HIV-1 directly kills CD4+ T cells by a Fas-independent mechanism

Making a Monkey out of Human Immunodeficiency Virus/Simian Immunodeficiency Virus Pathogenesis: Immune Cell Depletion Experiments as a Tool to Understand the Immune Correlates of Protection and Pathogenicity in HIV Infection

Understanding the underlying mechanisms of HIV pathogenesis is critical for designing successful HIV vaccines and cure strategies. However, achieving this goal is complicated by the virus's direct interactions with immune cells, the induction of persistent reservoirs in the immune system cells, and multiple strategies developed by the virus for immune evasion. Meanwhile, HIV and SIV infections induce a pandysfunction of the immune cell populations, making it difficult to untangle the various concurrent mechanisms of HIV pathogenesis. Over the years, one of the most successful approaches for dissecting the immune correlates of protection in HIV/SIV infection has been the in vivo depletion of various immune cell populations and assessment of the impact of these depletions on the outcome of infection in non-human primate models. Here, we present a detailed analysis of the strategies and results of manipulating SIV pathogenesis through in vivo depletions of key immune cells populations. Although each of these methods has its limitations, they have all contributed to our understanding of key pathogenic pathways in HIV/SIV infection.

CD4+ T cell count and HIV-1 viral load dynamics positively impacted by H. pylori infection in HIV-positive patients regardless of ART status in a high-burden setting

BACKGROUND

There is a widespread co-infection of HIV and Helicobacter pylori (H. pylori) globally, particularly in developing countries, and it has been suggested that this co-infection may affect the course of HIV disease. However, the interplay between H. pylori infection and HIV disease progression is not fully elucidated. In this study, we investigated the effect of H. pylori co-infection on CD4+ T cell count and HIV viral load dynamics in HIV-positive individuals in a high co-endemic setting.

METHODS

A comparative cross-sectional study was conducted among 288 HIV-positive and 175 HIV-negative individuals, both with and without H. pylori infection. Among HIV-positive participants, 195 were on antiretroviral therapy (ART) and 93 were ART-naïve. CD4+ T cell count and HIV-1 viral load were measured and compared between H. pylori-infected and -uninfected individuals, taking into account different HIV and ART status.

RESULT

Our study demonstrated that individuals infected with H. pylori had a significantly higher CD4+ T cell count compared to uninfected controls among both HIV-negative and HIV-positive participants, regardless of ART therapy. Conversely, HIV/H. pylori co-infected participants had lower HIV-1 viral load than those without H. pylori infection. Linear regression analysis further confirmed a positive association between H. pylori infection, along with other clinical factors such as BMI, ART, and duration of therapy, with CD4+ T cell count while indicating an inverse relationship with HIV-1 viral load in HIV-positive patients. Additionally, factors such as khat chewing, age and WHO clinical stage of HIV were associated with reduced CD4+ T cell count and increased HIV-1 viral load.

CONCLUSION

Our study demonstrates that H. pylori co-infection was associated with higher CD4+ T cell count and lower HIV-1 viral load in HIV-positive patients, regardless of ART status. These findings show a positive effect of H. pylori co-infection on the dynamics of HIV-related immunological and virological parameters. Further studies are needed to elucidate the underlying mechanisms of the observed effects.

Role of T Cells in Microbial Pathogenesis

The immune system functions as a sophisticated defense mechanism, shielding the body from harmful pathogenic invaders [...].

Analysis of the Contribution of 6-mer Seed Toxicity to HIV-1-Induced Cytopathicity

HIV-1 (HIV) infects CD4+ T cells, the gradual depletion of which can lead to AIDS in the absence of antiretroviral therapy (ART). Some cells, however, survive HIV infection and persist as part of the latently infected reservoir that causes recurrent viremia after ART cessation. Improved understanding of the mechanisms of HIV-mediated cell death could lead to a way to clear the latent reservoir. Death induced by survival gene elimination (DISE), an RNA interference (RNAi)-based mechanism, kills cells through short RNAs (sRNAs) with toxic 6-mer seeds (positions 2 to 7 of sRNA). These toxic seeds target the 3' untranslated region (UTR) of mRNAs, decreasing the expression of hundreds of genes critical for cell survival. In most cells under normal conditions, highly expressed cell-encoded nontoxic microRNAs (miRNAs) block access of toxic sRNAs to the RNA-induced silencing complex (RISC) that mediates RNAi, promoting cell survival. HIV has been shown to inhibit the biogenesis of host miRNAs in multiple ways. We now report that HIV infection of cells deficient in miRNA expression or function results in enhanced RISC loading of an HIV-encoded miRNA HIV-miR-TAR-3p, which can kill cells by DISE through a noncanonical (positions 3 to 8) 6-mer seed. In addition, cellular RISC-bound sRNAs shift to lower seed viability. This also occurs after latent HIV provirus reactivation in J-Lat cells, suggesting independence of permissiveness of cells to viral infection. More precise targeting of the balance between protective and cytotoxic sRNAs could provide new avenues to explore novel cell death mechanisms that could be used to kill latent HIV. IMPORTANCE Several mechanisms by which initial HIV infection is cytotoxic to infected cells have been reported and involve various forms of cell death. Characterizing the mechanisms underlying the long-term survival of certain T cells that become persistent provirus reservoirs is critical to developing a cure. We recently discovered death induced by survival gene elimination (DISE), an RNAi-based mechanism of cell death whereby toxic short RNAs (sRNAs) containing 6-mer seed sequences (exerting 6-mer seed toxicity) targeting essential survival genes are loaded into RNA-induced silencing complex (RISC) complexes, resulting in inescapable cell death. We now report that HIV infection in cells with low miRNA expression causes a shift of mostly cellular RISC-bound sRNAs to more toxic seeds. This could prime cells to DISE and is further enhanced by the viral microRNA (miRNA) HIV-miR-TAR-3p, which carries a toxic noncanonical 6-mer seed. Our data provide multiple new avenues to explore novel cell death mechanisms that could be used to kill latent HIV.

HIV-1 usurps transcription start site heterogeneity of host RNA polymerase II to maximize replication fitness

HIV-1 relies on host RNA polymeraseII (Pol II) to transcribe its genome and uses multiple transcription start sites (TSS), including three consecutive guanosines located near the U3-R junction, to generate transcripts containing three, two, and one guanosine at the 5' end, referred to as 3G, 2G, and 1G RNA, respectively. The 1G RNA is preferentially selected for packaging, indicating that these 99.9% identical RNAs exhibit functional differences and highlighting the importance of TSS selection. Here, we demonstrate that TSS selection is regulated by sequences between the CATA/TATA box and the beginning of R. Furthermore, we have generated two HIV-1 mutants with distinct 2-nucleotide modifications that predominantly express 3G RNA or 1G RNA. Both mutants can generate infectious viruses and undergo multiple rounds of replication in T cells. However, both mutants exhibit replication defects compared to the wild-type virus. The 3G-RNA-expressing mutant displays an RNA genome-packaging defect and delayed replication kinetics, whereas the 1G-RNA-expressing mutant exhibits reduced Gag expression and a replication fitness defect. Additionally, reversion of the latter mutant is frequently observed, consistent with sequence correction by plus-strand DNA transfer during reverse transcription. These findings demonstrate that HIV-1 maximizes its replication fitness by usurping the TSS heterogeneity of host RNA Pol II to generate unspliced RNAs with different specialized roles in viral replication. The three consecutive guanosines at the junction of U3 and R may also maintain HIV-1 genome integrity during reverse transcription. These studies reveal the intricate regulation of HIV-1 RNA and complex replication strategy.

Prolonged experimental CD4+ T-cell depletion does not cause disease progression in SIV-infected African green monkeys

CD4⁺ T-cell depletion is a hallmark of HIV infection, leading to impairment of cellular immunity and opportunistic infections, but its contribution to SIV/HIV-associated gut dysfunction is unknown. Chronically SIV-infected African Green Monkeys (AGMs) partially recover mucosal CD4⁺ T-cells, maintain gut integrity and do not progress to AIDS. Here we assess the impact of prolonged, antibody-mediated CD4 + T-cell depletion on gut integrity and natural history of SIV infection in AGMs. All circulating CD4⁺ T-cells and >90% of mucosal CD4⁺ T-cells are depleted. Plasma viral loads and cell-associated viral RNA in tissues are lower in CD4⁺-cell-depleted animals. CD4⁺-cell-depleted AGMs maintain gut integrity, control immune activation and do not progress to AIDS. We thus conclude that CD4⁺ T-cell depletion is not a determinant of SIV-related gut dysfunction, when gastrointestinal tract epithelial damage and inflammation are absent, suggesting that disease progression and resistance to AIDS are independent of CD4⁺ T-cell restoration in SIVagm-infected AGMs.

