The expression of Fas Ligand by macrophages and its upregulation by human immunodeficiency virus infection

The Spectrum of CAR Cellular Effectors: Modes of Action in Anti-Tumor Immunity

Chimeric antigen receptor-T cells have spearheaded the field of adoptive cell therapy and have shown remarkable results in treating hematological neoplasia. Because of the different biology of solid tumors compared to hematological tumors, response rates of CAR-T cells could not be transferred to solid entities yet. CAR engineering has added co-stimulatory domains, transgenic cytokines and switch receptors to improve performance and persistence in a hostile tumor microenvironment, but because of the inherent cell type limitations of CAR-T cells, including HLA incompatibility, toxicities (cytokine release syndrome, neurotoxicity) and high costs due to the logistically challenging preparation process for autologous cells, the use of alternative immune cells is gaining traction. NK cells and γδ T cells that do not need HLA compatibility or macrophages and dendritic cells with additional properties such as phagocytosis or antigen presentation are increasingly seen as cellular vehicles with potential for application. As these cells possess distinct properties, clinicians and researchers need a thorough understanding of their peculiarities and commonalities. This review will compare these different cell types and their specific modes of action seen upon CAR activation.

Elimination of oncogenic KRAS in genetic mouse models eradicates pancreatic cancer by inducing FAS-dependent apoptosis by CD8+ T cells

Oncogenic KRASG12D (KRAS∗) is critical for the initiation and maintenance of pancreatic ductal adenocarcinoma (PDAC) and is a known repressor of tumor immunity. Conditional elimination of KRAS∗ in genetic mouse models of PDAC leads to the reactivation of FAS, CD8+ T cell-mediated apoptosis, and complete eradication of tumors. KRAS∗ elimination recruits activated CD4+ and CD8+ T cells and promotes the activation of antigen-presenting cells. Mechanistically, KRAS∗-mediated immune evasion involves the epigenetic regulation of Fas death receptor in cancer cells, via methylation of its promoter region. Furthermore, analysis of human RNA sequencing identifies that high KRAS expression in PDAC tumors shows a lower proportion of CD8+ T cells and demonstrates shorter survival compared with tumors with low KRAS expression. This study highlights the role of CD8+ T cells in the eradication of PDAC following KRAS∗ elimination and provides a rationale for the combination of KRAS∗ targeting with immunotherapy to control PDAC.

Prime, shock and kill: BCL-2 inhibition for HIV cure

While modern HIV therapy can effectively suppress viral replication, the persistence of the latent reservoir posits the greatest hurdle to complete cure. The "shock and kill" strategy is under investigation for HIV therapy, aiming to reactivate latent HIV, and subsequently eliminate it through anti-retroviral therapy and host immune function. However, thus far, studies have yielded suboptimal results, stemming from a combination of ineffective latency reversal and poor immune clearance. Concomitantly, studies have now revealed the importance of the BCL-2 anti-apoptotic protein as a critical mediator of infected cell survival, reservoir maintenance and immune evasion in HIV. Furthermore, BCL-2 inhibitors are now recognized for their anti-HIV effects in pre-clinical studies. This minireview aims to examine the intersection of BCL-2 inhibition and current shock and kill efforts, hoping to inform future studies which may ultimately yield a cure for HIV.

Inhibition of myeloid PD-L1 suppresses osteoclastogenesis and cancer bone metastasis

Programmed death-ligand 1 (PD-L1) is predominantly expressed in the antigen-presenting cells (APCs) that are originated and are abundant in the bone marrow. The roles of PD-L1 in bone cell differentiation and cancer bone metastasis remain unclear. Here we show that PD-L1 antibody or PD-L1 conditional knockout in the hematopoietic or myeloid lineage suppresses osteoclast differentiation in vitro and in vivo. Bone metastases of breast cancer and melanoma are diminished by PD-L1 antibody or PD-L1 deletion in the myeloid lineage. Transcriptional profiling of bone marrow cells reveals that PD-L1 deletion in the myeloid cells upregulates immune-stimulatory genes, leading to increased macrophage M1 polarization, decreased M2 polarization, enhanced IFNγ signaling, and elevated T cell recruitment and activation. All these alterations result in heightened anti-tumor immunity in the cancer microenvironment. Our findings support PD-L1 antibody as a potent therapy for bone metastasis of breast cancer and melanoma by simultaneously suppressing osteoclast and enhancing immunity.

Dendritic Cell Vaccination of Glioblastoma: Road to Success or Dead End

Glioblastomas (GBM) are the most frequent and aggressive malignant primary brain tumor and remains a therapeutic challenge: even after multimodal therapy, median survival of patients is only 15 months. Dendritic cell vaccination (DCV) is an active immunotherapy that aims at inducing an antitumoral immune response. Numerous DCV trials have been performed, vaccinating hundreds of GBM patients and confirming feasibility and safety. Many of these studies reported induction of an antitumoral immune response and indicated improved survival after DCV. However, two controlled randomized trials failed to detect a survival benefit. This raises the question of whether the promising concept of DCV may not hold true or whether we are not yet realizing the full potential of this therapeutic approach. Here, we discuss the results of recent vaccination trials, relevant parameters of the vaccines themselves and of their application, and possible synergies between DCV and other therapeutic approaches targeting the immunosuppressive microenvironment of GBM.

Post-injury immunosuppression and secondary infections are caused by an AIM2 inflammasome-driven signaling cascade

Loss of lymphocytes, particularly T cell apoptosis, is a central pathological event after severe tissue injury that is associated with increased susceptibility for life-threatening infections. The precise immunological mechanisms leading to T cell death after acute injury are largely unknown. Here, we identified a monocyte-T cell interaction driving bystander cell death of T cells in ischemic stroke and burn injury. Specifically, we found that stroke induced a FasL-expressing monocyte population, which led to extrinsic T cell apoptosis. This phenomenon was driven by AIM2 inflammasome-dependent interleukin-1β (IL-1β) secretion after sensing cell-free DNA. Pharmacological inhibition of this pathway improved T cell survival and reduced post-stroke bacterial infections. As such, this study describes inflammasome-dependent monocyte activation as a previously unstudied cause of T cell death after injury and challenges the current paradigms of post-injury lymphopenia.

