Interleukin-1 beta converting enzyme-like protease involvement in Fas-induced and activation-induced peripheral blood T cell apoptosis in HIV infection. TNF-related apoptosis-inducing ligand can mediate activation-induced T cell death in HIV infection

Poly (ADP-ribose) polymerase-1 regulates HIV-1 replication in human CD4+ T cells

The cellular enzyme poly (ADP-ribose) polymerase-1 (PARP-1) regulates multiple processes that are potentially implicated in HIV-1 infection. However, the role of PARP-1 in HIV-1 infection remains controversial, with reports indicating or excluding that PARP-1 influence early steps of the HIV-1 life cycle. Most of these studies have been conducted with Vesicular Stomatitis virus Glycoprotein G (VSV-G)-pseudotyped, single-round infection HIV-1; limiting our understanding of the role of PARP-1 in HIV-1 replication. Therefore, we evaluated the effect of PARP-1 deficiency or inhibition in HIV-1 replication in human CD4+ T cells. Our data showed that PARP-1 knockout increased viral replication in SUP-T1 cells. Similarly, a PARP-1 inhibitor that targets PARP-1 DNA-binding activity enhanced HIV-1 replication. In contrast, inhibitors affecting the catalytic activity of the enzyme were inactive. In correspondence with the pharmacological studies, mutagenesis analysis indicated that the DNA-binding domain was required for the PARP-1 anti-HIV-1 activity, but the poly-ADP-ribosylation activity was dispensable. Our results also demonstrated that PARP-1 acts at the production phase of the viral life cycle since HIV-1 produced in cells lacking PARP-1 was more infectious than control viruses. The effect of PARP-1 on HIV-1 infectivity required Env, as PARP-1 deficiency or inhibition did not modify the infectivity of Env-deleted, VSV-G-pseudotyped HIV-1. Furthermore, virion-associated Env was more abundant in sucrose cushion-purified virions produced in cells lacking the enzyme. However, PARP-1 did not affect Env expression or processing in the producer cells. In summary, our data indicate that PARP-1 antagonism enhances HIV-1 infectivity and increases levels of virion-associated Env.

Importance

Different cellular processes counteract viral replication. A better understanding of these interfering mechanisms will enhance our ability to control viral infections. We have discovered a novel, antagonist effect of the cellular enzyme poly (ADP-ribose) polymerase-1 (PARP-1) in HIV-1 replication. Our data indicate that PARP-1 deficiency or inhibition augment HIV-1 infectivity in human CD4+ T cells, the main HIV-1 target cell in vivo . Analysis of the mechanism of action suggested that PARP-1 antagonism increases in the virus the amounts of the viral protein mediating viral entry to the target cells. These findings identify for the first time PARP-1 as a host factor that regulates HIV-1 infectivity, and could be relevant to better understand HIV-1 transmission and to facilitate vaccine development.

Mycoplasma gallisepticum infection triggered histopathological changes, oxidative stress and apoptosis in chicken thymus and spleen

Previous studies mainly focused on the inflammatory responses caused by Mycoplasma gallisepticum (MG) in the chicken respiratory mucosa, setting the stage for chronic infection and disease manifestation. However, the underlying mechanism is still unknown. Spleen and thymus are important immune organs, which play a critical role in eliciting protective immune responses to ensure healing process and elimination of harmful stimuli. In the present study, the effects of MG infection on chicken spleen and thymus were investigated. The results showed that MG infection reduced antioxidant activities and induced oxidative stress in the spleen and thymus tissues. Histological examination showed normal morphology of chicken spleen and thymus in control group compared to MG infection group. In contrast, increased number of necrotic and nuclear debris, lymphocytolysis, prominent reticuloepithelial cells and loose arrangement of cells in the spleen and thymus were seen in MG-infected chickens. Ultrastructural analysis indicated nuclear and mitochondrial damage including mitochondrial swelling, deformation of nuclear membrane and congestion of chromatin material in MG infection group. The mRNA and protein expression of apoptosis-related genes were significantly upregulated in the spleen and thymus of MG-infected chickens compared to control group. Moreover, Terminal deoxynucleotidyl transferase-mediated dUTP nick endlabeling (TUNEL) assay results suggested that MG infection increased the number of positive-stained nuclei in the spleen and thymus. Meanwhile, the mRNA expression of mitochondrial dynamics in the spleen and thymus were altered by MG infection. In summary, these results showed that MG induced oxidative stress and apoptosis, which could be the possible causes associated with the immune damage, structural impairment and disease pathogenesis of MG infection.

TRAILshort Protects against CD4 T Cell Death during Acute HIV Infection

CD4 T cells from HIV-1 infected patients die at excessive rates compared to those from uninfected patients, causing immunodeficiency. We previously identified a dominant negative ligand that antagonizes the TRAIL-dependent pathway of cell death, which we called TRAILshort. Because the TRAIL pathway has been implicated in CD4 T cell death occurring during HIV-1 infection, we used short hairpin RNA knockdown, CRISPR deletion, or Abs specific for TRAILshort to determine the effect of inhibiting TRAILshort on the outcome of experimental acute HIV infection in vitro. Strikingly, all three approaches to TRAILshort deletion/inhibition enhanced HIV-induced death of both infected and uninfected human CD4 T cells. Thus, TRAILshort impacts T cell dynamics during HIV infection, and inhibiting TRAILshort causes more HIV-infected and uninfected bystander cells to die. TRAILshort is, therefore, a host-derived, host-adaptive mechanism to limit the effects of TRAIL-induced cell death. Further studies on the effects of TRAILshort in other disease states are warranted.

HIV-1 Nef physically associate with CAMKIIδ - ASK-1 complex to inhibit p38MAPK signalling and apoptosis in infected cells

Human immunodeficiency type 1 virus accessory protein Nef is a key modulator of AIDS pathogenesis. With no enzymatic activity, Nef regulated functions in host cells largely depends on its ability to form multi-protein complex with the cellular proteins. Here, we identified Calcium (Ca²⁺)/Calmodulin dependent protein kinase II subunit delta (CAMKIIδ) as novel Nef interacting host protein. Further, we confirmed that Nef mediated [Ca²⁺]_I promote formation of Nef-CAMKIIδ - apoptosis signal-regulating kinase (ASK-1) heterotrimeric complex. The assembly of Nef with CAMKIIδ - ASK-1 inhibits the downstream p38MAPK phosphorylation resulting in abrogation of apoptosis. Further, using competitive peptide inhibitors against Nef binding domains to CAMKIIδ, identified in the present study and ASK-1, individually blocked physical interaction of Nef with CAMKIIδ-ASK-1 complex and restored p38MAPK phosphorylation and apoptosis. Altogether, our study indicates that HIV-Nef modulates cytosolic [Ca²⁺]_I and blocks CAMKIIδ - ASK-1 kinase activity to inhibit apoptosis of infected cells.

Both HIV-Infected and Uninfected Cells Express TRAILshort, Which Confers TRAIL Resistance upon Bystander Cells within the Microenvironment

TNF-related apoptosis-inducing ligand (TRAIL) was initially described to induce apoptosis of tumor cells and/or virally infected cells, although sparing normal cells, and has been implicated in the pathogenesis of HIV disease. We previously identified TRAILshort, a TRAIL splice variant, in HIV-infected patients and characterized it as being a dominant negative ligand to subvert TRAIL-mediated killing. Herein, using single-cell genomics we demonstrate that TRAILshort is produced by HIV-infected cells, as well as by uninfected bystander cells, and that the dominant stimulus which induces TRAILshort production are type I IFNs and TLR7, TLR8, and TLR9 agonists. TRAILshort has a short t1/2 by virtue of containing a PEST domain, which targets the protein toward the ubiquitin proteasome pathway for degradation. Further we show that TRAILshort binds preferentially to TRAIL receptors 1 and 2 with significantly reduced interaction with the decoy TRAIL receptors 3 and 4. Recombinant TRAILshort is sufficient to protect cells against TRAIL-induced killing, whereas immunodepletion of TRAILshort with a specific Ab restores TRAIL sensitivity. Importantly we show that TRAILshort is shed in microvesicles into the cellular microenvironment and therefore confers TRAIL resistance not only on the cell which produces it, but also upon neighboring bystander cells. These results establish a novel paradigm for understanding and overcoming TRAIL resistance, in particular how HIV-infected cells escape immune elimination by the TRAIL:TRAILshort receptor axis.