Clearance of HIV-1 or SIV reservoirs by promotion of apoptosis and inhibition of autophagy: Targeting intracellular molecules in cure-directed strategies

The reservoirs of the HIV display cellular properties resembling long-lived immune memory cells that could be exploited for viral clearance. Our interest in developing a cure for HIV stems from the studies of immunologic memory against infections. We and others have found that long-lived immune memory cells employ prosurvival autophagy and antiapoptotic mechanisms to protect their longevity. Here, we describe the rationale for the development of an approach to clear HIV-1 by selective elimination of host cells harboring replication-competent HIV (SECH). While reactivation of HIV-1 in the host cells with latency reversing agents (LRAs) induces viral gene expression leading to cell death, LRAs also simultaneously up-regulate prosurvival antiapoptotic molecules and autophagy. Mechanistically, transcription factors that promote HIV-1 LTR-directed gene expression, such as NF-κB, AP-1, and Hif-1α, can also enhance the expression of cellular genes essential for cell survival and metabolic regulation, including Bcl-xL, Mcl-1, and autophagy genes. In the SECH approach, we inhibit the prosurvival antiapoptotic molecules and autophagy induced by LRAs, thereby allowing maximum killing of host cells by the induced HIV-1 proteins. SECH treatments cleared HIV-1 infections in humanized mice in vivo and in HIV-1 patient PBMCs ex vivo. SECH also cleared infections by the SIV in rhesus macaque PBMCs ex vivo. Research efforts are underway to improve the efficacy and safety of SECH and to facilitate the development of SECH as a therapeutic approach for treating people with HIV.

Rapid Loss of CD4 T Cells by Pyroptosis during Acute SIV Infection in Rhesus Macaques

The mechanisms underlying depletion of CD4 T cells during acute HIV-1 infection are not well understood. Here we show that caspase-1-induced pyroptosis, a highly inflammatory programmed cell death pathway, is the dominant mechanism responsible for the rapid depletion of CD4 T cells in gut-associated lymphatic tissue (GALT), spleen, and lymph nodes during acute simian immunodeficiency virus (SIV) infection in rhesus macaques. Upregulation of interferon-gamma inducible factor 16, a host DNA sensor that triggers pyroptosis, was also observed in tissue-resident CD4 T cells and correlated with viral loads and CD4 T cell loss. In contrast, caspase-3-mediated apoptosis and viral cytotoxicity only accounted for a small fraction of CD4 T cell death. Other programmed cell death mechanisms, including mitochondria-induced caspase-independent cell death, necroptosis, and autophagy, did not significantly contribute to CD4 T cell depletion. These data support a model in which caspase-1-mediated pyroptosis is the principal mechanism that results in CD4 T cell loss in the GALT and lymphoid organs and release of proinflammatory cytokines. These findings contribute to our understanding of the pathogenesis of acute SIV infection and have important implications for the development of therapeutic strategies.

IMPORTANCE Different mechanisms for CD4 T cell depletion during acute HIV-1 infection have been proposed. In this study, we demonstrate that in early simian immunodeficiency virus infection, depletion of CD4 T cells is primarily due to pyroptosis. Other mechanisms may also contribute in a minor way to CD4 T cell depletion.

Wnt/β-Catenin Protects Lymphocytes from HIV-Mediated Apoptosis via Induction of Bcl-xL

HIV infection mediates the apoptosis of lymphocytes, the mechanism of which is multifaceted. Here, we evaluated the role of Wnt/β-catenin signaling in HIV-associated T cell apoptosis, as Wnt/β-catenin regulates the transcriptional activity of genes impacting apoptosis. We specifically investigated the role of the Wnt/β-catenin pathway in the HIV-associated apoptosis of CD4+ T cells and CD4dimCD8bright T cells, a population that is infected by HIV. We found that the induction of β-catenin, via a 6-bromoindirubin-3-oxime (BIO), significantly rescued HIV-infected CD4+ and CD4dimCD8bright T cells from apoptosis by >40−50%. Further, a small-molecule inhibitor of the Wnt/β-catenin pathway (PNU-74654) reversed BIO-mediated protection from HIV-associated apoptosis. BIO also induced Bcl-xL, an anti-apoptotic protein, and a target gene of Wnt/β-catenin, in CD4+ and CD4dimCD8bright T cells by approximately 3-fold. Inhibiting Bcl-xL by WEHI-539 abrogated β-catenin-mediated apoptotic protection in infected CD4+ and CD4dimCD8bright T cells. Collectively, these findings demonstrate that engaging Wnt/β-catenin signaling in HIV-infected T cells protects them from HIV-associated apoptosis by inducing Bcl-xL.

Prolonged viral suppression with anti-HIV-1 antibody therapy

HIV-1 infection remains a public health problem with no cure. Anti-retroviral therapy (ART) is effective but requires lifelong drug administration owing to a stable reservoir of latent proviruses integrated into the genome of CD4+ T cells1. Immunotherapy with anti-HIV-1 antibodies has the potential to suppress infection and increase the rate of clearance of infected cells2,3. Here we report on a clinical study in which people living with HIV received seven doses of a combination of two broadly neutralizing antibodies over 20 weeks in the presence or absence of ART. Without pre-screening for antibody sensitivity, 76% (13 out of 17) of the volunteers maintained virologic suppression for at least 20 weeks off ART. Post hoc sensitivity analyses were not predictive of the time to viral rebound. Individuals in whom virus remained suppressed for more than 20 weeks showed rebound viraemia after one of the antibodies reached serum concentrations below 10 µg ml-1. Two of the individuals who received all seven antibody doses maintained suppression after one year. Reservoir analysis performed after six months of antibody therapy revealed changes in the size and composition of the intact proviral reservoir. By contrast, there was no measurable decrease in the defective reservoir in the same individuals. These data suggest that antibody administration affects the HIV-1 reservoir, but additional larger and longer studies will be required to define the precise effect of antibody immunotherapy on the reservoir.

Single-Cell Sequencing Facilitates Elucidation of HIV Immunopathogenesis: A Review of Current Literature

Knowledge gaps remain in the understanding of HIV disease establishment and progression. Scientists continue to strive in their endeavor to elucidate the precise underlying immunopathogenic mechanisms of HIV-related disease, in order to identify possible preventive and therapeutic targets. A useful tool in the quest to reveal some of the enigmas related to HIV infection and disease is the single-cell sequencing (scRNA-seq) technique. With its proven capacity to elucidate critical processes in cell formation and differentiation, to decipher critical hematopoietic pathways, and to understand the regulatory gene networks that predict immune function, scRNA-seq is further considered to be a potentially useful tool to explore HIV immunopathogenesis. In this article, we provide an overview of single-cell sequencing platforms, before delving into research findings gleaned from the use of single cell sequencing in HIV research, as published in recent literature. Finally, we describe two important avenues of research that we believe should be further investigated using the single-cell sequencing technique.

Biomarkers of disease severity in patients with visceral leishmaniasis co-infected with HIV

Visceral leishmaniasis (VL) is caused by the protozoan Leishmania spp, transmitted by sand fly bites. VL is one of the deadliest tropical infection diseases, yet the coinfection with HIV virus drastically increases relapses, treatment failure and mortality. The concomitant action of these two pathogens leads to high cellular activation independently of the progression to AIDS. In addition, microbial translocation and bacterial infections are thought to contribute worsening the clinical picture. Identifying biomarkers associated with disease severity is of interest for clinical management of patients with VL-HIV/AIDS. Thus, we analyzed in the sera several markers including interleukins (IL-1β, IL-6, IL-8, and IL-17), interferon-γ (IFN- γ), tumor necrosis factor (TNF), lipopolysaccharide (LPS), soluble CD14 (sCD14), macrophage migration inhibitory factor (MIF) and intestinal fatty acid-binding protein (IFABP). These markers were compared with disease severity in 24 patients with VL/HIV presenting different clinical outcomes. Disease severity was defined by the probability of death calculated using a score set system derived by the Kala-Cal® software. Probability of death ranged from 3.7% to 97.9%, with median of 28.8%. Five patients died (20%). At the univariate analysis, disease severity was correlated with TNF, IFN-γ and sCD14. LPS was positively correlated with sCD14 specifically in patients with low CD4⁺ count (CD4⁺ T-cell <200 cells/mL). Most importantly, the multivariate analysis including LPS, CD4⁺count and sCD14 showed that sCD14 was the only independent predictor for disease severity and death. Altogether, our results indicated that sCD14 is a powerful marker of pathogenicity and death for patients with VL-HIV/AIDS.

