorescence reporter genes. This model virus exhibited similar transcriptional and functional properties of the antisense transcript as observed with a wild type HIV, and largely facilitated the generation of latently-infected T cells clones. We show that latently-infected cells can be divided into two types, those with and those without antisense transcription. Upon addition of latency reversal agents, only the cells that lack antisense transcripts are readily reactivated to transcribe HIV. Thus, antisense transcripts may exhibit a dominant suppressor activity and can lock an integrated provirus into a non-reactivatable state. These findings could have important implications for the development of strategies to eradicate HIV from infected individuals.

Pace of Coreceptor Tropism Switch in HIV-1-Infected Individuals after Recent Infection

HIV-1 entry into target cells influences several aspects of HIV-1 pathogenesis, including viral tropism, HIV-1 transmission and disease progression, and response to entry inhibitors. The evolution from CCR5- to CXCR4-using strains in a given human host is still unpredictable. Here we analyzed timing and predictors for coreceptor evolution among recently HIV-1-infected individuals. Proviral DNA was longitudinally evaluated in 66 individuals using Geno2pheno_[coreceptor] Demographics, viral load, CD4⁺ and CD8⁺ T cell counts, CCR5Δ32 polymorphisms, GB virus C (GBV-C) coinfection, and HLA profiles were also evaluated. Ultradeep sequencing was performed on initial samples from 11 selected individuals. A tropism switch from CCR5- to CXCR4-using strains was identified in 9/49 (18.4%) individuals. Only a low baseline false-positive rate (FPR) was found to be a significant tropism switch predictor. No minor CXCR4-using variants were identified in initial samples of 4 of 5 R5/non-R5 switchers. Logistic regression analysis showed that patients with an FPR of >40.6% at baseline presented a stable FPR over time whereas lower FPRs tend to progressively decay, leading to emergence of CXCR4-using strains, with a mean evolution time of 27.29 months (range, 8.90 to 64.62). An FPR threshold above 40.6% determined by logistic regression analysis may make it unnecessary to further determine tropism for prediction of disease progression related to emergence of X4 strains or use of CCR5 antagonists. The detection of variants with intermediate FPRs and progressive FPR decay over time not only strengthens the power of Geno2pheno in predicting HIV tropism but also indirectly confirms a continuous evolution from earlier R5 variants toward CXCR4-using strains.IMPORTANCE The introduction of CCR5 antagonists in the antiretroviral arsenal has sparked interest in coreceptors utilized by HIV-1. Despite concentrated efforts, viral and human host features predicting tropism switch are still poorly understood. Limited longitudinal data are available to assess the influence that these factors have on predicting tropism switch and disease progression. The present study describes longitudinal tropism evolution in a group of recently HIV-infected individuals to determine the prevalence and potential correlates of tropism switch. We demonstrated here that a low baseline FPR determined by the Geno2pheno_[coreceptor] algorithm can predict tropism evolution from CCR5 to CXCR4 coreceptor use.

N-terminally truncated POM121C inhibits HIV-1 replication

Recent studies have identified host cell factors that regulate early stages of HIV-1 infection including viral cDNA synthesis and orientation of the HIV-1 capsid (CA) core toward the nuclear envelope, but it remains unclear how viral DNA is imported through the nuclear pore and guided to the host chromosomal DNA. Here, we demonstrate that N-terminally truncated POM121C, a component of the nuclear pore complex, blocks HIV-1 infection. This truncated protein is predominantly localized in the cytoplasm, does not bind to CA, does not affect viral cDNA synthesis, reduces the formation of 2-LTR and diminished the amount of integrated proviral DNA. Studies with an HIV-1-murine leukemia virus (MLV) chimeric virus carrying the MLV-derived Gag revealed that Gag is a determinant of this inhibition. Intriguingly, mutational studies have revealed that the blockade by N-terminally-truncated POM121C is closely linked to its binding to importin-β/karyopherin subunit beta 1 (KPNB1). These results indicate that N-terminally-truncated POM121C inhibits HIV-1 infection after completion of reverse transcription and before integration, and suggest an important role for KPNB1 in HIV-1 replication.

Phosphorylation of the HIV-1 capsid by MELK triggers uncoating to promote viral cDNA synthesis

Regulation of capsid disassembly is crucial for efficient HIV-1 cDNA synthesis after entry, yet host factors involved in this process remain largely unknown. Here, we employ genetic screening of human T-cells to identify maternal embryonic leucine zipper kinase (MELK) as a host factor required for optimal uncoating of the HIV-1 core to promote viral cDNA synthesis. Depletion of MELK inhibited HIV-1 cDNA synthesis with a concomitant delay of capsid disassembly. MELK phosphorylated Ser-149 of the capsid in the multimerized HIV-1 core, and a mutant virus carrying a phosphorylation-mimetic amino-acid substitution of Ser-149 underwent premature capsid disassembly and earlier HIV-1 cDNA synthesis, and eventually failed to enter the nucleus. Moreover, a small-molecule MELK inhibitor reduced the efficiency of HIV-1 replication in peripheral blood mononuclear cells in a dose-dependent manner. These results reveal a previously unrecognized mechanism of HIV-1 capsid disassembly and implicate MELK as a potential target for anti-HIV therapy.

Phosphorylation of the HIV-1 capsid has long been known to regulate viral uncoating and cDNA synthesis processes, but the cellular kinases responsible for this have remained unidentified. Here, we report that a host cell kinase MELK dictates optimal capsid disassembly through phosphorylation of Ser-149 in the multimerized HIV-1 core, which leads to efficient viral cDNA synthesis in target cells. The phosphorylation-mimetic capsid mutation of Ser-149 caused aberrant capsid disassembly and too-early completion of reverse transcription, and impeded nuclear entry of HIV-1 cDNA, suggesting the importance of well-ordered capsid disassembly in the early stages of viral replication. This discovery will facilitate understanding of the functional link among virus uncoating, reverse transcription and nuclear entry, and is expected to contribute to developing a novel strategy for AIDS therapy.

