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

Increased platelet-CD8+ T-cell aggregates displaying high activation, exhaustion, and tendency to death correlate with disease progression in people with HIV-1

Platelets engage in HIV-1 infection by interacting with immune cells, which has been realized broadly. However, the potential interaction between platelets and CD8+ T cells remains unidentified. Here, treatment-naive individuals with HIV-1, complete immunological responders to antiretroviral therapy, and healthy controls were enrolled. First, we found that treatment-naive individuals with HIV-1 had low platelet numbers and high CD8+ T-cell counts when compared with complete immunological responders to antiretroviral therapy and healthy controls, leading to a low platelet/CD8+ T-cell ratio in peripheral blood, which could effectively differentiate the status of HIV-1 infection. Moreover, cytokines that may have been derived from platelets were higher in the plasma of people with HIV-1 despite viral suppression. Furthermore, we demonstrated that platelet-CD8+ T-cell aggregates were elevated in treatment-naive individuals with HIV-1, which positively correlated with HIV-1 viral load but negatively correlated with CD4+ T-cell count and CD4/CD8 ratio. Finally, we revealed that platelet-CD8+ T-cell aggregates correlate with enhanced activation/exhaustion and pyroptosis/apoptosis compared with free CD8+ T cells. Moreover, platelet-induced caspase 1 activation of CD8+ T cells correlated with IL-1β and IL-18 plasma levels. In brief, we reveal the importance of platelets in HIV-1 infection, which might secrete more cytokines and mediate CD8+ T-cell phenotypic characteristics by forming platelet-CD8+ T-cell aggregates, which are related to poor prognosis.

The Role of Bcl-xL Protein in Viral Infections

Bcl-xL represents a family of proteins responsible for the regulation of the intrinsic apoptosis pathway. Due to its anti-apoptotic activity, Bcl-xL co-determines the viability of various virally infected cells. Their survival may determine the effectiveness of viral replication and spread, dynamics of systemic infection, and viral pathogenesis. In this paper, we have reviewed the role of Bcl-xL in the context of host infection by eight different RNA and DNA viruses: hepatitis B virus (HBV), hepatitis C virus (HCV), human immunodeficiency virus (HIV), influenza A virus (IAV), Epstein-Barr virus (EBV), human T-lymphotropic virus type-1 (HTLV-1), Maraba virus (MRBV), Schmallenberg virus (SBV) and coronavirus (CoV). We have described an influence of viral infection on the intracellular level of Bcl-xL and discussed the impact of Bcl-xL-dependent cell survival control on infection-accompanying pathogenic events such as tissue damage or oncogenesis. We have also presented anti-viral treatment strategies based on the pharmacological regulation of Bcl-xL expression or activity.

Rapid in vitro generation of bona fide exhausted CD8+ T cells is accompanied by Tcf7 promotor methylation

Exhaustion is a dysfunctional state of cytotoxic CD8+ T cells (CTL) observed in chronic infection and cancer. Current in vivo models of CTL exhaustion using chronic viral infections or cancer yield very few exhausted CTL, limiting the analysis that can be done on these cells. Establishing an in vitro system that rapidly induces CTL exhaustion would therefore greatly facilitate the study of this phenotype, identify the truly exhaustion-associated changes and allow the testing of novel approaches to reverse or prevent exhaustion. Here we show that repeat stimulation of purified TCR transgenic OT-I CTL with their specific peptide induces all the functional (reduced cytokine production and polyfunctionality, decreased in vivo expansion capacity) and phenotypic (increased inhibitory receptors expression and transcription factor changes) characteristics of exhaustion. Importantly, in vitro exhausted cells shared the transcriptomic characteristics of the gold standard of exhaustion, CTL from LCMV cl13 infections. Gene expression of both in vitro and in vivo exhausted CTL was distinct from T cell anergy. Using this system, we show that Tcf7 promoter DNA methylation contributes to TCF1 downregulation in exhausted CTL. Thus this novel in vitro system can be used to identify genes and signaling pathways involved in exhaustion and will facilitate the screening of reagents that prevent/reverse CTL exhaustion.

The Role of the BCL-2 Family of Proteins in HIV-1 Pathogenesis and Persistence

Advances in HIV-1 therapy have transformed the once fatal infection into a manageable, chronic condition, yet the search for a widely applicable approach to cure remains elusive. The ineffectiveness of antiretroviral therapy (ART) in reducing the size of the HIV-1 latent reservoir has prompted investigation into the mechanisms of HIV-1 latency and immune escape. One of the major regulators of apoptosis, the BCL-2 protein, alongside its homologous family members, is a major target of HIV-1-induced change. Recent studies have now demonstrated the association of this protein with cells that support proviral forms in the setting of latency and have helped identify BCL-2 as a novel and promising therapeutic target for HIV-1 therapy directed at possible cure. This review aims to systematically review the interactions of HIV-1 with BCL-2 and its homologs and to examine the possibility of using BCL-2 inhibitors in the study and elimination of the latent reservoir.

Metabolic plasticity of HIV-specific CD8+ T cells is associated with enhanced antiviral potential and natural control of HIV-1 infection

Spontaneous control of human immunodeficiency virus (HIV) is generally associated with an enhanced capacity of CD8⁺ T cells to eliminate infected CD4⁺ T cells, but the molecular characteristics of these highly functional CD8⁺ T cells are largely unknown. In the present study, using single-cell analysis, it was shown that HIV-specific, central memory CD8⁺ T cells from spontaneous HIV controllers (HICs) and antiretrovirally treated non-controllers have opposing transcriptomic profiles. Genes linked to effector functions and survival are upregulated in cells from HICs. In contrast, genes associated with activation, exhaustion and glycolysis are upregulated in cells from non-controllers. It was shown that HIV-specific CD8⁺ T cells from non-controllers are largely glucose dependent, whereas those from HICs have more diverse metabolic resources that enhance both their survival potential and their capacity to develop anti-HIV effector functions. The functional efficiency of the HIV-specific CD8⁺ T cell response in HICs is thus engraved in their memory population and related to their metabolic programme. Metabolic reprogramming in vitro through interleukin-15 treatment abrogated the glucose dependency and enhanced the antiviral potency of HIV-specific CD8⁺ T cells from non-controllers.

