B7-H1 up-regulation impairs myeloid DC and correlates with disease progression in chronic HIV-1 infection

Biological Characteristics and Clinical Significance of Soluble PD-1/PD-L1 and Exosomal PD-L1 in Cancer

The immune checkpoint pathway consisting of the cell membrane-bound molecule programmed death protein 1 (PD-1) and its ligand PD-L1 has been found to mediate negative regulatory signals that effectively inhibit T-cell proliferation and function and impair antitumor immune responses. Considerable evidence suggests that the PD-1/PD-L1 pathway is responsible for tumor immune tolerance and immune escape. Blockage of this pathway has been found to reverse T lymphocyte depletion and restore antitumor immunity. Antagonists targeting this pathway have shown significant clinical activity in specific cancer types. Although originally identified as membrane-type molecules, several other forms of PD-1/PD-L1 have been detected in the blood of cancer patients, including soluble PD-1/PD-L1 (sPD-1/sPD-L1) and exosomal PD-L1 (exoPD-L1), increasing the composition and functional complications of the PD-1/PD-L1 signaling pathway. For example, sPD-1 has been shown to block the PD-1/PD-L immunosuppressive pathway by binding to PD-L1 and PD-L2, whereas the role of sPD-L1 and its mechanism of action in cancer remain unclear. In addition, many studies have investigated the roles of exoPD-L1 in immunosuppression, as a biomarker for tumor progression and as a predictive biomarker for response to immunotherapy. This review describes the molecular mechanisms underlying the generation of sPD-1/sPD-L1 and exoPD-L1, along with their biological activities and methods of detection. In addition, this review discusses the clinical importance of sPD-1/sPD-L1 and exoPD-L1 in cancer, including their predictive and prognostic roles and the effects of treatments that target these molecules.

Defects in Macrophage Reprogramming in Cancer Therapy: The Negative Impact of PD-L1/PD-1

Classically activated M1 macrophages and alternatively activated M2 macrophages are two polarized subsets of macrophages at the extreme ends of a constructed continuum. In the field of cancer research, M2 macrophage reprogramming is defined as the repolarization of pro-tumoral M2 to anti-tumoral M1 macrophages. It is known that colony-stimulating factor 1 (CSF1)/CSF1 receptor (CSF1R) and CSF2/CSF2R signaling play important roles in macrophage polarization. Targeting CSF1/CSF1R for M2 macrophage reprogramming has been widely performed in clinical trials for cancer therapy. Other targets for M2 macrophage reprogramming include Toll-like receptor 7 (TLR7), TLR8, TLR9, CD40, histone deacetylase (HDAC), and PI3Kγ. Although macrophages are involved in innate and adaptive immune responses, M1 macrophages are less effective at phagocytosis and antigen presenting, which are required properties for the activation of T cells and eradication of cancer cells. Similar to T and dendritic cells, the “functionally exhausted” status might be attributed to the high expression of programmed death-ligand 1 (PD-L1) or programmed cell death protein 1 (PD-1). PD-L1 is expressed on both M1 and M2 macrophages. Macrophage reprogramming from M2 to M1 might increase the expression of PD-L1, which can be transcriptionally activated by STAT3. Macrophage reprogramming or PD-L1/PD-1 blockade alone is less effective in the treatment of most cancers. Since PD-L1/PD-1 blockade could make up for the defect in macrophage reprogramming, the combination of macrophage reprogramming and PD-L1/PD-1 blockade might be a novel treatment strategy for cancer therapy.

Soluble PD-1: Predictive, Prognostic, and Therapeutic Value for Cancer Immunotherapy

Programmed death protein 1 (PD-1) interaction with PD-L1 deliver immunosuppressive environment for tumor growth, and its blockade with directed monoclonal antibodies (anti-PD-1/anti-PD-L1) has shown remarkable clinical outcome. Lately, their soluble counterparts, sPD-1 and sPD-L1, have been detected in plasma, and elevated levels have been associated with advanced disease, clinical stages, and worst prognosis for cancer patients. Elevated plasma levels of sPD-L1 have been correlated with worst prognosis in several studies and has displayed a persistent outlook. On the other hand, sPD-1 levels have been inconsistent in their predictive and prognostic ability. Pretherapeutic higher sPD-1 plasma levels have shown to predict advanced disease state and to a lesser extent worst prognosis. Any increase in sPD-1 plasma level post therapeutically have been correlated with improved survival for various cancers. In vitro and in vivo studies have shown sPD-1 ability to bind PD-L1 and PD-L2 and block PD-1/PD-L1 interaction. Local delivery of sPD-1 in cancer tumor microenvironment through local gene therapy have demonstrated an increase in tumor specific CD8+ T cell immunity and tumor growth reduction. It had also exhibited enhancement of T cell immunity induced by vaccination and other gene therapeutic agents. Furthermore, it may also lessen the inhibitory effect of circulating sPD-L1 and enhance the effects of mAb-based immunotherapy. In this review, we highlight various aspects of sPD-1 role in cancer prediction, prognosis, and anti-cancer immunity, as well as, its therapeutic value for local gene therapy or systemic immunotherapy in blocking the PD-1 and PD-L1 checkpoint interactions.

HIV-1 Tat - TLR4/MD2 interaction drives the expression of IDO-1 in monocytes derived dendritic cells through NF-κB dependent pathway

In the present study we showed that HIV-1 Tat protein stimulated the expression of Indoleamine 2,3 dioxygenase (IDO) -1 in human monocytes derived dendritic cells (MoDC) but not IDO-2 by acting directly at the cell membrane level. This induction of IDO-1 is dependent on the secondary structure of Tat protein, since stimulation with a chemically oxidized Tat protein loses its capacity to induce the production of IDO-1. Among the variety of candidate receptors described for Tat, we demonstrated that Tat protein interacted physically with TLR4/MD2 complex. Strikingly, blockade of Tat-TLR4 interaction by anti-TLR4 antibodies (clone HTA125), LPS-RS, a known TLR4 antagonist, or by soluble recombinant TLR4/MD2 complex inhibited strongly or totally the capacity of Tat to induce IDO-1 in MoDC while such treatments had no effect on IFN-γ-induced IDO-1. Furthermore, we showed that the activation of the transcription factor NF-κB by Tat is essential for the production of IDO-1 by human MoDC. Indeed, Tat activated NF-κB pathway in MoDC as demonstrated by the phosphorylation of p65 in Tat-treated MoDC. Further, we demonstrate that the stimulation of IDO-1 by Tat or by IFN-γ was totally or partially inhibited in the presence of NF-κB inhibitor respectively. These results suggest that Tat and IFN-γ act probably by two distinct mechanisms to induce the production of IDO-1. Our results clearly demonstrated that, although TLR4 pathway is necessary for Tat-induced IDO-1 in MoDC, it seems not to be sufficient since stable transfection of a functional TLR4/MD2 pathway in HEK or HeLa cell lines which are endogenously defectives for TLR4, did not restore the capacity of Tat to induce IDO-1 while IFN-γ treatment induces IDO-1 in HeLa cells independently of TLR4 pathway. These results suggest the involvement of additional stimuli in addition to TLR4 pathway which remain to be identified. Altogether our results demonstrated that, in human MoDC, HIV-1 Tat protein induced IDO-1 expression and activity in a NF-κB dependent-manner by recruiting TLR4 pathway.

The PD-1/PD-L1 Axis and Virus Infections: A Delicate Balance

Programmed cell death protein (PD-1) and its ligands play a fundamental role in the evasion of tumor cells from antitumor immunity. Less well appreciated is the fact that the PD-1/PD-L1 axis also regulates antiviral immune responses and is therefore modulated by a number of viruses. Upregulation of PD-1 and its ligands PD-L1 and PD-L2 is observed during acute virus infection and after infection with persistent viruses including important human pathogens such as human immunodeficiency virus (HIV), hepatitis C virus (HCV), and hepatitis B virus (HBV). Experimental evidence suggests that insufficient signaling through the PD-1 pathway promotes immunopathology during acute infection by exaggerating primary T cell responses. If chronic infection is established, however, high levels of PD-1 expression can have unfavorable immunological consequences. Exhaustion and suppression of antiviral immune responses can result in viral immune evasion. The role of the PD-1/PD-L1 axis during viral infections is further complicated by evidence that PD-L1 also mediates inflammatory effects in the acute phase of an immune response. In this review, we discuss the intricate interplay between viruses and the PD-1/PD-L1 axis.

