CD4 T lymphocytes are primed to express Fas ligand by CD4 cross-linking and to contribute to CD8 T-cell apoptosis via Fas/FasL death signaling pathway

Vaccines targeting helper T cells for cancer immunotherapy

There are compelling arguments for designing cancer vaccines specifically to induce CD4+ helper T cell responses. Recent studies highlight the crucial role of proliferating, activated effector memory Th1 CD4+ T cells in effective antitumor immunity and reveal that CD4+ T cells induce more durable immune-mediated tumor control than CD8+ T cells. CD4+ T cells promote antitumor immunity by numerous mechanisms including enhancing antigen presentation, co-stimulation, T cell homing, T cell activation, and effector function. These effects are mediated at sites of T cell priming and at the tumor microenvironment. Several cancer vaccine approaches induce durable CD4+ T cell responses and have promising clinical activity. Future work should further optimize vaccine adjuvants and combination therapies incorporating helper peptide vaccines.

Vitamin B12 deficiency

Vitamin B₁₂ (B12; also known as cobalamin) is a B vitamin that has an important role in cellular metabolism, especially in DNA synthesis, methylation and mitochondrial metabolism. Clinical B12 deficiency with classic haematological and neurological manifestations is relatively uncommon. However, subclinical deficiency affects between 2.5% and 26% of the general population depending on the definition used, although the clinical relevance is unclear. B12 deficiency can affect individuals at all ages, but most particularly elderly individuals. Infants, children, adolescents and women of reproductive age are also at high risk of deficiency in populations where dietary intake of B12-containing animal-derived foods is restricted. Deficiency is caused by either inadequate intake, inadequate bioavailability or malabsorption. Disruption of B12 transport in the blood, or impaired cellular uptake or metabolism causes an intracellular deficiency. Diagnostic biomarkers for B12 status include decreased levels of circulating total B12 and transcobalamin-bound B12, and abnormally increased levels of homocysteine and methylmalonic acid. However, the exact cut-offs to classify clinical and subclinical deficiency remain debated. Management depends on B12 supplementation, either via high-dose oral routes or via parenteral administration. This Primer describes the current knowledge surrounding B12 deficiency, and highlights improvements in diagnostic methods as well as shifting concepts about the prevalence, causes and manifestations of B12 deficiency.

Silymarin suppressed lung cancer growth in mice via inhibiting myeloid-derived suppressor cells

In this study, we investigated the antitumor activity of Silymarin in a mouse model of colon cancer xenograft of Lewis lung cancer (LLC) cells. Silymarin significantly suppressed tumor growth and induced apoptosis of cells in tumor tissues at a dose of 25 and 50mg/kg. Silymarin treatment enhanced the infiltration and function of CD8(+) T cells. In the meantime, Silymarin decreased the level of IL-10 while elevated the level of IL-2 and IFN-γ in the serum of tumor-bearing mice. Finally, Silymarin reduced the proportion of myeloid-derived suppressor cells (MDSC) in the tumor tissue and also the mRNA expressions of inducible nitric oxide synthases-2 (iNOS2), arginase-1 (Arg-1) and MMP9, which indicated that the function of MDSC in tumor tissues were suppressed. Altogether, our data here showed that Silymarin inhibited the MDSC and promoted the infiltration and function of CD8(+) T cells thus suppressed the growth of LLC xenografts, which provides evidence for the possible use of Silymarin against lung cancer.

Clinacanthus nutans (Burm. f.) Lindau Ethanol Extract Inhibits Hepatoma in Mice through Upregulation of the Immune Response

Clinacanthans nutans (Burm. f.) Lindau is a popular medicinal vegetable in Southern Asia, and its extracts have displayed significant anti-proliferative effects on cancer cells in vitro. However, the underlying mechanism for this effect has yet to be established. This study investigated the antitumor and immunomodulatory activity of C. nutans (Burm. f.) Lindau 30% ethanol extract (CN30) in vivo. CN30 was prepared and its main components were identified using high-performance liquid chromatography (HPLC) and mass spectrometry (LC/MS/MS). CN30 had a significant inhibitory effect on tumor volume and weight. Hematoxylin and eosin (H & E) staining and TUNEL assay revealed that hepatoma cells underwent significant apoptosis with CN30 treatment, while expression levels of proliferation markers PCNA and p-AKT were significantly decreased when treated with low or high doses of CN30 treatment. Western blot analysis of PAPR, caspase-3, BAX, and Bcl2 also showed that CN30 induced apoptosis in hepatoma cells. Furthermore, intracellular staining analysis showed that CN30 treatment increased the number of IFN-γ⁺ T cells and decreased the number of IL-4⁺ T cells. Serum IFN-γ and interleukin-2 levels also significantly improved. Our findings indicated that CN30 demonstrated antitumor properties by up-regulating the immune response, and warrants further evaluation as a potential therapeutic agent for the treatment and prevention of cancers.

Pathophysiologic and transcriptomic analyses of viscerotropic yellow fever in a rhesus macaque model