IL-15 regulates susceptibility of CD4+ T cells to HIV infection

HIV integrates into the host genome to create a persistent viral reservoir. Stimulation of CD4⁺ memory T lymphocytes with common γc-chain cytokines renders these cells more susceptible to HIV infection, making them a key component of the reservoir itself. IL-15 is up-regulated during primary HIV infection, a time when the HIV reservoir established. Therefore, we investigated the molecular and cellular impact of IL-15 on CD4⁺ T-cell infection. We found that IL-15 stimulation induces SAM domain and HD domain-containing protein 1 (SAMHD1) phosphorylation due to cell cycle entry, relieving an early block to infection. Perturbation of the pathways downstream of IL-15 receptor (IL-15R) indicated that SAMHD1 phosphorylation after IL-15 stimulation is JAK dependent. Treating CD4⁺ T cells with Ruxolitinib, an inhibitor of JAK1 and JAK2, effectively blocked IL-15-induced SAMHD1 phosphorylation and protected CD4⁺ T cells from HIV infection. Using high-resolution single-cell immune profiling using mass cytometry by TOF (CyTOF), we found that IL-15 stimulation altered the composition of CD4⁺ T-cell memory populations by increasing proliferation of memory CD4⁺ T cells, including CD4⁺ T memory stem cells (T_SCM). IL-15-stimulated CD4⁺ T_SCM, harboring phosphorylated SAMHD1, were preferentially infected. We propose that IL-15 plays a pivotal role in creating a self-renewing, persistent HIV reservoir by facilitating infection of CD4⁺ T cells with stem cell-like properties. Time-limited interventions with JAK1 inhibitors, such as Ruxolitinib, should prevent the inactivation of the endogenous restriction factor SAMHD1 and protect this long-lived CD4⁺ T-memory cell population from HIV infection.

Functional Expression of Programmed Death-Ligand 1 (B7-H1) by Immune Cells and Tumor Cells

The programmed death-1 (PD-1) and its ligand PD-L1 (B7-H1) signaling pathway has been the focus of much enthusiasm in the fields of tumor immunology and oncology with recent FDA approval of the anti-PD-1 antibodies pembrolizumab and nivolumab and the anti-PD-L1 antibodies durvalumab, atezolimuab, and avelumab. These therapies, referred to here as PD-L1/PD-1 checkpoint blockade therapies, are designed to block the interaction between PD-L1, expressed by tumor cells, and PD-1, expressed by tumor-infiltrating CD8⁺ T cells, leading to enhanced antitumor CD8⁺ T cell responses and tumor regression. The influence of PD-L1 expressed by tumor cells on antitumor CD8⁺ T cell responses is well characterized, but the impact of PD-L1 expressed by immune cells has not been well defined for antitumor CD8⁺ T cell responses. Although PD-L1 expression by tumor cells has been used as a biomarker in selection of patients for PD-L1/PD-1 checkpoint blockade therapies, patients whose tumor cells lack PD-L1 expression often respond positively to PD-L1/PD-1 checkpoint blockade therapies. This suggests that PD-L1 expressed by non-malignant cells may also contribute to antitumor immunity. Here, we review the functions of PD-L1 expressed by immune cells in the context of CD8⁺ T cell priming, contraction, and differentiation into memory populations, as well as the role of PD-L1 expressed by tumor cells in regulating antitumor CD8⁺ T cell responses.

Innate Lymphoid Cells Are Depleted Irreversibly during Acute HIV-1 Infection in the Absence of Viral Suppression

Innate lymphoid cells (ILCs) play a central role in the response to infection by secreting cytokines crucial for immune regulation, tissue homeostasis, and repair. Although dysregulation of these systems is central to pathology, the impact of HIV-1 on ILCs remains unknown. We found that human blood ILCs were severely depleted during acute viremic HIV-1 infection and that ILC numbers did not recover after resolution of peak viremia. ILC numbers were preserved by antiretroviral therapy (ART), but only if initiated during acute infection. Transcriptional profiling during the acute phase revealed upregulation of genes associated with cell death, temporally linked with a strong IFN acute-phase response and evidence of gut barrier breakdown. We found no evidence of tissue redistribution in chronic disease and remaining circulating ILCs were activated but not apoptotic. These data provide a potential mechanistic link between acute HIV-1 infection, lymphoid tissue breakdown, and persistent immune dysfunction.

Modulation of apoptosis and viral latency - an axis to be well understood for successful cure of human immunodeficiency virus

Human immunodeficiency virus (HIV) is the causative agent of the deadly disease AIDS, which is characterized by the progressive decline of CD4(+)T-cells. HIV-1-encoded proteins such as envelope gp120 (glycoprotein gp120), Tat (trans-activator of transcription), Nef (negative regulatory factor), Vpr (viral protein R), Vpu (viral protein unique) and protease are known to be effective in modulating host cell signalling pathways that lead to an alteration in apoptosis of both HIV-infected and uninfected bystander cells. Depending on the stage of the virus life cycle and host cell type, these viral proteins act as mediators of pro- or anti-apoptotic signals. HIV latency in viral reservoirs is a persistent phenomenon that has remained beyond the control of the human immune system. To cure HIV infections completely, it is crucial to reactivate latent HIV from cellular pools and to drive these apoptosis-resistant cells towards death. Several previous studies have reported the role of HIV-encoded proteins in apoptosis modulation, but the molecular basis for apoptosis evasion of some chronically HIV-infected cells and reactivated latently HIV-infected cells still needs to be elucidated. The current review summarizes our present understanding of apoptosis modulation in HIV-infected cells, uninfected bystander cells and latently infected cells, with a focus on highlighting strategies to activate the apoptotic pathway to kill latently infected cells.