NLRP3 inflammasome induces CD4+ T cell loss in chronically HIV-1-infected patients

Chronic HIV-1 infection is generally characterized by progressive CD4+ T cell depletion due to direct and bystander death that is closely associated with persistent HIV-1 replication and an inflammatory environment in vivo. The mechanisms underlying the loss of CD4+ T cells in patients with chronic HIV-1 infection are incompletely understood. In this study, we simultaneously monitored caspase-1 and caspase-3 activation in circulating CD4+ T cells, which revealed that pyroptotic and apoptotic CD4+ T cells are distinct cell populations with different phenotypic characteristics. Levels of pyroptosis and apoptosis in CD4+ T cells were significantly elevated during chronic HIV-1 infection, and decreased following effective antiretroviral therapy. Notably, the occurrence of pyroptosis was further confirmed by elevated gasdermin D activation in lymph nodes of HIV-1-infected individuals. Mechanistically, caspase-1 activation closely correlated with the inflammatory marker expression and was shown to occur through NLRP3 inflammasome activation driven by virus-dependent and/or -independent ROS production, while caspase-3 activation in CD4+ T cells was more closely related to T cell activation status. Hence, our findings show that NLRP3-dependent pyroptosis plays an essential role in CD4+ T cell loss in HIV-1-infected patients and implicate pyroptosis signaling as a target for anti-HIV-1 treatment.

The Hitchhiker Guide to CD4+ T-Cell Depletion in Lentiviral Infection. A Critical Review of the Dynamics of the CD4+ T Cells in SIV and HIV Infection

CD4⁺ T-cell depletion is pathognomonic for AIDS in both HIV and simian immunodeficiency virus (SIV) infections. It occurs early, is massive at mucosal sites, and is not entirely reverted by antiretroviral therapy (ART), particularly if initiated when T-cell functions are compromised. HIV/SIV infect and kill activated CCR5-expressing memory and effector CD4⁺ T-cells from the intestinal lamina propria. Acute CD4⁺ T-cell depletion is substantial in progressive, nonprogressive and controlled infections. Clinical outcome is predicted by the mucosal CD4⁺ T-cell recovery during chronic infection, with no recovery occurring in rapid progressors, and partial, transient recovery, the degree of which depends on the virus control, in normal and long-term progressors. The nonprogressive infection of African nonhuman primate SIV hosts is characterized by partial mucosal CD4⁺ T-cell restoration, despite high viral replication. Complete, albeit very slow, recovery of mucosal CD4+ T-cells occurs in controllers. Early ART does not prevent acute mucosal CD4⁺ T-cell depletion, yet it greatly improves their restoration, sometimes to preinfection levels. Comparative studies of the different models of SIV infection support a critical role of immune activation/inflammation (IA/INFL), in addition to viral replication, in CD4⁺ T-cell depletion, with immune restoration occurring only when these parameters are kept at bay. CD4⁺ T-cell depletion is persistent, and the recovery is very slow, even when both the virus and IA/INFL are completely controlled. Nevertheless, partial mucosal CD4⁺ T-cell recovery is sufficient for a healthy life in natural hosts. Cell death and loss of CD4⁺ T-cell subsets critical for gut health contribute to mucosal inflammation and enteropathy, which weaken the mucosal barrier, leading to microbial translocation, a major driver of IA/INFL. In turn, IA/INFL trigger CD4⁺ T-cells to become either viral targets or apoptotic, fueling their loss. CD4⁺ T-cell depletion also drives opportunistic infections, cancers, and comorbidities. It is thus critical to preserve CD4⁺ T cells (through early ART) during HIV/SIV infection. Even in early-treated subjects, residual IA/INFL can persist, preventing/delaying CD4⁺ T-cell restoration. New therapeutic strategies limiting mucosal pathology, microbial translocation and IA/INFL, to improve CD4⁺ T-cell recovery and the overall HIV prognosis are needed, and SIV models are extensively used to this goal.

Clearance of HIV infection by selective elimination of host cells capable of producing HIV

The RNA genome of the human immunodeficiency virus (HIV) is reverse-transcribed into DNA and integrated into the host genome, resulting in latent infections that are difficult to clear. Here we show an approach to eradicate HIV infections by selective elimination of host cells harboring replication-competent HIV (SECH), which includes viral reactivation, induction of cell death, inhibition of autophagy and the blocking of new infections. Viral reactivation triggers cell death specifically in HIV-1-infected T cells, which is promoted by agents that induce apoptosis and inhibit autophagy. SECH treatments can clear HIV-1 in >50% mice reconstituted with a human immune system, as demonstrated by the lack of viral rebound after withdrawal of treatments, and by adoptive transfer of treated lymphocytes into uninfected humanized mice. Moreover, SECH clears HIV-1 in blood samples from HIV-1-infected patients. Our results suggest a strategy to eradicate HIV infections by selectively eliminating host cells capable of producing HIV.

Identification of pathogenic TRAIL-expressing innate immune cells during HIV-1 infection in humanized mice by scRNA-Seq

Depletion of CD4+ T cells during HIV-1 infection is mostly mediated by inflammatory cells via indirect but not clearly defined mechanisms. In this report, we used single-cell RNA-Seq (scRNA-Seq) technology to study HIV-induced transcriptomic change in innate immune cells in lymphoid organs. We performed scRNA-Seq on hCD45+hCD3-hCD19- human leukocytes isolated from spleens of humanized NOD/Rag2-/-γc-/- (NRG) mice transplanted with human CD34+ hematopoietic stem progenitor cells (NRG-hu HSC mice). We identified major populations of innate immune cells, including plasmacytoid dendritic cells (pDCs), myeloid dendritic cells (mDCs), macrophages, NK cells, and innate lymphoid cells (ILCs). HIV-1 infection significantly upregulated genes involved in type I IFN inflammatory pathways in each of the innate immune subsets. Interestingly, we found that TRAIL was upregulated in the innate immune populations, including pDCs, mDCs, macrophages, NK cells, and ILCs. We further demonstrated that blockade of the TRAIL signaling pathway in NRG-hu HSC mice prevented HIV-1-induced CD4+ T cell depletion in vivo. In summary, we characterized HIV-induced transcriptomic changes of innate immune cells in the spleen at single-cell levels, identified the TRAIL+ innate immune cells, and defined an important role of the TRAIL signaling pathway in HIV-1-induced CD4+ T cell depletion in vivo.

IL6 and FAS/FASL gene polymorphisms may be associated with disease progression in HIV-1-positive ethnically mixed patients

The progression of AIDS depends on the complex host and virus interactions. The most important disease progression hallmarks are immune activation and apoptosis. In this study, we address the prevalence of polymorphisms related to proinflammatory and apoptotic genes, such as IFNG (+874T/A), TNF (308G/A), IL6 (-174G/C), IL8 (-251A/T), FAS (-670A/G), and FASL (-124A/G) in 160 ethnically mixed HIV-1-infected patients from multicentre cohorts with different clinical outcomes (13 elite controllers [EC], 66 slow long-term non-progressors [LTNPs], and 81 progressors [P]). The genotyping was accomplished by TaqMan-qPCR. Among all the polymorphisms analyzed in the cytokines, the IL6 -174G/C polymorphism showed a higher frequency of GG genotype in the LTNP and LTNP+EC groups as compared to the P group. Moreover, there was a significantly higher frequency of the G allele in the LTNP and LTNP+EC groups as compared to the P group. On the other hand, the levels of CD4⁺ T lymphocytes were higher among individuals showing the AA and AG genotypes for the FASL -124A/G polymorphism as compared to the GG genotype. Furthermore, the AG and AA genotypes were more frequent, as compared to the GG genotype, in individuals showing a lower viral load. In contrast, for the FAS -670A/G polymorphism, a significantly higher viral load was observed in individuals with the AG genotype as compared to the GG genotype. In conclusion, we found three genetic allelic variants of the IL6 -174G/C, FASL -124A/G, and FAS -670A/G polymorphisms that were related to disease progression and immunological and virological markers in cohorts of HIV-1-positive ethnically mixed patients.