Infection with yellow fever virus (YFV), an explosively replicating flavivirus, results in viral hemorrhagic disease characterized by cardiovascular shock and multi-organ failure. Unvaccinated populations experience 20 to 50% fatality. Few studies have examined the pathophysiological changes that occur in humans during YFV infection due to the sporadic nature and remote locations of outbreaks. Rhesus macaques are highly susceptible to YFV infection, providing a robust animal model to investigate host-pathogen interactions. In this study, we characterized disease progression as well as alterations in immune system homeostasis, cytokine production and gene expression in rhesus macaques infected with the virulent YFV strain DakH1279 (YFV-DakH1279). Following infection, YFV-DakH1279 replicated to high titers resulting in viscerotropic disease with ∼72% mortality. Data presented in this manuscript demonstrate for the first time that lethal YFV infection results in profound lymphopenia that precedes the hallmark changes in liver enzymes and that although tissue damage was noted in liver, kidneys, and lymphoid tissues, viral antigen was only detected in the liver. These observations suggest that additional tissue damage could be due to indirect effects of viral replication. Indeed, circulating levels of several cytokines peaked shortly before euthanasia. Our study also includes the first description of YFV-DakH1279-induced changes in gene expression within peripheral blood mononuclear cells 3 days post-infection prior to any clinical signs. These data show that infection with wild type YFV-DakH1279 or live-attenuated vaccine strain YFV-17D, resulted in 765 and 46 differentially expressed genes (DEGs), respectively. DEGs detected after YFV-17D infection were mostly associated with innate immunity, whereas YFV-DakH1279 infection resulted in dysregulation of genes associated with the development of immune response, ion metabolism, and apoptosis. Therefore, WT-YFV infection is associated with significant changes in gene expression that are detectable before the onset of clinical symptoms and may influence disease progression and outcome of infection.

Yellow fever virus causes ∼200,000 infections and 30,000 deaths annually in Africa and South America. Although this is an important human pathogen, the basis of yellow fever disease severity remains poorly understood. Rhesus macaques are susceptible to yellow fever and develop similar symptoms as severe as those observed in humans. In this study, we characterized disease progression in this model and observed a profound loss of lymphocytes that preceded the appearance of serum markers of virus-induced liver pathology. This change might provide an early indicator of fatal yellow fever. In addition, we also identified significant changes in gene expression in white blood cells that occur before any measurable disease symptoms and these genetic signatures may provide future targets for antiviral therapeutics and better diagnostics.

Human immunodeficiency virus-1 (HIV-1)-mediated apoptosis: new therapeutic targets

HIV has posed a significant challenge due to the ability of the virus to both impair and evade the host’s immune system. One of the most important mechanisms it has employed to do so is the modulation of the host’s native apoptotic pathways and mechanisms. Viral proteins alter normal apoptotic signaling resulting in increased viral load and the formation of viral reservoirs which ultimately increase infectivity. Both the host’s pro- and anti-apoptotic responses are regulated by the interactions of viral proteins with cell surface receptors or apoptotic pathway components. This dynamic has led to the development of therapies aimed at altering the ability of the virus to modulate apoptotic pathways. These therapies are aimed at preventing or inhibiting viral infection, or treating viral associated pathologies. These drugs target both the viral proteins and the apoptotic pathways of the host. This review will examine the cell types targeted by HIV, the surface receptors exploited by the virus and the mechanisms whereby HIV encoded proteins influence the apoptotic pathways. The viral manipulation of the hosts’ cell type to evade the immune system, establish viral reservoirs and enhance viral proliferation will be reviewed. The pathologies associated with the ability of HIV to alter apoptotic signaling and the drugs and therapies currently under development that target the ability of apoptotic signaling within HIV infection will also be discussed.

T-Cell Signaling in HIV-1 Infection

HIV exploits the T-cell signaling network to gain access to downstream cellular components, which serves as effective tools to break the cellular barriers. Multiple host factors and their interaction with viral proteins contribute to the complexity of HIV-1 pathogenesis and disease progression. HIV-1 proteins gp120, Nef, Tat and Vpr alter the T-cell signaling pathways by activating multiple transcription factors including NF-ĸB, Sp1 and AP-1. HIV-1 evades the immune system by developing a multi-pronged strategy. Additionally, HIV-1 encoded proteins influence the apoptosis in the host cell favoring or blocking T-cell apoptosis. Thus, T-cell signaling hijacked by viral proteins accounts for both viral persistence and immune suppression during HIV-1 infection. Here, we summarize past and present studies on HIV-1 T-cell signaling with special focus on the possible role of T cells in facilitating viral infection and pathogenesis

Phenotypic characterization of HIV-specific CD8+ T cells during early and chronic infant HIV-1 infection

Although CD8(+) T cells play an important role in the containment of adult HIV-1 replication, their role in infant HIV-1 infection is not as well understood. Impaired HIV-specific CD8(+) T cell responses may underlie the persistently high viral loads observed in infants. We examined the frequency and phenotype of infant HIV-specific CD8(+) T cells in 7 HIV-infected antiretroviral therapy-naïve infants during the first 2 years of life, using class I HLA tetramers and IFN-γ-ELISPOT. The frequency (0.088-3.9% of CD3(+)CD8(+) cells) and phenotype (CD27(+)CD28(-), CD45RA(+/-), CD57(+/-), HLA-DR(+), CD95(+)) of infant HIV-specific CD8(+) T cells were similar to reports in adults undergoing early infection. Unlike adults, at 23-24 months post-infection a high frequency of HIV-specific CD8(+) T cells expressed HLA-DR (mean 80%, range 68-85%) and CD95 (mean 88%, range 79-96%), suggesting sustained activation and vulnerability to apoptosis. Despite comparable expansion of HIV-specific CD8(+) T cells of a similar phenotype to adults during early infection, infant T cells failed to contain HIV-1 replication, and remained persistently activated and vulnerable to apoptosis during chronic infection.

Dendritic cells and CD28 costimulation are required to sustain virus-specific CD8+ T cell responses during the effector phase in vivo

Although much is known about the initiation of immune responses, much less is known about what controls the effector phase. CD8(+) T cell responses are believed to be programmed in lymph nodes during priming without any further contribution by dendritic cells (DCs) and Ag. In this study, we report the requirement for DCs, Ag, and CD28 costimulation during the effector phase of the CD8(+) T cell response. Depleting DCs or blocking CD28 after day 6 of primary influenza A virus infection decreases the virus-specific CD8(+) T cell response by inducing apoptosis, and this results in decreased viral clearance. Furthermore, effector CD8(+) T cells adoptively transferred during the effector phase fail to expand without DC, CD28 costimulation, and cognate Ag. The absence of costimulation also leads to reduced survival of virus-specific effector cells as they undergo apoptosis mediated by the proapoptotic molecule Bim. Finally, IL-2 treatment restored the effector response in the absence of CD28 costimulation. Thus, in contrast to naive CD8(+) T cells, which undergo an initial Ag-independent proliferation, effector CD8(+) T cells expanding in the lungs during the effector phase require Ag, CD28 costimulation, and DCs for survival and expansion. These requirements would greatly impair effector responses against viruses and tumors that are known to inhibit DC maturation and in chronic infections and aging where CD28(-/-) CD8(+) T cells accumulate.