Plinabulin, a Distinct Microtubule-Targeting Chemotherapy, Promotes M1-Like Macrophage Polarization and Anti-tumor Immunity

Reprogramming tumor infiltrating myeloid cells to elicit pro-inflammatory responses is an exciting therapeutic maneouver to improve anti-tumor responses. We recently demonstrated that a distinct microtubule-targeting drug, plinabulin—a clinical-stage novel agent—modulates dendritic cell maturation and enhances anti-tumor immunity. Here, we investigated the effects of plinabulin on macrophage polarization in vitro and in vivo. Plinabulin monotherapy induced significant tumor growth inhibition in mice bearing subcutaneous MC38 colon cancer. Importantly, the regressing tumors were characterized by an increase in M1-like/M2-like tumor-associated macrophages (TAM) ratio. The efficacy of plinabulin remained unaltered in T cell-deficient Rag2^−/− mice, suggesting an important role of macrophages in driving the drug's anti-tumor effect. Exposure of murine and healthy human macrophages to plinabulin induced polarization toward the M1 phenotype, including increased expression of co-stimulatory molecules CD80, CD86 and pro-inflammatory cytokines IL-1β, IL-6, and IL-12. M2-associated immunosuppressive cytokines IL-10 and IL-4 were reduced. This pro-inflammatory M1-like skewing of TAMs in response to plinabulin was dependent on the JNK pathway. Functionally, plinabulin-polarized human M1 macrophages directly killed HuT 78 tumor cells in vitro. Importantly, plinabulin induced a functional M1-like polarization of tumor infiltrating macrophages in murine tumors as well as in tumor samples from ovarian cancer patients, by preferentially triggering M1 proliferation. Our study uncovers a novel immunomodulatory effect of plinabulin in directly triggering M1 polarization and proliferation as well as promoting TAM anti-tumoral effector functions.

Apoptosis of tumor-infiltrating T lymphocytes: a new immune checkpoint mechanism

Immunotherapy based on checkpoint inhibitors is providing substantial clinical benefit, but only to a minority of cancer patients. The current priority is to understand why the majority of patients fail to respond. Besides T-cell dysfunction, T-cell apoptosis was reported in several recent studies as a relevant mechanism of tumoral immune resistance. Several death receptors (Fas, DR3, DR4, DR5, TNFR1) can trigger apoptosis when activated by their respective ligands. In this review, we discuss the immunomodulatory role of the main death receptors and how these are shaping the tumor microenvironment, with a focus on Fas and its ligand. Fas-mediated apoptosis of T cells has long been known as a mechanism allowing the contraction of T-cell responses to prevent immunopathology, a phenomenon known as activation-induced cell death, which is triggered by induction of Fas ligand (FasL) expression on T cells themselves and qualifies as an immune checkpoint mechanism. Recent evidence indicates that other cells in the tumor microenvironment can express FasL and trigger apoptosis of tumor-infiltrating lymphocytes (TIL), including endothelial cells and myeloid-derived suppressor cells. The resulting disappearance of TIL prevents anti-tumor immunity and may in fact contribute to the absence of TIL that is typical of "cold" tumors that fail to respond to immunotherapy. Interfering with the Fas-FasL pathway in the tumor microenvironment has the potential to increase the efficacy of cancer immunotherapy.

Tat is a multifunctional viral protein that modulates cellular gene expression and functions

The human immunodeficiency virus type I (HIV-1) has developed several strategies to condition the host environment to promote viral replication and spread. Viral proteins have evolved to perform multiple functions, aiding in the replication of the viral genome and modulating the cellular response to the infection. Tat is a small, versatile, viral protein that controls transcription of the HIV genome, regulates cellular gene expression and generates a permissive environment for viral replication by altering the immune response and facilitating viral spread to multiple tissues. Studies carried out utilizing biochemical, cellular, and genomic approaches show that the expression and activity of hundreds of genes and multiple molecular networks are modulated by Tat via multiple mechanisms.

Fas ligand-expressing lymphocytes enhance alveolar macrophage apoptosis in the resolution of acute pulmonary inflammation

Apoptosis of alveolar macrophages and their subsequent clearance by neighboring phagocytes are necessary steps in the resolution of acute pulmonary inflammation. We have recently identified that activation of the Fas death receptor on the cell surface of macrophages drives macrophage apoptosis. However, the source of the cognate ligand for Fas (FasL) responsible for induction of alveolar macrophage apoptosis is not defined. Given their known role in the resolution of inflammation and ability to induce macrophage apoptosis ex vivo, we hypothesized that T lymphocytes represented a critical source of FasL. To address this hypothesis, C57BL/6J and lymphocyte-deficient (Rag-1(-/-)) mice were exposed to intratracheal lipopolysaccharide to induce pulmonary inflammation. Furthermore, utilizing mice expressing nonfunctional FasL, we adoptively transferred donor lymphocytes into inflamed lymphocyte-deficient mice to characterize the effect of lymphocyte-derived FasL on alveolar macrophage apoptosis in the resolution of inflammation. Herein, evidence is presented that lymphocytes expressing FasL enhance alveolar macrophage apoptosis during the resolution of LPS-induced inflammation. Moreover, lymphocyte induction of alveolar macrophage apoptosis results in contraction of the alveolar macrophage pool, which occurs in a FasL-dependent manner. Specifically, FasL-expressing CD8(+) T lymphocytes potently induce alveolar macrophage apoptosis and contraction of the alveolar macrophage pool. Together, these studies identify a novel role for CD8(+) T lymphocytes in the resolution of acute pulmonary inflammation.