CD8 T cells targeting adapted epitopes in chronic HIV infection promote dendritic cell maturation and CD4 T cell trans-infection

HIV-1 frequently escapes from CD8 T cell responses via HLA-I restricted adaptation, leading to the accumulation of adapted epitopes (AE). We previously demonstrated that AE compromise CD8 T cell responses during acute infection and are associated with poor clinical outcomes. Here, we examined the impact of AE on CD8 T cell responses and their biological relevance in chronic HIV infection (CHI). In contrast to acute infection, the majority of AE are immunogenic in CHI. Longitudinal analyses from acute to CHI showed an increased frequency and magnitude of AE-specific IFNγ responses compared to NAE-specific ones. These AE-specific CD8 T cells also were more cytotoxic to CD4 T cells. In addition, AE-specific CD8 T cells expressed lower levels of PD1 and CD57, as well as higher levels of CD28, suggesting a more activated and less exhausted phenotype. During CHI, viral sequencing identified AE-encoding strains as the dominant quasispecies. Despite increased CD4 T cell cytotoxicity, CD8 T cells responding to AE promoted dendritic cell (DC) maturation and CD4 T cell trans-infection perhaps explaining why AE are predominant in CHI. Taken together, our data suggests that the emergence of AE-specific CD8 T cell responses in CHI confers a selective advantage to the virus by promoting DC-mediated CD4 T cell trans-infection.

HIV-1 infection remains a critical public health threat across the world. Over the past two decades, CD8 T cells have been clearly shown to exert immune pressure on HIV and drive viral adaptation. Previously, our group reported that such HLA-I associated adaptations can predict clinical outcomes and are beneficial to HIV-1 as CD8 T cells are unable to recognize epitopes with adaptation in acute HIV infection. However, it is still unclear how HIV-1 adaptation impacts CD8 T cells during chronic HIV infection. In this study, we observed an enhancement of CD8 T cell responses targeting adapted epitopes in chronic infection. Although these responses were cytotoxic, they also exhibited a “helper” effect by promoting viral infection of CD4 T cells via interaction with dendritic cells. This phenomenon may contribute to the persistence of adapted viruses. In summary, these findings present a novel mechanism of CD8 T cell driven HIV-1 adaptation.

CD8+ T cells with high TGF‑β1 expression cause lymph node fibrosis following HIV infection

Lymph node (LN) fibrosis resulting in cluster of differentiation (CD) 4+ T cell reduction following human immunodeficiency virus (HIV) infection is an important step in the pathogenesis of acquired immunodeficiency syndrome. The mechanisms mediating LN fibrosis following HIV infection have not been completely elucidated. In order to investigate the mechanism of LN fibrosis, the expression of transforming growth factor (TGF)‑β1 was determined in the LNs of HIV‑infected individuals by immunohistochemistry and fluorescence‑based flow cytometry. The effect of stimulated CD8+ T cells on collagen secretion by fibroblasts was detected using immunofluorescence staining and western blot analysis. The results demonstrated that the LNs of HIV‑infected individuals exhibited a significantly increased proportion of CD8+ T cells with high TGF‑β1 expression. These CD8+ T cells demonstrated increased CD38 and programmed cell death protein 1 expression and decreased CD127 expression compared with the controls. CD8+ T cells from the LNs of non‑HIV infected individuals expressed a high TGF‑β1 level following stimulation with phorbol‑12‑myristate 13‑acetate. These CD8+T cells subsequently induced the secretion of a large amount of type I collagen in human lymphatic fibroblasts. The results of the present study indicated that CD8+ T cells with high TGF‑β1 expression served an important role in LN fibrosis following HIV infection.

The anti-caspase inhibitor Q-VD-OPH prevents AIDS disease progression in SIV-infected rhesus macaques

Apoptosis has been proposed as a key mechanism responsible for CD4+ T cell depletion and immune dysfunction during HIV infection. We demonstrated that Q-VD-OPH, a caspase inhibitor, inhibits spontaneous and activation-induced death of T cells from SIV-infected rhesus macaques (RMs). When administered during the acute phase of infection, Q-VD-OPH was associated with (a) reduced levels of T cell death, (b) preservation of CD4+/CD8+ T cell ratio in lymphoid organs and in the gut, (c) maintenance of memory CD4+ T cells, and (d) increased specific CD4+ T cell response associated with the expression of cytotoxic molecules. Although therapy was limited to the acute phase of infection, Q-VD-OPH-treated RMs showed lower levels of both viral load and cell-associated SIV DNA as compared with control SIV-infected RMs throughout the chronic phase of infection, and prevented the development of AIDS. Overall, our data demonstrate that Q-VD-OPH injection in SIV-infected RMs may represent an adjunctive therapeutic agent to control HIV infection and delaying disease progression to AIDS.

Host and Viral Factors in HIV-Mediated Bystander Apoptosis

Human immunodeficiency virus (HIV) infections lead to a progressive loss of CD4 T cells primarily via the process of apoptosis. With a limited number of infected cells and vastly disproportionate apoptosis in HIV infected patients, it is believed that apoptosis of uninfected bystander cells plays a significant role in this process. Disease progression in HIV infected individuals is highly variable suggesting that both host and viral factors may influence HIV mediated apoptosis. Amongst the viral factors, the role of Envelope (Env) glycoprotein in bystander apoptosis is well documented. Recent evidence on the variability in apoptosis induction by primary patient derived Envs underscores the role of Env glycoprotein in HIV disease. Amongst the host factors, the role of C-C Chemokine Receptor type 5 (CCR5), a coreceptor for HIV Env, is also becoming increasingly evident. Polymorphisms in the CCR5 gene and promoter affect CCR5 cell surface expression and correlate with both apoptosis and CD4 loss. Finally, chronic immune activation in HIV infections induces multiple defects in the immune system and has recently been shown to accelerate HIV Env mediated CD4 apoptosis. Consequently, those factors that affect CCR5 expression and/or immune activation in turn indirectly regulate HIV mediated apoptosis making this phenomenon both complex and multifactorial. This review explores the complex role of various host and viral factors in determining HIV mediated bystander apoptosis.

HIV-1 Infection-Induced Suppression of the Let-7i/IL-2 Axis Contributes to CD4(+) T Cell Death

The mechanisms underlying HIV-1-mediated CD4⁺ T cell depletion are highly complicated. Interleukin-2 (IL-2) is a key cytokine that maintains the survival and proliferation of activated CD4⁺ T cells. IL-2 levels are disturbed during HIV-1 infection, but the underlying mechanism(s) requires further investigation. We have reported that cellular microRNA (miRNA) let-7i upregulates IL-2 expression by targeting the promoter TATA-box region, which functions as a positive regulator. In this study, we found that HIV-1 infection decreases the expression of let-7i in CD4⁺ T cells by attenuating its promoter activity. The reduced let-7i miRNA expression led to a decline in IL-2 levels. A let-7i mimic increased IL-2 expression and subsequently enhanced the resistance of CD4⁺ T cells to HIV-1-induced apoptosis. By contrast, the blockage of let-7i with a specific inhibitor resulted in elevated CD4⁺ T cell apoptosis during HIV-1 infection. Furthermore, by knocking down the expression of IL-2, we found that the let-7i-mediated CD4⁺ T cell resistance to apoptosis during HIV-1 infection was dependent on IL-2 signaling rather than an alternative CD95-mediated cell-death pathway. Taken together, our findings reveal a novel pathway for HIV-1-induced dysregulation of IL-2 cytokines and depletion of CD4⁺ T-lymphocytes.

Dissecting How CD4 T Cells Are Lost During HIV Infection

Although the replicative life cycle of HIV within CD4 T cells is understood in molecular detail, less is known about how this human retrovirus promotes the loss of CD4 T lymphocytes. It is this cell death process that drives clinical progression to acquired immune deficiency syndrome (AIDS). Recent studies have highlighted how abortive infection of resting and thus nonpermissive CD4 T cells in lymphoid tissues triggers a lethal innate immune response against the incomplete DNA products generated by inefficient viral reverse transcription in these cells. Sensing of these DNA fragments results in pyroptosis, a highly inflammatory form of programmed cell death, that potentially further perpetuates chronic inflammation and immune activation. As discussed here, these studies cast CD4 T cell death during HIV infection in a different light. Further, they identify drug targets that may be exploited to both block CD4 T cell demise and the chronic inflammatory response generated during pyroptosis.