HIV-1 susceptibility of transgenic rat-derived primary macrophage/T cells and a T cell line that express human receptors, CyclinT1 and CRM1 genes

We developed transgenic (Tg) rats that express human CD4, CCR5, CXCR4, CyclinT1, and CRM1 genes. Tg rat macrophages were efficiently infected with HIV-1 and supported production of infectious progeny virus. By contrast, both rat primary CD4+ T cells and established T cell lines expressing human CD4, CCR5, CyclinT1, and CRM1 genes were infected inefficiently, but this was ameliorated by inhibition of cyclophilin A. The infectivity of rat T cell-derived virus was lower than that of human T cell-derived virus.

Homeostatically Maintained Resting Naive CD4+ T Cells Resist Latent HIV Reactivation

Homeostatic proliferation (HSP) is a major mechanism by which long-lived naïve and memory CD4⁺ T cells are maintained in vivo and suggested to contribute to the persistence of the latent HIV-1 reservoir. However, while many in vitro latency models rely on CD4⁺ T cells that were initially differentiated via T-cell receptor (TCR) stimulation into memory/effector cells, latent infection of naïve resting CD4⁺ T cells maintained under HSP conditions has not been fully addressed. Here, we describe an in vitro HSP culture system utilizing the cytokines IL-7 and IL-15 that allows studying latency in naïve resting CD4⁺ T cells. CD4⁺ T cells isolated from several healthy donors were infected with HIV pseudotypes expressing GFP and cultured under HSP conditions or TCR conditions as control. Cell proliferation, phenotype, and GFP expression were analyzed by flow cytometry. RNA expression was quantified by qRT-PCR. Under HSP culture conditions, latently HIV-1 infected naïve cells are in part maintained in the non-dividing (= resting) state. Although a few HIV-1 provirus⁺ cells were present in these resting GFP negative cells, the estimated level of GFP transcripts per infected cell seems to indicate a block at the post-transcriptional level. Interestingly, neither TCR nor the prototypic HDAC inhibitor SAHA were able to reactivate HIV-1 provirus from these cells. This lack of reactivation was not due to methylation of the HIV LTR. These results point to a mechanism of HIV control in HSP-cultured resting naïve CD4⁺ T cells that may be distinct from that in TCR-stimulated memory/effector T cells.

Proliferation of functional human natural killer cells with anti-HIV-1 activity in NOD/SCID/Jak3(null) mice

Natural killer cells, a critical component of the innate immune system, eradicate both virus-infected cells and tumor cells through cytotoxicity and secretion of cytokines. Human NK cell research has largely been based on in vitro studies because of the lack of appropriate animal models. In this study, a selective proliferation model of functional human NK cells was established in NOD/SCID/Jak3(null) (NOJ) mice transplanted with peripheral blood mononuclear cells (PBMC) and K562 cells. The antiviral effects of NK cells were evaluated by challenging this mouse model with HIV-1. The percentage of intracellular p24(+) T cells and the amount of plasma p24 was decreased compared with NOJ mice transplanted with PBMC. Our findings indicate that NK cells have an anti-HIV-1 effect through direct cytotoxicity against HIV-1-infected cells. These mice provide an important model for evaluating human NK function against human infectious diseases such as HIV-1 and malignancies.

Understanding Factors That Modulate the Establishment of HIV Latency in Resting CD4+ T-Cells In Vitro

Developing robust in vitro models of HIV latency is needed to better understand how latency is established, maintained and reversed. In this study, we examined the effects of donor variability, HIV titre and co-receptor usage on establishing HIV latency in vitro using two models of HIV latency. Using the CCL19 model of HIV latency, we found that in up to 50% of donors, CCL19 enhanced latent infection of resting CD4+ T-cells by CXCR4-tropic HIV in the presence of low dose IL-2. Increasing the infectious titre of CXCR4-tropic HIV increased both productive and latent infection of resting CD4+ T-cells. In a different model where myeloid dendritic cells (mDC) were co-cultured with resting CD4+ T-cells, we observed a higher frequency of latently infected cells in vitro than CCL19-treated or unstimulated CD4+ T-cells in the presence of low dose IL-2. In the DC-T-cell model, latency was established with both CCR5- and CXCR4-tropic virus but higher titres of CCR5-tropic virus was required in most donors. The establishment of latency in vitro through direct infection of resting CD4+ T-cells is significantly enhanced by CCL19 and mDC, but the efficiency is dependent on virus titre, co-receptor usage and there is significant donor variability.

A novel dual luciferase assay for the simultaneous monitoring of HIV infection and cell viability

Human immunodeficiency virus type 1 (HIV-1) reporter cell lines are critical tools for drug development. However, one disadvantage of HIV-1 reporter cell lines is that reductions in reporter gene activity need to be normalized to cytotoxicity, i.e., live cell numbers. Here, we developed a dual luciferase assay based on a R. reniformis luciferase (hRLuc)-expressing R5-type HIV-1 (NLAD8-hRLuc) and a CEM cell line expressing CCR5 and firefly luciferase (R5CEM-FiLuc). The NLAD8-hRLuc reporter virus was replication competent in peripheral blood mononuclear cells. The level of hRLuc was correlated with p24 antigen levels (p<0.001, R=0.862). The target cell line, R5CEM-FiLuc, stably expressed the firefly luciferase (FiLuc) reporter gene and allowed the simultaneous monitoring of compound cytotoxicity. The dual reporter assay combining a NLAD8-hRLuc virus with R5CEM-FiLuc cells permitted the accurate determination of drug susceptibility for entry, reverse transcriptase, integrase, and protease inhibitors at different multiplicities of infection. This dual reporter assay provides a rapid and direct method for the simultaneous monitoring of HIV infection and cell viability.