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

Modeling of HIV-1 infection: insights to the role of monocytes/macrophages, latently infected T4 cells, and HAART regimes

A novel dynamic model covering five types of cells and three connected compartments, peripheral blood (PB), lymph nodes (LNs), and the central nervous system (CNS), is here proposed. It is based on assessment of the biological principles underlying the interactions between the human immunodeficiency virus type I (HIV-1) and the human immune system. The simulated results of this model matched the three well-documented phases of HIV-1 infection very closely and successfully described the three stages of LN destruction that occur during HIV-1 infection. The model also showed that LNs are the major location of viral replication, creating a pool of latently infected T4 cells during the latency period. A detailed discussion of the role of monocytes/macrophages is made, and the results indicated that infected monocytes/macrophages could determine the progression of HIV-1 infection. The effects of typical highly active antiretroviral therapy (HAART) drugs on HIV-1 infection were analyzed and the results showed that efficiency of each drug but not the time of the treatment start contributed to the change of the turnover of the disease greatly. An incremental count of latently infected T4 cells was made under therapeutic simulation, and patients were found to fail to respond to HAART therapy in the presence of certain stimuli, such as opportunistic infections. In general, the dynamics of the model qualitatively matched clinical observations very closely, indicating that the model may have benefits in evaluating the efficacy of different drug therapy regimens and in the discovery of new monitoring markers and therapeutic schemes for the treatment of HIV-1 infection.

Decidual macrophages are significantly increased in spontaneous miscarriages and over-express FasL: a potential role for macrophages in trophoblast apoptosis

Decidual macrophages (DM) are the second most abundant population in the fetal-maternal interface. Their role has been so far identified as being local immuno-modulators favoring the maternal tolerance to the fetus. Herein we investigated tissue samples from 11 cases of spontaneous miscarriages and from 9 cases of elective terminations of pregnancy. Using immunohistochemistry and dual immunofluorescence we have demonstrated that in spontaneous miscarriages the DM are significantly increased. Additionally, we noted a significant up-regulation of macrophage FasL expression. Our results further support a dual role for DM during pregnancy and miscarriages. We hypothesize that the baseline DM population in normal pregnancy is in line with an M2 phenotype supporting the ongoing gestation. In contrast, during spontaneous miscarriages, the increased FasL-expressing population could be a part of an M1 phenotype participating in Fas/FasL-related apoptosis. Our results highlight a new aspect of macrophage biology in pregnancy physiology and pathophysiology. Further studies with larger samples are needed to verify the current results and evaluate their clinical impact.

IL-7 promotes CD95-induced apoptosis in B cells via the IFN-γ/STAT1 pathway

Interleukin-7 (IL-7) concentrations are increased in the blood of CD4+ T cell depleted individuals, including HIV-1 infected patients. High IL-7 levels might stimulate T cell activation and, as we have shown earlier, IL-7 can prime resting T cell to CD95 induced apoptosis as well. HIV-1 infection leads to B cell abnormalities including increased apoptosis via the CD95 (Fas) death receptor pathway and loss of memory B cells. Peripheral B cells are not sensitive for IL-7, due to the lack of IL-7Ra expression on their surface; however, here we demonstrate that high IL-7 concentration can prime resting B cells to CD95-mediated apoptosis via an indirect mechanism. T cells cultured with IL-7 induced high CD95 expression on resting B cells together with an increased sensitivity to CD95 mediated apoptosis. As the mediator molecule responsible for B cell priming to CD95 mediated apoptosis we identified the cytokine IFN-γ that T cells secreted in high amounts in response to IL-7. These results suggest that the lymphopenia induced cytokine IL-7 can contribute to the increased B cell apoptosis observed in HIV-1 infected individuals.

Isolation of a TRAIL antagonist from the serum of HIV-infected patients

Virus-host interactions are characterized by the selection of adaptive mechanisms by which to evade pathogenic and defense mechanisms, respectively. In primary T cells infected with HIV, HIV infection up-regulates TNF-related apoptosis inducing ligand (TRAIL) and death-inducing TRAIL receptors, but blockade of TRAIL:TRAIL receptor interaction does not alter HIV-induced cell death. Instead, HIV infection results in a novel splice variant that we call TRAIL-short (TRAIL-s), which antagonizes TRAIL-R2. In HIV patients, plasma TRAIL-s concentration increases with increasing viral load and renders cells resistant to TRAIL-induced death. Knockdown of TRAIL-s abrogates this resistance. We propose that TRAIL-s is a novel adaptive mechanism of apoptosis resistance acquired by HIV-infected cells to avoid their elimination by TRAIL-dependent effector mechanism.

Herpes simplex virus type 1-induced FasL expression in human monocytic cells and its implications for cell death, viral replication, and immune evasion

Herpes simplex virus type 1 (HSV-1) is a ubiquitously occurring pathogen that infects humans early in childhood. The virus persists as a latent infection in dorsal root ganglia, especially of the trigeminal nerve, and frequently becomes reactivated in humans under conditions of stress. Monocytic cells constitute an important component of the innate and adaptive immune responses. We show here for the first time that HSV-1 stimulates human FasL promoter and induces de novo expression of FasL on the surface of human monocytic cells, including monocytes and macrophages. This virus-induced FasL expression causes death of monocytic cells growing in suspension, but not in monolayers (e.g., macrophages). The addition of a broad-spectrum caspase inhibitor, as well as anti-FasL antibodies, reduced cell death but increased viral replication in the virus-infected cell cultures. We also show here for the first time that the virus-induced de novo expression of FasL on the cell surface acts as an immune evasion mechanism by causing the death of interacting human CD4+ T cells, CD8+ T cells, and natural killer (NK) cells. Our study provides novel insights on FasL expression and cell death in HSV-infected human monocytic cells and their impact on interacting immune cells.