Disruption of Type I Interferon Induction by HIV Infection of T Cells

Our main objective of this study was to determine how Human Immunodeficiency Virus (HIV) avoids induction of the antiviral Type I Interferon (IFN) system. To limit viral infection, the innate immune system produces important antiviral cytokines such as the IFN. IFN set up a critical roadblock to virus infection by limiting further replication of a virus. Usually, IFN production is induced by the recognition of viral nucleic acids by innate immune receptors and subsequent downstream signaling. However, the importance of IFN in the defense against viruses has lead most pathogenic viruses to evolve strategies to inhibit host IFN induction or responses allowing for increased pathogenicity and persistence of the virus. While the adaptive immune responses to HIV infection have been extensively studied, less is known about the balance between induction and inhibition of innate immune defenses, including the antiviral IFN response, by HIV infection. Here we show that HIV infection of T cells does not induce significant IFN production even IFN I Interferon production. To explain this paradox, we screened HIV proteins and found that two HIV encoded proteins, Vpu and Nef, strongly antagonize IFN induction, with expression of these proteins leading to loss of expression of the innate immune viral RNA sensing adaptor protein, IPS-1 (IFN-β promoter stimulator-1). We hypothesize that with lower levels of IPS-1 present, infected cells are defective in mounting antiviral responses allowing HIV to replicate without the normal antiviral actions of the host IFN response. Using cell lines as well as primary human derived cells, we show that HIV targeting of IPS-1 is key to limiting IFN induction. These findings describe how HIV infection modulates IFN induction providing insight into the mechanisms by which HIV establishes infection and persistence in a host.

Reduction of immunity in HIV-infected individuals: can fibrosis induce hypoplasia in palatine and lingual tonsils of individuals with HIV infection?

The role of tonsils in oral immunity has been described. However, the pathogenesis of HIV infection in these organs is still unclear. The aim of this study is to perform histological and immunohistochemical analysis of the palatine and lingual tonsils of autopsied individuals with or without HIV infection. Twenty-six autopsied individuals with HIV infection (HI) (n=13) and without HIV infection (CO) (n=13) were selected. Palatine and lingual tonsil fragments were collected for histological and immunohistochemical analysis. We found in the HI group a higher frequency of hyaline degeneration in both palatine and lingual tonsils; smaller follicle areas, and a higher percentage of collagen in comparison with the CO group. In the HI group, there was higher density of blood vessels in palatine tonsils than in the CO group. In the HI group, there were significant positive correlations between palatine and lingual tonsils and the area of lymphoid follicles, and between the percentage of blood vessels and collagen in palatine tonsils. In addition, there was a significant negative correlation between the percentage of collagen and lymphoid follicle area in both palatine and lingual tonsils in the HI group. These findings suggest that the immune functions of these tonsils are prejudiced by fibrosis. Therapies to reduce the neoformation of collagen are required to improve immune function of organs against pathogens.

HIV integrase and the swan song of the CD4 T cells?

T cell apoptosis represents one pathophysiological mechanism associated with AIDS. Herein, we discuss the recent report published by A. Cooper et al. in Nature (June 2013) regarding HIV viral DNA integration-mediated apoptosis.

Genetic signatures of HIV-1 envelope-mediated bystander apoptosis

The envelope (Env) glycoprotein of HIV is an important determinant of viral pathogenesis. Several lines of evidence support the role of HIV-1 Env in inducing bystander apoptosis that may be a contributing factor in CD4(+) T cell loss. However, most of the studies testing this phenomenon have been conducted with laboratory-adapted HIV-1 isolates. This raises the question of whether primary Envs derived from HIV-infected patients are capable of inducing bystander apoptosis and whether specific Env signatures are associated with this phenomenon. We developed a high throughput assay to determine the bystander apoptosis inducing activity of a panel of primary Envs. We tested 38 different Envs for bystander apoptosis, virion infectivity, neutralizing antibody sensitivity, and putative N-linked glycosylation sites along with a comprehensive sequence analysis to determine if specific sequence signatures within the viral Env are associated with bystander apoptosis. Our studies show that primary Envs vary considerably in their bystander apoptosis-inducing potential, a phenomenon that correlates inversely with putative N-linked glycosylation sites and positively with virion infectivity. By use of a novel phylogenetic analysis that avoids subtype bias coupled with structural considerations, we found specific residues like Arg-476 and Asn-425 that were associated with differences in bystander apoptosis induction. A specific role of these residues was also confirmed experimentally. These data demonstrate for the first time the potential of primary R5 Envs to mediate bystander apoptosis in CD4(+) T cells. Furthermore, we identify specific genetic signatures within the Env that may be associated with the bystander apoptosis-inducing phenotype.

Assessing main death pathways in T lymphocytes from HIV infected individuals

Increased lymphocyte death is a hallmark of human immunodeficiency virus (HIV) infection. Although virological factors have been linked to this phenomenon, increased cell death rates are still observed in treated individuals in which viral replication is halted. To understand the nature of this remaining altered cell death, we have developed a simple and fast assay to assess major cell death pathways in lymphocytes isolated from HIV-infected individuals. The combination of three factors: (i) antibody staining to identify CD3(+) CD4(+) and CD3(+) CD8(+) cells, (ii) assessment of mitochondrial and plasma membrane function using DiOC6(3) or JC-1 probes and vital dyes, and (iii) caspase inhibition, allowed for the quantification of caspase-independent and -dependent cell death in CD4 and CD8 T cells. The latter mechanism was divided in intrinsic and extrinsic apoptotic pathways according to the sensitivity of the dissipation of mitochondrial membrane potential to Z-VAD-fmk or Q-VD-oPH treatment. Our data show similar results for both caspase inhibitors in treated infected individuals, whereas Q-VD-oPH showed a more potent inhibition in viremic individuals, yielding lower levels of intrinsic apoptosis. Comparison of DiOC6(3) and JC-1 probes yielded similar results in CD4 T cells, allowing for a clear definition of death mechanism in these cells. However, in CD8 T-cells, JC-1 showed heterogeneous staining and detected significantly lower levels of cell death with a higher contribution of intrinsic apoptosis. In conclusion, we provide a simple method to assess CD4 T-cell death mechanisms in HIV-infected individuals. The reasons and consequences of mitochondrial heterogeneity in CD8 T-cells require further evaluation.

DRAM triggers lysosomal membrane permeabilization and cell death in CD4(+) T cells infected with HIV

Productive HIV infection of CD4⁺ T cells leads to a caspase-independent cell death pathway associated with lysosomal membrane permeabilization (LMP) and cathepsin release, resulting in mitochondrial outer membrane permeabilization (MOMP). Herein, we demonstrate that HIV infection induces damage-regulated autophagy modulator (DRAM) expression in a p53-dependent manner. Knocking down the expression of DRAM and p53 genes with specific siRNAs inhibited autophagy and LMP. However, inhibition of Atg5 and Beclin genes that prevents autophagy had a minor effect on LMP and cell death. The knock down of DRAM gene inhibited cytochrome C release, MOMP and cell death. However, knocking down DRAM, we increased viral infection and production. Our study shows for the first time the involvement of DRAM in host-pathogen interactions, which may represent a mechanism of defense via the elimination of infected cells.

Lysosomes are acidic organelles capable of digesting macromolecules and regulating autophagy. In the context of host-pathogen interactions, productive viral infections are associated with lysosome membrane permeabilization (LMP) and programmed cell death (PCD). At a molecular level, the tumor suppressor protein 53 (p53), which is a key player in the detection of DNA damage, acts also as a sensor of pathogen replication. Activation of p53 has been considered to be an altruistic cell suicide mechanism that limits viral infection. Here, we provide new evidence that damage-regulated autophagy modulator (DRAM), a p53 target gene, regulates both LMP and PCD of HIV-infected CD4 T cells. Whereas the inhibition of DRAM or p53 prevents autophagy in infected cells, the inhibition of the autophagy machinery has a minor role in this context. As a consequence, the silencing of DRAM leads to increased HIV viral infection. This is the first report describing the role of DRAM in the context of host-pathogen interaction. Whereas it is to the advantage of the pathogens to preserve their hosts and thus facilitate their multiplication and dissemination, hosts have developed altruistic cellular processes to defend themself and limit the spread of the infectious agent in multicellular organisms. We propose that the ancestral DRAM protein represents a mechanism of self-defense, inducing elimination of infected cells through LMP.

Excessive conversion and impaired thymic output contribute to disturbed regulatory T-cell homeostasis in AIDS patients with low CD4 cell counts

OBJECTIVE

T regulatory (Treg) cells are a heterogeneous population that consists of CD4(+)CD25(low)CD45RA(+) [naive Treg (nTreg) cells] and CD4(+)CD25(high)CD45RA(-) [activated Treg (aTreg) cells] subsets. We investigated the effects of HIV infection and HAART on distinct Treg subsets.