Slit2N Inhibits Transmission of HIV-1 from Dendritic Cells to T-cells by Modulating Novel Cytoskeletal Elements

Dendritic cells are among the first cells to encounter sexually acquired human immunodeficiency virus (HIV-1), in the mucosa, and they can transmit HIV-1 to CD4(+) T-cells via an infectious synapse. Recent studies reveal that actin-rich membrane extensions establish direct contact between cells at this synapse and facilitate virus transmission. Genesis of these contacts involves signaling through c-Src and Cdc42, which modulate actin polymerization and filopodia formation via the Arp2/3 complex and Diaphanous 2 (Diaph2). We found that Slit2N, a ligand for the Roundabout (Robo) receptors, blocked HIV-1-induced signaling through Arp2/3 and Diaph2, decreased filopodial extensions on dendritic cells, and inhibited cell-to-cell transmission of HIV-1 in a Robo1-dependent manner. Employing proteomic analysis, we identified Flightless-1 as a novel, Robo1-interacting protein. Treatment with shRNAs reduced levels of Flightless-1 and demonstrated its role in efficient cell-to-cell transfer of HIV-1. These results suggest a novel strategy to limit viral infection in the host by targeting the Slit/Robo pathway with modulation of cytoskeletal elements previously unrecognized in HIV-1 transmission.

APOBEC3 Proteins in Viral Immunity

Apolipoprotein B editing complex 3 family members are cytidine deaminases that play important roles in intrinsic responses to infection by retroviruses and have been implicated in the control of other viruses, such as parvoviruses, herpesviruses, papillomaviruses, hepatitis B virus, and retrotransposons. Although their direct effect on modification of viral DNA has been clearly demonstrated, whether they play additional roles in innate and adaptive immunity to viruses is less clear. We review the data regarding the various steps in the innate and adaptive immune response to virus infection in which apolipoprotein B editing complex 3 proteins have been implicated.

HIV-1 Coreceptor CXCR4 Antagonists Promote Clonal Expansion of Viral Epitope-Specific CD8+ T Cells During Acute SIV Infection in Rhesus Monkeys In Vivo

BACKGROUND

The underlying molecular mechanisms and the kinetics of T cell receptor (TCR) repertoire selection during administration of CXCR4 or CCR5 inhibitors in infection of AIDS viruses in vivo have remained largely unexplored. Viral epitope-specific CD8(+) T lymphocytes play a dominant role in the control of HIV and simian immunodeficiency virus (SIV). We hypothesized that blockade of CXCR4 or CCR5 might influence the clonal expansion of epitope-specific CD8(+) T cells, contributing to antiviral immune responses in vivo.

METHODS

We measured frequencies of the dominant epitope p11C-specific CD8(+) T cells and analyzed the TCR repertoire of those cells in SIV-infected rhesus monkeys treated by CXCR4 or CCR5 inhibitors and vMIP-II, which binds multiple chemokine receptors.

RESULTS

A significantly increase in the levels of epitope-specific CD8(+) T cells was observed after blockade of CXCR4 or CCR5 compared with untreated control groups. Those CD8(+) T cells exhibited selected usage of TCR Vβ families and complementarity-determining region 3 (CDR3) segments. The clonal expansion of distinct Vβ populations could efficiently inhibit SIV replication in vitro, and CXCR4 inhibitor induced more expansion of epitope-specific CD8(+) T cells than CCR5 antagonist (P < 0.01), whereas vMIP-II treatment showed the most marked augmentation of p11C-specific CD8(+) T cells.

CONCLUSIONS

Antagonists of HIV coreceptors, particularly CXCR4, play an important role in the clonal expansion of SIV epitope-specific CD8(+) T cells in vivo, thus inhibitors of chemokine receptors such as CXCR4 or CCR5 may contribute to the ability of epitope-specific CD8(+) T cells to inhibit SIV or HIV infection.

Humanized mice dually challenged with R5 and X4 HIV-1 show preferential R5 viremia and restricted X4 infection of CCR5(+)CD4(+) T cells

CCR5-tropic (R5) immunodeficiency virus type 1 (HIV-1) strains are highly transmissible during the early stage of infection in humans, whereas CXCR4-tropic (X4) strains are less transmissible. This study aimed to explore the basis for early phase R5 and X4 HIV-1 infection in vivo by using humanized mice dually challenged with R5 HIV-1NLAD8-D harboring DsRed and X4 HIV-1(NL-E) harboring EGFP. Whereas R5 HIV-1 replicated well, X4 HIV-1 caused only transient viremia with variable kinetics; however, this was distinct from the low level but persistent viremia observed in mice challenged with X4 HIV-1 alone. Flow cytometric analysis of HIV-1-infected cells revealed that X4 HIV-1 infection of CCR5(+)CD4(+) T cells was significantly suppressed in the presence of R5 HIV-1. X4 HIV-1 was more cytopathic than R5 HIV-1; however, this was not the cause of restricted X4 HIV-1 infection because there were no significant differences in the mortality rates of CCR5(+) and CCR5(-) cells within the X4 HIV-1-infected cell populations. Taken together, these results suggest that restricted infection of CCR5(+)CD4(+) T cells by X4 HIV-1 (occurring via a still-to-be-identified mechanism) might contribute to the preferential transmission of R5 HIV-1 during the early phase of infection.