Cancer preventive effect of recombinant TRAIL by ablation of oncogenic inflammation in colitis-associated cancer rather than anticancer effect

The potential of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in inducing apoptosis is a hallmark in cancer therapeutics, after which its selective ability to achieve cell death pathways against cancer cells led to hope for recombinant TRAIL in cancer therapeutics. The present data from azoxymethane-initiated, dextran sulfate sodium-promoted colitis associated cancer (CAC) model strongly indicate the potential of rTRAIL in cancer prevention rather than in cancer therapeutics. Early treatment of rTRAIL significantly reduced colitis and CAC by inhibiting the recruitment of macrophages into the damaged mucosa and activating the scavenger activity with efferocytosis and the production of several growth factors. In contrast, late administration of rTRAIL as for anti-cancer effect did not decrease the initiation and development of CAC at all. Significant cancer preventing mechanisms of rTRAIL were identified. In the CAC model, anti-inflammation, regeneration, and efferocytosis was induced by treatment of TRAIL for 6 days, significant inhibitory activity was evident at 4 weeks and anti-oxidative and anti-inflammatory induction were noted at 12 weeks. Most importantly, TRAIL promoted tissue regeneration by enhancing the resolution of pathological inflammation through the activation of the NLRP3 inflammasome pathway. The results indicate that TRAIL reduces the induction of colitis and the initiation of CAC by inhibiting pro-inflammatory signaling and promoting tissue repair to maintain intestinal homeostasis through activation of the NLRP3 inflammasome. Therefore, TRAIL can be used as a chemopreventive agent against CAC, rather than as a therapeutic drug endowing apoptosis.

Early Loss of Splenic Tfh Cells in SIV-Infected Rhesus Macaques

Follicular T helper cells (Tfh), a subset of CD4 T lymphocytes, provide crucial help to B cells in the production of antigen-specific antibodies. Although several studies have analyzed the dynamics of Tfh cells in peripheral blood and lymph nodes (LNs) during Aids, none has yet addressed the impact of SIV infection on the dynamics of Tfh cells in the spleen, the primary organ of B cell activation. We show here a significant decrease in splenic Tfh cells in SIVmac251-infected rhesus macaques (RMs) during the acute phase of infection, which persists thereafter. This profound loss is associated with lack of sustained expression of the Tfh-defining transcription factors, Bcl-6 and c-Maf but with higher expression of the repressors KLF2 and Foxo1. In this context of Tfh abortive differentiation and loss, we found decreased percentages of memory B cell subsets and lower titers of SIV-specific IgG. We further demonstrate a drastic remodeling of the lymphoid architecture of the spleen and LNs, which disrupts the crucial cell-cell interactions necessary to maintain memory B cells and Tfh cells. Finally, our data demonstrated the early infection of Tfh cells. Paradoxically, the frequencies of SIV DNA were higher in splenic Tfh cells of RMs progressing more slowly suggesting sanctuaries for SIV in the spleen. Our findings provide important information regarding the impact of HIV/SIV infection on Tfh cells, and provide new clues for future vaccine strategies.

Minocycline attenuates HIV-1 infection and suppresses chronic immune activation in humanized NOD/LtsZ-scidIL-2Rγ(null) mice

More than a quarter of a century of research has established chronic immune activation and dysfunctional T cells as central features of chronic HIV infection and subsequent immunodeficiency. Consequently, the search for a new immunomodulatory therapy that could reduce immune activation and improve T-cell function has been increased. However, the lack of small animal models for in vivo HIV study has hampered progress. In the current study, we have investigated a model of cord blood haematopoietic progenitor cells (CB-HPCs) -transplanted humanized NOD/LtsZ-scidIL-2Rγ(null) mice in which progression of HIV infection is associated with widespread chronic immune activation and inflammation. Indeed, HIV infection in humanized NSG mice caused up-regulation of several T-cell immune activation markers such as CD38, HLA-DR, CD69 and co-receptor CCR5. T-cell exhaustion markers PD-1 and CTLA-4 were found to be significantly up-regulated on T cells. Moreover, increased plasmatic levels of lipopolysaccharide, sCD14 and interleukin-10 were also observed in infected mice. Treatment with minocycline resulted in a significant decrease of expression of cellular and plasma immune activation markers, inhibition of HIV replication and improved T-cell counts in HIV-infected humanized NSG mice. The study demonstrates that minocycline could be an effective, low-cost adjunctive treatment to regulate chronic immune activation and replication of HIV.

Type I interferon upregulates Bak and contributes to T cell loss during human immunodeficiency virus (HIV) infection

The role of Type I interferon (IFN) during pathogenic HIV and SIV infections remains unclear, with conflicting observations suggesting protective versus immunopathological effects. We therefore examined the effect of IFNα/β on T cell death and viremia in HIV infection. Ex vivo analysis of eight pro- and anti-apoptotic molecules in chronic HIV-1 infection revealed that pro-apoptotic Bak was increased in CD4+ T cells and correlated directly with sensitivity to CD95/Fas-mediated apoptosis and inversely with CD4+ T cell counts. Apoptosis sensitivity and Bak expression were primarily increased in effector memory T cells. Knockdown of Bak by RNA interference inhibited CD95/Fas-induced death of T cells from HIV-1-infected individuals. In HIV-1-infected patients, IFNα-stimulated gene expression correlated positively with ex vivo T cell Bak levels, CD95/Fas-mediated apoptosis and viremia and negatively with CD4+ T cell counts. In vitro IFNα/β stimulation enhanced Bak expression, CD95/Fas expression and CD95/Fas-mediated apoptosis in healthy donor T cells and induced death of HIV-specific CD8+ T cells from HIV-1-infected patients. HIV-1 in vitro sensitized T cells to CD95/Fas-induced apoptosis and this was Toll-like receptor (TLR)7/9- and Type I IFN-dependent. This sensitization by HIV-1 was due to an indirect effect on T cells, as it occurred in peripheral blood mononuclear cell cultures but not purified CD4+ T cells. Finally, peak IFNα levels and viral loads correlated negatively during acute SIV infection suggesting a potential antiviral effect, but positively during chronic SIV infection indicating that either the virus drives IFNα production or IFNα may facilitate loss of viral control. The above findings indicate stage-specific opposing effects of Type I IFNs during HIV-1 infection and suggest a novel mechanism by which these cytokines contribute to T cell depletion, dysregulation of cellular immunity and disease progression.

Dynamics of physical interaction between HIV-1 Nef and ASK1: identifying the interacting motif(s)

FasL mediated preferential apoptosis of bystander CTLs while protection of infected CD4(+)T cells remains one of the hallmarks of immune evasion during HIV infection. The property of infected host cells to evade cell-autonomous apoptosis emanates from ability of HIV-1Nef-protein to physically interact with ASK-1 and thereby inhibit its enzymatic activity. The specific domains of HIV-1Nef through which it may interact with ASK1 and thereby impair the ASK1 activity remain unidentified so far and represent a major challenge towards developing clear understanding about the dynamics of this interaction. Using mammalian two hybrid screen in association with site directed mutagenesis and competitive inhibitor peptides, we identified constituent minimal essential domain (152 DEVGEANN 159) through which HIV-1Nef interacts with ASK1 and inhibits its function. Furthermore our study also unravels a novel alternate mechanism underlying HIV-1 Nef mediated ASK1 functional modulation, wherein by potentiating the inhibitory ser(967) phosphorylation of ASK1, HIV-1Nef negatively modulated ASK1 function.