METHODS

HIV-infected adult patients naive to HAART (n=57), patients with acute HIV infection (n=13), and healthy controls (n=92) were recruited for a cross-sectional study. Patients receiving HAART were followed up in a longitudinal study.

RESULTS

Compared with healthy controls, we observed a reduced proportion of nTreg cells and an elevated frequency of aTreg cells in peripheral blood from HAART-naïve patients. Moreover, nTreg cells showed a decreased CD31(+) frequency, whereas aTreg cells showed an increased CD31(+) frequency, indicating impaired thymic output and excessive conversion from nTreg to aTreg cells. nTreg and aTreg cells both displayed higher levels of Ki67(+), reflecting hyperproliferation. The longitudinal study showed that HAART successfully recovered nTreg but not aTreg cell frequency. Higher baseline naïve CD4 T-cell percentages were associated with faster reconstitution of nTreg cell frequency as well as CD4(+) T-cell count.

CONCLUSION

Our data suggest that the disturbed homeostasis of Treg cells in HIV-infected patients is probably caused by excessive conversion from nTreg to aTreg cells, and impaired thymic output of nTreg cells. nTreg cells can be recovered by HAART, which was associated with baseline naive CD4(+) T-cell percentages, indicating that reconstitution of nTreg cells may benefit from earlier antiretroviral treatment.

Intracellular events and cell fate in filovirus infection

Marburg and Ebola viruses cause a severe hemorrhagic disease in humans with high fatality rates. Early target cells of filoviruses are monocytes, macrophages, and dendritic cells. The infection spreads to the liver, spleen and later other organs by blood and lymph flow. A hallmark of filovirus infection is the depletion of non-infected lymphocytes; however, the molecular mechanisms leading to the observed bystander lymphocyte apoptosis are poorly understood. Also, there is limited knowledge about the fate of infected cells in filovirus disease. In this review we will explore what is known about the intracellular events leading to virus amplification and cell damage in filovirus infection. Furthermore, we will discuss how cellular dysfunction and cell death may correlate with disease pathogenesis.

The HR2 polymorphism N140I in the HIV-1 gp41 combined with the HR1 V38A mutation is associated with a less cytopathic phenotype

Background

Resistance to the fusion inhibitor enfuvirtide (ENF) is achieved by changes in the gp41 subunit of the HIV envelope glycoprotein (Env). Specific ENF-associated mutational pathways correlate with immunological recovery, even after virological failure, suggesting that the acquisition of ENF resistance alters gp41 pathogenicity. To test this hypothesis, we have characterized the expression, fusion capability, induction of CD4⁺ T cell loss and single CD4⁺ T cell death of 48 gp41 proteins derived from three patients displaying different amino acids (N, T or I) at position 140 that developed a V38A mutation after ENF-based treatment.

Results

In all cases, intra-patient comparison of Env isolated pre- or post-treatment showed comparable values of expression and fusogenic capacity. Furthermore, Env with either N or T at position 140 induced comparable losses of CD4⁺ T-cells, irrespective of the residue present at position 38. Conversely, Env acquiring the V38A mutation in a 140I background induced a significantly reduced loss of CD4⁺ T cells and lower single-cell death than did their baseline controls. No altered ability to induce single-cell death was observed in the other clones.

Conclusions

Overall, primary gp41 proteins with both V38A and N140I changes showed a reduced ability to induce single cell death and deplete CD4⁺ T cells, despite maintaining fusion activity. The specificity of this phenotype highlights the relevance of the genetic context to the cytopathic capacity of Env and the role of ENF-resistance mutations in modulating viral pathogenicity in vivo, further supporting the hypothesis that gp41 is a critical mediator of HIV pathogenesis.

Viremic HIV infected individuals with high CD4 T cells and functional envelope proteins show anti-gp41 antibodies with unique specificity and function

BACKGROUND

CD4 T-cell decay is variable among HIV-infected individuals. In exceptional cases, CD4 T-cell counts remain stable despite high plasma viremia. HIV envelope glycoprotein (Env) properties, namely tropism, fusion or the ability to induce the NK ligand NKp44L, or host factors that modulate Env cytopathic mechanisms may be modified in such situation.

METHODS

We identified untreated HIV-infected individuals showing non-cytopathic replication (VL>10,000 copies/mL and CD4 T-cell decay<50 cells/µL/year, Viremic Non Progressors, VNP) or rapid progression (CD4 T-cells<350 cells/µL within three years post-infection, RP). We isolated full-length Env clones and analyzed their functions (tropism, fusion activity and capacity to induce NKp44L expression on CD4 cells). Anti-Env humoral responses were also analyzed.

RESULTS

Env clones isolated from VNP or RP individuals showed no major phenotypic differences. The percentage of functional clones was similar in both groups. All clones tested were CCR5-tropic and showed comparable expression and fusogenic activity. Moreover, no differences were observed in their capacity to induce NKp44L expression on CD4 T cells from healthy donors through the 3S epitope of gp41. In contrast, anti- Env antibodies showed clear functional differences: plasma from VNPs had significantly higher capacity than RPs to block NKp44L induction by autologous viruses. Consistently, CD4 T-cells isolated from VNPs showed undetectable NKp44L expression and specific antibodies against a variable region flanking the highly conserved 3S epitope were identified in plasma samples from these patients. Conversely, despite continuous antigen stimulation, VNPs were unable to mount a broad neutralizing response against HIV.

CONCLUSIONS

Env functions (fusion and induction of NKp44L) were similar in viremic patients with slow or rapid progression to AIDS. However, differences in humoral responses against gp41 epitopes nearby 3S sequence may contribute to the lack of CD4 T cell decay in VNPs by blocking the induction of NKp44L by gp41.

Replication-independent expression of anti-apoptosis marker genes in human peripheral blood mononuclear cells infected with the wild-type HIV-1 and reverse transcriptase variants

Clinical trials with highly-active antiretroviral therapy (HAART) have shown that a substantial number of patients continue to show a decrease in viral load and/or increase or stable CD4(+) T-cell numbers even in the presence of multidrug resistant (MDR) viruses. We compared replication capacity (RC) and expression of anti-apoptosis marker genes (AAMGs) in human peripheral blood mononuclear (PBM) cells infected with NL4-3 (wild-type; WT) and mutant viruses. Replication kinetics assays showed a significant decrease in RC of all mutant viruses in comparison to the WT virus. The viruses containing patient-derived MDR RT without the K65R mutation (PSD5.2) replicated efficiently in comparison to the viruses with MDR RT containing the K65R mutation (PSD5.1), or the single mutations K65R and M184V. Compared with WT, a significant decrease in RCs of viruses: K65R (RC=0.39±0.02; p≤0.0001), M184V (RC=0.72±0.04; p≤0.0001), PSD5.1 (RC=0.32±0.04; p≤0.0001), and PSD5.2 (RC=0.90±0.04; p=0.002) was observed on day 10. RT-PCR-based apoptosis array was performed on total cellular RNA. Recombinant virus PSD5.2 showed a 1.5- to 6-fold upregulation in 8 AAMGs (AKT1, BAG3, BCL2A1, BFAR, BIRC2, BNIP1, BNIP3, and CFLAR) on day 1 and day 7 post-infection with respect to WT virus. PSD5.1 showed upregulation of only one gene (BAG1) on day 1 (1.75-fold) and day 7 (1.97-fold). Point mutant K65R showed a 1.5- to 4-fold upregulation of six AAMGs on day 7. Viruses with the M184V mutation showed upregulation of only one gene (BAG1). These observations indicate that the upregulation of specific AAMGs may not be dependent on the RCs of HIV-I variants, and that the possible interaction among mutated RT residues and viral and/or host proteins may induce CD4(+) T-cell-protective anti-apoptosis proteins.

Single amino acid change in gp41 region of HIV-1 alters bystander apoptosis and CD4 decline in humanized mice

Background

The mechanism by which HIV infection leads to a selective depletion of CD4 cells leading to immunodeficiency remains highly debated. Whether the loss of CD4 cells is a direct consequence of virus infection or bystander apoptosis of uninfected cells is also uncertain.

Results

We have addressed this issue in the humanized mouse model of HIV infection using a HIV variant with a point mutation in the gp41 region of the Env glycoprotein that alters its fusogenic activity. We demonstrate here that a single amino acid change (V38E) altering the cell-to-cell fusion activity of the Env minimizes CD4 loss in humanized mice without altering viral replication. This differential pathogenesis was associated with a lack of bystander apoptosis induction by V38E virus even in the presence of similar levels of infected cells. Interestingly, immune activation was observed with both WT and V38E infection suggesting that the two phenomena are likely not interdependent in the mouse model.