TLR7 induces anergy in human CD4(+) T cells

The recognition of microbial patterns by Toll-like receptors (TLRs) is critical for activation of the innate immune system. Although TLRs are expressed by human CD4(+) T cells, their function is not well understood. Here we found that engagement of TLR7 in CD4(+) T cells induced intracellular calcium flux with activation of an anergic gene-expression program dependent on the transcription factor NFATc2, as well as unresponsiveness of T cells. As chronic infection with RNA viruses such as human immunodeficiency virus type 1 (HIV-1) induces profound dysfunction of CD4(+) T cells, we investigated the role of TLR7-induced anergy in HIV-1 infection. Silencing of TLR7 markedly decreased the frequency of HIV-1-infected CD4(+) T cells and restored the responsiveness of those HIV-1(+) CD4(+) T cells. Our results elucidate a previously unknown function for microbial pattern-recognition receptors in the downregulation of immune responses.

Myeloid dendritic cells induce HIV-1 latency in non-proliferating CD4+ T cells

Latently infected resting CD4⁺ T cells are a major barrier to HIV cure. Understanding how latency is established, maintained and reversed is critical to identifying novel strategies to eliminate latently infected cells. We demonstrate here that co-culture of resting CD4⁺ T cells and syngeneic myeloid dendritic cells (mDC) can dramatically increase the frequency of HIV DNA integration and latent HIV infection in non-proliferating memory, but not naïve, CD4⁺ T cells. Latency was eliminated when cell-to-cell contact was prevented in the mDC-T cell co-cultures and reduced when clustering was minimised in the mDC-T cell co-cultures. Supernatants from infected mDC-T cell co-cultures did not facilitate the establishment of latency, consistent with cell-cell contact and not a soluble factor being critical for mediating latent infection of resting CD4⁺ T cells. Gene expression in non-proliferating CD4⁺ T cells, enriched for latent infection, showed significant changes in the expression of genes involved in cellular activation and interferon regulated pathways, including the down-regulation of genes controlling both NF-κB and cell cycle. We conclude that mDC play a key role in the establishment of HIV latency in resting memory CD4⁺ T cells, which is predominantly mediated through signalling during DC-T cell contact.

Current antiretroviral drugs significantly prolong life and reduce morbidity but are unable to cure HIV. While on treatment, the virus is able to hide in resting memory T cells in a silent or “latent” form. These latently infected cells are rare and thus are hard to study using blood from HIV-infected individuals on treatment. Therefore, it is very important to have laboratory models that can closely mimic what is going on in the body. We have developed a novel model of HIV latency in the laboratory. Using this model we have shown that the presence of dendritic cells, an important type of immune cell that can regulate T cell activation, at the time of infection allows for the infection of resting T cells and the establishment of latency. We have demonstrated that this is predominantly mediated by direct cell-to-cell interactions. Further exploration of the mechanisms behind HIV latency could lead to new ways to treat and possibly eradicate HIV.

Neuroprotective maraviroc monotherapy in simian immunodeficiency virus-infected macaques: reduced replicating and latent SIV in the brain

OBJECTIVE

HIV-associated neurocognitive deficits remain a challenge despite suppressive combined antiretroviral therapy. Given the association between HIV-induced central nervous system (CNS) disease and replication of HIV in immune-activated macrophages, CCR5 antagonists may attenuate CNS disease by modulating inflammatory signaling and by limiting viral replication.

DESIGN

To establish whether initiating CCR5 inhibition during early infection altered CNS disease progression, outcomes were compared between simian immunodeficiency virus (SIV)-infected macaques treated with maraviroc (MVC) versus untreated SIV-infected macaques.

METHODS

Six SIV-infected rhesus macaques were treated with MVC monotherapy for 5 months beginning 24 days postinoculation; 22 SIV-infected animals served as untreated controls. SIV RNA levels in plasma, cerobrospinal fluid, and brain, and CNS expression of TNFα and CCL2 were measured by qRT-PCR. Immunostaining for CD68 and amyloid precursor protein in the brain was measured by image analysis. Plasma sCD163 was measured by ELISA.

RESULTS

SIV RNA and proviral DNA levels in brain were markedly lower with MVC treatment, demonstrating CCR5 inhibition reduces CNS replication of SIV and may reduce the CNS latent viral reservoir. MVC treatment also lowered monocyte and macrophage activation, represented by CNS CD68 immunostaining and plasma sCD163 levels, and reduced both TNFα and CCL2 RNA expression in brain. Treatment also reduced axonal amyloid precursor protein immunostaining to levels present in uninfected animals, consistent with neuroprotection.

CONCLUSION

CCR5 inhibitors may prevent neurologic disorders in HIV-infected individuals by reducing inflammation and by limiting viral replication in the brain. Furthermore, CCR5 inhibitors may reduce the latent viral reservoir in the CNS. Adding CCR5 inhibitors to combined antiretroviral regimens may offer multiple neuroprotective benefits.

Dectin-1/TLR2 and NOD2 agonists render dendritic cells susceptible to infection by X4-using HIV-1 and promote cis-infection of CD4(+) T cells

HIV-1 pathogenesis is intimately linked with microbial infections and innate immunity during all stages of the disease. While the impact of microbial-derived products in transmission of R5-using virus to CD4⁺ T cells by dendritic cells (DCs) has been addressed before, very limited data are available on the effect of such compounds on DC-mediated dissemination of X4-tropic variant. Here, we provide evidence that treatment of DCs with dectin-1/TLR2 and NOD2 ligands increases cis-infection of autologous CD4⁺ T cells by X4-using virus. This phenomenon is most likely associated with an enhanced permissiveness of DCs to productive infection with X4 virus, which is linked to increased surface expression of CXCR4 and the acquisition of a maturation profile by DCs. The ensuing DC maturation enhances susceptibility of CD4⁺ T cells to productive infection with HIV-1. This study highlights the crucial role of DCs at different stages of HIV-1 infection and particularly in spreading of viral strains displaying a X4 phenotype.