Increased metallothionein gene expression, zinc, and zinc-dependent resistance to apoptosis in circulating monocytes during HIV viremia

Circulating monocytes exhibit an apoptotic resistance phenotype during HIV viremia in association with increased MT expression. MTs are known to play an important role in zinc metabolism and immune function. We now show, in a cross-sectional study using peripheral monocytes, that expression of MT1 isoforms E, G, H, and X is increased significantly in circulating monocyte cells from HIV+ subjects during chronic viremic episodes as compared with uninfected subjects. This increase in expression is also observed during acute viremia following interruption of suppressive ART. Circulating monocytes from HIV+ donors were also found to have elevated zinc importer gene Zip8 expression in conjunction with elevated intracellular zinc levels in contrast to CD4(+)T-lymphocytes. In vitro HIV-1 infection studies with elutriated MDM confirm a direct relation between HIV-1 infection and increased MDM MT1 (isoform G) gene expression and increased intracellular zinc levels. A direct link between elevated zinc levels and apoptosis resistance was established using a cell-permeable zinc chelator TPEN, which reversed apoptosis resistance effectively in monocytes from HIV-infected to levels comparable with uninfected controls. Taken together, increases in MT gene expression and intracellular zinc levels may contribute directly to maintenance of an immune-activated monocyte by mediating an increased resistance to apoptosis during active HIV-1 viremia.

Immunomodulation by hepatitis C virus-derived proteins: targeting human dendritic cells by multiple mechanisms

Hepatitis C virus (HCV) has the ability to persist in the majority of infected people. Strong, multispecific and sustained T-cell response is correlated with viral clearance. The mechanisms of chronicity by HCV are unclear. HCV could restrain the immune system and establish chronic infection by modulating dendritic cell (DC) function, T-cell function or both. DC dysfunction has been postulated to be either due to direct HCV infection or by the presence of HCV proteins. In this report, for the first time, we have examined whether soluble HCV proteins can impair DC function or directly inhibit T-cell responses in the cells obtained from healthy uninfected people. Our studies revealed that different HCV proteins used distinct mechanisms to down-regulate DC functions. Individual HCV proteins, Core, NS3, NS4, NS5 as well as fused Polyprotein (Core-NS3-NS4) were found to impair functions of both immature DCs and mature DCs by regulating the expression of co-stimulatory and antigen presentation molecules, strikingly reducing IL-12 secretion, inducing the expression of FasL to mediate apoptosis, interfering with allo-stimulatory capacity, inhibiting toll-like receptor signaling and inhibiting nuclear translocation of NFkappaB in DCs. Interestingly, HCV proteins did not directly inhibit T-cell proliferation. Our findings clearly demonstrate that HCV proteins impair T-cell responses indirectly by inhibiting DCs that could result in a sub-optimal cellular immune response allowing for persistent HCV infections. These studies delineate important mechanisms by which initial DC dysfunction can establish contributing to chronicity. Our data are in agreement with earlier observations that DCs are impaired in HCV infected people.

CD95 co-stimulation blocks activation of naive T cells by inhibiting T cell receptor signaling

CD95 is a multifunctional receptor that induces cell death or proliferation depending on the signal, cell type, and cellular context. Here, we describe a thus far unknown function of CD95 as a silencer of T cell activation. Naive human T cells triggered by antigen-presenting cells expressing a membrane-bound form of CD95 ligand (CD95L) or stimulated by anti-CD3 and -CD28 antibodies in the presence of recombinant CD95L had reduced activation and proliferation, whereas preactivated, CD95-sensitive T cells underwent apoptosis. Triggering of CD95 during T cell priming interfered with proximal T cell receptor signaling by inhibiting the recruitment of ζ-chain–associated protein of 70 kD, phospholipase-γ, and protein kinase C-θ into lipid rafts, thereby preventing their mutual tyrosine protein phosphorylation. Subsequently, Ca²⁺ mobilization and nuclear translocation of transcription factors NFAT, AP1, and NF-κB were strongly reduced, leading to impaired cytokine secretion. CD95-mediated inhibition of proliferation in naive T cells could not be reverted by the addition of exogenous interleukin-2 and T cells primed by CD95 co-stimulation remained partially unresponsive upon secondary T cell stimulation. HIV infection induced CD95L expression in primary human antigeen-presenting cells, and thereby suppressed T cell activation, suggesting that CD95/CD95L-mediated silencing of T cell activation represents a novel mechanism of immune evasion.

A new function of the Fas-FasL pathway in macrophage activation

Upon infection with the protozoan parasite Leishmania major, susceptible BALB/c mice develop unhealing lesions associated with the maturation of CD4(+)Th2 cells secreting IL-4. In contrast, resistant C57BL/6 mice heal their lesions, because of expansion and secretion of IFN-gamma of CD4(+) Th1 cells. The Fas-FasL pathway, although not involved in Th cell differentiation, was reported to be necessary for complete resolution of lesions. We investigate here the role of IFN-gamma and IL-4 on Fas-FasL nonapoptotic signaling events leading to the modulation of macrophage activation. We show that addition of FasL and IFN-gamma to BMMø led to their increased activation, as reflected by enhanced secretion of TNF, IL-6, NO, and the induction of their microbicidal activity, resulting in the killing of intracellular L. major. In contrast, the presence of IL-4 decreased the synergy of IFN-gamma/FasL significantly on macrophage activation and the killing of intracellular L. major. These results show that FasL synergizes with IFN-gamma to activate macrophages and that the tight regulation by IFN-gamma and/or IL-4 of the nonapoptotic signaling events triggered by the Fas-FasL pathway affects significantly the activation of macrophages to a microbicidal state and may thus contribute to the pathogenesis of L. major infection.