Conclusions

We conclude that Env fusion activity is one of the determinants of HIV pathogenesis and it may be possible to attenuate HIV by targeting gp41.

Abortive HIV infection mediates CD4 T cell depletion and inflammation in human lymphoid tissue

The mechanism by which CD4 T cells are depleted in HIV-infected hosts remains poorly understood. In ex vivo cultures of human tonsil tissue, CD4 T cells undergo a pronounced cytopathic response following HIV infection. Strikingly, >95% of these dying cells are not productively infected but instead correspond to bystander cells. We now show that the death of these "bystander" cells involves abortive HIV infection. Inhibitors blocking HIV entry or early steps of reverse transcription prevent CD4 T cell death while inhibition of later events in the viral life cycle does not. We demonstrate that the nonpermissive state exhibited by the majority of resting CD4 tonsil T cells leads to accumulation of incomplete reverse transcripts. These cytoplasmic nucleic acids activate a host defense program that elicits a coordinated proapoptotic and proinflammatory response involving caspase-3 and caspase-1 activation. While this response likely evolved to protect the host, it centrally contributes to the immunopathogenic effects of HIV.

Apoptosis: a clinically useful measure of antiretroviral drug toxicity?

Antiretroviral therapy (ART) has improved life expectancy with HIV infection, but long-term toxicities associated with these medications are now a major global disease burden. There is a clear need to develop useful methods for monitoring patients on antiretroviral drugs for early signs of toxicity. Assays with predictive utility -- allowing therapy to be changed before serious end organ damage occurs -- would be ideal. Attempts to develop biochemical methods of monitoring ART toxicity have concentrated on the mitochondrial toxicity of nucleoside analogue reverse transcriptase inhibitors and have not generally lead to assays with widespread clinical applications. For example, plasma lactate and peripheral blood measurements of mitochondrial DNA associate with exposure to potentially toxic nucleoside analogue reverse transcriptase inhibitors but have not reliably predicted clinical toxicity. Better assays are needed, including markers of toxicity from additional drug classes. Apoptosis may be a potential marker of ART toxicity. Increased apoptosis has been demonstrated both in vitro and in vivo in association with various antiretroviral drug classes and a range of clinical toxicities. However, quantifying apoptosis on biopsy specimens of tissue (such as adipose tissue) is impractical for patient monitoring. Novel assays have been described that can quantify apoptosis using minute tissue samples and initial results from clinical samples suggest peripheral blood may have utility in predicting ART toxicities. The limitations and potential of such techniques for monitoring patients for drug side effects will be discussed.

Nef-mediated MHC class I down-regulation unmasks clonal differences in virus suppression by SIV-specific CD8(+) T cells independent of IFN-gamma and CD107a responses

CD8(+) T lymphocytes (CTL) play a role in controlling HIV/SIV infection. CTL antiviral activity is dependent on recognition of antigenic peptides associated with MHC class I molecules on infected target cells, and CTL activation can be impaired by Nef-mediated down-regulation of MHC class I molecules. We tested the ability of a series of rhesus macaque CD8(+) T-cell clones specific for the SIV Gag CM9 peptide to suppress SIV infection of autologous CD4(+) T cells. We used a set of SIV(mac)239 viruses with either wild-type Nef or Nef mutations that impair MHC class I down-regulation. All CTL clones efficiently suppressed virus replication in cells infected with mutant viruses with altered Nef function, phenotypically MHC class I(high) or MHC class I(intermediate). However, the ability of the clones to suppress virus replication was variably reduced in the presence of wild-type Nef (MHC class I(low)) despite the observations that all CTL clones showed similar IFN-gamma responses to titrated amounts of cognate peptide as well as to SIV-infected cells. In addition, the CTL clones showed variable CD107a (CTL degranulation marker) responses that did not correlate with their capacity to suppress virus replication. Thus, the clonal differences are not attributable to TCR avidity or typical effector responses, and point to a potential as yet unknown mechanism for CTL-mediated suppression of viral replication. These data emphasize that current assays for evaluating CTL responses in infected or vaccinated individuals do not fully capture the complex requirements for effective CTL-mediated control of virus replication.

Role of HIV Gp41 mediated fusion/hemifusion in bystander apoptosis

Mechanisms of HIV-mediated CD4+ T cell loss leading to immunodeficiency are amongst the most extensively studied yet unanswered questions in HIV biology. The level of CD4+ T cell depletion in HIV infected patients far exceeds the number of infected T cells, suggesting an indirect mechanism of HIV pathogenesis termed bystander cell death. Evidence is accumulating that the HIV envelope glycoprotein (Env) is a major determinant of HIV pathogenesis and plays a critical role in bystander cell death. The complex structure and function of HIV Env makes the determination of the mechanism of Env mediated apoptosis more complex than previously thought. This review will examine the complex relationship between HIV Env phenotype, coreceptor expression and immune activation in determining HIV pathogenesis. We review data here corresponding to the role of HIV Env hemifusion activity in HIV pathogenesis and how it interplays with other AIDS associated factors such as chemokine receptor expression and immune activation.

Implication of p38 mitogen-activated protein kinase isoforms (alpha, beta, gamma and delta) in CD4+ T-cell infection with human immunodeficiency virus type I

The CD4(+) T-cell reduction characteristic of human immunodeficiency virus type 1 (HIV-1) infection is thought to result, in addition to infected T-cell death, mainly from uninfected bystander T-cell apoptosis. Nevertheless, the immunological and virological mechanisms leading to T-cell death during HIV-1 infection are not yet fully understood. In the present study, we analysed the individual implication of the p38 mitogen-activated protein kinase (MAPK) isoforms (p38alpha, p38beta, p38gamma and p38delta) during apoptosis induced by HIV-1, taking into account that HIV-1 replication is known to be blocked by p38 inhibitors. For this purpose, we used the SupT1 cell line, where death induced by HIV-1 mainly occurs by uninfected bystander cell apoptosis. A variety of SupT1-based cell lines were constructed constitutively expressing, under the control of cytomegalovirus promoter (PCMV), each dominant-negative (dn) p38 isoform and each wild-type p38 isoform as a control. An enhanced green fluorescent protein marker gene, under the control of the HIV-1 promoter, was inserted in all of them. These cell lines were infected with HIV-1 and analysed by flow cytometry. We found that survival in SupT1-based cell lines infected by HIV-1 was increased by the p38alphadn, p38gammadn and p38deltadn isoforms, but not by the p38betadn isoform. HIV-1 replication was delayed most by p38deltadn and to a lesser extent by p38alphadn and p38gammadn. Moreover, these three isoforms, p38alphadn, p38gammadn and p38deltadn, reduced apoptosis induced by HIV-1. These results suggest that, in SupT1-based cell lines, p38alpha, p38gamma and p38delta, but not p38beta, are implicated in both HIV-1 induced replication and apoptosis in infected and uninfected bystander cells.

The role of collagen deposition in depleting CD4+ T cells and limiting reconstitution in HIV-1 and SIV infections through damage to the secondary lymphoid organ niche

The hallmark of HIV/SIV infections is the progressive depletion of CD4+ T cells that ultimately renders the host incapable of defending against AIDS defining opportunistic infections and malignancies. Although many potential mechanisms have been proposed to explain CD4+ T cell loss, we review here the growing evidence that fibrotic 'scarring' and consequent damage to the lymphatic tissue niche contributes to CD4+ T cell decline and limits CD4+ T cell re-population with retroviral therapy.