Up-regulation of a death receptor renders antiviral T cells susceptible to NK cell-mediated deletion

Hepatic NK cells eliminate HBV-specific T cells dependent on TRAIL and TRAIL-R2 interactions to limit antiviral immunity in chronic infection.

Antiviral T cell responses in hepatotropic viral infections such as hepatitis B virus (HBV) are profoundly diminished and prone to apoptotic deletion. In this study, we investigate whether the large population of activated NK cells in the human liver contributes to this process. We show that in vitro removal of NK cells augments circulating CD8⁺ T cell responses directed against HBV, but not against well-controlled viruses, in patients with chronic hepatitis B (CHB). We find that NK cells can rapidly eliminate HBV-specific T cells in a contact-dependent manner. CD8⁺ T cells in the liver microcirculation are visualized making intimate contact with NK cells, which are the main intrahepatic lymphocytes expressing TNF-related apoptosis-inducing ligand (TRAIL) in CHB. High-level expression of the TRAIL death receptor TRAIL-R2 is found to be a hallmark of T cells exposed to the milieu of the HBV-infected liver in patients with active disease. Up-regulation of TRAIL-R2 renders T cells susceptible to caspase-8–mediated apoptosis, from which they can be partially rescued by blockade of this death receptor pathway. Our findings demonstrate that NK cells can negatively regulate antiviral immunity in chronic HBV infection and illustrate a novel mechanism of T cell tolerance in the human liver.

TNF-related apoptosis-inducing ligand (TRAIL) exerts therapeutic efficacy for the treatment of pneumococcal pneumonia in mice

Apoptotic death of alveolar macrophages observed during lung infection with Streptococcus pneumoniae is thought to limit overwhelming lung inflammation in response to bacterial challenge. However, the underlying apoptotic death mechanism has not been defined. Here, we examined the role of the TNF superfamily member TNF-related apoptosis-inducing ligand (TRAIL) in S. pneumoniae-induced macrophage apoptosis, and investigated the potential benefit of TRAIL-based therapy during pneumococcal pneumonia in mice. Compared with WT mice, Trail(-/-) mice demonstrated significantly decreased lung bacterial clearance and survival in response to S. pneumoniae, which was accompanied by significantly reduced apoptosis and caspase 3 cleavage but rather increased necrosis in alveolar macrophages. In WT mice, neutrophils were identified as a major source of intraalveolar released TRAIL, and their depletion led to a shift from apoptosis toward necrosis as the dominant mechanism of alveolar macrophage cell death in pneumococcal pneumonia. Therapeutic application of TRAIL or agonistic anti-DR5 mAb (MD5-1) dramatically improved survival of S. pneumoniae-infected WT mice. Most importantly, neutropenic mice lacking neutrophil-derived TRAIL were protected from lethal pneumonia by MD5-1 therapy. We have identified a previously unrecognized mechanism by which neutrophil-derived TRAIL induces apoptosis of DR5-expressing macrophages, thus promoting early bacterial killing in pneumococcal pneumonia. TRAIL-based therapy in neutropenic hosts may represent a novel antibacterial treatment option.

The prevalence of six species of Mycoplasmataceae in an HIV/AIDS population in Jiangsu Province, China

This study employed culture and polymerase chain reaction (PCR) to examine the prevalence of Ureaplasma urealyticum, Mycoplasma hominis, Mycoplasma genitalium, Mycoplasma fermentans, Mycoplasma penetrans and Mycoplasma pirum in 210 HIV/AIDS patients, 455 sexually transmitted infection (STI) clinic attendees and 245 healthy volunteers from first-void urine specimens for men and endocervical swabs for women. U. urealyticum and M. hominis were detected in 107 (51.0%) and 69 (32.9%) patients in the HIV/AIDS group. At least one of the other four organisms was detected in 34 (16.2%) HIV/AIDS patients, 29 (6.4%) STI clinic attendees and six (2.5%) healthy volunteers. This study showed that U. urealyticum, M. hominis and M. fermentans were significantly more prevalent in HIV/AIDS patients, as were other mycoplasmas. Our results suggest a possible role for co-infection.

Microbial Translocation and B Cell Dysfunction in Human Immunodeficiency Virus Disease

The gut mucosal barrier disrupted in HIV disease, resulting in increased systemic exposure to microbial products such as Lipo Polys Accharide (LPS). The association of enhanced microbial translocation and B cell dysfunction in HIV disease is not fully understood. High dose and short term exposure of microbial Toll-Like Receptor (TLR) agonists were used as vaccine adjuvants, however, low dose and long term exposure of TLR agonists could be harmful. The characteristics of B cell dysfunction in HIV disease included B cell, especially memory B cell depletion, enhanced levels of autoimmune antibodies and impaired vaccine or antigen responsiveness. This review discusses and explores the possibility of the effect of microbial translocation on memory B cell depletion and impaired vaccine responses in HIV infection. By determining the mechanisms of B cell depletion and perturbations in HIV disease, it may be possible to design interventions that can improve immune responses to vaccines, reduce selected opportunistic infections and perhaps slow disease progression.

Mechanisms of HIV envelope-induced T lymphocyte apoptosis

Infection by the human immunodeficiency virus (HIV) is characterized by a progressive depletion of CD4 T lymphocytes, which leads to dysfunction of the immune system. Although a variety of mechanisms may contribute to the gradual T cell decline that occurs in HIV-infected patients, abnormal apoptosis of infected or bystander T lymphocytes is an important event leading to immunodeficiency. The HIV envelope glycoprotein plays a crucial role in HIV associated apoptosis through both death receptor-mediated and mitochondria-dependent pathways. This review summarizes current knowledge of Env-mediated T lymphocyte apoptosis.

A placental protective role for trophoblast-derived TNF-related apoptosis-inducing ligand (TRAIL)

Recent studies show that apoptosis, programmed cell death, plays an important role in the normal development of the human placenta and that an altered balance between proliferation and apoptosis of villous trophoblasts is associated with abnormal pregnancies. The TNF-related apoptosis-inducing ligand (TRAIL) is a molecule belonging to TNF superfamily. The role of TRAIL and its Death Receptor 5 (DR5) in regulating villous trophoblast cell turnover in normal and pathologic pregnancies remains to be explored. In order to elucidate the role of TRAIL in the regulation of placental growth, primary cytotrophoblast cells were isolated from normal term placentas (n=13) and cultured for 18 and 66h to generate mononucleate and multinucleate trophoblasts, respectively. The protein expression and localisation of TRAIL and DR5 were determined by Western blotting and immunofluorescence. Secreted sTRAIL was also measured by ELISA. Trophoblast apoptosis was measured by TUNEL in the presence of recombinant TRAIL (rTRAIL), and DR5 relocalisation was assessed by immunostaining after 18h exposure to TNFalpha. We demonstrated that TRAIL protein expression and the secretion of soluble TRAIL (sTRAIL) were down-regulated in syncytialised villous trophoblasts and that sTRAIL was independent of biochemical differentiation, as TRAIL-neutralizing antibody (2E5) failed to influence hCG production. TRAIL immunoreactivity was detected in mono- and multinucleated trophoblast cells and localised to the cytoplasm and cellular membranes -- more intense staining was associated with apoptotic nuclei. rTRAIL failed to induce apoptosis in trophoblasts cells owing to the nuclear localisation of DR5. However, TNFalpha treatment caused the redistribution of intracellular DR5 to the cell surface, potentiating apoptotic susceptibly to exogenously administered rTRAIL. These findings highlight a mechanism by which TRAIL and DR5 serve to protective trophoblasts in normal development, but may be activated in conditions of excessive TNFalpha.