Depletion of resident alveolar macrophages does not prevent Fas-mediated lung injury in mice

Activation of the Fas/Fas ligand (FasL) system in the lungs results in a form of injury characterized by alveolar epithelial apoptosis and neutrophilic inflammation. Studies in vitro show that Fas activation induces apoptosis in alveolar epithelial cells and cytokine production in alveolar macrophages. The main goal of this study was to determine the contribution of alveolar macrophages to Fas-induced lung inflammation in mice, by depleting alveolar macrophages using clodronate-containing liposomes. Liposomes containing clodronate or PBS were instilled by intratracheal instillation. After 24 h, the mice received intratracheal instillations of the Fas-activating monoclonal antibody Jo2 or an isotype control antibody and were studied 18 h later. The Jo2 MAb induced increases in bronchoalveolar lavage fluid (BALF) total neutrophils, lung caspase-3 activity, and BALF total protein and worsened histological lung injury in the macrophage-depleted mice. Studies in vitro showed that Fas activation induced the release of the cytokine KC in a mouse lung epithelial cell line, MLE-12. These results suggest that the lung inflammatory response to Fas activation is not primarily dependent on resident alveolar macrophages and may instead depend on cytokine release by alveolar epithelial cells.

A replication-competent adenovirus-human immunodeficiency virus (Ad-HIV) tat and Ad-HIV env priming/Tat and envelope protein boosting regimen elicits enhanced protective efficacy against simian/human immunodeficiency virus SHIV89.6P challenge in rhesus macaques

We previously demonstrated that replication-competent adenovirus (Ad)-simian immunodeficiency virus (SIV) recombinant prime/protein boost regimens elicit potent immunogenicity and strong, durable protection of rhesus macaques against SIV(mac251). Additionally, native Tat vaccines have conferred strong protection against simian/human immunodeficiency virus SHIV(89.6P) challenge of cynomolgus monkeys, while native, inactivated, or vectored Tat vaccines have failed to elicit similar protective efficacy in rhesus macaques. Here we asked if priming rhesus macaques with replicating Ad-human immunodeficiency virus (HIV) tat and boosting with the Tat protein would elicit protection against SHIV(89.6P). We also evaluated a Tat/Env regimen, adding an Ad-HIV env recombinant and envelope protein boost to test whether envelope antibodies would augment acute-phase protection. Further, expecting cellular immunity to enhance chronic viremia control, we tested a multigenic group: Ad-HIV tat, -HIV env, -SIV gag, and -SIV nef recombinants and Tat, Env, and Nef proteins. All regimens were immunogenic. A hierarchy was observed in enzyme-linked immunospot responses (with the strongest response for Env, followed by Gag, followed by Nef, followed by Tat) and antibody titers (with the highest titer for Env, followed by Tat, followed by Nef, followed by Gag). Following intravenous SHIV(89.6P) challenge, all macaques became infected. Compared to controls, no protection was seen in the Tat-only group, confirming previous reports for rhesus macaques. However, the multigenic group blunted acute viremia by approximately 1 log (P = 0.017), and both the multigenic and Tat/Env groups reduced chronic viremia by 3 and 4 logs, respectively, compared to controls (multigenic, P = 0.0003; Tat/Env, P < 0.0001). The strikingly greater reduction in the Tat/Env group than in the multigenic group (P = 0.014) was correlated with Tat and Env binding antibodies. Since prechallenge anti-Env antibodies lacked SHIV(89.6P)-neutralizing activity, other functional anti-Env and anti-Tat activities are under investigation, as is a possible synergy between the Tat and Env immunogens.

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.

Genomic cloning of feline Fas ligand gene and characterization of the transcription regulatory region

The feline Fas ligand gene was molecularly cloned from a feline genomic library and its genomic organization was determined. The feline Fas ligand gene contained four exons and spanned approximately 10 kb in the genome, and thus had the same structure as the human Fas ligand gene. The promoter region of the feline Fas ligand gene was further characterized by deletion mapping. The region between nucleotides -459 and -172 relative to the ATG codon was essential for the promoter activity when transfected into human and feline lymphoid cells. The characterization of the feline Fas ligand gene in the present study will be useful for further investigations of the regulatory mechanism of feline Fas ligand expression.

Human immunodeficiency virus infection and acquired immunodeficiency syndrome dementia complex: role of cells of monocyte-macrophage lineage

The entry of human immunodeficiency virus (HIV) into the central nervous system (CNS) causes both the establishment of a lifelong viral reservoir in the brain and symptoms of neurological dysfunction that have an AIDS dementia complex (ADC) clinical appearance. Neurological dysfunction in ADC patients still remains an unresolved problem. However, ADC pathogenesis may be a multistep process that starts with HIV invasion of CNS by crossing the blood-brain barrier (BBB). It progresses by developing a chronic inflammatory status that can cause dysfunction in neurons and astrocytes that result in apoptotic death. Monocytes-macrophages (M/M) may play an important role by concealing the HIV transfer across the BBB. Furthermore, HIV-infected M/M could produce and release neurotoxic factors. In this review the main mediators and cells involved in pathogenesis and development of ADC are highlighted. A better understanding of the mechanisms involved in this process may help in a successful therapeutic approach to the neuropathogenesis of HIV infection.

Apoptosis in SIV infection

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

Nelfinavir: a review of its use in the management of HIV infection

Nelfinavir (Viracept) is an orally administered protease inhibitor. In combination with other antiretroviral drugs (usually nucleoside reverse transcriptase inhibitors [NRTIs]), nelfinavir produces substantial and sustained reductions in viral load in patients with HIV infection. Nelfinavir may be used in the treatment of adults, adolescents and children aged >or=2 years with HIV infection. It can also be used in pregnancy. Resistance to nelfinavir may develop, but the most common mutation (D30N, appearing mainly in HIV-1 subtype B) does not confer resistance to other protease inhibitors, thereby conserving these agents for later use. Although less effective than lopinavir/ritonavir, the preferred first-line treatment in US guidelines, nelfinavir is positioned as an alternative agent for the treatment of adults and adolescents with HIV infection and is an option for those unable to tolerate other protease inhibitors. Nelfinavir also has a role in the management of pregnant patients as well as paediatric patients with HIV infection.