HIV-1 causes CD4 cell death through DNA-dependent protein kinase during viral integration

Human immunodeficiency virus-1 (HIV-1) has infected more than 60 million people and caused nearly 30 million deaths worldwide, ultimately the consequence of cytolytic infection of CD4(+) T cells. In humans and in macaque models, most of these cells contain viral DNA and are rapidly eliminated at the peak of viraemia, yet the mechanism by which HIV-1 induces helper T-cell death has not been defined. Here we show that virus-induced cell killing is triggered by viral integration. Infection by wild-type HIV-1, but not an integrase-deficient mutant, induced the death of activated primary CD4 lymphocytes. Similarly, raltegravir, a pharmacologic integrase inhibitor, abolished HIV-1-induced cell killing both in cell culture and in CD4(+) T cells from acutely infected subjects. The mechanism of killing during viral integration involved the activation of DNA-dependent protein kinase (DNA-PK), a central integrator of the DNA damage response, which caused phosphorylation of p53 and histone H2AX. Pharmacological inhibition of DNA-PK abolished cell death during HIV-1 infection in vitro, suggesting that processes which reduce DNA-PK activation in CD4 cells could facilitate the formation of latently infected cells that give rise to reservoirs in vivo. We propose that activation of DNA-PK during viral integration has a central role in CD4(+) T-cell depletion, raising the possibility that integrase inhibitors and interventions directed towards DNA-PK may improve T-cell survival and immune function in infected individuals.

HIV-1 Trans Infection of CD4(+) T Cells by Professional Antigen Presenting Cells

Since the 1990s we have known of the fascinating ability of a complex set of professional antigen presenting cells (APCs; dendritic cells, monocytes/macrophages, and B lymphocytes) to mediate HIV-1 trans infection of CD4(+) T cells. This results in a burst of virus replication in the T cells that is much greater than that resulting from direct, cis infection of either APC or T cells, or trans infection between T cells. Such APC-to-T cell trans infection first involves a complex set of virus subtype, attachment, entry, and replication patterns that have many similarities among APC, as well as distinct differences related to virus receptors, intracellular trafficking, and productive and nonproductive replication pathways. The end result is that HIV-1 can sequester within the APC for several days and be transmitted via membrane extensions intracellularly and extracellularly to T cells across the virologic synapse. Virus replication requires activated T cells that can develop concurrently with the events of virus transmission. Further research is essential to fill the many gaps in our understanding of these trans infection processes and their role in natural HIV-1 infection.

Binding of HIV-1 gp120 to DC-SIGN promotes ASK-1-dependent activation-induced apoptosis of human dendritic cells

During disease progression to AIDS, HIV-1 infected individuals become increasingly immunosuppressed and susceptible to opportunistic infections. It has also been demonstrated that multiple subsets of dendritic cells (DC), including DC-SIGN(+) cells, become significantly depleted in the blood and lymphoid tissues of AIDS patients, which may contribute to the failure in initiating effective host immune responses. The mechanism for DC depletion, however, is unclear. It is also known that vast quantities of viral envelope protein gp120 are shed from maturing HIV-1 virions and form circulating immune complexes in the serum of HIV-1-infected individuals, but the pathological role of gp120 in HIV-1 pathogenesis remains elusive. Here we describe a previously unrecognized mechanism of DC death in chronic HIV-1 infection, in which ligation of DC-SIGN by gp120 sensitizes DC to undergo accelerated apoptosis in response to a variety of activation stimuli. The cultured monocyte-derived DC and also freshly-isolated DC-SIGN(+) blood DC that were exposed to either cross-linked recombinant gp120 or immune-complex gp120 in HIV(+) serum underwent considerable apoptosis after CD40 ligation or exposure to bacterial lipopolysaccharide (LPS) or pro-inflammatory cytokines such as TNFα and IL-1β. Furthermore, circulating DC-SIGN(+) DC that were isolated directly from HIV-1(+) individuals had actually been pre-sensitized by serum gp120 for activation-induced exorbitant apoptosis. In all cases the DC apoptosis was substantially inhibited by DC-SIGN blockade. Finally, we showed that accelerated DC apoptosis was a direct consequence of excessive activation of the pro-apoptotic molecule ASK-1 and transfection of siRNA against ASK-1 significantly prevented the activation-induced excessive DC death. Our study discloses a previously unknown mechanism of immune modulation by envelope protein gp120, provides new insights into HIV immunopathogenesis, and suggests potential therapeutic approaches to prevent DC depletion in chronic HIV infection.

HIV-1 infected individuals become increasingly immunocompromised and susceptible to opportunistic infection during disease progression, which is associated with significant reduction of the dendritic cell number in the peripheral blood or secondary lymphoid tissues. Because dendritic cells are the most powerful antigen-presenting cells, their survival is critical for host defence and inadequate dendritic cell number will fail to induce effective host immune responses. Here we describe a mechanism that may at least partly explain why dendritic cells become significantly depleted in chronic HIV-1 infection. We found that after binding of the HIV-1 envelope protein gp120 to the dendritic cell surface protein DC-SIGN, the subsequent activation by CD40 ligation, or by exposure to bacterial product lipopolysaccharide or pro-inflammatory cytokines such as TNF-α and IL-1β, will lead to overexpression of pro-apoptotic molecule ASK-1, resulting in excessive dendritic cell death. We also confirmed that DC-SIGN(+) dendritic cells in the blood of HIV-1 infected individuals have actually been pre-sensitized by viral gp120, which exists in vast amount in the blood, for activation-induced exorbitant death. Our study thus reveals a previously unknown pathway for dendritic cell depletion and provides clues for potential therapeutic approaches to prevent DC depletion in chronic HIV infection.