Differential association of programmed death-1 and CD57 with ex vivo survival of CD8+ T cells in HIV infection

Recent studies have revealed the critical role of programmed death-1 (PD-1) in exhaustion of HIV- and SIV-specific CD8(+) T cells. In this study, we show that high expression of PD-1 correlates with increased ex vivo spontaneous and CD95/Fas-induced apoptosis, particularly in the "effector-memory" CD8(+) T cell population from HIV(+) donors. High expression of PD-1 was linked to a proapoptotic phenotype characterized by low expression of Bcl-2 and IL7-R alpha, high expression of CD95/Fas and high mitochondrial mass. Expression of PD-1 and CD57 was differentially associated with the maturation status of CD8(+) T cells in HIV infection. CD57 was linked to higher apoptosis resistance, with cells expressing a PD-1(L)CD57(H) phenotype exhibiting lower levels of cell death. The majority of HIV-specific CD8(+) T cells were found to express a PD-1(H)CD57(L) or PD-1(H)CD57(H) phenotype. No correlation was found between PD-1 expression and ex vivo polyfunctionality of either HIV- or CMV-specific CD8(+) T cells. Contrary to CD57, high expression of PD-1 was characterized by translocation of PD-1 into the area of CD95/Fas-capping, an early necessary step of CD95/Fas-induced apoptosis. Thus, our data further support the role of PD-1 as a preapoptotic factor for CD8(+) T cells in HIV infection.

The Biology of TRAIL and the Role of TRAIL-Based Therapeutics in Infectious Diseases

TNF-related apoptosis inducing ligand (TRAIL) is a key mediator of the innate immune response to infection. While TRAIL-mediated apoptosis plays an essential role in the clearance of virus-infected cells, its physiologic role also includes immunosurveilance for cancer cells. Therapeutics that induce TRAIL-mediated apoptosis in cancer cells remain a focus of ongoing investigation in clinical trials, and much has been learned from these studies regarding the efficacy and toxicity of these interventions. These data, combined with data from numerous preclinical studies that detail the important and multifaceted role of TRAIL during infection with human immunodeficiency virus and other viruses, suggest that therapeutic exploitation of TRAIL signaling offers a novel and efficacious strategy for the management of infectious diseases.

Tat-induced FOXO3a is a key mediator of apoptosis in HIV-1-infected human CD4+ T lymphocytes

The high mutation rate of HIV is linked to the generation of viruses expressing proteins with altered function whose impact on disease progression is unknown. We investigated how HIV-1 viruses lacking Env, Vpr, and Nef affect CD4(+) T cell survival. We found that in the absence of these proteins, HIV-1-infected CD4(+) primary T cells progress to the G(0) phase of the cell cycle and to cell death, indicating that viruses expressing inactive forms of these proteins can contribute to the CD4(+) T cell decline as the wild-type virus, suggesting that other HIV proteins are responsible for inducing apoptosis. Apoptosis in these cells is triggered by the alteration of the Egr1-PTEN-Akt (early growth response-1/phosphate and tensin homolog deleted on chromosome 10/Akt) and p53 pathways, which converge on the FOXO3a (Forkhead box transcription factor O class 3a) transcriptional activator. The FOXO3a target genes Fas ligand and TRAIL, involved in the extrinsic apoptotic pathway, and PUMA, Noxa, and Bim, which are part of the intrinsic apoptotic pathway, were also up-regulated, indicating that HIV infection leads to apoptosis by the engagement of multiple apoptotic pathways. RNAi-mediated knockdown of Egr1 and FOXO3a resulted in reduced apoptosis in HIV-infected HeLa and CD4(+) T cells, providing further evidence for their critical role in HIV-induced apoptosis and G(0) arrest. We tested the possibility that Tat is responsible for the T cell apoptosis observed with these mutant viruses. The induction of Egr1 and FOXO3a and its target genes was observed in Jurkat cells transduced by Tat alone. Tat-dependent activation of the Egr1-PTEN-FOXO3a pathway provides a mechanism for HIV-1-associated CD4(+) T cell death.

Effect of antiretroviral therapy on apoptosis markers and morphology in peripheral lymph nodes of HIV-infected individuals

BACKGROUND

CD4+ T cell depletion and destruction and the involution of the lymphoid tissue are hallmarks of HIV infection. Although the underlying mechanisms are still unclear, apoptosis appears to play a central role. The objective of this study was to investigate the effect of antiretroviral therapy on the lymph node tissue, particularly with respect to morphology and apoptosis.

PATIENTS AND METHODS

Between 1997 and 1999, two inguinal lymph nodes were excised from 31 previously untreated individuals who were in an early stage of HIV infection, the first one prior to treatment and the second after 16 to 20 months of treatment. Paraffin sections were investigated for lymph node architecture, distribution of cellular and viral markers, apoptosis, and expression of apoptotic key molecules which indirectly reflect apoptotic processes.

RESULTS

After 16-20 months of antiretroviral therapy, a significant decrease in highly activated HIV-driven immune response was observed in the lymph node tissue as a marked reduction in follicular hyperplasia, a normalization of the follicular dendritic cell network, a significant increase in the number of CD4+ T cells, and a significant decrease in the number of CD8+ T cells. The expression of several proapoptotic (Fas, TRAIL, and active caspase 3) and antiapoptotic (Bcl-2 and IL-7Ralpha) molecules that were reconstituted in the tissues during therapy resembled their expression in lymph nodes of HIV-negative individuals. Limitations of the study are (a) the lack of untreated patients in the late stages, (b) for ethical reasons, the lack of a control group with untreated patients, and (c) for methodological reasons, the restriction of sequential measurements of apotpotic markers to one-third of the patients.

CONCLUSION

Antiretroviral therapy initiated in the early stages in HIV infection may halt the irreversible destruction of the lymph node tissue and may partially normalize apoptotic processes.

Apoptosis signaling pathways and lymphocyte homeostasis

It has been almost three decades since the term "apoptosis" was first coined to describe a unique form of cell death that involves orderly, gene-dependent cell disintegration. It is now well accepted that apoptosis is an essential life process for metazoan animals and is critical for the formation and function of tissues and organs. In the adult mammalian body, apoptosis is especially important for proper functioning of the immune system. In recent years, along with the rapid advancement of molecular and cellular biology, great progress has been made in understanding the mechanisms leading to apoptosis. It is generally accepted that there are two major pathways of apoptotic cell death induction: extrinsic signaling through death receptors that leads to the formation of the death-inducing signaling complex (DISC), and intrinsic signaling mainly through mitochondria which leads to the formation of the apoptosome. Formation of the DISC or apoptosome, respectively, activates initiator and common effector caspases that execute the apoptosis process. In the immune system, both pathways operate; however, it is not known whether they are sufficient to maintain lymphocyte homeostasis. Recently, new apoptotic mechanisms including caspase-independent pathways and granzyme-initiated pathways have been shown to exist in lymphocytes. This review will summarize our understanding of the mechanisms that control the homeostasis of various lymphocyte populations.

Human CD5 protects circulating tumor antigen-specific CTL from tumor-mediated activation-induced cell death

We previously characterized several tumor-specific T cell clones from PBL and tumor-infiltrating lymphocytes of a lung cancer patient with identical TCR rearrangements and similar lytic potential, but with different antitumor response. A role of the TCR inhibitory molecule CD5 to impair reactivity of peripheral T cells against the tumor was found to be involved in this process. In this report, we demonstrate that CD5 also controls the susceptibility of specific T cells to activation-induced cell death (AICD) triggered by the tumor. Using a panel of tumor-infiltrating lymphocytes and PBL-derived clones expressing different levels of CD5, our results indicate that T lymphocyte AICD in response to the cognate tumor is inversely proportional to the surface expression level of CD5. They also suggest a direct involvement of CD5 in this process, as revealed by an increase in tumor-mediated T lymphocyte AICD following neutralization of the molecule with specific mAb. Mechanistically, our data indicate that down-regulation of FasL expression and subsequent inhibition of caspase-8 activation are involved in CD5-induced T cell survival. These results provide evidence for a role of CD5 in the fate of peripheral tumor-specific T cells and further suggest its contribution to regulate the extension of CTL response against tumor.