Dysfunctional HIV-specific CD8+ T cell proliferation is associated with increased caspase-8 activity and mediated by necroptosis

Decreased HIV-specific CD8(+) T cell proliferation is a hallmark of chronic infection, but the mechanisms of decline are unclear. We analyzed gene expression profiles from antigen-stimulated HIV-specific CD8(+) T cells from patients with controlled and uncontrolled infection and identified caspase-8 as a correlate of dysfunctional CD8(+) T cell proliferation. Caspase-8 activity was upregulated in HIV-specific CD8(+) T cells from progressors and correlated positively with disease progression and programmed cell death-1 (PD-1) expression, but negatively with proliferation. In addition, progressor cells displayed a decreased ability to upregulate membrane-associated caspase-8 activity and increased necrotic cell death following antigenic stimulation, implicating the programmed cell death pathway necroptosis. In vitro necroptosis blockade rescued HIV-specific CD8(+) T cell proliferation in progressors, as did silencing of necroptosis mediator RIPK3. Thus, chronic stimulation leading to upregulated caspase-8 activity contributes to dysfunctional HIV-specific CD8(+) T cell proliferation through activation of necroptosis and increased cell death.

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.

Involvement of GRIM-19 in apoptosis induced in H5N1 virus-infected human macrophages

The fatal H5N1 infection has a high mortality rate among infected patients. The pathogenesis of H5N1 viral infection is associated with the ability of the virus to induce apoptotic cell death. However, the molecular mechanism of apoptosis induced by H5N1 remains unclear. In the present study we demonstrate that H5N1 virus is able to up-regulate the expression of gene associated with retinoid and interferon induced mortality-19 (GRIM-19) in human monocyte-derived macrophages (hMDMs). GRIM-19 has been identified as a novel gene with apoptotic effects in virus-infected cells. The percentage of apoptotic cells is significantly decreased in H5N1-infected GRIM-19 depleted hMDMs, which is also associated with a decrease of BH3-interacting domain death agonist cleavage and apoptosis-inducing factor (AIF) release to the cytosol. These results suggested the involvement of GRIM-19 in apoptosis induced by H5N1 virus. Furthermore, neutralizing-IFN-β Ab is able to suppress GRIM-19 expression in H5N1-infected cells resulting in a decrease in apoptotic cell number, indicating that IFN-β secreted by H5N1-infected hMDMs regulates GRIM-19 expression leading to apoptosis. Altogether, the results presented here provide additional insight on the regulatory mechanism of H5N1 viral-induced apoptotic cell death in hMDMs.

HIV-specific CD8+ T cells from elite controllers are primed for survival

HIV-specific cytotoxic T lymphocytes (CTL) are preferentially primed for apoptosis, and this may represent a viral escape mechanism. We hypothesized that HIV-infected individuals that control virus to undetectable levels without antiretroviral therapy (ART) (elite controllers [EC]) have the capacity to upregulate survival factors that allow them to resist apoptosis. To address this, we performed cross-sectional and longitudinal analysis of proapoptotic (cleaved caspase-3) and antiapoptotic (Bcl-2) markers of cytomegalovirus (CMV) and HIV-specific CD8 T cells in a cohort of HIV-infected subjects with various degrees of viral control on and off ART. We demonstrated that HIV-specific CTL from EC are more resistant to apoptosis than those with pharmacologic control (successfully treated patients [ST]), despite similar in vivo conditions. Longitudinal analysis of chronically infected persons starting ART revealed that the frequency of HIV-specific T cells prone to death decreased, suggesting that this phenotype is partially reversible even though it never achieves the levels present in EC. Elucidating the apoptotic factors contributing to the survival of CTL in EC is paramount to our development of effective HIV-1 vaccines. Furthermore, a better understanding of cellular markers that can be utilized to predict response durability in disease- or vaccine-elicited responses will advance the field.

The cytokine network of acute HIV infection: a promising target for vaccines and therapy to reduce viral set-point?

Cytokines play a central role in the pathogenesis of many diseases, including HIV infection. However, the role of the cytokine network in early HIV infection is only now starting to be elucidated. A number of studies conducted in recent years have indicated that cytokines of the acute/early stages of HIV and SIV infection can impact viral set-point months later, and this is of critical importance since viral set-point during chronic HIV infection affects virus transmission and disease progression. This raises the question whether modulating the cytokine environment during acute/early HIV infection can be a target for novel approaches to develop a vaccine and therapeutics. In this review we focus on the kinetics and function of cytokines during acute HIV and SIV infection and how these may impact viral set-point. We also discuss unresolved questions that are essential for our understanding of the role of acute infection cytokines in HIV infection and that, if answered, may suggest novel therapeutic and vaccine strategies to control the worldwide HIV pandemic.

T cells expanded in presence of IL-15 exhibit increased antioxidant capacity and innate effector molecules

Persistence of effector cytotoxic T lymphocytes (CTLs) during an immunological response is critical for successfully controlling a viral infection or tumor growth. Various cytokines are known to play an important part in regulating the immune response. The IL-2 family of cytokines that includes IL-2 and IL-15 are known to function as growth and survival factors for antigen-experienced T cells. IL-2 and IL-15 possess similar properties, including the ability to induce T cell proliferation. Whereas long-term IL-2 exposure has been shown to promote apoptosis and limit CD8(+) memory T cell survival and proliferation, it is widely believed that IL-15 can inhibit apoptosis and helps maintain a memory CD8(+) T-cell population. However, mechanisms for superior outcomes for IL-15 as compared to IL-2 are still under investigation. Our data shows that human T cells cultured in the presence of IL-15 exhibit increased expression of anti-oxidant molecules glutathione reductase (GSR), thioredoxin reductase 1 (TXNDR1), peroxiredoxin (PRDX) and superoxide dismutase (SOD). An increased expression of cell-surface thiols, intracellular glutathione, and thioredoxins was also noted in IL-15 cultured T cells. Additionally, IL-15 cultured T cells showed an increase in cytolytic effector molecules. Apart from increased level of Granzyme A and Granzyme B, IL-15 cultured T cells exhibited increased accumulation of reactive oxygen (ROS) and reactive nitrogen species (RNS) as compared to IL-2 cultured T cells. Overall, this study suggests that T cells cultured in IL-15 show increased persistence not only due to levels of anti-apoptotic proteins, but also due to increased anti-oxidant levels, which is complimented by increased cytolytic effector functions.