Programmed cell death-1 contributes to the establishment and maintenance of HIV-1 latency

OBJECTIVE

In HIV-infected individuals on antiretroviral therapy (ART), latent HIV is enriched in CD4 T cells expressing immune checkpoint molecules, in particular programmed cell death-1 (PD-1). We therefore assessed the effect of blocking PD-1 on latency, both in vitro and in vivo.

METHODS

HIV latency was established in vitro following coculture of resting CD4+ T cells with myeloid dendritic cells. Expression of PD-1 was quantified by flow cytometry, and latency assessed in sorted PD-1high and PD-1low/-nonproliferating CD4+ memory T cells. The role of PD-1 in the establishment of latency was determined by adding anti-PD-1 (pembrolizumab) to cocultures before and after infection. In addition, a single infusion of anti-PD-1 (nivolumab) was administered to an HIV-infected individual on ART with metastatic melanoma, and cell-associated HIV DNA and RNA, and plasma HIV RNA were quantified.

RESULTS

HIV latency was significantly enriched in PD-1high compared with PD-1low/- nonproliferating, CD4 memory T cells. Sorting for an additional immune checkpoint molecule, T-cell immunoglobulin domain and mucin domain-3, in combination with PD-1, further enriched for latency. Blocking PD-1 prior to HIV infection, in vitro, resulted in a modest but significant decrease in latently infected cells in all donors (n = 6). The administration of anti-PD-1 to an HIV-infected individual on ART resulted in a significant increase in cell-associated HIV RNA in CD4 T cells, without significant changes in HIV DNA or plasma HIV RNA, consistent with reversal of HIV latency.

CONCLUSION

PD-1 contributes to the establishment and maintenance of HIV latency and should be explored as a target, in combination with other immune checkpoint molecules, to reverse latency.

Human Immunodeficiency Virus Type-1 Myeloid Derived Suppressor Cells Inhibit Cytomegalovirus Inflammation through Interleukin-27 and B7-H4

HIV/CMV co-infected persons despite prolonged viral suppression often experience persistent immune activation, have an increased frequency of myeloid derived suppressor cells (MDSC) and are at increased risk for cardiovascular disease. We examined how HIV MDSC control CD4⁺ T cell IFNγ response to a CMVpp65 peptide pool (CMVpp65). We show that HIV/CMV co-infected persons with virologic suppression and recovered CD4⁺ T cells compared to HIV(−)/CMV(+) controls exhibit an increase in CD4⁺CX3CR1⁺IFNγ⁺ cells in response to CMVpp65; MDSC depletion further augmented CD4⁺CX3CR1⁺IFNγ⁺ cells and IFNγ production. IL-2 and IFNγ in response to CMVpp65 were enhanced with depletion of MDSC expanded in presence of HIV (HIV MDSC), but decreased with culture of HIV MDSC with autologous PBMCs. CMVpp65 specific CD4⁺CX3CR1⁺IFNγ⁺ cells were also decreased in presence of HIV MDSC. HIV MDSC overexpressed B7-H4 and silencing B7-H4 increased the production of IL-2 and IFNγ from autologous cells; a process mediated through increased phosphorylated (p)-Akt upon stimulation with CMVpp65. Additionally, IL-27 regulated the expression of B7-H4 on HIV MDSC, and controlled CMV-specific T cell activity by limiting CMVpp65-IFNγ production and expanding CD4⁺IL-10⁺ regulatory T cells. These findings provide new therapeutic targets to control the chronic immune activation and endothelial cell inflammation observed in HIV-infected persons.

Increased expression of negative regulators of cytokine signaling during chronic HIV disease cause functionally exhausted state of dendritic cells

Mechanisms of functional impairment of dendritic cells (DCs) during chronic HIV-1 infection are not well understood. In order to understand this phenomenon, we aimed to study the expression of negative regulators of cytokine signaling and correlate with DC exhaustion during chronic HIV-1 disease. Monocyte-derived DCs (mo-DCs) from 27 HIV-1 infected patients (CD4+ T-cell counts: 429±44 cells/μL, plasma viral load: Log₁₀3.9±1.0copies/ml) and 19 healthy controls (HCs) were stimulated ex vivo with TLR4 agonist, lipopolysaccharide (LPS) for 2days to evaluate their functional fitness. The expression of a set of genes associated with cytokine signaling was evaluated in a custom designed PCR array by Real-Time PCR. The mo-DCs from HIV-1 infected patients depicted functional exhaustion as evident by decreased allo-stimulation index (mean±SD: 10±6 vs. 24±16) (p<0.05), decreased cytokine production (pg/ml) (IL-12: 4.6±16 vs. 25±85; TNF-α: 128±279 vs. 286±544; IL-10: 6±12 vs. 13±20; IL-8: 10,688±11,748 vs. 17,470±125,049) and retained endocytosis (1.1±0.3 vs. 1.0±0.29) (p<0.05) even after LPS-stimulation, as compared to HCs. Significantly upregulated expression of SOCS-1 (mean±SD fold change: 2.2±2vs.0.8±0.6), SOCS-3 (6.3±7.4vs.1.4±0.4), PIAS-1 (1.6±0.1vs.1.0±0.3) and SHP-1 (0.8±0.4vs.0.4±0.2) correlated positively with PD-L1 expression in these DCs (Spearman's coefficient, SOCS-1: 0.63, SOCS-3: 1.0 and PIAS-1: 0.7) (p<0.05). The expression of these molecules trended positively with plasma viral load and negatively with CD4+ T-cell counts. These findings suggest that the upregulation of negative regulatory factors during chronic HIV disease have profound down-modulatory effects on DC functions and establishment of an overall exhausted state. Understanding mechanisms causing upregulation of these factors may lead to the design of new generation therapeutics based on silencing of their gene expression.

Soluble programmed cell death receptor-1 (sPD-1): a potential biomarker with anti-inflammatory properties in human and experimental acute respiratory distress syndrome (ARDS)

Background

Acute respiratory distress syndrome (ARDS) remains a common organ dysfunction in the critically ill patient. Mechanisms for its development have focused on immune mediated causes, aspects of our understanding are not complete, and we lack biomarkers.

Design, setting, and subjects

Blood and bronchial alveolar lavage fluid (BAL) from humans (n = 10–13) with ARDS and controls (n = 5–10) as well as a murine model of ARDS (n = 5–6) with controls (n = 6–7) were studied.

Methods

ARDS was induced in mice by hemorrhagic shock (day 1) followed by poly-microbial sepsis (day 2). Samples were then collected on the third day after the animals were euthanized. Ex vivo experiments used splenocytes from animals with ARDS cultured with and without soluble programmed death receptor-1 (sPD-1).

Results

Levels of sPD-1 are increased in both the serum (11,429.3 pg/mL(SD 2133.3) vs. 8061.4(SD 4187.8), p = 0.036) and bronchial alveolar lavage (BAL) fluid (6,311.1 pg/mL(SD 3758.0) vs. 90.7 pg/mL(SD 202.8), p = 0.002) of humans with ARDS. Similar results are seen in the serum (9396.1 pg/mL(SD 1546.0) vs. 3464.5 pg/mL(SD 2511.8), p = 0.001) and BAL fluid (2891.7 pg/mL(SD 868.1) vs. 1385.9 pg/mL(SD 927.8), p = 0.012) of mice. sPD-1 levels in murine blood (AUC = 1(1–1), p = 0.006), murine BAL fluid (AUC = 0.905(0.717–1.093), p = 0.015), and human BAL (AUC = 1(1–1), p = 0.001) fluid predicted ARDS. To assess the importance of sPD-1 in ARDS, ex vivo experiments were undertaken. BAL fluid from mice with ARDS dampens the TNF-α production compared to cells cultured with BAL lacking sPD-1 (2.7 pg/mL(SD 3.8) vs. 52.38 pg/mL(SD 25.1), p = 0.002).