Expansion of activated memory CD4+ T cells affects infectivity of CCR5-tropic HIV-1 in humanized NOD/SCID/JAK3null mice

Humanized mice reconstituted with human hematopoietic cells have been developed as an experimental animal model for human immunodeficiency virus type 1 (HIV-1) infection. Myeloablative irradiation is usually performed to augment the engraftment of donor hematopoietic stem cells (HSCs) in recipient mice; however, some mouse strains are susceptible to irradiation, making longitudinal analysis difficult. We previously attempted to construct humanized NOD/SCID/JAK3^null (hNOJ) mice, which were not irradiated prior to human HSC transplantation. We found that, over time, many of the reconstituted CD4⁺ T cells expanded with an activated effector memory phenotype. Therefore, the present study used hNOJ mice that were irradiated (hNOJ (IR+)) or not (hNOJ (IR−)) prior to human HSC transplantation to examine whether the development and cellularity of the reconstituted CD4⁺ T cells were influenced by the degree of chimerism, and whether they affected HIV-1 infectivity. Indeed, hNOJ (IR+) mice showed a greater degree of chimerism than hNOJ (IR−) mice. However, the conversion of CD4⁺ T cells to an activated effector memory phenotype, with a high percentage of cells showing Ki-67 expression, occurred in both hNOJ (IR+) and hNOJ (IR−) mice, probably as a result of lymphopenia-induced homeostatic expansion. Furthermore, when hNOJ (IR+) and hNOJ (IR−) mice, which were selected as naïve- and memory CD4⁺ T cell subset-rich groups, respectively, were infected with CCR5-tropic HIV-1 in vivo, virus replication (as assessed by the plasma viral load) was delayed; however, the titer subsequently reached a 1-log higher level in memory-rich hNOJ (IR−) mice than in naïve-rich hNOJ (IR+) mice, indicating that virus infectivity in hNOJ mice was affected by the different status of the reconstituted CD4⁺ T cells. Therefore, the hNOJ mouse model should be used selectively, i.e., according to the specific experimental objectives, to gain an appropriate understanding of HIV-1 infection/pathogenesis.

The molecular basis of HIV entry

Infection by HIV starts when the virus attaches to a susceptible cell. For viral replication to continue, the viral envelope must fuse with a cellular membrane, thereby delivering the viral core to the cytoplasm, where the RNA genome is reverse-transcribed. The key players in this entry by fusion are the envelope glycoprotein, on the viral side, and CD4 and a co-receptor, CCR5 or CXCR4, on the cellular side. Here, the interplay of these molecules is reviewed from cell-biological, structural, mechanistic, and modelling-based perspectives. Hypotheses are evaluated regarding the cellular compartment for entry, the transfer of virus through direct cell-to-cell contact, the sequence of molecular events, and the number of molecules involved on each side of the virus-cell divide. An emerging theme is the heterogeneity among the entry mediators on both sides, a diversity that affects the efficacy of entry inhibitors, be they small-molecule ligands, peptides or neutralizing antibodies. These insights inform rational strategies for therapy as well as vaccination.

HIV-1 infection ex vivo accelerates measles virus infection by upregulating signaling lymphocytic activation molecule (SLAM) in CD4+ T cells

Measles virus (MV) infection in children harboring human immunodeficiency virus type 1 (HIV-1) is often fatal, even in the presence of neutralizing antibodies; however, the underlying mechanisms are unclear. Therefore, the aim of the present study was to examine the interaction between HIV-1 and wild-type MV (MVwt) or an MV vaccine strain (MVvac) during dual infection. The results showed that the frequencies of MVwt- and MVvac-infected CD4(+) T cells within the resting peripheral blood mononuclear cells (PBMCs) were increased 3- to 4-fold after HIV-1 infection, and this was associated with a marked upregulation of signaling lymphocytic activation molecule (SLAM) expression on CD4(+) T cells but not on CD8(+) T cells. SLAM upregulation was induced by infection with a replication-competent HIV-1 isolate comprising both the X4 and R5 types and to a lesser extent by a pseudotyped HIV-1 infection. Notably, SLAM upregulation was observed in HIV-infected as well as -uninfected CD4(+) T cells and was abrogated by the removal of HLA-DR(+) cells from the PBMC culture. Furthermore, SLAM upregulation did not occur in uninfected PBMCs cultured together with HIV-infected PBMCs in compartments separated by a permeable membrane, indicating that no soluble factors were involved. Rather, CD4(+) T cell activation mediated through direct contact with dendritic cells via leukocyte function-associated molecule 1 (LFA-1)/intercellular adhesion molecule 1 (ICAM-1) and LFA-3/CD2 was critical. Thus, HIV-1 infection induces a high level of SLAM expression on CD4(+) T cells, which may enhance their susceptibility to MV and exacerbate measles in coinfected individuals.