Blood myeloid dendritic cells from HIV-1-infected individuals display a proapoptotic profile characterized by decreased Bcl-2 levels and by caspase-3+ frequencies that are associated with levels of plasma viremia and T cell activation in an exploratory study

Reduced frequencies of myeloid and plasmacytoid dendritic cell (DC) subsets (mDCs and pDCs, respectively) have been observed in the peripheral blood of HIV-1-infected individuals throughout the course of disease. Accumulation of DCs in lymph nodes (LNs) may partly account for the decreased numbers observed in blood, but increased DC death may also be a contributing factor. We used multiparameter flow cytometry to evaluate pro- and antiapoptotic markers in blood mDCs and pDCs from untreated HIV-1-infected donors, from a subset of infected donors before and after receiving antiretroviral therapy (ART), and from uninfected control donors. Blood mDCs, but not pDCs, from untreated HIV-1-infected donors expressed lower levels of antiapoptotic Bcl-2 than DCs from uninfected donors. A subset of HIV-1-infected donors had elevated frequencies of proapoptotic caspase-3(+) blood mDCs, and positive correlations were observed between caspase-3(+) mDC frequencies and plasma viral load and CD8(+) T-cell activation levels. Caspase-3(+) mDC frequencies, but not mDC Bcl-2 expression, were reduced with viral suppression on ART. Apoptosis markers on DCs in blood and LN samples from a cohort of untreated, HIV-1-infected donors with chronic disease were also evaluated. LN mDCs displayed higher levels of Bcl-2 and lower caspase-3(+) frequencies than did matched blood mDCs. Conversely, LN pDCs expressed lower Bcl-2 levels than their blood counterparts. In summary, blood mDCs from untreated HIV-1-infected subjects displayed a proapoptotic profile that was partially reversed with viral suppression, suggesting that DC death may be a factor contributing to blood DC depletion in the setting of chronic, untreated HIV disease.

Contribution of astrocyte-derived IL-15 to CD8 T cell effector functions in multiple sclerosis

The contribution of local factors to the activation of immune cells infiltrating the CNS of patients with multiple sclerosis (MS) remains to be defined. The cytokine IL-15 is pivotal in the maintenance and activation of CD8 T lymphocytes, a prominent lymphocyte population found in MS lesions. We investigated whether astrocytes are a functional source of IL-15 sufficient to enhance CD8 T lymphocyte responses and whether they provide IL-15 in the inflamed CNS of patients with MS. We observed that human astrocytes in primary cultures increased surface IL-15 levels upon activation with combinations of proinflammatory cytokines. Expanded human myelin autoreactive CD8 T lymphocytes cultured with such activated astrocytes displayed elevated lytic enzyme content, NKG2D expression, and Ag-specific cytotoxicity. These functional enhancements were abrogated by anti-IL-15-blocking Abs. Immunohistochemical analysis of brain tissue sections obtained from patients with MS demonstrated colocalization for IL-15 and the astrocyte marker glial fibrillary acidic protein within white matter lesions. The majority of astrocytes (80-90%) present in demyelinating MS lesions expressed IL-15, whereas few astrocytes in normal control brain sections had detectable IL-15. IL-15 could be detected in the majority of Iba-1-expressing microglia in the control sections, albeit in lower numbers when compared with microglia/macrophages in MS lesions. Furthermore, infiltrating CD8 T lymphocytes in MS lesions were in close proximity to IL-15-expressing cells. Astrocyte production of IL-15 resulting in the activation of CD8 T lymphocytes ascribes a role for these cells as contributors to the exacerbation of tissue damage during MS pathogenesis.

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.

New insights on the role of apoptosis and autophagy in HIV pathogenesis

Viruses manipulate host cells to ensure their own survival and, at late stages of the viral life cycle, they kill the infected target cell to ensure their propagation. In addition, some viruses induce a bystander killing, a viral strategy to escape from the host's innate and cognate defense systems. In HIV-infection, the disabling of the immune system is initially due to the preferential depletion by apoptosis of virus-specific CD4(+) T cells in lymphoid tissues, followed by the destruction of non-infected bystander cells. Both the extrinsic and the intrinsic pathways are activated, and this is the consequence of systemic immune activation. This review presents recent developments showing that the gastrointestinal tract is the major reservoir of infected cells and the site of rapid and profound loss of CD4 T cells, and that microbial translocation from the gastrointestinal tract is the cause of immune activation. Furthermore, apoptosis mechanisms involved in HIV-induced neuropathological disorders are discussed, including the role of syncytia that involve the sequential activation of ATM, p38MAPK and p53. Finally, HIV-associated dementia (HAD) was recently found in monkey models to be linked to inhibition of autophagy in neurons, suggesting that homeostasis of autophagy is a reliable security factor for neurons, and challenging the development of new therapeutics aimed at boosting neuronal autophagy to prevent HAD.

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Distinctive in vitro effects of T-cell growth cytokines on cytomegalovirus-stimulated T-cell responses of HIV-infected HAART recipients

Functional immune reconstitution is limited after HAART, maintaining the interest in adjunctive immune-modulators. We compared in vitro the effects of the gamma-chain T-cell growth cytokines IL-2, IL-4, IL-7 and IL-15 on cytomegalovirus-stimulated cell-mediated immunity. IL-2 and IL-15 increased cytomegalovirus-specific lymphocyte proliferation in HAART recipients, whereas IL-4 and IL-7 did not. The boosting effect of IL-2 and IL-15 on proliferation correlated with their ability to prevent late apoptosis. However, IL-2 increased the frequency of cells in early apoptosis, whereas IL-15 increased the frequency of fully viable cells. Both IL-2 and IL-15 increased cytomegalovirus-induced CD4+ and CD8+ T-cell proliferation and the synthesis of Th1 and pro-inflammatory cytokines and chemokines. However, only IL-2 increased the frequency of regulatory T cells and Th2 cytokine production, both of which have the potential to attenuate antiviral immune responses. Overall, compared to other gamma-chain cytokines, IL-15 had the most favorable profile for boosting antiviral cell-mediated immunity.