Conclusions

This suggests sPD-1 is elevated in critical illness and may represent a potential biomarker for ARDS. In addition, sPD-1 has an anti-inflammatory mechanism in conditions of marked stress and aids in the resolution of severe inflammation. sPD-1 could be used to not only diagnose ARDS, but may be a potential therapy.

HIV-1 and hijacking of the host immune system: the current scenario

Human immunodeficiency virus (HIV) infection is a major health burden across the world which leads to the development of acquired immune deficiency syndrome (AIDS). This review article discusses the prevalence of HIV, its major routes of transmission, natural immunity, and evasion from the host immune system. HIV is mostly prevalent in Sub-Saharan Africa and low income countries. It is mostly transmitted by sharing syringe needles, blood transfusion, and sexual routes. The host immune system is categorized into three main types; the innate, the adaptive, and the intrinsic immune system. Regarding the innate immune system against HIV, the key players are mucosal membrane, dendritic cells (DCs), complement system, interferon, and host Micro RNAs. The major components of the adaptive immune system exploited by HIV are T cells mainly CD4+ T cells and B cells. The intrinsic immune system confronted by HIV involves (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G) APOBEC3G, tripartite motif 5-α (TRIM5a), terherin, and (SAM-domain HD-domain containing protein) SAMHD1. HIV-1 efficiently interacts with the host immune system, exploits the host machinery, successfully replicates and transmits from one cell to another. Further research is required to explore evasion strategies of HIV to develop novel therapeutic approaches against HIV.

Lymph nodes from HIV-infected individuals harbor mature dendritic cells and increased numbers of PD-L1+ conventional dendritic cells

The immune response induced by dendritic cells (DC) during the HIV infection has been of remarkable interest because of the therapeutic potential of DC for vaccine development. However, their beneficial or detrimental contribution in HIV infection remains unclear. The activation state of DC in lymph nodes (LN) is essential to induce T cell responses against HIV. In the present study, we characterized the immunophenotype and function of conventional (cDC) and plasmacytoid (pDC) dendritic cells from peripheral blood (PB) and LN of HIV(+) individuals. We observed that the frequency of PB pDC was decreased and exhibited an immature phenotype; whereas in the LN, activated pDC accumulated (CD40(+) and CD83(+)). In addition, the frequency of PB cDC from HIV(+) individuals was decreased and exhibited an immature phenotype, whereas LN harbored activated and mature cDC (CD40(+), CD83(+), CD80(+) and CD86(+)). However, an increased number of PD-L1(+) cDC was also observed in the LN. Moreover, pDC and cDC were able to produce inflammatory cytokines (IFN-α, TNF-α and IL-12) after TLR stimulation. These findings suggests that LN cDC expressing PD-L1 from HIV(+) individuals may negatively impact the generation of HIV-specific T cells and that DC might be contributing to tissue chronic immune activation.

A lack of Fas/FasL signalling leads to disturbances in the antiviral response during ectromelia virus infection

Ectromelia virus (ECTV) is an orthopoxvirus (OPV) that causes mousepox, the murine equivalent of human smallpox. Fas receptor-Fas ligand (FasL) signaling is involved in apoptosis of immune cells and virus-specific cytotoxicity. The Fas/FasL pathway also plays an important role in controlling the local inflammatory response during ECTV infection. Here, the immune response to the ECTV Moscow strain was examined in Fas (-) (lpr), FasL (-) (gld) and C57BL6 wild-type mice. During ECTV-MOS infection, Fas- and FasL mice showed increased viral titers, decreased total numbers of NK cells, CD4(+) and CD8(+) T cells followed by decreased percentages of IFN-γ expressing NK cells, CD4(+) and CD8(+) T cells in spleens and lymph nodes. At day 7 of ECTV-MOS infection, Fas- and FasL-deficient mice had the highest regulatory T cell (Treg) counts in spleen and lymph nodes in contrast to wild-type mice. Furthermore, at days 7 and 10 of the infection, we observed significantly higher numbers of PD-L1-expressing dendritic cells in Fas (-) and FasL (-) mice in comparison to wild-type mice. Experiments in co-cultures of CD4(+) T cells and bone-marrow-derived dendritic cells showed that the lack of bilateral Fas-FasL signalling led to expansion of Tregs. In conclusion, our results demonstrate that during ECTV infection, Fas/FasL can regulate development of tolerogenic DCs and Tregs, leading to an ineffective immune response.

Role of Regulatory T Cells and Inhibitory Molecules in the Development of Immune Exhaustion During Human Immunodeficiency Virus Type 1 Infection

One of the key hallmarks of chronic human immunodeficiency virus type 1 (HIV-1) infection is the persistent immune activation triggered since early stages of the infection, followed by the development of an exhaustion phenomena, which leads to the inability of immune cells to respond appropriately to the virus and other pathogens, constituting the acquired immunodeficiency syndrome (AIDS); this exhausting state is characterized by a loss of effector functions of immune cells such as proliferation, production of cytokine, as well as cytotoxic potential and it has been attributable to an increased response of regulatory T cells and recently also to the expression in different cell populations of inhibitory molecules, such as programmed death receptor-1 (PD-1), cytotoxic T lymphocyte antigen-4 (CTLA-4), T cell immunoglobulin-3 (Tim-3), and lymphocyte activation gene-3 (LAG-3). The importance of these molecules relies on the possibility to restore the immune response once these molecules are blocked, constituting a potential therapeutic target for treatment during HIV infection. In this regard, we explored the available data evaluating the functional role of Treg cells and inhibitory molecules during the infection in both blood and gut-associated lymphoid tissue (GALT) and their contribution to the development of immune exhaustion and progression to AIDS, as well as their therapeutic potential.

HIV-1 Tat Protein Induces Production of Proinflammatory Cytokines by Human Dendritic Cells and Monocytes/Macrophages through Engagement of TLR4-MD2-CD14 Complex and Activation of NF-κB Pathway

We recently reported that the human immunodeficiency virus type-1 (HIV-1) Tat protein induced the expression of programmed death ligand-1 (PD-L1) on dendritic cells (DCs) through a TLR4 pathway. However, the underlying mechanisms by which HIV-1 Tat protein induces the abnormal hyper-activation of the immune system seen in HIV-1 infected patients remain to be fully elucidated. In the present study, we report that HIV-1 Tat protein induced the production of significant amounts of the pro-inflammatory IL-6 and IL-8 cytokines by DCs and monocytes from both healthy and HIV-1 infected patients. Such production was abrogated in the presence of anti-TLR4 blocking antibodies or soluble recombinant TLR4-MD2 as a decoy receptor, suggesting TLR4 was recruited by Tat protein. Tat-induced murine IL-6 and CXCL1/KC a functional homologue of human IL-8 was abolished in peritoneal macrophages derived from TLR4 KO but not from Wt mice, confirming the involvement of the TLR4 pathway. Furthermore, the recruitment of TLR4-MD2-CD14 complex by Tat protein was demonstrated by the activation of TLR4 downstream pathways including NF-κB and SOCS-1 and by down-modulation of cell surface TLR4 by endocytosis in dynamin and lipid-raft-dependent manners. Collectively, these findings demonstrate, for the first time, that HIV-1 Tat interacts with TLR4-MD2-CD14 complex and activates the NF-κB pathway, leading to overproduction of IL-6 and IL-8 pro-inflammatory cytokines by myeloid cells from both healthy and HIV-1 infected patients. This study reveals a novel mechanism by which HIV-1, via its early expressed Tat protein, hijacks the TLR4 pathway, hence establishing abnormal hyper-activation of the immune system.