Human and simian immunodeficiency virus-mediated upregulation of the apoptotic factor TRAIL occurs in antigen-presenting cells from AIDS-susceptible but not from AIDS-resistant species

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections lead to AIDS in humans and rhesus macaques (RM), while they are asymptomatic in species naturally infected with SIV, such as chimpanzees, sooty mangabeys (SM), and African green monkeys (AGM). Differential CD4(+) T-cell apoptosis may be responsible for these species-specific differences in susceptibility to disease. To identify factors that influence the different apoptotic responses of these species, we analyzed virus-infected human and nonhuman primate peripheral blood mononuclear cells (PBMC). We found that the apoptotic factor TRAIL was present at higher levels in human and RM PBMC cultures and was mediating, at least in part, CD4(+) T-cell apoptosis in these cultures. The species-specific increase in TRAIL and death receptor expression observed with cultures also occurred in vivo in SIV-infected RM but not in SIV-infected SM. In human and RM myeloid immature dendritic cells and macrophages, the virus-induced expression of TRAIL and other interferon-inducible genes, which did not occur in the same cells from chimpanzee, SM, and AGM, was Tat dependent. Our results link the differential induction of TRAIL in human and nonhuman primate cells to species-specific differences in disease susceptibility.

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.

Apoptotic Cells Induce Tolerance by Generating Helpless CD8+ T Cells That Produce TRAIL

The decision to generate a productive immune response or immune tolerance following pathogenic insult often depends on the context in which T cells first encounter Ag. The presence of apoptotic cells favors the induction of tolerance, whereas immune responses generated with necrotic cells promote immunity. We have examined the tolerance induced by injection of apoptotic cells, a system in which cross-presentation of Ag associated with the dead cells induces CD8+ regulatory (or suppressor) T cells. We observed that haptenated apoptotic cells induced CD8+ suppressor T cells without priming CD4+ T cells for immunity. These CD8+ T cells transferred unresponsiveness to naive recipients. In contrast, haptenated necrotic cells stimulated immunity, but induced CD8+ suppressor T cells when CD4+ T cells were absent. We further found that CD8+ T cells induced by these treatments displayed a "helpless CTL" phenotype and suppress the immune response by producing TRAIL. Animals deficient in TRAIL were resistant to tolerance induction by apoptotic cells. Thus, the outcome of an immune response taking place in the presence of cell death can be determined by the presence of CD4+-mediated Th cell function.

Interleukin 7 reduces the levels of spontaneous apoptosis in CD4+ and CD8+ T cells from HIV-1-infected individuals

Apoptosis has been suggested as one of the major mechanisms of CD4+ T cell depletion during the course of HIV type 1 (HIV-1) infection. Here, we show that interleukin 7 (IL-7), a nonredundant cytokine that plays essential roles in the generation and homeostasis of the T cell compartment of the immune system, exerts strong antiapoptotic effects ex vivo on both CD4+ and CD8+ T cells derived from HIV-1-infected subjects. The level of IL-7-mediated reduction of apoptosis was inversely correlated with the number of circulating CD4+ T cells, indicating a higher sensitivity to IL-7 effects in patients with more advanced disease. The antiapoptotic effect of IL-7 was uncoupled from the induction of cellular proliferation or endogenous HIV-1 replication. These results provide a further rationale for consideration of IL-7 as an agent of immune reconstitution in HIV-1 infection.

RCAS1 induced by HIV-Tat is involved in the apoptosis of HIV-1 infected and uninfected CD4+ T cells

HIV-1 infection is known to lead to a massive depletion of CD4(+) T cells, and the Fas/FasL and TRAIL/TRAIL-receptor systems have been reported to be one of the mechanisms of CD4(+) T cell apoptosis in HIV-1 infection. RCAS1 (a receptor-binding cancer antigen expressed on SiSo cells) is also an apoptosis-associated protein that induces apoptosis in receptor positive-cells including T cells, and NK cells. To investigate the role of RCAS1 in HIV-1 infection, we stimulated CD4(+) T cells, monocytes, and several cell lines by HIV-Tat protein and thus showed that Tat significantly increased the mRNA transcription levels and the secretion of soluble RCAS1 in CD4(+) T cells and monocytes. We also showed that the apoptosis induced by HIV-Tat was blocked by inhibiting the expression of RCAS1, using small interfering RNA (siRNA), which was specific for RCAS1. These results indicate that RCAS1 is one of the mechanisms of CD4(+) T cell depletion induced by HIV infection.

Apoptosis in SIV infection

Pathogenic human immunodeficiency virus (HIV)/Simian immunodeficiency virus (SIV) infection is associated with increased T-cell apoptosis. In marked contrast to HIV infection in humans and SIV infection in macaques, the SIV infection of natural host species is typically nonpathogenic despite high levels of viral replication. In these nonpathogenic primate models, no observation of T-cell apoptosis was observed, suggesting that either SIV is less capable of directly inducing apoptosis in natural hosts (likely as a result of coevolution/coadaptation with the host) or, alternatively, that the indirect T-cell apoptosis plays the key role in determining the HIV-associated T-cell depletion and progression to acquired immune deficiency syndrome (AIDS). Understanding the molecular and cellular mechanisms responsible for the disease-free equilibrium in natural hosts for SIV infection, including those determining the absence of high levels of T-cell apoptosis, is likely to provide important clues regarding the mechanisms of AIDS pathogenesis in humans.

Differential expression of IFN-alpha and TRAIL/DR5 in lymphoid tissue of progressor versus nonprogressor HIV-1-infected patients

Loss of CD4+ T cells, the hallmark of HIV pathogenesis, was suggested to be partly due to apoptosis. We recently reported that IFN-alpha produced by HIV-1-activated plasmacytoid dendritic cells (pDCs) contributes to CD4+ T cell apoptosis by the TNF-related apoptosis-inducing ligand (TRAIL)/death receptor (DR)5 pathway. Here, we show that HIV-1-induced intracellular expression of IFN-alpha in pDCs is coupled to increased expression of IFN regulatory factor 7 and MyD88 by pDCs in vivo and in vitro. Expression of IFN-alpha was increased in lymphoid tonsillar tissue (LT) of patients with progressive (HIV(prog)) compared with nonprogressive (HIV(NP)) HIV-1 disease and to uninfected controls. LT from HIV(prog) exhibited higher TRAIL and DR5 mRNA levels than LT from HIV(NP) or controls. TRAIL mRNA levels in LT correlated with plasma viral load. We show that HIV-1 induces IFN-alpha and the TRAIL/DR5 apoptotic pathway in LT, suggesting a role for these cytokines in HIV-1 immunopathogenesis.

Regulation of neural cell survival by HIV-1 infection

Infection by the lentivirus, human immunodeficiency virus type 1 (HIV-1), results in a variety of syndromes involving both the central (CNS) and the peripheral (PNS) nervous systems. Productive HIV-1 infection of the CNS is chiefly detectable in perivascular macrophages and microglia. HIV-1 encoded transcripts and proteins have also been detected in the PNS; however, productive viral replication appears to be sparse and restricted to the macrophage cell population. Despite the absence of productive infection of neurons, HIV-1 infection has been associated with neuronal loss in distinct regions of the brain. Neuronal cell loss may occur through both necrosis and apoptosis, although neuronal apoptosis appears to be a feature of AIDS, as only rare apoptotic neurons have been demonstrated in a few pre-AIDS cases. Although there is no clear consensus as to the underlying mechanism of HIV-induced neuropathogenesis, two complementary concepts predominate. Firstly, HIV-1 encoded proteins injure neurons directly without requiring the intermediary functions of nonneuronal cells. Alternatively, neuronal apoptosis may result indirectly from the secretion of neurotoxic host molecules by resident brain macrophages or microglia in response to HIV-1 infection, stimulation by viral proteins or immune activation. Herein, we review the neurological disorders and their underlying mechanisms associated with HIV infection, focusing on HIV-associated dementia (HAD) and HIV sensory neuropathy (HIV-SN). The evidence that neuronal loss in HIV-1-infected individuals may be due to neuronal apoptosis is then discussed. This review also summarizes the current data supporting both the direct and indirect mechanisms by which neuronal death may occur during infection with HIV-1 or the closely related lentiviruses SIV and FIV. Lastly, strategies are examined for treating or preventing HAD by targeting specific neurotoxic mechanisms.