Subcellular expression pattern and role of IL-15 in pneumococci induced lung epithelial apoptosis

Streptococcus pneumoniae is the leading causative agent of community-acquired pneumonia. Induction of apoptosis in pulmonary epithelial cells by bacteria during pneumonia might be harmful to the host. Interleukin-15 (IL-15) has been demonstrated as an effective inhibitor of apoptosis and is expressed in lung epithelium on the mRNA and protein level. Therefore, we characterized the sub-cellular expression pattern of the short and long IL-15 isoforms in lung epithelial cells in vitro as well as its role in pneumococci-related lung epithelial cell apoptosis. We found an expression pattern for both IL-15 signal peptides in the pulmonary epithelial cell lines A549 and Beas-2B. Moreover, a strong co-localization of IL-15 and IL-15Ralpha was detected on cell surfaces. Compared to pro-inflammatory cytokine stimulation, neither IL-15 nor its trimeric receptor complex was up-regulated after pneumococcal infection. However, overexpression of IL-15 isoforms revealed IL-15LSP and IL-15Vkl as inhibitors of pneumococci induced apoptosis in pulmonary epithelial cells. Thus, IL-15 may act as an anti-apoptotic molecule in pneumococci infection, thereby suggesting IL-15 as a benefical cytokine in pulmonary host defense against infection.

IL-15 treatment during acute simian immunodeficiency virus (SIV) infection increases viral set point and accelerates disease progression despite the induction of stronger SIV-specific CD8+ T cell responses

In this study, we examined the effect of in vivo treatment of acutely SIV-infected Mamu-A*01+ rhesus macaques with IL-15. IL-15 treatment during acute infection increased viral set point by 3 logs and accelerated the development of simian AIDS in two of six animals with one developing early minimal lesion SIV meningoencephalitis. Although IL-15 induced a 2- to 3-fold increase in SIV-specific CD8+ T cell and NK cell numbers at peak viremia and reduced lymph node (LN) SIV-infected cells, this had no impact on peak viremia and did not lower viral set point. At viral set point, however, activated SIV-specific CD8+ T cells and NK cells were reduced in the blood of IL-15-treated animals and LN SIV-infected cells were increased. Week 30 LN from IL-15-treated animals had significantly increased Gag-specific CD8+ T cell numbers, whereas total cell, lymphocyte, and CD4+ T cell numbers were reduced. IL-15 treatment significantly reduced anti-SIV Ab concentrations at week 3 and viral set point. IL-15 increased Ki-67+CD4+ T cells at week 1 of treatment and reduced blood CCR5+ and CD45RA-CD62L- CD4+ T cells. The frequency of day 7 Ki-67+CD4+ T cells strongly correlated with viral set point. These findings suggest that CD4+ T cell activation during acute infection determines subsequent viral set point and IL-15 treatment by increasing such activation elevates viral set point. Finally, IL-15-treated acutely SIV-infected primates may serve as a useful model to investigate the poorly understood mechanisms that control viral set point and disease progression in HIV infection.

Actin integrity is indispensable for CD95/Fas-induced apoptosis of HIV-specific CD8+ T cells

We have recently provided data suggesting a potential role for mitochondria and Bcl-2-family molecules in apoptosis sensitivity of HIV-specific CD8+ T cells. Here, we report on the role of filamentous (F) actin in this process. Disruption of actin by cytochalasin D (cytD) or lantrunculin A remarkably reduced CD95/Fas-induced apoptosis of HIV-specific CD8+ T cells while their spontaneous apoptosis was unaffected. This inhibition cannot be attributed to changes of CD95/Fas distribution or levels in these cells. Furthermore, cytD treatment reduced CD95/Fas-induced apoptosis of CD8+ T cells from HIV+ patients independently of their differentiation status. CD95/Fas-induced apoptosis of both CD38+ and CD38- HIV-specific CD8+ T cells was inhibited by cytD treatment indicating that actin mediates this apoptotic process independently of the activation level of these cells. CytD was found to reduce the activation of caspase-8 induced by short treatment of purified CD8+ T cells from HIV+ patients with anti-CD95/Fas. Our data reveal actin as a critical mediator of HIV-specific CD8+ T cell apoptosis; further analysis of the molecular mechanisms governing this process may potentially contribute to design new therapies targeting the enhancement of the immune system in HIV infection.

Live attenuated lentivirus infection elicits polyfunctional simian immunodeficiency virus Gag-specific CD8+ T cells with reduced apoptotic susceptibility in rhesus macaques that control virus replication after challenge with pathogenic SIVmac239

HIV-specific CD8+ T cells that secrete multiple cytokines in response to Ag stimulation are associated with the control of virus replication during chronic HIV infection. To determine whether the presence of polyfunctional CD8+ T cell responses distinguishes protected and unprotected monkeys in a live attenuated lentivirus model, SIV Gag peptide-specific CD8+ T cell responses of simian HIV (SHIV) 89.6-vaccinated, SIVmac239-challenged rhesus macaques were compared in two monkeys that controlled challenge virus replication and two that did not. The ratio of Bcl-2+ Gag-specific CD8+ T cells to caspase-3+ Gag-specific CD8+ T cells was higher in the vaccinated-protected animals compared with unprotected monkeys. In addition, polyfunctional SIV-specific CD8+ T cells were consistently detected through 12 wk postchallenge in the protected animals but not in the unprotected animals. In the unprotected monkeys, there was an increased frequency of CD8+ T cells expressing markers associated with effector memory T cells. Further, there was increased annexin V expression in central memory T cells of the unprotected animals before challenge. Thus, monkeys that control viral replication after live attenuated SHIV infection have polyfunctional SIV-specific CD8+ T cells with an increased survival potential. Importantly, the differences in the nature of the SIV-specific CD8+ T cell response in the protected and unprotected animals are present during acute stages postchallenge, before different antigenic levels are established. Thus, the polyfunctional capacity and increased survival potential of CD8+ SIV-specific T cells may account for live attenuated, SHIV89.6-mediated protection from uncontrolled SIV replication.