Role of PD-1 co-inhibitory pathway in HIV infection and potential therapeutic options

Virus-specific CD8⁺ T cells play an important role in controlling viral infections including human immunodeficiency virus (HIV) infection. However, during chronic HIV infection, virus-specific CD8⁺ T cells undergo functional exhaustion, lose effector functions and fail to control viral infection. HIV-specific CD8 T cells expressing high levels of co-inhibitory molecule programmed death-1 (PD-1) during the chronic infection and are characterized by lower proliferation, cytokine production, and cytotoxic abilities. Although, antiretroviral therapy has resulted in dramatic decline in HIV replication, there is no effective treatment currently available to eradicate viral reservoirs or restore virus-specific T or B-cell functions that may complement ART in order to eliminate the virus. In recent years, studies in mice and non-human primate models of HIV infection demonstrated the functional exhaustion of virus-specific T and B cells could be reversed by blockade of interaction between PD-1 and its cognate ligands (PD-L1 and PD-L2). In this review, we discuss recent advances in our understanding of PD-1 pathway in HIV/SIV infection and discuss the beneficial effects of PD-1 blockade during chronic HIV/SIV infection and its potential role as immunotherapy for HIV/AIDS.

Programming T cell Killers for an HIV Cure: Teach the New Dogs New Tricks and Let the Sleeping Dogs Lie

Despite the success of combination antiretroviral therapy (cART), a latent viral reservoir persists in HIV-1-infected persons. Unfortunately, endogenous cytotoxic T lymphocytes (CTLs) are unable to control viral rebound when patients are removed from cART. A "kick and kill" strategy has been proposed to eradicate this reservoir, whereby infected T cells are induced to express viral proteins via latency-inducing drugs followed by their elimination by CTLs. It has yet to be determined if stimulation of existing HIV-1-specific CTL will be sufficient, or if new CTLs should be primed from naïve T cells. In this review, we propose that dendritic cells (DCs), the most potent antigen presenting cells, act as dog trainers and can induce T cells (the dogs) to do magnificent tricks. We propose the hypothesis that an HIV-1 cure will require targeting of naïve T cells and will necessitate "teaching new dogs new tricks" while avoiding activation of potentially dysfunctional endogenous memory CTLs (letting the sleeping dogs lie).

The PD-1/PD-Ls pathway and autoimmune diseases

The programmed death (PD)-1/PD-1 ligands (PD-Ls) pathway, is a new member of the B7/CD28 family, and consists of the PD-1 receptor and its ligands PD-L1 (B7-H1, CD274) and PD-L2 (B7-DC, CD273). Recently, it is reported that PD-1, PD-L1 and PD-L2 also have soluble forms aside from their membrane bound forms. The soluble forms increase the diversity and complexity of PD-1/PD-Ls pathway in both composition and function. The PD-1/PD-Ls pathway is broadly expressed and exerts a wider range of immunoregulatory roles in T-cell activation and tolerance compared with other B7/CD28 family members. Studies show that the PD-1/PD-Ls pathway regulates the induction and maintenance of peripheral tolerance and protects tissues from autoimmune attack in physiological conditions. In addition, it is also involved in various diseases mediated by T cells, such as autoimmunity, tumor immunity, chronic viral infections, and transplantation immunity. In this review, we will summarize the relevance of the soluble forms and the latest researches on the role of PD-1/PD-Ls pathway in autoimmune diseases.

HIV-1 Tat protein induces PD-L1 (B7-H1) expression on dendritic cells through tumor necrosis factor alpha- and toll-like receptor 4-mediated mechanisms

Chronic human immunodeficiency virus type 1 (HIV-1) infection is associated with induction of T-cell coinhibitory pathways. However, the mechanisms by which HIV-1 induces upregulation of coinhibitory molecules remain to be fully elucidated. The aim of the present study was to determine whether and how HIV-1 Tat protein, an immunosuppressive viral factor, induces the PD-1/PD-L1 coinhibitory pathway on human dendritic cells (DCs). We found that treatment of DCs with whole HIV-1 Tat protein significantly upregulated the level of expression of PD-L1. This PD-L1 upregulation was observed in monocyte-derived dendritic cells (MoDCs) obtained from either uninfected or HIV-1-infected patients as well as in primary myeloid DCs from HIV-negative donors. In contrast, no effect on the expression of PD-L2 or PD-1 molecules was detected. The induction of PD-L1 on MoDCs by HIV-1 Tat (i) occurred in dose- and time-dependent manners, (ii) was mediated by the N-terminal 1-45 fragment of Tat, (iii) did not require direct cell-cell contact but appeared rather to be mediated by soluble factor(s), (iv) was abrogated following neutralization of tumor necrosis factor alpha (TNF-α) or blocking of Toll-like receptor 4 (TLR4), (v) was absent in TLR4-knockoout (KO) mice but could be restored following incubation with Tat-conditioned medium from wild-type DCs, (vi) impaired the capacity of MoDCs to functionally stimulate T cells, and (vii) was not reversed functionally following PD-1/PD-L1 pathway blockade, suggesting the implication of other Tat-mediated coinhibitory pathways. Our results demonstrate that HIV-1 Tat protein upregulates PD-L1 expression on MoDCs through TNF-α- and TLR4-mediated mechanisms, functionally compromising the ability of DCs to stimulate T cells. The findings offer a novel potential molecular target for the development of an anti-HIV-1 treatment.

IMPORTANCE

The objective of this study was to investigate the effect of human immunodeficiency virus type 1 (HIV-1) Tat on the PD-1/PD-L1 coinhibitory pathway on human monocyte-derived dendritic cells (MoDCs). We found that treatment of MoDCs from either healthy or HIV-1-infected patients with HIV-1 Tat protein stimulated the expression of PD-L1. We demonstrate that this stimulation was mediated through an indirect mechanism, involving tumor necrosis factor alpha (TNF-α) and Toll-like receptor 4 (TLR4) pathways, and resulted in compromised ability of Tat-treated MoDCs to functionally stimulate T-cell proliferation.

Latency-associated protein Acr1 impairs dendritic cell maturation and functionality: a possible mechanism of immune evasion by Mycobacterium tuberculosis

Mycobacterium tuberculosis (M. tuberculosis) in latently infected individuals survives and thwarts the attempts of eradication by the immune system. During latency, Acr1 is predominantly expressed by the bacterium. However, whether M. tuberculosis exploits its Acr1 in impairing the host immunity remains widely unexplored. Hence, currently we have investigated the role of Acr1 in influencing the differentiation and function of dendritic cells (DCs), which play a cardinal role in innate and adaptive immunity. Therefore, for the first time, we have revealed a novel mechanism of mycobacterial Acr1 in inhibiting the maturation and differentiation of DCs by inducing tolerogenic phenotype by modulating the expression of PD-L1; Tim-3; indoleamine 2, 3-dioxygenase (IDO); and interleukin 10. Furthermore, Acr1 interferes in the differentiation of DCs by targeting STAT-6 and STAT-3 pathways. Continuous activation of STAT-3 inhibited the translocation of NF-κB in Acr1-treated DCs. Furthermore, Acr1 also augmented the induction of regulatory T cells. These DCs displayed decline in their antigen uptake capacity and reduced ability to help T cells. Interestingly, M. tuberculosis exhibited better survival in Acr1-treated DCs. Thus, this study provides a crucial insight into a strategy adopted by M. tuberculosis to survive in the host by impairing the function of DCs.

Enhancing immunogenicity and cross-reactivity of HIV-1 antigens by in vivo targeting to dendritic cells

Current retroviral treatments have reduced AIDS to a chronic disease for most patients. However, given drug-related side effects, the emergence of drug-resistant strains and the persistence of viral replication, the development of alternative treatments is a pressing need. This review focuses on recent developments in HIV immunotherapy treatments, with particular emphasis on current vaccination strategies for optimizing the induction of an effective immune response by the recruitment of dendritic cells. In addition to cell-based therapies, targeted strategies aiming to deliver synthetic HIV peptides to dendritic cell-specific receptors in vivo will be discussed.