Modulation of the immune response in the nervous system by rabies virus

Rabies virus (RABV) is a pathogen well-adapted to the nervous system, where it infects neurons. RABV is transmitted by the bite of an infected animal. It enters the nervous system via a motor neuron through the neuromuscular junction, or via a sensory nerve through nerve spindles. It then travels from one neuron to the next, along the spinal cord to the brain and the salivary glands. The virions are then excreted in the saliva of the animal and can be transmitted to another host by bite. Thus preservation of neuronal network integrity is crucial for the virus to be transmitted. Successful invasion of the nervous system by RABV seems to be the result of a subversive strategy based on the survival of infected neurons. This strategy includes protection against virus-mediated apoptosis and destruction of T cells that invade the CNS in response to infection.

Enhanced cellular immunity to SIV Gag following co-administration of adenoviruses encoding wild-type or mutant HIV Tat and SIV Gag

Among candidate antigens for human immunodeficiency virus (HIV) prophylactic vaccines, the regulatory protein Tat is a critical early target, but has a potential for immune suppression. Adenovirus (Ad) recombinants encoding wild-type HIV Tat (Tat-wt) and a transdominant negative mutant HIV Tat (Tat22) were constructed and administered to mice separately or together with Ad-SIVgag. Immunogenicity and effects on immune responses to the co-administered Gag immunogen were evaluated. Wild-type and mutant Tat recombinants elicited similar Tat-specific cellular and humoral immune responses. Co-administration of either Tat immunogen with Ad-SIVgag induced modest but significant enhancement of Gag-specific interferon-gamma secreting T cells and lymphoproliferative responses. Neither the Ad-recombinant encoding Tat-wt nor Tat22 suppressed induction of anti-Tat or anti-Gag antibodies. Based on the immune responses observed in mice, both recombinants appear to be suitable vaccine candidates. Their contribution to protective efficacy remains to be determined in a non-human primate model.

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

BACKGROUND

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

OBJECTIVE

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

DESIGN

In vitro assessment of TRAIL receptor expression and TRAIL sensitivity.

METHODS

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

RESULTS

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

CONCLUSIONS

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

Death ligand-mediated apoptosis in HIV infection

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

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.

Foot-and-mouth disease virus (FMDV) causes an acute disease that can be lethal for adult laboratory mice

Foot-and-mouth disease virus (FMDV) is a picornavirus that causes an acute vesicular disease of cloven-hoofed animals. This virus continues to be threat to livestock worldwide with outbreaks causing severe economic losses. However, very little is known about FMDV pathogenesis, partially due to the inconveniences of working with cattle and swine, the main natural hosts of the virus. Here we demonstrate that C57BL/6 and BALB/C adult mice are highly susceptible to FMDV infection when the virus is administered subcutaneously or intraperitoneally. The first clinical signs are ruffled fur, apathy, humped posture, and wasting, which are followed by neurological signs such as hind-limb paralysis. Within 2-3 days of disease onset, the animals die. Virus is found in all major organs, indicating a systemic infection. Mice developed microvesicles near the basal layer of the epithelium, event that precedes the vesiculation characteristics of FMD. In addition, a lymphoid depletion in spleen and thymus and severe lymphopenia is observed in the infected mice. When these mice were immunized with conventional inactivated FMDV vaccine, they were protected (100% of vaccinated animals) against challenge with a lethal dose of FMDV. The data indicate that this mouse model may facilitate the study of FMDV pathogenesis, and the development of new effective vaccines for FMD.

Up-regulation of Fas ligand (FasL) in the central nervous system: a mechanism of immune evasion by rabies virus

Following its injection into the hindlimbs of mice, CVS, a highly pathogenic strain of rabies virus, invades the spinal cord and brain resulting in the death of the animal. In contrast, central nervous system (CNS) invasion by PV, a strain of attenuated pathogenicity, is restricted to the spinal cord and mice infected with this virus survive. Lymphocytes display transient migration into the infected CNS in fatal rabies and sustained migration in nonfatal rabies. The transient migration of T cells in fatal rabies is associated with an increase in T-cell apoptosis. We found that the early production of Fas ligand (FasL) mRNAs was up-regulated only in fatal rabies. FasL is produced by several neuronal cells and mainly in infected neurons. In mice lacking FasL (gld), infection with the neuroinvasive rabies virus strain was less severe, and the number of CD3 T cells undergoing apoptosis was smaller than that in normal mice. These data provide strong evidence that fatal rabies virus infection involves the early triggering of FasL production leading to the destruction of migratory T cells by the Fas/FasL apoptosis pathway. This mechanism could be in part responsible for the fact that T cells cannot control neuroinvasive rabies infection. Thus, rabies virus seems to use an immunosubversive strategy that takes advantage of the immune privilege status of the CNS.

Increased Fas-mediated apoptosis in polymorphonuclear cells from HIV-infected patients

Neutrophils represent an important line of innate host defence against invading microorganisms and their functional detriment during HIV infection, including accelerated spontaneous cell death, has been shown to contribute to AIDS development. Neutrophils are susceptible to apoptosis via Fas and an interaction between Fas and FasL was suggested originally as a mechanism to explain constitutive neutrophil apoptosis. We have explored some intracellular pathways leading to PMN apoptosis from 28 HIV-infected patients and 24 healthy volunteers. As previously reported, accelerated spontaneous apoptosis was observed in HIV+ patients, but this did not correlate with viral load. Furthermore, an increase in the level of spontaneous apoptosis was detected in neutrophils from HIV-infected patients following inhibition of ERK, suggesting an impairment of this kinase pathway during the early stages of infection which may contribute to PMN dysfunction. An elevated susceptibility to undergo apoptosis was observed following cross-linking of Fas, which correlated both with viral load and co-expression of Fas/FasL surface molecules. Different mechanisms for spontaneous and Fas-induced apoptosis are proposed which together contribute to the neutropenia and secondary infections observed during the progression to AIDS.