Early establishment and antigen dependence of simian immunodeficiency virus-specific CD8+ T-cell defects

Differentiation and survival defects of human immunodeficiency virus (HIV)-specific CD8(+) T cells may contribute to the failure of HIV-specific CD8(+) T cells to control HIV replication. It is not known, however, whether simian immunodeficiency virus (SIV)-infected rhesus macaques show comparable defects in these virus-specific CD8(+) T cells or when such defects are established during infection. Peripheral blood cells from acutely and chronically infected rhesus macaques were stained ex vivo for memory subpopulations and examined by in vitro assays for apoptosis sensitivity. We show here that SIV-specific CD8(+) T cells from chronically SIV infected rhesus macaques show defects comparable to those observed in HIV infection, namely, a skewed CD45RA(-) CD62L(-) effector memory phenotype, reduced Bcl-2 levels, and increased levels of spontaneous and CD95-induced apoptosis of SIV-specific CD8(+) T cells. Longitudinal studies showed that the survival defects and phenotype are established early in the first few weeks of SIV infection. Most importantly, they appear to be antigen driven, since most probably the loss of epitope recognition due to viral escape results in the reversal of the phenotype and reduced apoptosis sensitivity, something we observed also for animals treated with antiretroviral therapy. These findings further support the use of SIV-infected rhesus macaques to investigate the phenotypic changes and apoptotic defects of HIV-specific CD8(+) T cells and indicate that such defects of HIV-specific CD8(+) T cells are the result of chronic antigen stimulation.

Increased mitochondrial mass characterizes the survival defect of HIV-specific CD8(+) T cells

What governs the increased apoptosis sensitivity of HIV-specific CD8(+) T cells is poorly understood. Here, we examined the involvement of mitochondria in this apoptosis. Remarkably higher mitochondrial mass (MM) was found in HIV-specific compared with CMV-specific CD8(+) T cells from HIV(+) patients and this could not be attributed to their different differentiation status. MM(High) phenotype characterized those CD8(+) T cells from HIV(+) patients that are sensitive to spontaneous and CD95/Fas-induced apoptosis. CD38 expression did not correlate with high MM, whereas Bcl-2 levels were significantly reduced in both CD38(+) and CD38(-) HIV-specific CD8(+) T cells. Although CD38(+) HIV-specific CD8(+) T cells were more susceptible to apoptosis, CD38 expression does not explain on its own the selective apoptosis sensitivity of HIV-specific CD8(+) T cells, as CD38(-) HIV-specific CD8(+) T cells were more apoptotic than CD38(+) CMV-specific ones. Proapoptotic HIV-specific CD8(+) T cells were CD38(+)Bcl-2(Low)MM(High). Copolarization of mitochondria with CD95/Fas capping, very early in CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells, suggests that mitochondria act as an amplification step for this apoptosis. Thus, an extensive mitochondrial network contributes to apoptosis sensitivity of CD8(+) T cells and, when this occurs together with reduced levels of Bcl-2 and chronic activation, determines the proapoptotic state of HIV-specific CD8(+) T cells.

Interleukin-15 responses to aging and unloading-induced skeletal muscle atrophy

Interleukin-15 (IL-15) mRNA is constitutively expressed in skeletal muscle. Although IL-15 has proposed hypertrophic and anti-apoptotic roles in vitro, its role in skeletal muscle cells in vivo is less clear. The purpose of this study was to determine if skeletal muscle aging and unloading, two conditions known to promote muscle atrophy, would alter basal IL-15 expression in skeletal muscle. We hypothesized that IL-15 mRNA expression would increase as a result of both aging and muscle unloading and that muscle would express the mRNA for a functional trimeric IL-15 receptor (IL-15R). Two models of unloading were used in this study: hindlimb suspension (HS) in rats and wing unloading in quail. The absolute muscle wet weight of plantaris and soleus muscles from aged rats was significantly less when compared with muscles from young adult rats. Although 14 days of HS resulted in reduced muscle mass of plantaris and soleus muscles from young adult animals, this effect was not observed in muscles from aged animals. A significant aging times unloading interaction was observed for IL-15 mRNA in both rat soleus and plantaris muscles. Patagialis (PAT) muscles from aged quail retained a significant 12 and 6% of stretch-induced hypertrophy after 7 and 14 days of unloading, respectively. PAT muscles from young quail retained 15% hypertrophy at 7 days of unloading but regressed to control levels following 14 days of unloading. A main effect of age was observed on IL-15 mRNA expression in PAT muscles at 14 days of overload, 7 days of unloading, and 14 days of unloading. Skeletal muscle also expressed the mRNAs for a functional IL-15R composed of IL-15Ralpha, IL-2/15R-beta, and -gammac. Based on these data, we speculate that increases in IL-15 mRNA in response to atrophic stimuli may be an attempt to counteract muscle mass loss in skeletal muscles of old animals. Additional research is warranted to determine the importance of the IL-15/IL-15R system to counter muscle wasting.

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.