Donor lymphocyte infusion following allogeneic hematopoietic stem cell transplantation

INTRODUCTION

Allogeneic hematopoietic stem cell transplantation (SCT) is the treatment of choice for many malignant hematological disorders. Following recent improvements in non-relapse-related mortality rates, relapse has become the commonest cause of treatment failure. Infusion of donor lymphocytes can potentially enhance immune-mediated antitumor activity and offers a salvage option for some patients. This paper reviews the current literature on the efficacy of this therapeutic strategy.

AREAS COVERED

The biology of adoptive cellular therapy with allogeneic immune cells to treat relapse across a spectrum of diseases in both the full intensity and reduced intensity hematopoietic SCT settings is explored. The review discusses the current limitations of the approach and reviews several new experimental strategies which aim to segregate the desired graft-versus-tumor effect from the deleterious effects of more widespread graft-versus-host reactivity.

EXPERT OPINION

Durable responses to DLI have been noted in chronic myeloid leukemia and responses have also been described in acute leukemia, multiple myeloma and chronic lymphoproliferative disorders. The new challenge in transplantation is to optimize DLI therapy in order to further improve patient outcomes.

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.

Kinetics of T lymphocyte apoptosis and the cellular immune response in SIVmac239-infected rhesus macaques

BACKGROUND

Although increased apoptosis is a central feature of AIDS, little is known about its kinetics or relationship to the early host response in acute HIV/SIV infection.

METHODS

Ex vivo apoptosis in freshly isolated peripheral blood and lymph node lymphocytes was monitored longitudinally in SIVmac239-infected rhesus macaques by flow-cytometric detection of active caspase-3, cleaved poly (ADP-ribose) polymerase, and fragmented DNA.

RESULTS

Increased apoptosis of multiple lymphocyte subsets was observed in the first 2 weeks following SIV infection. Apoptosis of CD4+ T lymphocytes was of low magnitude but peaked earlier than other T lymphocyte subsets. A 10- to 36-fold increase in CD8+ T lymphocyte apoptosis coincided temporally with onset of the SIV-specific cellular immune response and enrichment of caspase-3-positive cells within recently proliferating, activated CD8+ T lymphocytes.

CONCLUSIONS

The virus-specific T lymphocyte response to primary infection and generalized non-specific immune activation contribute to the pathogenesis of apoptosis in acute SIV infection.

Late signals from CD27 prevent Fas-dependent apoptosis of primary CD8+ T cells

The role of costimulation has previously been confined to the very early stages of the CD8+ T cell response. In this study, we demonstrate the requirement for CD27 costimulation during the later phase, but not programming of the primary CD8+ T cell response to influenza virus and reveal a novel mechanism of action for CD27 costimulation. CD27 signals, during the later phase of the primary CD8+ T cell response, prevent apoptosis of Ag-specific CD8+ T cells. Blocking CD27L (CD70) on days 6 and 8 after infection reduces the number of NP(366-374)-specific CD8+ T cells, increases their sensitivity to CD95/Fas-mediated apoptosis, and up-regulates FasL on CD4+ T cells. This reduction of NP(366-374)-specific CD8+ T cells requires the presence of CD4+ T cells and Fas signaling. Lack of CD27 signals also decreases the quality of memory CD8+ T cell responses. Memory CD8+ T cells, which express surface CD27 similar to naive cells, however, do not require CD27 costimulation during a secondary response. Thus, CD27 acts indirectly to regulate primary Ag-specific CD8+ T cell responses by preventing apoptosis of CD8+ T cells during the later phase of the primary response and is required for optimal quality of memory cells, but is not required during normally primed secondary CD8+ T cell responses.

[Expression Fas and FasL antigen protein of lymphocytes T and B in hypertrophied adenoid in children with otitis media with effusion]

INTRODUCTION

Apoptosis of lymphocytes can be induced by different factors mainly however the activation of antigen Fas (CD95) and his ligand FasL (CD95L) route. Apoptosis induced by Fas-FasL has large significance in elimination of lymphocytes T in final phase immune response designate AICD (activation induced cell death). Lymphocytes B undergo apoptosis induced by Fas-FasL in germinal center, which has special meaning in elimination of cells about low specificity and weak affinity of receptor BCR ( B-cell antigen receptor).

AIM OF STUDY

The aim of this study was evaluation of percentage of apoptotic lymphocytes and expression of Fas and FasL in CD4+, CD8+, CD19+ lymphocytes in hypertrophied adenoid and hypertrophied adenoid and otitis media with effusion.

METHODS

CD4+Fas+, CD8+Fas+, CD19+Fas+CD4+FasL+, CD8+FasL+, CD19+FasL+ cells subpopulation were identified using monoclonal antibodies and flow cytometry method.

RESULTS

The percentage of lymphocytes CD4+ Fas+, CD8+Fas+, CD19+Fas+ was higher in hypertrophied adenoid and otitis media with effusion compored to the control group. Their was no significant difference the percentage CD4+ Fas+, CD8+Fas+, CD19+Fas+CD4+FasL+, CD8+FasL+, CD19+FasL+ in hypertrophied adenoid and otitis media with effusion then the control group. The percentage of apoptotic lymphocytes was higher in hypertrophied adenoid and otitis media with effusion compored to the control group.