The glutamine-rich region of the HIV-1 Tat protein is involved in T-cell apoptosis

Human immunodeficiency virus (HIV) infection and the progression to AIDS are characterized by the depletion of CD4(+) T-cells. HIV-1 infection leads to apoptosis of uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated, in part, by the HIV-1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells. We chemically synthesized two 86-residue subtype D Tat proteins, Ug05RP and Ug11LTS, from two Ugandan patients who were clinically categorized as either rapid progressor or long-term survivor, with non-conservative mutations located essentially in the glutamine-rich region. Structural heterogeneities were revealed by CD, which translate into differing trans-activational and apoptotic effects. CD data analysis and molecular modeling indicated that the short alpha-helix observed in subtype D Tat proteins from rapid progressor patients such as Tat Mal and Tat Ug05RP is not present in Ug11LTS. We show that Tat Ug05RP is more efficient than Tat Ug11LTS in its trans-activational role and in inducing apoptosis in binding tubulin via the mitochondrial pathway. The glutamine-rich region of Tat appears to be involved in the Tat-mediated apoptosis of T-cells.

HIV‐1 Nef equips dendritic cells to reduce survival and function of CD8 + T cells: a mechanism of immune evasion

The accessory HIV-1 Nef protein is a crucial determinant for viral replication and pathogenesis. During HIV infection, loss of immune control in the setting of a strong and broad HIV-specific T-lymphocyte response, leads to a lethal outcome through AIDS. Moreover, dysfunction of dendritic cells (DCs) may contribute to the immune suppression associated with AIDS progression. We recently demonstrated that exogenous Nef selectively activates immature DCs manipulating their phenotypical, morphological, and functional developmental program. Here, we tracked whether Nef, targeting DCs, could be involved in the dysregulation of CD8+ T cell responses. We found that Nef inhibits the capacity of DCs to prime alloreactive CD8+ T cell responses down-regulating their proliferation and functional competence. This coincides with the induction of CD8+ T cell apoptosis. Nef oversees apoptotic killing of CD8+ T cells up-regulating TNF-alpha and FasL production by DCs and interfering with the death receptor pathway in CD8+ T cells and thus activating caspase 8. Our findings suggest that Nef may contribute to the immune evasion associated with HIV-1 infection, subverting DC biology. This may help explain the pleiotropic function that Nef plays during infection and makes this protein an attractive target for preventive and therapeutic intervention.

Strong T cell type-1 immune responses to HIV-1 Tat (1–72) protein-coated nanoparticles

A significant emphasis has been placed on the development of adjuvants and/or delivery systems to improve both antibody production and cell-mediated immune responses. We previously reported on a novel anionic nanoparticle, which led to enhanced humoral and T helper type-1 (Th1) biased immune responses in mice when coated with cationized model antigen. Tat (1-72) is a conserved regulatory HIV-1 protein. It was hypothesized that HIV vaccine strategies employing Tat (1-72) may be a promising approach. Although previous reports have suggested that Tat (1-86) may be immunosuppressive, it was demonstrated in this present study that Tat (1-72) was not immunosuppressive when co-administered to mice with ovalbumin (OVA). Tat (1-72) was coated on novel anionic nanoparticles. BALB/c mice were immunized with Tat (5 microg)-coated nanoparticles (15 microg) by subcutaneous injection on days 0 and 14. Antibody and cytokine release were determined on day 28 and compared to Tat (5 microg) adjuvanted with Alum (15 microg) as a Th2 control, Tat (5 microg) adjuvanted with Lipid A (50 microg) as a Th1 control. Immunization of BALB/c mice with Tat-coated nanoparticles resulted in antibody levels (IgG and IgM) comparable to those elicited from Tat and Alum. However, Tat-coated nanoparticles led to a Th1 biased immune response. The IFN-gamma release from splenocytes with Tat-coated nanoparticles was comparable to that from mice immunized with Tat and Lipid A, and 3.3-fold greater than that from mice immunized with Tat and Alum. These studies warrant further investigation of these nanoparticles to enhance both antibody and cellular-based immune responses.

IFN-gamma inhibits the proliferation of allergen-activated T lymphocytes from atopic, asthmatic patients by inducing Fas/FasL-mediated apoptosis

The defect in interferon-gamma (IFN-gamma) production that results in a T helper cell type 2-dominated response may be responsible for a decrease in the apoptosis of allergen-activated T cells in asthma. We investigated the effect of recombinant IFN-gamma on proliferation, Fas/Fas ligand (FasL) expression, and apoptosis in allergen-stimulated peripheral blood mononuclear cells obtained from atopic, asthmatic patients and nonatopic, control subjects. The addition of IFN-gamma at the start of cultures markedly inhibited the proliferative response to a specific allergen in cells from all asthmatic patients, whereas no change was observed in cells from nonatopic, control subjects. IFN-gamma induced an increase in the expression of Fas and FasL by allergen-stimulated CD4+ T cells from asthmatic patients and caused the apoptosis of these cells. A Fas-blocking monoclonal antibody prevented the inhibitory effect of IFN-gamma on allergen-induced proliferation. These results suggest that IFN-gamma inhibits the proliferation of allergen-stimulated CD4+ T cells from atopic, asthmatic patients by inducing the surface expression of Fas and FasL, which in turn triggers their apoptotic program. The defect in IFN-gamma production involved in the allergic, immune response may therefore be responsible for a decrease in apoptosis of allergen-activated T lymphocytes in the airways of atopic, asthmatic patients.

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

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

Fascination with bacteria-triggered cell death: the significance of Fas-mediated apoptosis during bacterial infection in vivo

Increasing evidence indicates that bacterial pathogens have developed mechanisms to modulate the apoptotic signaling cascade of host cells and thereby cause disease. The Fas death receptor pathway is one of the most extensively investigated apoptotic signaling pathways. In this review we discuss the role of Fas signaling during the interplay between bacterial pathogens and the host in vivo.