PD-1 is a regulator of virus-specific CD8+ T cell survival in HIV infection

Here, we report on the expression of programmed death (PD)-1 on human virus-specific CD8⁺ T cells and the effect of manipulating signaling through PD-1 on the survival, proliferation, and cytokine function of these cells. PD-1 expression was found to be low on naive CD8⁺ T cells and increased on memory CD8⁺ T cells according to antigen specificity. Memory CD8⁺ T cells specific for poorly controlled chronic persistent virus (HIV) more frequently expressed PD-1 than memory CD8⁺ T cells specific for well-controlled persistent virus (cytomegalovirus) or acute (vaccinia) viruses. PD-1 expression was independent of maturational markers on memory CD8⁺ T cells and was not directly associated with an inability to produce cytokines. Importantly, the level of PD-1 surface expression was the primary determinant of apoptosis sensitivity of virus-specific CD8⁺ T cells. Manipulation of PD-1 led to changes in the ability of the cells to survive and expand, which, over several days, affected the number of cells expressing cytokines. Therefore, PD-1 is a major regulator of apoptosis that can impact the frequency of antiviral T cells in chronic infections such as HIV, and could be manipulated to improve HIV-specific CD8⁺ T cell numbers, but possibly not all functions in vivo.

Gammaherpesvirus persistence alters key CD8 T-cell memory characteristics and enhances antiviral protection

In herpesvirus infections, the virus persists for life but is contained through T-cell-mediated immune surveillance. How this immune surveillance operates is poorly understood. Recent studies of other persistent infections have indicated that virus persistence is associated with functional deficits in the CD8(+) T-cell response. To test whether this is the case in a herpesvirus infection, we used a mutant murine gammaherpesvirus that is defective in its ability to persist in the host. By comparing the immune response to this virus with a revertant virus that can persist, we were able to dissect the changes in the antiviral CD8(+) T-cell response that are induced by virus persistence. Surprisingly, persistently infected mice controlled a secondary challenge infection more rapidly than nonpersistently infected mice, indicating enhanced rather than diminished effector functions. Consistent with this, virus-specific CD8 T cells from these mice exhibited faster upregulation of the cytotoxic mediator granzyme B. Another unexpected finding was that CD8(+) T cells from neither infection responded efficiently to homeostatic cytokines. The unresponsiveness of the memory cells from the nonpersistently infected mice appears to be linked to the prolonged replication of virus within the lungs. Other changes seen in different chronic infection models were also observed, such as changes in Bcl-2 levels, interleukin-2 production, and the immunodominance hierarchy. These data show persistence of gammaherpesvirus type 68 alters the properties of CD8(+) T cells and illustrates that immune surveillance does not require CD8 T cells with the same attributes as "classical" memory CD8(+) T cells.

Differential sensitivity of naïve and memory subsets of human CD8+ T cells to TNF-alpha-induced apoptosis

In this investigation, we have examined the relative sensitivity of human naïve, central memory (T(CM)), and two types of effector memory CD8+ T cells (T(EM) and T(EMRA)) to TNF-alpha-induced apoptosis. Our data show that naïve and T(CM) CD8+ T cells were sensitive, whereas T(EM) and T(EMRA) CD8+ T cells were relatively resistant to TNF-alpha-induced apoptosis. The apoptosis profile correlated with the activation of caspase-8 and caspase-3. However, no correlation was observed between relative sensitivity of four CD8+ T cell subsets to apoptosis and the expression of TNFR-I or TNFR-II. T(EM) and T(EMRA) CD8+ T cells displayed increased phosphorylation of IKKalpha/beta and IkappaB and increased NF-kappaB activity as compared to naïve and T(CM) CD8+ T cells. Bcl-2, Bcl-x(L) and FLIP(L) expression was higher and Bax expression was lower in T(EM) and T(EMRA) CD8+ T cells as compared to naïve and T(CM) CD8+ T cells. These data suggest that signaling molecules downstream of TNFRs may be responsible for differential sensitivity among subsets of CD8+ T cells to TNF-alpha-induced apoptosis.

Primary human T lymphocytes engineered with a codon-optimized IL-15 gene resist cytokine withdrawal-induced apoptosis and persist long-term in the absence of exogenous cytokine

IL-15 is a common gamma-chain cytokine that has been shown to be more active than IL-2 in several murine cancer immunotherapy models. Although T lymphocytes do not produce IL-15, murine lymphocytes carrying an IL-15 transgene demonstrated superior antitumor activity in the immunotherapy of B16 melanoma. Thus, we sought to investigate the biological impact of constitutive IL-15 expression by human lymphocytes. In this report we describe the generation of a retroviral vector encoding a codon-optimized IL-15 gene. Alternate codon usage significantly enhanced the translational efficiency of this tightly regulated gene in retroviral vector-transduced cells. Activated human CD4+ and CD8+ human lymphocytes expressed IL-15Ralpha and produced high levels of cytokine upon retroviral transduction with the IL-15 vector. IL-15-transduced lymphocytes remained viable for up to 180 days in the absence of exogenous cytokine. IL-15 vector-transduced T cells showed continued proliferation after cytokine withdrawal and resistance to apoptosis while retaining specific Ag recognition. In the setting of adoptive cell transfer, IL-15-transduced lymphocytes may prolong lymphocyte survival in vivo and could potentially enhance antitumor activity.

Interleukin-2 reconstitutes defective human immunodeficiency virus (HIV), and cytomegalovirus (CMV) specific CD8+ T cell proliferation in HIV infection

Recent studies indicate that a defective proliferative response of HIV-specific CD8+ T cells is associated with the lack of virologic control in chronic HIV infection in humans. The possible mechanisms that might be responsible for the reduced proliferative potential of HIV-specific CD8+ T cells and conditions conducive to the proliferation of CD8+ T cells were examined in 14 HIV-infected individuals and 7 HIV-uninfected controls using CFSE labeling and flow cytometry techniques, and analyzed data using 2 quantitative measurements: the percentages of proliferating CD8+ T cells (Tp), and the maximum number of cell divisions (Dm) after stimulation. It was found that CD8+ T cells from HIV-infected and -uninfected subjects proliferated equally well after polyclonal stimulation by phylohemagglutinin A (PHA); both groups reached a Tp of 92%-96% and a Dm of 5-8. However, in HIV-infected subjects, proliferation of HIV- and CMV-specific CD8+ T cells was significantly reduced compared to proliferation of CMV- specific CD8+ T cells from HIV-uninfected subjects. These defective proliferative responses of HIV- and CMV-specific CD8+ T cells were restored by the addition of IL-2 at the time of stimulation. These results may have implications for the design of immune modulation strategies in vivo.