Regulation of TNF-related apoptosis-inducing ligand on primary CD4+ T cells by HIV-1: role of type I IFN-producing plasmacytoid dendritic cells

TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily, was suggested to contribute to HIV-1 pathogenesis by inducing CD4+ T cell death characteristic of AIDS. We previously reported HIV-1-mediated, TRAIL-induced apoptosis in primary CD4+ T cells in vitro and observed elevated levels of plasma TRAIL in HIV-1-infected patients. The present study elucidates the unresolved mechanism by which HIV-1 induces TRAIL expression on primary CD4+ T cells. We demonstrate that the expression of TRAIL by primary CD4+ T cells is regulated by IFN-alpha that is produced by HIV-1-stimulated plasmacytoid dendritic cells (pDCs). We also found that IFN-induced TRAIL is mediated by signal transducers and activators of transcription 1 and 2. We show that IFN-alpha production by HIV-1-activated pDCs is blocked by an early viral entry inhibitor of CD4-gp120 binding, but not by inhibitors of viral coreceptor binding. Our in vitro data are supported by the demonstration that anti-IFN-alpha and -beta Abs inhibit apoptosis and TRAIL expression in CD4+ T cells from HIV-1-infected patients. Our findings suggest a potential unique role of pDCs in the immunopathogenesis of HIV-1 infection by inducing the death molecule TRAIL.

CD4+ T-cell death induced by infectious and noninfectious HIV-1: role of type 1 interferon-dependent, TRAIL/DR5-mediated apoptosis

It has been proposed that direct and indirect mechanisms contribute to the unresolved issue of CD4(+) T-cell depletion that results from HIV-1 infection. We recently reported that plasma levels of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) are elevated in HIV-1-infected patients and that they correlate with viral load. The present study investigates the expression of TRAIL death receptor 5 (DR5) in the peripheral-blood mononuclear cells (PBMCs) of HIV-1-infected patients and its role in CD4(+) T-cell death. DR5 expression was elevated and associated with the apoptotic marker annexin V. Apoptosis was reduced in CD4(+) T cells when cultured with anti-DR5 antibody. CD4(+), but not CD8(+), T cells from uninfected donors expressed TRAIL, DR5, and activated caspase-3 when cultured with infectious or noninfectious HIV-1, resulting in preferential apoptosis of CD4(+) T cells. TRAIL, caspase-3 expression, and apoptosis were type 1 interferon (IFN) dependent. Induction of apoptosis and DR5 expression required glycoprotein 120 (gp120)-CD4 interaction. Finally, we analyzed DR5 expression by CD4(+) T cells in highly active antiretroviral therapy (HAART)-treated patients. The decreased viral loads and increased CD4 counts of HAART-responsive patients were associated with a decrease in DR5 mRNA expression by CD4(+) T lymphocytes. We propose a novel model in which a type 1 IFN-regulated TRAIL /DR5 mechanism induces apoptosis of HIV-1-exposed CD4(+) T cells.

Acquired T-cell sensitivity to TRAIL mediated killing during HIV infection is regulated by CXCR4-gp120 interactions

BACKGROUND

Sensitivity towards apoptosis induced by ligation of the tumor necrosis factor family of death receptors is controlled in part by death receptor expression. Whereas cellular activation enhances Fas receptor expression and induces Fas sensitivity, such cellular activation neither alters TRAIL receptor expression nor induces TRAIL sensitivity. Cells infected by HIV acquire sensitivity to TRAIL induced death, although the mechanisms by which this is achieved are undefined.

OBJECTIVE

To define the mechanism by which cells from HIV infected patients acquire sensitivity to TRAIL mediated killing.

DESIGN

In vitro assessment of TRAIL receptor expression and TRAIL sensitivity.

METHODS

Treatment of Jurkat T cells, peripheral blood lymphocytes from HIV negative donors, or human osteogenic seroma (HOS) cells expressing CD4, CXCR4 or CCR5 with T tropic gp120, M tropic gp120, or agonistic antibodies against CD4, CXCR4 or CCR5. TRAIL receptors were measured by flow cytometry or reverse transcription-PCR and TRAIL sensitivity was assessed by incubation with recombinant TRAIL followed by Annexin V fluorescein isothiocyanate/Propidium Iodide (PI) staining.

RESULTS

Treatment of uninfected Jurkat T cells, as well as primary T cells with gp120 results in the upregulation of TRAIL death receptor expression and acquired sensitivity to TRAIL mediated cell death. The increase in TRAIL death receptor expression and acquisition of TRAIL sensitivity requires the chemokine coreceptor CXCR4 but not CCR5 or the CD4 receptor.

CONCLUSIONS

These results indicate that chemokine receptor interactions regulate TRAIL receptor expression and provide an explanation for the acquired T cell sensitivity to TRAIL mediated killing death during HIV infection.

HIV-1 Tat protein concomitantly down-regulates apical caspase-10 and up-regulates c-FLIP in lymphoid T cells: a potential molecular mechanism to escape TRAIL cytotoxicity

In this study, we showed the existence of a positive correlation between the amount of human immunodeficiency virus-type 1 (HIV-1) RNA in HIV-1 seropositive subjects and the plasma levels of TRAIL. Since it has been previously demonstrated that HIV-1 Tat protein up-regulates the expression of TRAIL in monocytic cells whereas tat-expressing lymphoid cells are more resistant to TRAIL cytotoxicity, we next investigated the effect of Tat on the expression/activity of both apical caspase-8 and -10, which play a key role in mediating the initial phases of apoptosis by TRAIL, and c-FLIP. Jurkat lymphoblastoid human T cell lines stably transfected with a plasmid expressing wild-type (HIV-1) tat gene showed normal levels of caspase-8 but significantly decreased levels of caspase-10 at both mRNA and protein levels with respect to Jurkat transfected with the control plasmid or with a mutated (cys22) non-functional tat cDNA. A significant decrease of caspase-10 expression/activity was also observed in transient transfection experiments with plasmid carrying tat cDNA. Moreover, c-FLIP(L) and c-FLIP(S) isoforms were up-regulated in tat-expressing cells at both mRNA and protein level in comparison with control cells. Taken together, these results provide a molecular basis to explain the resistance of tat-expressing Jurkat cells to apoptosis induced by TRAIL and, possibly, to other death-inducing ligands.

Death ligand-mediated apoptosis in HIV infection

Apoptosis has been suggested to cause severe CD4+ T cell depletion in patients infected with HIV. This review focuses on the biological events involved in death ligand-induced apoptosis during HIV infection. Among these ligands, TRAIL appears critical in HIV-infection. Death ligand-induced apoptosis might be a major pathogenic event in many virus-induced diseases including AIDS and the clarification of its mechanism will aid in the development of therapeutic strategies.