Fluorescent Reporter Signals, EGFP, and DsRed, Encoded in HIV-1 Facilitate the Detection of Productively Infected Cells and Cell-Associated Viral Replication Levels

Flow cytometric analysis is a reliable and convenient method for investigating molecules at the single cell level. Previously, recombinant human immunodeficiency virus type 1 (HIV-1) strains were constructed that express a fluorescent reporter, either enhanced green fluorescent protein, or DsRed, which allow the monitoring of HIV-1-infected cells by flow cytometry. The present study further investigated the potential of these recombinant viruses in terms of whether the HIV-1 fluorescent reporters would be helpful in evaluating viral replication based on fluorescence intensity. When primary CD4⁺ T cells were infected with recombinant viruses, the fluorescent reporter intensity measured by flow cytometry was associated with the level of CD4 downmodulation and Gag p24 expression in infected cells. Interestingly, some HIV-1-infected cells, in which CD4 was only moderately downmodulated, were reporter-positive but Gag p24-negative. Furthermore, when the activation status of primary CD4⁺ T cells was modulated by T cell receptor-mediated stimulation, we confirmed the preferential viral production upon strong stimulation and showed that the intensity of the fluorescent reporter within a proportion of HIV-1-infected cells was correlated with the viral replication level. These findings indicate that a fluorescent reporter encoded within HIV-1 is useful for the sensitive detection of productively infected cells at different stages of infection and for evaluating cell-associated viral replication at the single cell level.

Penicillium marneffei-stimulated dendritic cells enhance HIV-1 trans-infection and promote viral infection by activating primary CD4+ T cells

Penicillium marneffei (P. marneffei) is considered an indicator pathogen of AIDS, and the endemicity and clinical features of P. marneffei have been described. While, how the co-infection of P. marneffei exacerbate deterioration of the immune response remains poorly understood. Here we isolated P. marneffei from the cutaneous lesions of AIDS patients and analyzed its effects on HIV-1-dendritic cells (DCs) interaction. We demonstrated that the monocyte-derived dendritic cells (MDDCs) could be activated by both thermally dimorphic forms of P. marneffei for significantly promoting HIV-1 trans-infection of CD4⁺ T cells, while these activated MDDCs were refractory to HIV-1 infection. Mechanistically, P. marneffei-activated MDDCs endocytosed large amounts of HIV-1 and sequestrated the internalized viruses into tetrapasnin CD81⁺ compartments potentially for proteolysis escaping. The activated MDDCs increased expression of intercellular adhesion molecule 1 and facilitated the formation of DC-T-cell conjunctions, where much more viruses were recruited. Moreover, we found that P. marneffei-stimulated MDDCs efficiently activated resting CD4⁺ T cells and induced more susceptible targets for viral infection. Our findings demonstrate that DC function and its interaction with HIV-1 have been modulated by opportunistic pathogens such as P. marneffei for viral dissemination and infection amplification, highlighting the importance of understanding DC-HIV-1 interaction for viral immunopathogenesis elucidation.

Thymic plasmacytoid dendritic cells are susceptible to productive HIV-1 infection and efficiently transfer R5 HIV-1 to thymocytes in vitro

Background

HIV-1 infection of the thymus contributes to the defective regeneration and loss of CD4⁺ T cells in HIV-1-infected individuals. As thymic dendritic cells (DC) are permissive to infection by HIV-1, we examined the ability of thymic DC to enhance infection of thymocytes which may contribute to the overall depletion of CD4⁺ T cells. We compared productive infection in isolated human thymic and blood CD11c⁺ myeloid DC (mDC) and CD123⁺ plasmacytoid DC (pDC) using enhanced green fluorescent protein (EGFP) CCR5 (R5)-tropic NL(AD8) and CXCR4 (X4)-tropic NL4-3 HIV-1 reporter viruses. Transfer of productive HIV-1 infection from thymic mDC and pDC was determined by culturing these DC subsets either alone or with sorted thymocytes.

Results

Productive infection was observed in both thymic pDC and mDC following exposure to R5 HIV-1 and X4 HIV-1. Thymic pDC were more frequently productively infected by both R5 and X4 HIV-1 than thymic mDC (p = 0.03; n = 6). Thymic pDC efficiently transferred productive R5 HIV-1 infection to both CD3^hi (p = 0.01; mean fold increase of 6.5; n = 6) and CD3^lo thymocytes (mean fold increase of 1.6; n = 2). In comparison, transfer of productive infection by thymic mDC was not observed for either X4 or R5 HIV-1.

Conclusions

The capacity of thymic pDC to efficiently transfer R5 HIV-1 to both mature and immature thymocytes that are otherwise refractory to R5 virus may represent a pathway to early infection and impaired production of thymocytes and CD4⁺ T cells in HIV-1-infected individuals.