CONCLUSIONS

The susceptibility the individual subpopulation of lymphocytes in hypertrophied adenoid seems to influence on change of relations of quantitative lymphocytes and upset the immunological function of tonsils which can influence on course otitis media with effusion at children.

An optimized immunomagnetic bead-based negative selection protocol for CD4 T-cell isolation from mouse lymph nodes and spleen

This study describes an immunomagnetic bead-based methodology for optimal purification of mouse CD4+ T cells. It reproducibly yields highly pure CD4+ T cells from mouse lymph nodes (95-99%) and spleen (93-96%) with no residual antigen-presenting cell (APC) function in the purified population. The recovery of the starting CD4+ T-cell population is consistently high ( > 70%) and many samples can be simultaneously processed in a short period of time. The key factors responsible for improved purity are combinations of monoclonal antibodies that were found, through trial and error, to yield T cells of maximal purity achievable by non-flow sort-based negative selection. These cocktails efficiently target unwanted cell subsets with antibodies against multiple surface markers expressed by non-CD4+ T cells. Because immunomagnetic bead-based protocols do not require the expensive and cumbersome processes required by flow sort-based purification, the methodology described here should find widespread use.

Increased proportion of Fas positive CD8+ cells in peripheral blood of patients with COPD

Chronic obstructive pulmonary disease (COPD) is characterised by chronic inflammation in pulmonary tissue and is also associated with systemic effects. The objective of this study was determination of lymphocyte subpopulation and the expression of Fas receptor on lymphocytes derived from peripheral blood of patients with stable COPD (n=18) and a control group: asymptomatic smokers (n=12) and non-smokers (n=12). Flow cytometry method with monoclonal antibodies was used for evaluation of lymphocyte subsets: CD4+ and CD8+ and the expression of Fas (CD95) on T lymphocytes. We found an elevated proportion of CD8+ cells in the blood of COPD patients. Proportion of Fas+ T lymphocytes was significantly higher in patients with COPD when compared with asymptomatic smokers and non-smokers (mean: 84.4% vs. 71.6% vs. 61.0% for Fas+/ CD4+ and 88.1% vs. 73.8% vs. 58.3% for Fas+/CD8+ lymphocytes). The proportion of Fas positive CD8+ cells significantly correlated with the degree of airway obstruction and hypoxemia. The significant correlations of Fas positive CD4+ and Fas positive CD8+ with smoking history expressed as pack years smoked were observed. Our observation of an elevated proportion of circulating lymphocytes bearing Fas receptor may play a role in induction of these cells' apoptosis and indicate the role of Fas/ FasL pathway in the changes in proportion of lymphocyte subpopulations in patients with COPD.

Dendritic cells infected with vpr-positive human immunodeficiency virus type 1 induce CD8+ T-cell apoptosis via upregulation of tumor necrosis factor alpha

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) plays a crucial role in viral replication and pathogenesis by inducing cell cycle arrest, apoptosis, translocation of preintegration complex, potentiation of glucocorticoid action, impairment of dendritic cell (DC) maturation, and T-cell activation. Recent studies involving the direct effects of Vpr on DCs and T cells indicated that HIV-1 containing Vpr selectively impairs phenotypic maturation, cytokine network, and antigen presentation in DCs and dysregulates costimulatory molecules and cytokine production in T cells. Here, we have further investigated the indirect effect of HIV-1 Vpr(+) virus-infected DCs on the bystander CD8(+) T-cell population. Our results indicate that HIV-1 Vpr(+) virus-infected DCs dysregulate CD8(+) T-cell proliferation and induce apoptosis. Vpr-containing virus-infected DC-mediated CD8(+) T-cell killing occurred in part through enhanced tumor necrosis factor alpha production by infected DCs and subsequent induction of death receptor signaling and activation of the caspase 8-dependent pathway in CD8(+) T cells. Collectively, these results provide evidence that Vpr could be one of the important contributors to the host immune escape by HIV-1 through its ability to dysregulate both directly and indirectly the DC biology and T-cell functions.

A subset of functional effector-memory CD8+ T lymphocytes in human immunodeficiency virus-infected patients

CD8(+) T cells provide protective immune responses via both cytolytic and non-cytolytic mechanisms in subjects infected with human immunodeficiency virus (HIV). In the present study, we investigated the CD28 expression of CD8(+) T cells present in the peripheral blood lymphocyte subset isolated from chronically HIV-infected subjects. Using flow cytometric analysis, a continuous spectrum of CD28 intensity ranging from negative to high, which could be separated into CD28-negative, intermediate (int) and high, was seen for CD8(+) T cells. Our study focused mostly on the CD28(int) CD8(+) T cells. The CD28(int) CD8(+) T cells are CD57(-) CD27(+) CD45RO(+) CD45RA(-) CCR7(low) CD62L(int). The proliferative capacity of CD28(int) CD8(+) T cells was intermediate between those of CD28(-) CD8(+) T cells and CD28(high) CD8(+) T cells. The CD28(int) CD8(+) T cells are specific for HIV, cytomegalovirus (CMV) and Epstein-Barr virus (EBV) antigens as measured by human leucocyte antigen pentamer binding and produce both intracellular interferon-gamma and tumour necrosis factor-alpha in response to their cognate viral peptides. The CD28(int) CD8(+) T cells have HIV-specific, CMV-specific and EBV-specific cytotoxic activity in response to their cognate viral peptides. These findings indicate that a subset of functional effector-memory CD8(+) T cells specific for HIV, CMV and EBV antigens may contribute to an efficient immune response in HIV-infected subjects.

CD8 apoptosis may be a predictor of T cell number normalization after immune reconstitution in HIV

Background

As part of the Houston Vanguard study, a subset of 10 patients randomized to receive IL-2 therapy were compared to 4 patients randomized to not receive IL-2, for markers of T cell activation and death during the first three cycles of IL-2. All patients were treated with combination antiretroviral therapy (ART) and were virally suppressed. The purpose of the study was to examine the role of CD8+ T cell death in responses to ART and IL-2 therapy.