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.

Shear stress induces apoptosis in vascular smooth muscle cells via an autocrine Fas/FasL pathway

Endothelial lesions may lead to the exposure of vascular smooth muscle cells (VSMCs) to the blood flow. In such circumstances VSMCs are exposed to shear stress, an extraordinary mechanical stimulus for this type of cells. Rat VSMCs are cultivated in normal tissue culture plates (statically) or in a cone-plate viscometer (dynamically). Dynamic cultivation leads to a great increase of apoptosis. Immunofluorescence reveals the shear-stress-dependent expression of fas. Apoptosis can be induced by addition of fas ligand-a process which can be blocked by antibodies against either fas or fas ligand. Conditioned medium of dynamically cultivated VSMCs contains fas ligand as the only active apoptosis inducing activity. Apoptosis can be blocked by caspase inhibitors. So the exposure of VSMCs to shear stress leads to apoptosis by the establishment of an autocrine loop of fas and fas ligand-a potential mechanism for the prevention of narrowing of vessel diameter by VSMC proliferation.

Alveolar Macrophage Apoptosis Contributes to Pneumococcal Clearance in a Resolving Model of Pulmonary Infection

The role of alveolar macrophages (AM) in host defense against pulmonary infection has been difficult to establish using in vivo models. This may reflect a reliance on models of fulminant infection. To establish a unique model of resolving infection, with which to address the function of AM, C57BL/6 mice received low-dose intratracheal administration of pneumococci. Administration of low doses of pneumococci produced a resolving model of pulmonary infection characterized by clearance of bacteria without features of pneumonia. AM depletion in this model significantly increased bacterial outgrowth in the lung. Interestingly, a significant increase in the number of apoptotic AM was noted with the low-dose infection as compared with mock infection. Caspase inhibition in this model decreased AM apoptosis and increased the number of bacteremic mice, indicating a novel role for caspase activation in pulmonary innate defense against pneumococci. These results suggest that AM play a key role in clearance of bacteria from the lung during subclinical infection and that induction of AM apoptosis contributes to the microbiologic host defense against pneumococci.

Monocytes treated with human immunodeficiency virus Tat kill uninfected CD4(+) cells by a tumor necrosis factor-related apoptosis-induced ligand-mediated mechanism

The human immunodeficiency virus (HIV) Tat protein has a critical role in viral transcription, but this study focuses on its additional role as an extracellular effector of lymphocyte cell death. It is well known that Tat induces tumor necrosis factor-related apoptosis-induced ligand (TRAIL) in peripheral blood mononuclear cells (PBMC), and we show that the majority of TRAIL is produced by the monocyte subset of PBMC. Human monocytes and U937 monoblastoid cells did not take up soluble HIV Tat-86, as T cells did, yet produced more TRAIL than did T cells. TRAIL secretion was induced by Tat and by a cysteine-rich peptide of Tat but not by sulfhydryl-modified Tat toxoid. Although there was only a slight increase in cell surface expression of TRAIL on monocytes, sufficient TRAIL was secreted to be toxic for T cells. The cytotoxicity of Tat-stimulated monocyte medium could be blocked by a TRAIL-neutralizing antibody. T cells treated with Tat did not secrete enough TRAIL to mediate cell death in our assay. Remarkably, uninfected T cells are more susceptible to TRAIL than are HIV-infected T cells. The production of TRAIL by Tat-stimulated monocytes provides a mechanism by which HIV infection can destroy uninfected bystander cells.

Apoptosis as an HIV strategy to escape immune attack

Viruses have evolved numerous mechanisms to evade the host immune system and one of the strategies developed by HIV is to activate apoptotic programmes that destroy immune effectors. Not only does the HIV genome encode pro-apoptotic proteins, which kill both infected and uninfected lymphocytes through either members of the tumour-necrosis factor family or the mitochondrial pathway, but it also creates a state of chronic immune activation that is responsible for the exacerbation of physiological mechanisms of clonal deletion. This review discusses the molecular mechanisms by which HIV manipulates the apoptotic machinery to its advantage, assesses the functional consequences of this process and evaluates how new therapeutics might counteract this strategy.

The role of intrahepatic immune effector cells in inflammatory liver injury and viral control during chronic hepatitis B infection

Cytotoxic T lymphocytes (CTL) and Kupffer cells play an important role in the immune control of hepatitis B virus (HBV), but may also induce liver injury during infection. We investigated the intrahepatic immune response in liver biopsies of chronic HBV patients in relation to inflammatory liver injury and viral control. Forty-seven liver biopsies from patients with chronic HBV with varying degrees of inflammation (ALT values) were selected. Acute hepatitis and normal liver specimens served as controls. Immune effector cells, cytotoxic effector molecules and cytokine producing cells were quantified after immunohistochemical staining in lobular and portal areas of the biopsies. The intralobular number of CD8+ T-lymphocytes was significantly decreased in biopsies of patients with high ALT (r = -0.54; P < 0.001). Higher ALT-values were correlated with increased numbers of granzyme+ cells in portal areas (r = 0.65; P < 0.001) and higher numbers of intralobular Fas-L+ cells (r = 0.32; P = 0.05). Fas-L was expressed on Kupffer and lymphoid cells. More intralobular CD8+ T-lymphocytes were found in HBeAg- than in HBeAg+ patients (P = 0.002). But IFN-gamma and TNF-alpha producing cells were observed sporadically in chronic HBV patients. Hence, in chronic HBV infection, low viral replication and HBeAg negativity is related to increased presence of intralobular CD8+ T-lymphocytes. Persistence of the virus may be caused by the absence of cells producing anti-viral cytokines in the liver. Inflammatory liver injury during chronic HBV infection is probably not the result of increased numbers of infiltrating CD8+ T-lymphocytes, but of Fas-L expression by Kupffer cells and increased cytolytic activity of cells in portal areas.