Molecular signatures of T-cell inhibition in HIV-1 infection

Cellular immune responses play a crucial role in the control of viral replication in HIV-infected individuals. However, the virus succeeds in exploiting the immune system to its advantage and therefore, the host ultimately fails to control the virus leading to development of terminal AIDS. The virus adopts numerous evasion mechanisms to hijack the host immune system. We and others recently described the expression of inhibitory molecules on T cells as a contributing factor for suboptimal T-cell responses in HIV infection both in vitro and in vivo. The expression of these molecules that negatively impacts the normal functions of the host immune armory and the underlying signaling pathways associated with their enhanced expression need to be discussed. Targets to restrain the expression of these molecular markers of immune inhibition is likely to contribute to development of therapeutic interventions that augment the functionality of host immune cells leading to improved immune control of HIV infection. In this review, we focus on the functions of inhibitory molecules that are expressed or secreted following HIV infection such as BTLA, CTLA-4, CD160, IDO, KLRG1, LAG-3, LILRB1, PD-1, TRAIL, TIM-3, and regulatory cytokines, and highlight their significance in immune inhibition. We also highlight the ensemble of transcriptional factors such as BATF, BLIMP-1/PRDM1, FoxP3, DTX1 and molecular pathways that facilitate the recruitment and differentiation of suppressor T cells in response to HIV infection.

Dysregulated immunophenotypic attributes of plasmacytoid but not myeloid dendritic cells in HIV-1 infected individuals in the absence of highly active anti-retroviral therapy

Dendritic cells (DC) in HIV-1-infected individuals are decreased and their dysfunction has been implicated in HIV-1 immunopathogenesis. The mechanism of their dysfunction remains unclear, thus we analysed the expression of membrane molecules associated with immune regulation and DC activation in myeloid (mDC) and plasmacytoid DC (pDC) in therapy-naive and highly active anti-retroviral therapy (HAART)-treated HIV-1(+) patients. DC from healthy controls, untreated HIV-1(+) and HAART-treated patients were assessed by flow cytometry for expression of: anergy and apoptosis inducing molecules [programmed death (PD)-1 and its ligands PD-L1 and PD-L2], inhibitory and regulatory T cell-inducing molecules [immunoglobulin-like transcript (ILT)-3 and ILT-4], interferon (IFN)-α inhibitory receptor (ILT-7) and co-stimulatory molecules (CD80, CD83, and CD86). pDC from untreated HIV-1(+) patients expressed significantly lower levels of ILT-7 compared to healthy controls, while HAART-treated patients showed normal expression. pDC were also found to express moderately higher levels of PD-L1 and ILT-3 and lower levels of PD-L2 receptors in untreated patients compared to controls and HAART-treated patients. No significant changes were observed in mDC. There were no associations between the percentages and levels of expression of these molecules by pDC and viral load or CD4 T cell count. In conclusion, pDC but not mDC from HIV-1(+) patients with active viraemia display higher levels of apoptosis and T regulatory-inducing molecules and may be predisposed to chronically produce IFN-α through down-regulation of ILT-7. HAART restored normal expression levels of these receptors.

Monocyte activation is a feature of common variable immunodeficiency irrespective of plasma lipopolysaccharide levels

Common variable immunodeficiency disorders (CVID), the most frequent cause of symptomatic primary immunodeficiency, are defined by impaired antibody production. Notwithstanding, T cell activation and granulomatous manifestations represent the main causes of CVID morbidity even in patients receiving immunoglobulin (Ig) G replacement therapy. Additionally, gut pathology is a frequent feature of CVID. In this study, we investigated monocyte imbalances and their possible relationship with increased microbial translocation in CVID patients. Monocyte subsets were defined according to CD14 and CD16 expression levels and evaluated in terms of human leucocyte antigen D-related (HLA-DR), CD86 and programmed death-1 molecule ligand 1 (PD-L1) expression by flow cytometry, in parallel with the quantification of plasma lipopolysaccharide (LPS) and serum levels of soluble CD14 (sCD14), LPS-binding protein (LBP) and anti-LPS antibodies. CVID patients (n=31) featured significantly increased levels of serum sCD14 and an expansion of CD14(bright) CD16(+) monocytes in direct correlation with T cell and B cell activation, the latter illustrated by the frequency of the CD21(low) CD38(low) subset. Such alterations were not observed in patients lacking B cells due to congenital agammaglobulinaemia (n=4). Moreover, we found no significant increase in circulating LPS or LBP levels in CVID patients, together with a relative preservation of serum anti-LPS antibodies, in agreement with their presence in commercial IgG preparations. In conclusion, CVID was associated with monocyte imbalances that correlated directly with T cell activation markers and with B cell imbalances, without an association with plasma LPS levels. The heightened monocyte activated state observed in CVID may represent an important target for complementary therapeutic strategies.

Role of PD-1 in HIV pathogenesis and as target for therapy

Major advances in Antiretroviral Therapy (ART) have resulted in a dramatic decline in HIV-related deaths. However, no current treatment regimen leads to viral eradication or restoration of HIV-specific immune responses capable of durable viral control after cessation of ART. Thus, there is a need for novel interventions that could complement ART in order to eliminate virus or reach a state of "functional cure." It has been shown in murine models and humans that the negative co-signaling molecule programmed-death 1 (PD-1) plays an active and reversible role in mediating T-cell exhaustion in chronic infections. This review summarizes recent advances in our understanding of the PD-1 pathway in HIV infection, and the lessons learned from studies in the SIV model and cancer. We discuss the potential of immunotherapeutic interventions targeting PD-1 in order to augment immune responses or facilitate viral eradication. We also present the challenges to therapies targeting immunoregulatory networks.

Myeloid dendritic cells isolated from tissues of SIV-infected Rhesus macaques promote the induction of regulatory T cells

OBJECTIVE

To determine whether the ability of primary myeloid dendritic cells (mDCs) to induce regulatory T cells (Treg) is affected by chronic simian immunodeficiency virus (SIV) infection.

DESIGN

Modulation of dendritic cell activity with the aim of influencing Treg frequency may lead to new treatment options for HIV and strategies for vaccine development.

METHODS

Eleven chronically infected SIV(+) Rhesus macaques were compared with four uninfected animals. Immature and mature mDCs were isolated from mesenteric lymph nodes and spleen by cell sorting and cultured with purified autologous non-Treg (CD4(+)CD25(-) T cells). CD25 and FOXP3 up-regulation was used to assess Treg induction.

RESULTS

The frequency of splenic mDC and plasmacytoid dendritic cell was lower in infected animals than in uninfected animals; their frequency in the mesenteric lymph nodes was not significantly altered, but the percentage of mature mDCs was increased in the mesenteric lymph nodes of infected animals. Mature splenic or mesenteric mDCs from infected animals were significantly more efficient at inducing Treg than mDCs from uninfected animals. Mature mDCs from infected macaques induced more conversion than immature mDCs. Splenic mDCs were as efficient as mesenteric mDCs in this context and CD103 expression by mDCs did not appear to influence the level of conversion.

CONCLUSIONS

Tissue mDCs from SIV-infected animals exhibit an enhanced capability to induce Treg and may contribute to the accumulation of Treg in lymphoid tissues during progressive infection. The activation status of dendritic cell impacts this process but the capacity to induce Treg was not restricted to mucosal dendritic cells in infected animals.

HIV impairment of immune responses in dendritic cells

Dendritic cells and their subsets are diverse populations of immune cells in the skin and mucous membranes that possess the ability to sense the presence of microbes and orchestrate an efficient and adapted immune response. Dendritic cells (DC) have the unique ability to act as a bridge between the innate and adaptive immune responses. These cells are composed of a number of subsets behaving with preferential and specific features depending on their location and surrounding environment. Langerhans cells (LC) or dermal DC (dDC) are readily present in mucosal areas. Other DC subsets such as plasmacytoid DC (pDC), myeloid DC (myDC), or monocyte-derived DC (MDDC) are thought to be recruited or differentiated in sites of pathogenic challenge. Upon HIV infection, DC and their subsets are likely among the very first immune cells to encounter incoming pathogens and initiate innate and adaptive immune responses. However, as evidenced during HIV infection, some pathogens have evolved subtle strategies to hijack key cellular machineries essential to generate efficient antiviral responses and subvert immune responses for spread and survival.In this chapter, we review recent research aimed at investigating the involvement of DC subtypes in HIV transmission at mucosal sites, concentrating on HIV impact on cellular signalling and trafficking pathways in DC leading to DC-mediated immune response alterations and viral immune evasion. We also address some aspects of DC functions during the chronic immune pathogenesis and conclude with an overview of the current and novel therapeutic and prophylactic strategies aimed at improving DC-mediated immune responses, thus to potentially tackle the early events of mucosal HIV infection and spread.