Dengue virus-induced apoptosis in hepatic cells is partly mediated by Apo2 ligand/tumour necrosis factor-related apoptosis-inducing ligand

Although hepatic injury is reported in cases with dengue haemorrhagic fever and dengue shock syndrome, its mechanism remains poorly understood. Several findings suggest that dengue virus (DEN) induces apoptosis of hepatocytes in vivo. In this work, DEN type 2 (DEN-2) strain NGC was shown to induce apoptosis in the hepatic cell line HepG2, and infection of HepG2 cells was found to induce Apo2 ligand (Apo2L, also known as tumour necrosis factor-related apoptosis-inducing ligand or TRAIL) expression. Furthermore, Apo2L/TRAIL induced apoptosis in HepG2 cells, which expressed the Apo2L/TRAIL receptor DR5/TRAIL-R2 on their surface. Analysis of the Apo2L/TRAIL promoter revealed that this gene was activated by DEN-2 infection, whose responsive element was overlapping NF-kappaB- and Sp1-binding sites located at nt -75 to -65. The proteasome inhibitor N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal (LLnL) inhibited Apo2L/TRAIL mRNA expression, and LLnL and anti-Apo2L/TRAIL antibody inhibited DEN-2-induced apoptosis. It was proposed that DEN infection promotes apoptosis partly through the induction of Apo2L/TRAIL expression.

Following a TRAIL: update on a ligand and its five receptors

Identification of tumour necrosis factor apoptosis inducing ligand (TRAIL), a TNF family ligand, sparked a torrent of research, following an initial observation that it could kill tumour cells, but spare normal cells. Almost a decade after its discovery, and with five known receptors, the true physiological role of TRAIL is still debated and its anti-tumorigenic properties limited by potential toxicity. This review takes a comprehensive look at the story of this enigmatic ligand, addressing its remaining potential as a therapeutic and providing an overview of the TRAIL receptors themselves.

Resistance to Apo2 ligand (Apo2L)/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis and constitutive expression of Apo2L/TRAIL in human T-cell leukemia virus type 1-infected T-cell lines

Adult T-cell leukemia (ATL), a CD4+-T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1), is difficult to cure, and novel treatments are urgently needed. Apo2 ligand (Apo2L; also tumor necrosis factor-related apoptosis-inducing ligand [TRAIL]) has been implicated in antitumor therapy. We found that HTLV-1-infected T-cell lines and primary ATL cells were more resistant to Apo2L-induced apoptosis than uninfected cells. Interestingly, HTLV-1-infected T-cell lines and primary ATL cells constitutively expressed Apo2L mRNA. Inducible expression of the viral oncoprotein Tax in a T-cell line up-regulated Apo2L mRNA. Analysis of the Apo2L promoter revealed that this gene is activated by Tax via the activation of NF-kappaB. The sensitivity to Apo2L was not correlated with expression levels of Apo2L receptors, intracellular regulators of apoptosis (FLICE-inhibitory protein and active Akt). NF-kappaB plays a crucial role in the pathogenesis and survival of ATL cells. The resistance to Apo2L-induced apoptosis was reversed by N-acetyl-L-leucinyl-L-leucinyl-lLnorleucinal (LLnL), an NF-kappaB inhibitor. LLnL significantly induced the Apo2L receptors DR4 and DR5. Our results suggest that the constitutive activation of NF-kappaB is essential for Apo2L gene induction and protection against Apo2L-induced apoptosis and that suppression of NF-kappaB may be a useful adjunct in clinical use of Apo2L against ATL.

Influence of expressed TRAIL on biophysical properties of the human leukemic cell line Jurkat

The cDNA fragment of human TRAIL (TNF-related apoptosis inducing ligand) was cloned into RevTet-On, a Tet-regulated and high-level gene expression system. The gene expression system was constructed in a human leukemic cell line: Jurkat. By using RevTet-On TRAIL gene expression system in Jurkat as a cell model, we studied the influence of TRAIL gene on the changes of cellular apoptosis before and after the TRAIL gene expression, which was induced by adding tetracycline derivative doxycycline (Dox). The results indicated that the cellular apoptosis ratio was largely dependent on the trail gene expression level. Moreover, it was found that the apoptosis-inducing TRAIL could cause significant changes in the biophysical properties of Jurkat cells. The cell surface charge density decreased, the membrane fluidity declined, the elastic coefficients K1 increased, and the proportion of a-helix in membrane protein secondary structure decreased. Thus, the apoptosis-inducing TRAIL gene caused significant changes on the biomechanic properties of Jurkat cells.

HIV-specific CD8+ T cells exhibit markedly reduced levels of Bcl-2 and Bcl-xL

Human immunodeficiency virus-specific CD8(+) T cells are highly sensitive to spontaneous and CD95/Fas-induced apoptosis, and this sensitivity may impair their ability to control HIV infection. To elucidate the mechanism behind this sensitivity, in this study we examined the levels of antiapoptotic molecules Bcl-2 and Bcl-x(L) in HIV-specific CD8(+) T cells from HIV-infected individuals. Bcl-2 expression was markedly decreased in HIV-specific CD8(+) T cells compared with CMV-specific and total CD8(+) T cells from HIV-infected individuals as well as total CD8(+) T cells from healthy donors. CD8(+) T cell Bcl-2 levels inversely correlated with spontaneous and CD95/Fas-induced apoptosis of CD8(+) T cells from HIV-infected individuals. HIV-specific CD8(+) T cells also had significantly lower levels of Bcl-x(L) compared with CMV-specific CD8(+) T cells. Finally, IL-15 induces both Bcl-2 and Bcl-x(L) expression in HIV-specific and total CD8(+) T cells, and this correlated with apoptosis inhibition and increased survival in both short- and long-term cultures. Our data indicate that reduced Bcl-2 and Bcl-x(L) may play an important role in the increased sensitivity to apoptosis of HIV-specific CD8(+) T cells and suggest a possible mechanism by which IL-15 increases their survival.

TRAIL and viral infection

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family that can induce apoptosis when binding to either of two receptors bearing an intracellular death domain. The physiologic function of the TRAIL system, which also comprises three receptors not mediating a death signal has just begun to be elucidated. Expression of TRAIL, mostly upon stimulation by interferons, in different cytotoxic immune cells suggested it has a role as an important effector molecule in immune surveillance. In addition to its ability to induce apoptosis in transformed tumor cells, TRAIL has attracted attention for its possibly critical role in the defense against viral infection. Viruses may induce TRAIL expression in host and?or immune cells and sensitize host cells toward TRAIL-mediated apoptosis. On the other hand, viruses have evolved a variety of strategies to prevent TRAIL-mediated host cell death early in infection, which may contribute to allowing their replication and the spread of viral progeny. The knowledge of the molecular mechanisms leading to modification of TRAIL sensitivity in virus-host cell interactions may also impact upon future (virus-based) strategies to increase TRAIL sensitivity of tumor cells.

Regulation of soluble and surface-bound TRAIL in human T cells, B cells, and monocytes

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF/nerve growth factor superfamily that, apart from inducing cell death in susceptible cells, displays immunoregulatory functions influencing, for instance, T cell proliferation. It can be found in two forms: membrane-bound and soluble protein. The regulation of these is still not fully understood. In this study, we have analyzed the regulation of TRAIL surface expression and secretion in human T cells, B cells, and monocytes in response to specific stimuli. T cells, B cells, and monocytes were cultured in the presence of phytohemagglutinin (PHA)+interleukin (IL-2), anti-CD40+IL-4, and lipopolysaccharide (LPS), respectively. In particular, not only PHA+IL-2 but also LPS were able to induce secretion of soluble TRAIL, but did not enhance the expression of surface-bound TRAIL. Simultaneously, we investigated the effect of the pleiotropic stimulus interferon (IFN)-beta, known to target all leukocyte subsets, on TRAIL. Predominantly, monocytes were affected by IFN-beta, causing both release of soluble TRAIL and upregulation of the surface-bound form. IFN-beta, however, did not cause any upregulation of TRAIL in T cells. Our data serve as a basis to better understand the complex regulation of TRAIL in human peripheral immune cells and might help to clarify the role of the TRAIL system in immunopathology.