Methods

Lymphocytes were examined at Day 0, 5 and 30 days during three cycles of IL-2 therapy. CD25, CD38, HLA-DR expression and annexin (cell death) were examined on CD4 and CD8 subpopulations. Follow up studies examined CD4 levels and CD4:CD8 reconstitution after 6 years using both univariant and multivariate analyses.

Results

Human lymphocytes responded to IL-2 therapy by upregulation of CD25 on CD4⁺ T cells, leading to an increase in CD4 cell counts. CD8⁺ T cells did not increase CD25 expression, but upregulated activation antigens (CD38 and DR) and had increased death. At baseline, 7 of the 14 patients had high CD8⁺ T cell apoptosis (mean 17.0% ± 6.0). We did an exploratory analysis of immune status after six years, and found that baseline CD8⁺ T cell apoptosis was correlated with CD4 cell count gain beginning two years post enrollment. Patients with low levels of CD8⁺ T cell apoptosis at baseline (mean 2.2% ± 2.1) had significantly higher CD4 cell counts and more normalized CD4:CD8 ratios than patients with high CD8⁺ T cell apoptosis (mean CD4 cell counts 1,209 ± 164 vs 754 ± 320 cells/mm³; CD4:CD8 ratios 1.55 vs. 0.70, respectively).

Conclusion

We postulate that CD8⁺ T cell apoptosis may reflect inherent activation status, which continues in some patients even though viral replication is suppressed which influences the ability of CD4⁺ T cells to rebound. Levels of CD8⁺ T cell apoptosis may therefore be an independent predictor of immune status, which should be shown in a prospective study.

No Life Without Death

Apoptosis-programed cell death-is the most common form of death in the body. Once apoptosis is induced, proper execution of the cell death program requires the coordinated activation and execution of multiple molecular processes. Here, we describe the pathways and the basic components of the death-inducing machinery. Since apoptosis is a key regulator of tissue homeostasis, an imbalance of apoptosis results in severe diseases like cancer, autoimmunity, and AIDS.

Vaccine-delivered HIV envelope inhibits CD4(+) T-cell activation, a mechanism for poor HIV vaccine responses

The human immunodeficiency virus (HIV) causes impairment of the immune system in part by targeting CD4(+) T cells for infection and dysfunction. HIV envelope (Env) present on free virions and infected cells causes dysfunction of uninfected bystander CD4(+) T cells via interaction with both CD4 and coreceptors. Env is commonly used as part of a cocktail of HIV antigens in current vaccines. In DNA and viral vector vaccine approaches, antigen-presenting cells (APCs) and non-APCs in the vicinity of the vaccine delivery site and draining lymph node express vaccine-derived antigens. The studies here demonstrate that cell-surface expression of Env on APCs and non-APCs as part of the vaccine action causes an inhibition of antigen-induced CD4(+) T-cell activation and proliferation mediated by CD4 binding and suggests a potential mechanism for reduced activity of Env-containing HIV vaccines. Similar studies using a functional Env lacking CD4 binding circumvented suppression, suggesting an alternative and potentially superior approach to HIV vaccine design.

Apoptosis induced in HIV-1-exposed, resting CD4+T cells subsequent to signaling through homing receptors is Fas/Fas ligand-mediated

The hallmark of HIV-1 disease is the gradual disappearance of CD4+ T cells from the blood. The mechanism of this depletion, however, is still unclear. Evidence suggests that lymphocytes die in lymph nodes, not in blood, and that uninfected bystander cells are the predominant cells dying. Our and others' previous studies showed that the lymph node homing receptor, CD62 ligand (CD62L), and Fas are up-regulated on resting CD4+ T cells after HIV-1 binding and that these cells home to lymph nodes at an enhanced rate. During the homing process, signals are induced through various homing receptors, which in turn, induced many of the cells to undergo apoptosis after they entered the lymph nodes. The purpose of this study was to determine how the homing process induces apoptosis in HIV-1-exposed, resting CD4+ T cells. We found that signaling through CD62L up-regulated FasL. This resulted in apoptosis of only HIV-1-presignaled, resting CD4+ T cells, not normal CD4+ T cells. This homing receptor-induced apoptosis could be blocked by anti-FasL antibodies or soluble Fas, demonstrating that the Fas-FasL interaction caused the apoptotic event.

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.

Molecular interactions of human immunodeficiency virus type 1 with primary human oral keratinocytes