Increase of cells expressing PD-L1 in bovine leukemia virus infection and enhancement of anti-viral immune responses in vitro via PD-L1 blockade

The inhibitory receptor programmed death-1 (PD-1) and its ligand, programmed death-ligand 1 (PD-L1) are involved in immune evasion mechanisms for several pathogens causing chronic infections. Blockade of the PD-1/PD-L1 pathway restores anti-virus immune responses, with concomitant reduction in viral load. In a previous report, we showed that, in bovine leukemia virus (BLV) infection, the expression of bovine PD-1 is closely associated with disease progression. However, the functions of bovine PD-L1 are still unknown. To investigate the role of PD-L1 in BLV infection, we identified the bovine PD-L1 gene, and examined PD-L1 expression in BLV-infected cattle in comparison with uninfected cattle. The deduced amino acid sequence of bovine PD-L1 shows high homology to the human and mouse PD-L1. The proportion of PD-L1 positive cells, especially among B cells, was upregulated in cattle with the late stage of the disease compared to cattle at the aleukemic infection stage or uninfected cattle. The proportion of PD-L1 positive cells correlated positively with prediction markers for the progression of the disease such as leukocyte number, virus load and virus titer whilst on the contrary, it inversely correlated with the degree of interferon-gamma expression. Blockade of the PD-1/PD-L1 pathway in vitro by PD-L1-specific antibody upregulated the production of interleukin-2 and interferon-gamma, and correspondingly, downregulated the BLV provirus load and the proportion of BLV-gp51 expressing cells. These data suggest that PD-L1 induces immunoinhibition in disease progressed cattle during chronic BLV infection. Therefore, PD-L1 would be a potential target for developing immunotherapies against BLV infection.

The PD-1/PD-L1 (B7-H1) pathway in chronic infection-induced cytotoxic T lymphocyte exhaustion

Cytotoxic CD8 T lymphocytes (CTLs) play a pivotal role in the control of infection. Activated CTLs, however, often lose effector function during chronic infection. PD-1 receptor and its ligand PD-L1 of the B7/CD28 family function as a T cell coinhibitory pathway and are emerging as major regulators converting effector CTLs into exhausted CTLs during chronic infection with human immunodeficiency virus, hepatitis B virus, hepatitis C virus, and other pathogens capable of establishing chronic infections. Importantly, blockade of the PD-1/PD-L1 pathway is able to restore functional capabilities to exhausted CTLs and early clinical trials have shown promise. Further research will reveal how chronic infection induces upregulation of PD-1 on CTLs and PD-L1 on antigen-presenting cells and other tissue cells and how the PD-1/PD-L1 interaction promotes CTLs exhaustion, which is crucial for developing effective prophylactic and therapeutic vaccination against chronic infections.

Mechanism of T cell exhaustion in a chronic environment

T cell exhaustion develops under conditions of antigen-persistence caused by infection with various chronic pathogens, such as human immunodeficiency virus (HIV) and mycobacterium tuberculosis (TB), or by the development of cancer. T cell exhaustion is characterized by stepwise and progressive loss of T cell function, which is probably the main reason for the failed immunological control of chronic pathogens and cancers. Recent observations have detailed some of the intrinsic and extrinsic factors that influence the severity of T cell exhaustion. Duration and magnitude of antigenic activation of T cells might be associated with up-regulation of inhibitory receptors, which is a major intrinsic factor of T cell exhaustion. Extrinsic factors might include the production of suppressive cytokines, T cell priming by either non-professional antigen-presenting cells (APCs) or tolerogenic dendritic cells (DCs), and alteration of regulatory T (Treg) cells. Further investigation of the cellular and molecular processes behind the development of T cell exhaustion can reveal therapeutic targets and strategies for the treatment of chronic infections and cancers. Here, we report the properties and the mechanisms of T cell exhaustion in a chronic environment.

Systemic immune activation in HIV infection is associated with decreased MDC responsiveness to TLR ligand and inability to activate naive CD4 T-cells

Background

HIV infection is characterized by ineffective anti-viral T-cell responses and impaired dendritic cell (DC) functions, including response to Toll-Like Receptor (TLR) ligands. Because TLR responsiveness may affect a host's response to virus, we examined TLR ligand induced Myeloid and Plasmacytoid DC (MDC and PDC) activation of naïve T-cells in HIV+ subjects.

Methods

Freshly purified MDC and PDC obtained from HIV+ subjects and healthy controls were cultured in the presence and absence of TLR ligands (poly I∶C or R-848). We evaluated indices of maturation/activation (CD83, CD86, and HLA-DR expression), cytokine secretion (IFN-alpha and IL-6), and ability to activate allogeneic naïve CD4 T-cells to secrete IFN-gamma and IL-2.

Results

MDC from HIV+ subjects had increased spontaneous IL-6 production and increased CD83 and CD86 expression when compared to MDC of controls. MDC IL-6 expression was associated with plasma HIV level. At the same time, poly I∶C induced HLA-DR up-regulation on MDC was reduced in HIV+ persons when compared to controls. The latter finding was associated with impaired ability of MDC from HIV+ subjects to activate allogeneic naïve CD4 T-cells. PDC from HIV+ persons had increased spontaneous and TLR ligand induced IL-6 expression, and increased HLA-DR expression at baseline. The latter was associated with an intact ability of HIV PDC to activate allogeneic naïve CD4 T-cells.

Conclusion

These results have implications for the ability of the HIV+ host to form innate and adaptive responses to HIV and other pathogens.

B and T lymphocyte attenuator down-regulation by HIV-1 depends on type I interferon and contributes to T-cell hyperactivation

BACKGROUND

Nonspecific T-cell hyperactivation is the main driving force for human immunodeficiency virus (HIV)-1 disease progression, but the reasons why the excess immune response is not properly shut off are poorly defined.

METHODS

Eighty-five HIV-1-infected individuals were enrolled to characterize B and T lymphocyte attenuator (BTLA) expression and function. Infection and blockade assays were used to dissect the factors that influenced BTLA signaling in vitro.

RESULTS

BTLA expression on overall CD4(+) and CD8(+) T cells was progressively decreased in HIV-1 infection, which was directly correlated with disease progression and CD4(+) T-cell differentiation and activation. BTLA(+)CD4(+) T cells from HIV-1-infected patients also displayed an altered immune status, which was indicated by reduced expression of naive markers but increased activation and exhaustion markers. Cross-linking of BTLA can substantially decrease CD4(+) T-cell activation in vitro. This responsiveness of CD4(+) T cells to BTLA-mediated inhibitory signaling was further found to be impaired in HIV-1-infected patients. Furthermore, HIV-1 NL4-3 down-regulated BTLA expression on CD4(+) T cells dependent on plasmacytoid dendritic cell (pDC)-derived interferon (IFN)-α. Blockade of IFN-α or depletion of pDCs prevents HIV-1-induced BTLA down-regulation.

CONCLUSIONS

HIV-1 infection potentially impairs BTLA-mediated signaling dependent on pDC-derived IFN-α, which may contribute to broad T-cell hyperactivation induced by chronic HIV-1 infection.

Phagocytosis, a potential mechanism for myeloid-derived suppressor cell regulation of CD8+ T cell function mediated through programmed cell death-1 and programmed cell death-1 ligand interaction

CD8(+) T cells become exhausted, inducing cell surface protein programmed cell death-1 (PD-1) as chronic virus diseases or tumors progress, but underlying mechanisms of this are unclear. We previously showed that M-CSF is important for developing tolerogenic dendritic cells (DCs) from human CD14(+) monocytes. In this article, we identify M-CSF-derived DCs (M-DCs) after stimulation with IL-10 as myeloid-derived suppressor cells with additional tolerogenic activities to CD8(+) T cells. IL-10 increased PD-1 ligand expression on M-DC, and IL-10-stimulated M-DCs (M-DC/IL-10) induced expression of PD-1 on, and apoptosis of, CD8(+) T cells and phagocytosed CD8(+) T cells. Enhanced phagocytic activity of M-DC/IL-10 required IFN-γ, which further increased PD-1 ligand and PD-2 ligand expression on M-DC/IL-10. IFN-γ-stimulated M-DC/IL-10 cells were phenotypically macrophage-like cells with little or no expression of CD86, a costimulatory molecule, but with high expression levels of CD14, CD200R, and CD80. No phagocytic activity was detected with GM-CSF-derived DCs. We propose that phagocytosis by IFN-γ-stimulated M-DC/IL-10 cells, which may be DCs or, alternatively, a unique subset of macrophages, may be a mechanism by which IFN-γ-producing CD8(+) T cells are tolerized after type 1 immune responses to chronic virus or tumor, and that IFN-γ links effector CD8(+) T cells to their phagocytic clearance.