Commitment to apoptosis in CD4(+) T lymphocytes productively infected with human immunodeficiency virus type 1 is initiated by lysosomal membrane permeabilization, itself induced by the isolated expression of the viral protein Nef

Primary CD4(+) T lymphocytes, supporting in vitro human immunodeficiency virus type 1 (HIV-1) replication, are destined to die by apoptosis. We explored the initial molecular events that act upstream from mitochondrial dysfunction in CD4(+) T lymphocytes exposed to the HIV-1(LAI) strain. We tracked by immunofluorescence the cells expressing the p24 viral antigen and used Percoll density gradients to isolate a nonapoptotic CD4(+) T-cell subset with a high inner mitochondrial transmembrane potential (DeltaPsim) but no outer mitochondrial membrane (OMM) rupture. In most p24(+) (but not bystander p24(-)) cells of this subset, the lysosomes were undergoing limited membrane permeabilization, allowing the lysosomal efflux of cathepsins (Cat) to the cytosol. This was also induced by HIV-1 isolates from infected patients. Using pepstatin A to inhibit Cat-D enzymatic activity and Cat-D small interfering RNA to silence the Cat-D gene, we demonstrate that once released into the cytosol, Cat-D induces the conformational change of Bax and its insertion into the OMM. Inhibition of Cat-D activity/expression also conferred a transient survival advantage upon productively HIV-1-infected cells, indicating that Cat-D is an early death factor. The transfection of activated CD4(+) T lymphocytes with a Nef expression vector rapidly induced the permeabilization of lysosomes and the release of Cat-D, with these two events preceding OMM rupture. These results reveal a previously undocumented mechanism in which Nef acts as an internal cytopathic factor and strongly suggest that this viral protein may behave similarly in the context of productive HIV-1 infection in CD4(+) T lymphocytes.

HIV-1 immunopathogenesis: how good interferon turns bad

The hallmark of acquired immunodeficiency syndrome (AIDS) is the progressive loss of CD4+ T cells that results from infection with human immunodeficiency virus type-1 (HIV-1). Despite 25 years of AIDS research, questions remain concerning the mechanisms responsible for HIV-induced CD4+ T cell depletion. Here we briefly review the in vitro and in vivo literature concerning the protective role of interferon-alpha (IFN-alpha) in HIV/AIDS. We then develop a laboratory- and clinically supported model of CD4+ T cell apoptosis in which either infectious or noninfectious HIV-1 induces the production of type I interferon by plasmacytoid dendritic cells (pDC). The interferon produced binds to its receptor on primary CD4+ T cells resulting in membrane expression of the TNF-related apoptosis-inducing ligand (TRAIL) death molecule. The binding of infectious or noninfectious HIV-1 to CD4 on these T cells results in expression of the TRAIL death receptor 5 (DR5), leading to the selective death of HIV-exposed CD4+ T cells.

Macrophage reservoirs or how HIV fuels the disease

Evaluation of: Brown A, Zhang H, Lopez P, Pardo CA, Gartner S:In vitro modeling of the HIV-macrophage reservoir. J. Leukoc. Biol. 80, 1127–1135 (2006). The main obstacle to viral eradication in HIV-infected patients is the presence of chronically infected latent reservoir cells, such as macrophages, and latently infected CD4+ T lymphocytes. New in vitro models might be useful to better understand the mechanisms involved in the formation of HIV reservoirs. In the above report, Brown and colleagues present evidence that macrophages represent a heterologous population of mononuclear phagocytes based on distinct profiles of viral replication, allowing in vitro modeling of the HIV-macrophage reservoir.

Lymphatic tissue fibrosis is associated with reduced numbers of naive CD4+ T cells in human immunodeficiency virus type 1 infection

The organized structure of lymphatic tissues (LTs) constitutes a microenvironment referred to as a niche that plays a critical role in immune system homeostasis by promoting cellular interactions and providing access to cytokines and growth factors on which cells are dependent for survival, proliferation, and differentiation. In chronic human immunodeficiency virus type 1 (HIV-1) infection, immune activation and inflammation result in collagen deposition and disruption of this LT niche. We have previously shown that these fibrotic changes correlate with a reduction in the size of the total population of CD4+ T cells. We now show that this reduction is most substantial within the naïve CD4+ T-cell population and is in proportion to the extent of LT collagen deposition in HIV-1 infection. Thus, the previously documented depletion of naïve CD4+ T cells in LTs in HIV-1 infection may be a consequence not only of a decreased supply of thymic emigrants or chronic immune activation but also of the decreased ability of those cells to survive in a scarred LT niche. We speculate that LT collagen deposition might therefore limit repopulation of naïve CD4+ T cells with highly active antiretroviral therapy, and thus, additional treatments directed to limiting or reversing inflammatory damage to the LT niche could potentially improve immune reconstitution.

Constitutive activation of STAT5 supersedes the requirement for cytokine and TCR engagement of CD4+ T cells in steady-state homeostasis

The transcription factor STAT5 is one of several signaling mediators activated via common gamma-chain cytokine receptors. As such, it plays an important role in lymphocyte survival and proliferation during normal homeostasis as well as under lymphopenic conditions. Transgenic mice expressing a constitutively activated form of STAT5b have been shown previously to contain increased numbers of peripheral CD4+CD25- T cells. To define the mechanism(s) for this occurrence, we have used adoptive transfer studies to examine the effects of STAT5 activity on steady-state CD4+ T cell homeostasis. We observed that constitutive STAT5 signaling induced 4- to 7-fold increased levels of basal steady-state proliferation, which was accompanied by a comparable increase in T cell recovery. Most strikingly, steady-state CD4 T cell proliferation occurred independently of both MHC class II and IL-15. These observations demonstrate that the STAT5-driven pathway is important to lymphocyte homeostasis and can supersede the need for both TCR engagement and cytokine stimulation. This suggests that the need for TCR stimulation to induce common gamma-chain cytokine receptor expression, and thus STAT5 activation, is a key factor in maintaining normal CD4+ T cell homeostasis.

CXCR4 tropic human immunodeficiency virus type 1 induces an apoptotic cascade in immature infected thymocytes that resembles thymocyte negative selection

HIV-1 often replicates in the thymus of infected individuals, causing thymocyte depletion and thymic dysfunction. Nevertheless, the mechanisms by which thymocyte depletion occurs are not clear. Here we report that HIV-1 infection induced apoptosis primarily in productively infected thymocytes; aldrithiol-2 or Efavirenz treatment largely abrogated HIV-1-induced apoptosis. Moreover, X4-HIV-1 induced apoptosis primarily in immature CD4+ CD8+ (DP) thymocytes whereas most mature CD4 or CD8 single-positive (SP) thymocytes were resistant to X4 HIV-1-induced apoptosis despite infection. Consistent with this, we observed significant induction of several genes involved in negative selection of DP thymocytes. Furthermore, treatment of thymocytes with cycloheximide abrogated HIV-1-induced apoptosis, implying a requirement for de novo protein synthesis. Our results suggest that HIV-1-induced apoptosis of thymocytes requires the activation of caspases and the participation of mitochondrial apoptosis effectors, which serve to amplify the apoptotic signal, a process similar to that elaborated during thymocyte negative selection.

HIV gp41-induced apoptosis is mediated by caspase-3-dependent mitochondrial depolarization, which is inhibited by HIV protease inhibitor nelfinavir

Apoptotic loss of CD4+ T cells has been proposed as a mechanism of T cell depletion in human immunodeficiency virus (HIV) infections resulting in immunodeficiency. The Env glycoprotein has been implicated in apoptosis of uninfected bystander cells via gp120 binding to CD4/CXC chemokine receptor 4 as well as the fusion/hemifusion process mediated by gp41. Using an in vitro model of coculture of Env-expressing cells as effectors and CD4+ T cells as targets, we find that apoptosis mediated by Env glycoprotein in bystander cells in fact correlates with gp41-induced hemifusion. Further, the apoptotic pathway initiated by this interaction involves caspase-3-dependent mitochondrial depolarization and reactive oxygen species production. HIV gp41-induced mitochondrial depolarization is inhibited by protease inhibitor nelfinavir but not by other HIV protease inhibitors or inhibitors of calpain and cathepsin. This "kiss of death" (hemifusion) signaling pathway is independent of p38 mitogen-activated protein kinase and p53, making it distinct from the apoptosis seen in syncytia. We also show that virion-induced apoptosis is gp41-dependent. Our findings provide new insights into the mechanism via which HIV gp41 mediates apoptosis in bystander cells.

New kinetic models for the hepatitis C virus

Viral kinetic modeling has played an important role in the analysis of HCV RNA decay after the initiation of antiviral therapy. Models have provided a means of evaluating the antiviral effectiveness of therapy, of estimating parameters such as the rate of virion clearance and the rate of clearance of hepatitis C virus (HCV)-infected cells, and they have suggested mechanisms of action for both interferon and ribavirin. Nevertheless, the models that were originally formulated were unable to explain all of the observed HCV RNA profiles. We provide an update on the state of HCV kinetic modeling and discuss new models that have taken into consideration the different pharmacokinetics of standard and pegylated forms of interferon, allow for changes in drug effectiveness as drug concentrations fall between dosing intervals, and that have incorporated alanine aminotransferase kinetics and aspects of immune responses to provide a more comprehensive picture of the biology underlying changes in HCV RNA during therapy.