Infection of the oral mucosa of human immunodeficiency virus type 1 (HIV-1)-infected individuals remains an under-evaluated and somewhat enigmatic process. Nonetheless, it is of profound importance in the ongoing AIDS pandemic, based on its potential as a site of person-to-person transmission of the virus as well as a location of HIV-1 pathogenesis and potential reservoir of disease in the setting of virally suppressive highly active antiretroviral therapy. We utilized molecular and virological techniques to analyze HIV-1 infection of primary human mucosal cells and also evaluated the proapoptotic potential of selected HIV-1 proteins in primary isolated human oral keratinocytes. Primary isolated human oral keratinocytes were plated on 0.4 microM polyethylenetetraphthalate cell culture inserts to form an in vitro oral mucosal layer. The strength of this layer in forming a barrier was determined by measuring trans-epithelial electrical current passage across the monolayer. The oral keratinocyte monolayers had trans-epithelial electrical resistance of approximately 176 to 208 omega. For viral infectivity assays, the macrophage-tropic (R5) HIV-1 strains, YU-2 and ADA, and T-cell-line-tropic (X4), NL4-3 virions, incubated with or without deoxynucleoside triphosphates (dNTPs) and/or the polyamines spermine and spermidine, were used to infect oral keratinocytes. Of importance, polyamines and dNTPs have been shown to enhance natural endogenous reverse transcription (NERT), a step essential for early lentiviral infection, and are abundantly present in human semen. The infectivities of HIV-1 strains YU-2, ADA, and NL4-3 for these primary keratinocytes were dramatically increased by the addition of physiological concentrations of dNTPs, spermine, and spermidine. Binding and viral internalization assay studies showed no differences in these oral mucosal cells, with or without NERT-altering agents. It was also observed that the recombinant, cell-free HIV-1 proteins Nef, Tat, and gp120 (R5) induced apoptosis in primary oral keratinocytes compared with the results seen with nontreated cells or cells treated with glutathione S-transferase protein as a control under similar conditions. Microarray analyses suggested that HIV-1 gp120 and Tat induce apoptosis in primary human oral keratinocytes via the Fas/FasL apoptotic pathway, whereas induction of apoptosis by Nef occurs through both Fas/FasL and mitochondrial apoptotic pathways. Thus, these findings suggest molecular mechanisms by which semen in particular, as well as other bodily fluids such as cervicovaginal secretions, could increase oral transmission of HIV-1 via increasing infectivity in confluent and low-replicating oral keratinocytes. As well, the induction of apoptosis in human oral keratinocytes with relevant HIV-1-specific proteins suggests another potential complementary mechanism by which the oral mucosa barrier may be disrupted during HIV-1 infection in vivo.

Distinct mechanisms of CD4+ and CD8+ T-cell activation and bystander apoptosis induced by human immunodeficiency virus type 1 virions

Apoptosis of uninfected bystander T cells contributes to T-cell depletion during human immunodeficiency virus type 1 (HIV-1) infection. HIV-1 envelope/receptor interactions and immune activation have been implicated as contributors to bystander apoptosis. To better understand the relationship between T-cell activation and bystander apoptosis during HIV-1 pathogenesis, we investigated the effects of the highly cytopathic CXCR4-tropic HIV-1 variant ELI6 on primary CD4(+) and CD8(+) T cells. Infection of primary T-cell cultures with ELI6 induced CD4(+) T-cell depletion by direct cell lysis and bystander apoptosis. Exposure of primary CD4(+) and CD8(+) T cells to nonreplicating ELI6 virions induced bystander apoptosis through a Fas-independent mechanism. Bystander apoptosis of CD4(+) T cells required direct contact with virions and Env/CXCR4 binding. In contrast, the apoptosis of CD8(+) T cells was triggered by a soluble factor(s) secreted by CD4(+) T cells. HIV-1 virions activated CD4(+) and CD8(+) T cells to express CD25 and HLA-DR and preferentially induced apoptosis in CD25(+)HLA-DR(+) T cells in a CXCR4-dependent manner. Maximal levels of binding, activation, and apoptosis were induced by virions that incorporated MHC class II and B7-2 into the viral membrane. These results suggest that nonreplicating HIV-1 virions contribute to chronic immune activation and T-cell depletion during HIV-1 pathogenesis by activating CD4(+) and CD8(+) T cells, which then proceed to die via apoptosis. This mechanism may represent a viral immune evasion strategy to increase viral replication by activating target cells while killing immune effector cells that are not productively infected.

Interleukin 7 increases human immunodeficiency virus type 1 LAI-mediated Fas-induced T-cell death

Fas-mediated T-cell death is known to occur during human immunodeficiency virus (HIV) infection. In this study, we found that HIV type 1 LAI (HIV-1(LAI)) primes CD8(+) T cells from healthy donors for apoptosis, which occurs after Fas ligation. This effect is counteracted by a broad caspase inhibitor (zVAD-fmk). Fas-mediated cell death does not depend on CD8(+) T-cell infection, because it occurred in the presence of reverse transcriptase inhibitors. However, purified CD8(+) T cells are sensitive to Fas only in the presence of soluble CD4. Finally, we found that interleukin 7 (IL-7) increases Fas-mediated CD4(+) and CD8(+) T-cell death induced by HIV-1(LAI). Since high levels of IL-7 are a marker of poor prognosis during HIV infection, our data suggest that enhancement of Fas-mediated T-cell death by HIV-1(LAI) and IL-7 is one of the mechanisms involved in progression to AIDS.

CXCR4-mediated T cell apoptosis in human immunodeficiency virus infection

Mechanisms of CXCR4-mediated T lymphocyte apoptosis in human immunodeficiency virus (HIV) infection are poorly understood. The authors used peripheral blood mononuclear cells isolated from HIV type 1-infected subjects and assessed both CD4(+) and CD8(+) T cell apoptosis in the presence and absence of CXCR4 blockade by AMD3100. Both CD4(+) and CD8(+) T cell apoptosis could be inhibited by CXCR4 blockade, mostly in acquired immunodeficiency syndrome subjects and more weakly in asymptomatic HIV-positive subjects, and depended only partially on the syncytium-inducing/non-syncytium-inducing viral envelope phenotype. Immune activation of CD8(+), but not CD4(+), T cells was CXCR4-dependent, resulting in increased T cell apoptosis. In the presence of monocyte-derived macrophages, CXCR4-mediated apoptosis targeted mostly CD8(+) T cells, with CD4(+) T cells being more weakly affected. Several immune and viral factors thus play a role in CXCR4-mediated T cell apoptosis in HIV infection: CD4/CD8 phenotype, viral envelope phenotype, T cell activation and T cell-macrophage intercellular contacts.