HIV-1 envelope, integrins and co-receptor use in mucosal transmission of HIV

It is well established that HIV-1 infection typically involves an interaction between the viral envelope protein gp120/41 and the CD4 molecule followed by a second interaction with a chemokine receptor, usually CCR5 or CXCR4. In the early stages of an HIV-1 infection CCR5 using viruses (R5 viruses) predominate. In some viral subtypes there is a propensity to switch to CXCR4 usage (X4 viruses). The receptor switch occurs in ~ 40% of the infected individuals and is associated with faster disease progression. This holds for subtypes B and D, but occurs less frequently in subtypes A and C. There are several hypotheses to explain the preferential transmission of R5 viruses and the mechanisms that lead to switching of co-receptor usage; however, there is no definitive explanation for either. One important consideration regarding transmission is that signaling by R5 gp120 may facilitate transmission of R5 viruses by inducing a permissive environment for HIV replication. In the case of sexual transmission, infection by HIV requires the virus to breach the mucosal barrier to gain access to the immune cell targets that it infects; however, the immediate events that follow HIV exposure at genital mucosal sites are not well understood. Upon transmission, the HIV quasispecies that is replicating in an infected donor contracts through a “genetic bottleneck”, and often infection results from a single infectious event. Many details surrounding this initial infection remain unresolved. In mucosal tissues, CD4⁺ T cells express high levels of CCR5, and a subset of these CD4⁺/CCR5^high cells express the integrin α₄β₇, the gut homing receptor. CD4⁺/CCR5^high/ α4β7^high T cells are highly susceptible to infection by HIV-1 and are ideal targets for an efficient productive infection at the point of transmission. In this context we have demonstrated that the HIV-1 envelope protein gp120 binds to α₄β₇ on CD4⁺ T cells. On CD4⁺/CCR5^high/ α4β7^high T cells, α₄β₇ is closely associated with CD4 and CCR5. Furthermore, α₄β₇ is ~3 times the size of CD4 on the cell surface, that makes it a prominent receptor for an efficient virus capture. gp120-α₄β₇ interactions mediate the activation of the adhesion-associated integrin LFA-1. LFA-1 facilitates the formation of virological synapses and cell-to-cell spread of HIV-1. gp120 binding to α₄β₇ is mediated by a tripeptide located in the V1/V2 domain of gp120. Of note, the V1/V2 domain of gp120 has been linked to variations in transmission fitness among viral isolates raising the intriguing possibility that gp120-α₄β₇ interactions may be linked to transmission fitness. Although many details remain unresolved, we hypothesize that gp120-α₄β₇ interactions play an important role in the very early events following sexual transmission of HIV and may have important implication in the design of vaccine strategies for the prevention of acquisition of HIV infection

Asymmetric HIV-1 co-receptor use and replication in CD4(+) T lymphocytes

Susceptibility to infection by the human immunodeficiency virus type-1 (HIV-1), both in vitro and in vivo, requires the interaction between its envelope (Env) glycoprotein gp120 Env and the primary receptor (R), CD4, and Co-R, either CCR5 or CXCR4, members of the chemokine receptor family. CCR5-dependent (R5) viruses are responsible for both inter-individual transmission and for sustaining the viral pandemics, while CXCR4-using viruses, usually dualtropic R5X4, emerge in ca. 50% of individuals only in the late, immunologically suppressed stage of disease. The hypothesis that such a major biological asymmetry is explained exclusively by the availability of cells expressing CCR5 or CXCR4 is challenged by several evidences. In this regard, binding of the HIV-1 gp120 Env to the entry R complex, i.e. CD4 and a chemokine R, leads to two major events: virion-cell membrane fusion and a cascade of cell signaling. While the fusion/entry process has been well defined, the role of R/Co-R signaling in the HIV-1 life cycle has been less characterized. Indeed, depending on the cellular model studied, the capacity of HIV-1 to trigger a flow of events favoring either its own latency or replication remains a debated issue. In this article, we will review the major findings related to the role of HIV R/Co-R signaling in the steps following viral entry and leading to viral spreading in CD4⁺ T lymphocytes.

Semen protects CD4+ target cells from HIV infection but promotes the preferential transmission of R5 tropic HIV

Sexual intercourse is the major means of HIV transmission, yet the impact of semen on HIV infection of CD4(+) T cells remains unclear. To resolve this conundrum, we measured CD4(+) target cell infection with X4 tropic HIV IIIB and HC4 and R5 tropic HIV BaL and SF162 after incubation with centrifuged seminal plasma (SP) from HIV-negative donors and assessed the impact of SP on critical determinants of target cell susceptibility to HIV infection. We found that SP potently protects CD4(+) T cells from infection with X4 and R5 tropic HIV in a dose- and time-dependent manner. SP caused a diminution in CD4(+) T cell surface expression of the HIVR CD4 and enhanced surface expression of the HIV coreceptor CCR5. Consequently, SP protected CD4(+) T cells from infection with R5 tropic HIV less potently than it protected CD4(+) T cells from infection with X4 tropic HIV. SP also reduced CD4(+) T cell activation and proliferation, and the magnitude of SP-mediated suppression of target cell CD4 expression, activation, and proliferation correlated closely with the magnitude of the protection of CD4(+) T cells from infection with HIV. Taken together, these data show that semen protects CD4(+) T cells from HIV infection by restricting critical determinants of CD4(+) target cell susceptibility to HIV infection. Further, semen contributes to the selective transmission of R5 tropic HIV to CD4(+) target cells.

SDF-1/CXCL12 production by mature dendritic cells inhibits the propagation of X4-tropic HIV-1 isolates at the dendritic cell-T-cell infectious synapse

An efficient mode of HIV-1 infection of CD4 lymphocytes occurs in the context of infectious synapses, where dendritic cells (DCs) enhance HIV-1 transmission to lymphocytes. Emergence of CXCR4-using (X4) HIV-1 strains occurs late in the course of HIV-1 infection, suggesting that a selective pressure suppresses the switch from CCR5 (R5) to X4 tropism. We postulated that SDF-1/CXCL12 chemokine production by DCs could be involved in this process. We observed CXCL12 expression by DCs in vivo in the parafollicular compartment of lymph nodes. The role of mature monocyte-derived dendritic cells (mMDDCs) in transmitting R5 and X4 HIV-1 strains to autologous lymphocytes was studied using an in vitro infection system. Using this model, we observed a strong enhancement of lymphocyte infection with R5, but not with X4, viruses. This lack of DC-mediated enhancement in the propagation of X4 viruses was proportional to CXCL12 production by mMDDCs. When CXCL12 activity was inhibited with specific neutralizing antibodies or small interfering RNAs (siRNAs), the block to mMDDC transfer of X4 viruses to lymphocytes was removed. These results suggest that CXCL12 production by DCs resident in lymph nodes represents an antiviral mechanism in the context of the infectious synapse that could account for the delayed appearance of X4 viruses.