Apoptosis of uninfected cells induced by HIV envelope glycoproteins

Apoptosis, or programmed cell death, is a key event in biologic homeostasis but is also involved in the pathogenesis of many human diseases including human immunodeficiency virus (HIV) infection. Although multiple mechanisms contribute to the gradual T cell decline that occurs in HIV-infected patients, programmed cell death of uninfected bystander T lymphocytes, including CD4+ and CD8+ T cells, is an important event leading to immunodeficiency. The HIV envelope glycoproteins (Env) play a crucial role in transducing this apoptotic signal after binding to its receptors, the CD4 molecule and a coreceptor, essentially CCR5 and CXCR4. Depending on Env presentation, the receptor involved and the complexity of target cell contact, apoptosis induction is related to death receptor and/or mitochondria-dependent pathways. This review summarizes current knowledge of Env-mediated cell death leading to T cell depletion and clinical complications and covers the sometimes conflicting studies that address the possible mechanisms of T cell death.

Differential effects of interleukin-7 and interleukin-15 on NK cell anti-human immunodeficiency virus activity

The ability of interleukin-7 (IL-7) and IL-15 to expand and/or augment effector cell functions may be of therapeutic benefit to human immunodeficiency virus (HIV)-infected patients. The functional effects of these cytokines on innate HIV-specific immunity and their impact on cells harboring HIV are unknown. We demonstrate that both IL-7 and IL-15 augment natural killer (NK) function by using cells (CD3(-) CD16(+) CD56(+)) from both HIV-positive and -negative donors. Whereas IL-7 enhances NK function through upregulation of Fas ligand, the effect of IL-15 is mediated through upregulation of tumor necrosis factor-related apoptosis-inducing ligand. The difference in these effector mechanisms is reflected by the ability of IL-15-treated but not IL-7-treated NK cells to reduce the burden of replication-competent HIV in autologous peripheral blood mononuclear cells (PBMC) (infectious units per million for control NK cells, 6.79; for IL-7-treated NK cells, 236.17; for IL-15-treated cells, 1.01; P = 0.01 versus control). In addition, the treatment of PBMC with IL-15-treated but not IL-7-treated NK cells causes undetectable HIV p24 (five of five cases), HIV RNA (five of five cases), or HIV DNA (three of five cases). These results support the concept of adjuvant immunotherapy of HIV infection with either IL-7 or IL-15 but suggest that the NK-mediated antiviral effect of IL-15 may be superior.

Amino acid residues Tyr-67, Asn-72, and Asp-73 of the TVB receptor are important for subgroup E avian sarcoma and leukosis virus interaction

The chicken TVB(S1) protein serves as the cellular receptor for the cytopathic subgroups B and D avian sarcoma and leukosis viruses (ASLVs) as well as for the non-cytopathic subgroup E ASLV. Previous studies had mapped the subgroup B viral interaction determinants to a region that was located between residues 32 and 46 of TVB(S1) [J. Virol. 76 (2002) 5404]. To gain a greater insight into ASLV Env-receptor interactions and the possible role of these interactions in viral cytopathic effects, we employed a homolog-scanning mutagenesis approach to identify amino acid residues important for subgroup E viral receptor function by exchanging amino acid residues between TVB(S1) and its human homolog, DR5. These studies identified residues Tyr-67, Asn-72, and Asp-73 of TVB(S1) as important subgroup E viral interaction determinants. Intriguingly, these three residues are conserved between TVB(S1) and DR5, demonstrating that the human protein contains critical subgroup E viral interaction determinants, but in this context, they cannot support viral entry. These data confirm that the molecular determinants of the TVB receptor required for subgroup E viral entry are completely distinct from those used by subgroup B viruses.

Effects of HIV-1 Vpr on Neuroinvasion and Neuropathogenesis

Human immunodeficiency virus type I (HIV-1) infection leads to penetration of the central nervous system (CNS) in virtually all infected individuals and HIV-1-induced encephalopathy in a significant number of untreated patients. The molecular mechanisms by which HIV-1 enters the CNS and yields CNS dysfunction are still unclear. Our laboratories and others have begun to explore the direct effects of prioritized HIV-1-specific proteins on diverse human CNS cell types. One of these proteins, the accessory HIV-1 protein Vpr, is a critical moiety in these studies, and will be discussed in this article.

T-cell and neuronal apoptosis in HIV infection: implications for therapeutic intervention

The pathogenesis of HIV infection involves the selective loss of CD4+ T cells contributing to immune deficiency. Although loss of T cells leading to immune dysfunction in HIV infection is mediated in part by viral infection, there is a much larger effect on noninfected T cells undergoing apoptosis in response to activation stimuli. In the subset of patients with HIV dementia complex, neuronal injury, loss, and apoptosis are observed. Viral proteins, gp120 and Tat, exhibit proapoptotic activities when applied to T cell and neuronal cultures by direct and indirect mechanisms. The pathways leading to cell death involve the activation of one or more death receptor pathways (i.e., TNF-alpha, Fas, and TRAIL receptors), chemokine receptor signaling, cytokine dysregulation, caspase activation, calcium mobilization, and loss of mitochondrial membrane potential. In this review, the mechanisms involved in T-cell and neuronal apoptosis, as well as antiapoptotic pathways potentially amenable to therapeutic application, are discussed.

Caspase-dependent and -independent T-cell death pathways in pathogenic simian immunodeficiency virus infection: relationship to disease progression

Studies of human immunodeficiency virus (HIV) and nonhuman primate models of pathogenic and nonpathogenic simian immunodeficiency virus (SIV) infections have suggested that enhanced ex vivo CD4 T-cell death is a feature of pathogenic infection in vivo. However, the relative contributions of the extrinsic and intrinsic pathways to programmed T-cell death in SIV infection have not been studied. We report here that the spontaneous death rate of CD4+ T cells from pathogenic SIVmac251-infected rhesus macaques ex vivo is correlated with CD4 T-cell depletion and plasma viral load in vivo. CD4+ T cells from SIVmac251-infected macaques showed upregulation of the death ligand (CD95L) and of the proapoptotic proteins Bim and Bak, but not of Bax. Both CD4+ and CD8+ T cells from SIVmac251-infected macaques underwent caspase-dependent death following CD95 ligation. The spontaneous death of CD4+ and CD8+ T cells was not prevented by a decoy CD95 receptor or by a broad-spectrum caspase inhibitor (zVAD-fmk), suggesting that this form of cell death is independent of CD95/CD95L interaction and caspase activation. IL-2 and IL-15 prevented the spontaneous death of CD4+ and CD8+ T cells, whereas IL-10 prevented only CD8 T-cell death and IL-7 had no effect on T-cell death. Our results indicate that caspase-dependent and caspase-independent pathways are involved in the death of T cells in pathogenic SIVmac251-infected primates.

Pathogenesis of Ebola hemorrhagic fever in cynomolgus macaques: evidence that dendritic cells are early and sustained targets of infection

Ebola virus (EBOV) infection causes a severe and fatal hemorrhagic disease that in many ways appears to be similar in humans and nonhuman primates; however, little is known about the development of EBOV hemorrhagic fever. In the present study, 21 cynomolgus monkeys were experimentally infected with EBOV and examined sequentially over a 6-day period to investigate the pathological events of EBOV infection that lead to death. Importantly, dendritic cells in lymphoid tissues were identified as early and sustained targets of EBOV, implicating their important role in the immunosuppression characteristic of EBOV infections. Bystander lymphocyte apoptosis, previously described in end-stage tissues, occurred early in the disease-course in intravascular and extravascular locations. Of note, apoptosis and loss of NK cells was a prominent finding, suggesting the importance of innate immunity in determining the fate of the host. Analysis of peripheral blood mononuclear cell gene expression showed temporal increases in tumor necrosis factor-related apoptosis-inducing ligand and Fas transcripts, revealing a possible mechanism for the observed bystander apoptosis, while up-regulation of NAIP and cIAP2 mRNA suggest that EBOV has evolved additional mechanisms to resist host defenses by inducing protective transcripts in cells that it infects. The sequence of pathogenetic events identified in this study should provide new targets for rational prophylactic and chemotherapeutic interventions.