Apoptosis of bystander T cells induced by human immunodeficiency virus type 1 with increased envelope/receptor affinity and coreceptor binding site exposure

Apoptosis of uninfected bystander CD4(+) T cells contributes to T-cell depletion during human immunodeficiency virus type 1 (HIV-1) pathogenesis. The viral and host mechanisms that lead to bystander apoptosis are not well understood. To investigate properties of the viral envelope glycoproteins (Env proteins) that influence the ability of HIV-1 to induce bystander apoptosis, we used molecularly cloned viruses that differ only in specific amino acids in Env. The ability of these strains to induce bystander apoptosis was tested in herpesvirus saimiri-immortalized primary CD4(+) T cells (CD4/HVS), which resemble activated primary T cells. Changes in Env that increase affinity for CD4 or CCR5 or increase coreceptor binding site exposure enhanced the capacity of HIV-1 to induce bystander apoptosis following viral infection or exposure to nonreplicating virions. Apoptosis induced by HIV-1 virions was inhibited by CD4, CXCR4, and CCR5 antibodies or by the CXCR4 inhibitor AMD3100, but not the fusion inhibitor T20. HIV-1 virions with mutant Envs that bind CXCR4 but are defective for CD4 binding or membrane fusion induced apoptosis, whereas CXCR4 binding-defective mutants did not. These results demonstrate that HIV-1 virions induce apoptosis through a CXCR4- or CCR5-dependent pathway that does not require Env/CD4 signaling or membrane fusion and suggest that HIV-1 variants with increased envelope/receptor affinity or coreceptor binding site exposure may promote T-cell depletion in vivo by accelerating bystander cell death.

CD40 Ligand Dysregulation in HIV Infection: HIV Glycoprotein 120 Inhibits Signaling Cascades Upstream of CD40 Ligand Transcription

IL-12 production and up-regulation of CD40 ligand (CD40L) expression are impaired in the PBMC of HIV-infected donors, and exogenous CD40L rescues IL-12 production by such cells. In this study, we implicate dysregulation of CD40L expression in the IL-12 defect associated with HIV by demonstrating that induction of CD40L expression by anti-CD3/CD28 stimulation was directly correlated with the IL-12 productive capacity of PBMC. Further, we demonstrate marked decreases in the induction of CD40L protein and mRNA following anti-CD3/CD28 stimulation in HIV-infected donors compared with uninfected donors, with a tight association between these two levels. Inhibition of CD40L up-regulation was selective, as induction of CD69 or OX40 was not as severely affected. Increased instability of CD40L mRNA did not constitute a major mechanism in CD40L dysregulation, thus suggesting a potential defect in the signaling cascades upstream of transcription. The mechanisms by which HIV infection affects the induction of CD40L expression appear to involve HIV gp120-mediated engagement of CD4. Indeed, anti-CD4 mAb or inactivated HIV virions that harbor a conformationally intact gp120 significantly inhibited CD40L up-regulation at both the protein and mRNA levels. This inhibition was due to the native, virion-associated gp120, as coculture with soluble CD4 or heat treatment of inactivated HIV abolished their effect. These in vitro models mirror the CD40L defect seen in cells from HIV-infected donors and thus provide a suitable model to investigate HIV-induced CD40L dysregulation. Clear elucidation of mechanism(s) may well lead to the development of novel immunotherapeutic approaches to HIV infection.

The multiple roles of Fas ligand in the pathogenesis of infectious diseases

Fas ligand (FasL) is a type II transmembrane protein that plays a critical role in immune homeostasis by binding to its receptor Fas (CD95) and inducing apoptosis. Fas/FasL dysregulation contributes to infectious disease pathogenesis. Microorganisms may inhibit Fas signal transduction to prolong intracellular survival and prevent killing by immune effector cells. FasL may be upregulated in directly infected cells to enhance killing of responding immune cells and facilitate immune evasion. The host response to infection may aim to induce apoptosis in directly infected cells, but immune cells that target directly infected cells can induce Fas-mediated apoptosis of uninfected bystander cells. FasL also contributes to the generation and regulation of the inflammatory response in infection. The multiple roles of FasL in infectious disease pathogenesis are discussed in the context of viral, bacterial and parasitic infections.

Intrinsic and extrinsic pathways signaling during HIV-1 mediated cell death

Infection with human immunodeficiency virus (HIV) is characterized by the gradual depletion of CD4+ T lymphocytes. The incorporation of the concept of apoptosis as a rationale to explain progressive T cell depletion has led to growing research in this field during the last 10 years. In parallel, the biochemical pathways implicated in programmed cell death have been extensively studied. Thus, the influence of mitochondrial control in the two major apoptotic pathways-the extrinsic and intrinsic pathways-is now well admitted. In this review, we summarized our current knowledge of the different pathways involved in the death of T cells in the course of HIV infection.

Mechanisms of CD4+ T lymphocyte cell death in human immunodeficiency virus infection and AIDS

AIDS, caused by the retroviruses human immunodeficiency virus type 1 and type 2 (HIV-1 and HIV-2), has reached pandemic proportions. Therefore, it is critical to understand how HIV causes AIDS so that appropriate therapies can be formulated. Primarily, HIV infects and kills CD4(+) T lymphocytes, which function as regulators and amplifiers of the immune response. In the absence of effective anti-retroviral therapy, the hallmark decrease in CD4(+) T lymphocytes during AIDS results in a weakened immune system, impairing the body's ability to fight infections or certain cancers such that death eventually ensues. The major mechanism for CD4(+) T cell depletion is programmed cell death (apoptosis), which can be induced by HIV through multiple pathways. Death of HIV-infected cells can result from the propensity of infected lymphocytes to form short-lived syncytia or from an increased susceptibility of the cells to death. However, the apoptotic cells appear to be primarily uninfected bystander cells and are eradicated by two different mechanisms: either a Fas-mediated mechanism during activation-induced cell death (AICD), or as a result of HIV proteins (Tat, gp120, Nef, Vpu) released from infected cells stimulating apoptosis in uninfected bystander cells. There is also evidence that as AIDS progresses cytokine dysregulation occurs, and the overproduction of type-2 cytokines (IL-4, IL-10) increases susceptibility to AICD whereas type-1 cytokines (IL-12, IFN-gamma) may be protective. Clearly there are multiple causes of CD4(+) T lymphocyte apoptosis in AIDS and therapies that block or decrease that death could have significant clinical benefit.