Human immunodeficiency virus type 1 Tat induces B7-H1 expression via ERK/MAPK signaling pathway

In HIV-infected subjects, B7-H1 synthesis and expression are up-regulated, and the degree of dysregulation correlates with the severity of disease. HIV-1 Tat protein, the viral transactivating factor, represents a key target for the host immune response. However, the relationship between B7-H1 and Tat protein has not been addressed. Here, we chose human endothelial cells which provide costimulatory signals sufficiently to influence T cells. We used recombinant pcDNA3.1(+)-Tat plasmid to transfect human endothelial cells ECV304 to establish stable Tat-expressed cell strain, and found that HIV-1 Tat was able to induce B7-H1 expression in ECV304 cells by Real-time PCR and flow cytometry analysis, and inhibited lymphocyte proliferation in co-culture system. Moreover, by using pharmacological inhibitor of ERK pathway, HIV-1 Tat induces B7-H1 expression via ERK/MAPK signaling pathway was corroborated. In summary, our results indicate that HIV-1 Tat could induce B7-H1 synthesis in ECV304 cells through ERK/MAPK signaling pathway.

A divergent myeloid dendritic cell response at virus set-point predicts disease outcome in SIV-infected rhesus macaques

BACKGROUND

The mechanism for loss of myeloid dendritic cells (mDCs) from the circulation in HIV-infected individuals and its relationship to disease progression is not understood.

METHODS

A longitudinal analysis of the mDC response in blood and lymph nodes during the first 12 weeks of infection was performed in a cohort of SIVmac251-infected rhesus macaques with different disease outcomes.

RESULTS

Monkeys that rapidly progressed to disease or had long-term stable infection had significant losses or increases, respectively, in blood mDCs that were inversely correlated with virus load at set-point. The loss of mDCs from progressor animals was associated with evidence of an increase in CCR7/CCL19-dependent mDC recruitment to lymph nodes and an increase in mDC apoptosis.

CONCLUSIONS

mDC recruitment to and death within inflamed lymph nodes may contribute to disease progression in SIV infection, whereas mobilization without increased recruitment to lymph nodes may promote disease control.

The STATus of PD-L1 (B7-H1) on tolerogenic APCs

Expression by DCs of co-inhibitory molecules such as programmed death ligand-1 (PD-L1/B7-H1/CD274), a member of the B7 superfamily, is crucial for the downregulation of T-cell responses and the maintenance of immune homeostasis. Exposure of immature DCs to danger-associated molecular patterns (DAMPS) or pathogen-associated molecular patterns (PAMPs) generally results in their maturation and acquisition of immunostimulatory function. However, exposure of DCs to TLR ligands early during their differentiation can inhibit further differentiation and confer tolerogenic properties on these APCs. A report in this issue of The European Journal of Immunology reveals that early inhibition of human DC differentiation from blood monocytes by TLR agonists is associated with a tolerogenic phenotype and Treg generation. The tolerogenic function of these APCs is dependent on MAPK-induced IL-6 and IL-10 production, which drives STAT-3-mediated PD-L1 expression. These observations link IL-10 and IL-6 to PD-L1 expression, providing a new dimension to the anti-inflammatory properties of these cytokines. These findings also have implications for understanding the inherent function of DCs in non-lymphoid tissues such as the liver and lung, where they are exposed to PAMPs that are found constitutively in the local microenvironment.

PD-1 negatively regulates interleukin-12 expression by limiting STAT-1 phosphorylation in monocytes/macrophages during chronic hepatitis C virus infection

Hepatitis C virus (HCV) is remarkably efficient at evading host immunity to establish chronic infection. During chronic HCV infection, interleukin-12 (IL-12) produced by monocytes/macrophages (M/Mφ) is significantly suppressed. Programmed death-1 (PD-1), an inhibitory receptor on immune cells, plays a pivotal role in suppressing T-cell responses during chronic viral infection. To determine whether PD-1 regulates IL-12 production by M/Mφ during chronic HCV infection, we examined the expressions of PD-1, its ligand PDL-1, and their relationship with IL-12 production in M/Mφ from HCV-infected, HCV-resolved, and healthy subjects by flow cytometry. Toll-like receptor (TLR) -mediated IL-12 production by M/Mφ was selectively suppressed, while PD-1/PDL-1 expressions were up-regulated, in HCV-infected subjects compared with HCV-resolved or healthy subjects. Up-regulation of PD-1 was inversely associated with the degree of IL-12 inhibition in HCV infection. Interestingly, the reduced response of M/Mφ from HCV-infected individuals to TLR ligands appeared not to be the result of a lack of the ability to sense pathogen, but to an impaired activation of intracellular janus kinase/signal transducer and activator of transfection (STAT) pathway as represented by inhibited STAT-1 phosphorylation in M/Mφ from HCV-infected individuals compared with HCV-negative subjects. Successful HCV treatment with pegylated interferon/ribavirin or blocking PD-1/PDL-1 engagement ex vivo led to reduced PD-1 expression and improved IL-12 production as well as STAT-1 activation in M/Mφ from HCV-infected individuals. These results suggest that the PD-1 inhibitory pathway may negatively regulate IL-12 expression by limiting STAT-1 phosphorylation in M/Mφ during chronic HCV infection.

Acute depletion of activated memory B cells involves the PD-1 pathway in rapidly progressing SIV-infected macaques

Rapid progression to AIDS is a significant problem, especially in developing countries, where the majority of HIV-infected individuals reside. As rapid disease progression is also frequently observed in SIV-infected macaques, they represent a valuable tool to investigate the pathogenesis of this condition in humans. Here, we have shown that pathogenic SIV infection in rhesus macaques resulted in a rapid depletion (as early as week 2) of activated memory B (CD21-CD27+; mBAct) cells that was strongly associated with rapid disease progression. This depletion was progressive and sustained in rapid progressors, but less severe and transient in typical progressors. Because of the rapid and sustained depletion of mBAct cells, rapid progressors failed to develop SIV-specific Ab responses, showed a decline in non-SIV-specific Ab titers, and succumbed faster to intestinal bacterial infections. Depletion of mBAct cells was strongly associated with preferential depletion of mBAct cells expressing programmed death-1 (PD-1), and in vitro blockade of PD-1 improved their survival. Furthermore, in vivo PD-1 blockade in SIV-infected macaques enhanced Ab responses to non-SIV as well as SIV Ags. Our results identify depletion of mBAct cells as a very early predictor of rapid disease progression in pathogenic SIV infection and suggest an important role for the PD-1 pathway in depletion of mBAct cells and impaired humoral immune responses in SIV-infected macaques.

Dendritic cell inhibition is connected to exhaustion of CD8+ T cell polyfunctionality during chronic hepatitis C virus infection

Although chronic viral infections have evolved mechanisms to interfere with aspects of pathogen recognition by dendritic cells (DCs), the role that these APCs play in virus-specific T cell exhaustion is unclear. Herein we report that NS3-dependent suppression of Toll/IL-1 domain-containing adapter-inducing IFN-beta- and IFN-beta promoter stimulator-1- but not MyD88-coupled pathogen-recognition receptor-induced synthesis of proinflammatory cytokines (IL-12 and TNF-alpha) from DCs by hepatitis C virus (HCV) is a distinctive feature of a subgroup of chronically infected patients. The result is decreased CD8(+) T cell polyfunctional capacities (production of IFN-gamma, IL-2, TNF-alpha, and CD107a mobilization) that is confined to HCV specificities and that relates to the extent to which HCV inhibits DC responses in infected subjects, despite comparable plasma viral load, helper T cell environments, and inhibitory programmed death 1 receptor/ligand signals. Thus, subjects in whom pathogen-recognition receptor signaling in DCs was intact exhibited enhanced polyfunctionality (i.e., IL-2-secretion and CD107a). In addition, differences between HCV-infected patients in the ability of CD8(+) T cells to activate multiple functions in response to HCV did not apply to CD8(+) T cells specific for other immune-controlled viruses (CMV, EBV, and influenza). Our findings identify reversible virus evasion of DC-mediated innate immunity as an additional important factor that impacts the severity of polyfunctional CD8(+) T cell exhaustion during a chronic viral infection.