HIV-1 infection initiates changes in the expression of a wide array of genes in U937 promonocytes and HUT78 T cells

Human monocytes/macrophages (M/M) are the major targets for human immunodeficiency virus type 1 (HIV-1) infection. To characterize the global effects of acute HIV-1 infection on gene expression in M/M, the expression levels of 550 host cell RNA transcripts in U937 human promonocytes at 2-3 days after HIV-1 infection were assessed using cDNA microarray analysis and were compared to those in the infected HUT78, a CD4+ T cell line. Confirmed by semiquantitative RT-PCR, our results showed that 12 genes were up-regulated and 26 genes were down-regulated in the infected U937 cells at 2-3 days post-infection, whereas 8 genes were up-regulated and 20 genes were down-regulated in the infected HUT78 cells at 2-3 days post-infection. These genes encode a host of proteins with divergent functions in a variety of cellular processes including apoptosis (FAS, Fas ligand, PIN, HSP90beta, bcl-2, bcl-x), cell signal transduction (Ras, RGS1, IRF-1, STAT3), receptor-mediated signaling transduction (CD71, CD69, CD3delta), cell cycle and growth (c-myc, cytokines, kinase), transcriptional regulation (EWS, CREB-2), and chemotaxis (beta-chemokines, RANTES), supporting the general effects of HIV-1 infection on cells of different origin. Although most identified genes were regulated similarly in both infected cell lines, differences in gene regulation, such as c-myc, CD71, CD69, and beta-chemokines, between the two infected cell lines were also identified in this study. These differences may further our understanding of the pathogenicity of HIV and enable the discovery of novel therapeutic approach for AIDS.

Uncoupled anti-HIV and immune-enhancing effects when combining IFN-alpha and IL-7

Cytokine-based therapies have been examined for purging viral reservoirs and immunomodulation in HIV infection. However, single cytokines did not result in either HIV eradication or an efficient HIV-specific immune response. We hypothesize that cytokines with distinct biologic effects need to be combined for immunotherapy of HIV infection. In this study, we investigated the anti-HIV activity and immune-enhancing effects of the combination of IFN-alpha and IL-7. In human lymphocyte aggregate cultures infected ex vivo with the X4 HIV strain NL4-3, IFN-alpha/IL-7 potently inhibited HIV replication and preserved CD4(+) T cells, probably by up-regulating Bcl-2. IFN-alpha/IL-7 also strongly inhibited R5 HIV replication. Furthermore, in allogeneic MLRs, IFN-alpha/IL-7 increased T cell proliferation and IFN-gamma production. IFN-alpha alone also had strong anti-HIV activity, but neither preserved CD4(+) T cells nor increased T cell responses in MLRs. IL-7 alone maintained T cells and enhanced T cell activation in MLRs, but only moderately inhibited or increased HIV replication. Thus, coadministration of IFN-alpha/IL-7 combines the potent anti-HIV activity of IFN-alpha with the beneficial effects of IL-7 on T cell survival and function. We speculate that IFN-alpha will block viral replication, activate APCs, and up-regulate MHC molecules, thus allowing IL-7 to display its effects for generating an efficient immune response. In this scenario, the known reactivation of latent HIV by IL-7 may be advantageous.

CD8+ T cell dynamics during primary simian immunodeficiency virus infection in macaques: relationship of effector cell differentiation with the extent of viral replication

Immunological and virological events that occur during the earliest stages of HIV-1 infection are now considered to have a major impact on subsequent disease progression. We observed changes in the frequencies of CD8(bright) T cells expressing different chemokine receptors in the peripheral blood and lymph nodes of rhesus macaques during the acute phase of the pathogenic SIVmac251 infection; the frequency of CD8(bright) T cells expressing CXCR4 decreased, while the frequency of those expressing CCR5 increased. These reciprocal changes in chemokine receptor expression were associated with changes in the proportion of cycling (Ki67(+)) CD8(bright) T cells, and with the pattern of CD8(bright) T cell differentiation as defined by expression of CCR7 and CD45RA. In contrast, during the primary phase of the attenuated SIVmac251Deltanef infection, no major change was observed. Whereas during the acute phase of the infection with pathogenic SIV (2 wk postinfection) no correlate of disease protection was identified, once the viral load set points were established (2 mo postinfection), we found that the levels of cycling and of CCR5- and CXCR4-positive CD8(bright) T cells were correlated with the extent of viral replication and therefore with SIV-infection outcome. Our data reveal that, during primary SIV infection, despite intense CD8 T cell activation and an increase in CCR5 expression, which are considered as essential for optimal effector function of CD8(+) T cells, these changes are associated with a poor prognosis for disease progression to AIDS.

1α,25-Dihydroxyvitamin D3 or analogue treated dendritic cells modulate human autoreactive T cells via the selective induction of apoptosis

Epidemiological evidence indicates that the vitamin D status after birth modulates the risk for development of type 1 diabetes mellitus (T1DM). We previously demonstrated that the biologically active form of vitamin D, 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3), as well as its analogue TX527 permanently alter the morphology and T cell stimulatory function of human dendritic cells (DC). Here, we studied the mechanism of T cell modulation by 1,25(OH)2D3 or analogue treated DC. By using CFSE-labelled autoreactive T cells, we observed that T cell proliferation is hampered upon coculture with modulated DCs, i.e. T cells underwent fewer cycles of cell divisions when compared to T cells stimulated by nontreated DCs. Moreover, 1,25(OH)2D3 or analogue modulated DCs induced significantly higher numbers of early apoptotic (annexin V+/PI-) and/or late apoptotic (annexin V+/PI+) T cells. Apoptosis was selectively induced in T cells activated by modulated DC, since other T cells present in the same cultures, either resting or activated by control untreated DC, were unaffected. Thus, in vitro preconditioning of DC with 1,25(OH)2D3 or analogue yields regulatory DC that may interfere with ongoing autoimmunity in vivo without affecting T cells with other specificities.

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.