Mitochondria in HIV-1-induced apoptosis

It is now well admitted that HIV infection leading to AIDS is associated with an abnormal susceptibility of T cells to undergo apoptosis. Recent progress in research into programmed cell death has resulted in the identification of the principal pathways involved in this process. Thus the "extrinsic" as well as the "intrinsic" pathways converge to the mitochondria considered as the main sensor of programmed cell death. This review summarizes our knowledge of the influence of mitochondrial control on T cell death during HIV and SIV infections.

Apoptosis in HIV-1 infection

Apoptosis is a key event in biologic homeostasis with particular importance to the immune system. It is an active energy-dependent process that is tightly regulated and controlled by a variety of signal transduction pathways. Apoptosis modulation plays a part in the pathogenesis of many human diseases, including HIV infection. Although multiple mechanisms may contribute to the decline in CD4 T-lymphocyte numbers observed, apoptosis is a significant factor. Alterations in levels of apoptosis are observed in both directly infected and uninfected bystander cells and a variety of pathways of apoptosis induction have been implicated. Apoptosis induction is related to death receptor and mitochondrial-induced pathways in specific circumstances. These events have been linked to individual HIV proteins and have been demonstrated to be altered by antiretroviral therapy.

Promises and pitfalls in the reconstitution of immunity in patients who have HIV-1 infection

Recent studies suggest that immunologic containment of HIV-1 is possible and the immune correlates of this control are being defined. There have been significant advances in our ability to elicit virus-specific immune responses, which have led to viral control in animal models. Although viral escape from the immune system is an important challenge, new strategies for therapeutic and preventative HIV-1 vaccination are being vigorously pursued in human trials.

Increased CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells

Why HIV-specific CD8(+) T cells ultimately fail to clear or control HIV infection is not known. We show here that HIV-specific CD8(+) T cells exhibit increased sensitivity to CD95/Fas-induced apoptosis. This apoptosis is 3-fold higher compared to CMV-specific CD8(+) T cells from the same patients. HIV-specific CD8(+) T cells express the CD45RA(-)CD62L(-) but lack the CD45RA(+)CD62L(-) T cell effector memory (T(EM)) phenotype. This skewing is not found in CMV- and EBV-specific CD8(+) T cells in HIV-infected individuals. CD95/Fas-induced apoptosis is much higher in the CD45RA(-)CD62L(-) T(EM) cells. However, cytotoxicity and IFNgamma production by HIV-specific CD8(+) T cells is not impaired. Our data suggest that the survival and differentiation of HIV-specific CD8(+) T cells may be compromised by CD95/Fas apoptosis induced by FasL-expressing HIV-infected cells.

CD4(+) and CD8(+) T cell death during human immunodeficiency virus infection in vitro

We have evaluated the death of CD4(+) and CD8(+) T cells during in vitro human immunodeficiency virus (HIV) infection of peripheral blood mononuclear cells (PBMC) and tonsilar tissue. Acute infections with several X4 and R5 HIV isolates induced a decrease in cell viability that was higher in infections with X4 viruses and correlated with an increased rate of CD4(+) T-cell death. In CD4(+) T cells, the primary X4 isolate AOM induced higher levels of death than the laboratory X4 isolates IIIB and NL4-3 or the R5 isolates BaL and MDM. An effect on CD8(+) T-cell viability was exclusively observed in infections by X4 viruses, including the NL4-3 strain, in both PBMC and tonsilar tissue. This effect was dependent on the env gene of the infecting isolate and required productive HIV replication in CD4(+) but not in CD8(+) T cells. Our results suggest that X4 and R5 HIV isolates depleted CD4(+) T cells to a different extent and that CD8(+) T-cell viability may also be affected by mechanisms other than those acting in CD4(+) T cells.

Lack of coreceptor allows survival of chronically stimulated double-negative alpha/beta T cells: implications for autoimmunity

Lymphoproliferative diseases are characterized by massive accumulation of CD4(-)CD8(-)B220(+) (double-negative [DN]) T cells in peripheral organs. Although evidence indicates these cells are derived from mature autoreactive alpha/beta T cells, the significance of coreceptor downregulation is not known. In this study, we examined the role CD4 coreceptor plays in the survival of repeatedly stimulated T cells. CD4(+/+) and CD4(-/-) T cells from AND T cell receptor (TCR) transgenic mice exhibited similar phenotypes after antigenic stimulation, but the CD4(-/-) T cells survived in much larger numbers than the CD4(+/+) cells upon primary and secondary major histocompatibility complex (MHC)/peptide stimulation. Enhanced survival of CD4(-/-) T cells was due to decreased apoptosis rather than enhanced proliferation. Similarly, circumvention of the CD4/MHC interaction by using a surrogate TCR ligand that does not engage CD4 led to significant enhancement of CD4(+/+) cells than when stimulated with MHC/peptide. Finally, we generated DN B220(+) T cells using an in vitro model system and showed they were more tolerant to chronic stimulation than CD4(+/+) cells. Together, these results indicate that coreceptor engagement controls expansion of normal T cells. In the absence of coreceptor, T cells survive chronic stimulation and express B220 as seen in autoimmune lymphoproliferative diseases.

Macrophages and T-cell apoptosis in HIV infection: a leading role for accessory cells?

Recent studies indicate that macrophages modulate T-cell apoptosis in HIV infection. Macrophages have been shown to trigger apoptosis of uninfected bystander T cells and to protect HIV-infected T cells from apoptosis. This article raises the possibility that macrophages, via modulation of T-cell apoptosis, play a crucial role in both immune suppression and the formation of viral reservoirs during HIV infection.