Human langerhans cells are more efficient than CD14(-)CD1c(+) dermal dendritic cells at priming naive CD4(+) T cells

Few data are available regarding the role of human skin dendritic cells (DCs) in driving T-cell responses. In this study we analyzed the relative capacity of Langerhans cells (LCs) and dermal CD14(-)CD1c(+) DCs (DDCs) to trigger naive CD4(+) T-cell proliferation and differentiation. DC subsets were purified after a 2-day migration from epidermis and dermis of the same skin sample. Migratory LCs showed far more activated phenotype than CD1c(+)DDCs and distinct expression of new molecules of the B7 family; when compared with LCs, CD1c(+)DDCs showed higher PD-L1 and lower inducible co-stimulator ligand (ICOS-L) expression. As expected, CD1c(+)DDCs showed lower allostimulatory property than LCs, a process that was partly reversed by anti-PD-L1 mAb. LCs were significantly more efficient than CD1c(+)DDCs at inducing allogeneic naive CD4(+) T cells to secrete both T helper cell 1 (Th1; IFN-gamma and tumor necrosis factor-alpha ) and Th2 (IL-4 and IL-5) cytokines. Moreover, anti-PD-L1 mAb increased the production of IFN-gamma by both LC- and CD1c(+)DDC-stimulated T cells. Globally, these results argue for a preponderant role of human LCs in inducing naive CD4(+) T-cell priming. Low expression of co-stimulatory molecules together with high expression of PD-L1 might limit the efficiency of CD1c(+)DDCs at inducing naive CD4(+) T-cell proliferation and secretion of cytokines.

ERK/p38 MAP-kinases and PI3K are involved in the differential regulation of B7-H1 expression in DC subsets

Regulatory molecules of the B7-H-family expressed by DC are important for immune homeostasis, but their regulation is largely unknown. When investigating the pathways regulating B7-H1 expression in monocyte-derived DC (MoDC), freshly isolated myeloid DC (mDC) and plasmacytoid DC, respectively, we showed that in MoDC and mDC B7-H1 expression was upregulated by a standard cytokine cocktail, poly I:C or LPS. MoDC utilize ERK and PI3K pathways to upregulate B7-H1 in response to cytokines, whereas p38 kinase was predominantly utilized in response to poly I:C. In mDC, ERK and p38 pathways are involved in B7-H1 regulation, and similar to MoDC, mainly p38 signaling was required for poly I:C-induced expression of B7-H1. Plasmacytoid DC responded only to CpG with upregulation of B7-H1 and in addition to p38 also utilized the PI3K and ERK pathways to regulate B7-H1 expression. As a functional consequence of B7-H1 expression on DC, we demonstrate downmodulation of IFN-gamma production in T cells. Thus, the differential regulation of B7-H1 on DC subsets may suppress immune responses variably, depending on the target DC population. Further analysis of the regulatory mechanisms may facilitate the development of new immunosuppressive therapies, utilizing the regulation of B7-H1 expression on DC.

Cryopreservation decreases receptor PD-1 and ligand PD-L1 coinhibitory expression on peripheral blood mononuclear cell-derived T cells and monocytes

The B7-CD28 immunoglobulin superfamily of costimulatory and coinhibitory ligands and their cell receptors play a critical role in modulating immune responses. Imbalances in these immune regulatory signals occur in pathological conditions characterized by chronic antigenic stimulation. Clinical studies often rely on the use of cryopreserved peripheral blood mononuclear cells (PBMC) to evaluate cellular immune responses. The impact of cryopreservation on these coinhibitory ligands and their cell receptors is unknown. In our studies, cryopreservation significantly reduced the expression of both PD-1 and PD-L1 on PBMC-derived CD3+/CD8+ T cells and CD45+/CD14+ monocytes obtained from adult control subjects. Blockade of PD-1, PD-L1, and PD-L2 using both freshly isolated and cryopreserved PBMC led to higher levels of phytohemagglutinin (PHA) and Candida-induced gamma interferon (IFN-gamma), interleukin-2 (IL-2), and tumor necrosis factor alpha (TNF-alpha) with no effect on IL-10 production. Coinhibitory signaling blockade of freshly isolated, PHA-stimulated PBMC from normal adult controls and human immunodeficiency virus (HIV)-infected subjects led to increased production of IL-4 and IL-5. Candida-stimulated PBMC preferentially induced IFN-gamma and TNF-alpha production, with reduced production of IL-2 and IL-10. This is in contrast to high levels of IFN-gamma, IL-2, and TNF-alpha production with PHA-stimulated cells. The effects of coinhibitory blockade on PHA and Candida-induced lymphoproliferation were varied, with freshly isolated PBMC from adult control subjects and HIV-infected patients yielding higher levels of lymphoproliferation in response to PD-1/PD-L1 blockade. Immune function studies employing cryopreserved cells may lead to increased T-cell effector cytolytic and regulatory immune responses.

siRNA knockdown of PD-L1 and PD-L2 in monocyte-derived dendritic cells only modestly improves proliferative responses to Gag by CD8(+) T cells from HIV-1-infected individuals

INTRODUCTION

Due to their capacity to elicit and regulate immunity, dendritic cells (DCs) are important targets to improve vaccination. Knowing that programmed death-1 (PD-1) high virus-specific T cells become functionally exhausted during chronic exposure to human immunodeficiency virus-1 (HIV-1), the development of a therapeutic DC-based HIV-1 vaccine might include strategies that downregulate PD-L1 and PD-L2 counter-receptors.

METHODS

After showing that monocyte-derived DCs rapidly upregulated PD-L1 and PD-L2 expression upon maturation with a variety of stimuli, e.g., Toll-like receptor ligands and cytokines, we determined that PD-L1 and PD-L2 expression could be knocked down by electroporation of a single small interfering RNA (siRNA) sequence twice at the monocyte and immature stages of DC development. This knockdown approached completion and was specific and lasting for several days.

RESULTS

We then added the PD-L1 and PD-L2 silenced monocyte-derived DCs to peripheral blood mononuclear cells from HIV-1-infected individuals along with pools of 15-mer HIV-1 Gag p24 peptides. However, in cultures from six patients, there was only a modest enhancing effect of PD-L1 and PD-L2 silencing on CD8(+) T cell proliferative responses to the DCs.

DISCUSSION

These findings suggest that, in monocyte-derived DCs, additional strategies than PD-L1 or PD-L2 blockade will be needed to improve the function of PD-1 high T cells.

PD-1 and CTLA-4 inhibitory cosignaling pathways in HIV infection and the potential for therapeutic intervention

The balance between proinflammatory mechanisms and the dampening of excessive immune activation is critical for successful clearance of a pathogen without harm to the host. In particular, molecules of the B7:CD28 family play a critical role in regulating T cell activation and peripheral tolerance. Chronic pathogens like HIV, which is characterized by ongoing viral replication despite detectable virus-specific T cell responses, and cancer cells have exploited these pathways to attenuate Ag-specific T cell immunity. This review summarizes evidence that molecules of the B7:CD28 family, PD-1, CTLA-4, and their ligands, play an active and reversible role in virus-specific T cell exhaustion associated with HIV infection in humans and in the SIV model in macaques. We discuss the potential for immunotherapeutic interventions based on manipulation of these inhibitory networks, the promising data obtained with blockade of the PD-1 pathway in animal models, and the challenges to such therapies.