David Olagnier

Cellular oxidative stress response controls the antiviral and apoptotic programs in dengue virus-infected dendritic cells.

Dengue virus (DENV) is a re-emerging arthropod borne flavivirus that infects more than 300 million people worldwide, leading to 50,000 deaths annually. Because dendritic cells (DC) in the skin and blood are the first target cells for DENV, we sought to investigate the early molecular events involved in the host response to the virus in primary human monocyte-derived dendritic cells (Mo-DC). Using a genome-wide transcriptome analysis of DENV2-infected human Mo-DC, three major responses were identified within hours of infection - the activation of IRF3/7/STAT1 and NF-κB-driven antiviral and inflammatory networks, as well as the stimulation of an oxidative stress response that included the stimulation of an Nrf2-dependent antioxidant gene transcriptional program. DENV2 infection resulted in the intracellular accumulation of reactive oxygen species (ROS) that was dependent on NADPH-oxidase (NOX). A decrease in ROS levels through chemical or genetic inhibition of the NOX-complex dampened the innate immune responses to DENV infection and facilitated DENV replication; ROS were also essential in driving mitochondrial apoptosis in infected Mo-DC. In addition to stimulating innate immune responses to DENV, increased ROS led to the activation of bystander Mo-DC which up-regulated maturation/activation markers and were less susceptible to viral replication. We have identified a critical role for the transcription factor Nrf2 in limiting both antiviral and cell death responses to the virus by feedback modulation of oxidative stress. Silencing of Nrf2 by RNA interference increased DENV-associated immune and apoptotic responses. Taken together, these data demonstrate that the level of oxidative stress is critical to the control of both antiviral and apoptotic programs in DENV-infected human Mo-DC and highlight the importance of redox homeostasis in the outcome of DENV infection.

Double-walled carbon nanotubes trigger IL-1β release in human monocytes through Nlrp3 inflammasome activation.

Because of their outstanding physical properties, carbon nanotubes (CNTs) are promising new materials in the field of nanotechnology. It is therefore imperative to assess their adverse effects on human health. Monocytes/macrophages that recognize and eliminate the inert particles constitute the main target of CNTs. In this article, we report our finding that double-walled CNTs (DWCNTs) synergize with Toll-like receptor agonists to enhance IL-1β release in human monocytes. We show that DWCNTs-induced IL-1β secretion is exclusively linked to caspase-1 and to Nlrp3 inflammasome activation in human monocytes. We also establish that this activation requires DWCNTs phagocytosis and potassium efflux, but not reactive oxygen specied (ROS) generation. Moreover, inhibition of lysosomal acidification or cathepsin-B activation reduces DWCNT-induced IL-1β secretion, suggesting that Nlrp3 inflammasome activation occurs via lysosomal destabilization. Thus, DWCNTs present a health hazard due to their capacity to activate Nlrp3 inflammasome, recalling the inflammation caused by asbestos and hence demonstrating that they should be used with caution.

IL-13 induces expression of CD36 in human monocytes through PPARgamma activation.

The class B scavenger receptor CD36 is a component of the pattern recognition receptors on monocytes that recognizes a variety of molecules. CD36 expression in monocytes depends on exposure to soluble mediators. We demonstrate here that CD36 expression is induced in human monocytes following exposure to IL‐13, a Th2 cytokine, via the peroxisome proliferator‐activated receptor (PPAR)γ pathway. Induction of CD36 protein was paralleled by an increase in CD36 mRNA. The PPARγ pathway was demonstrated using transfection of a PPARγ expression plasmid into the murine macrophage cell line RAW264.7, expressing very low levels of PPARγ, and in peritoneal macrophages from PPARγ‐conditional null mice. We also show that CD36 induction by IL‐13 via PPARγ is dependent on phospholipase A2 activation and that IL‐13 induces the production of endogenous 15‐deoxy‐Δ12,14‐prostaglandin J2, an endogenous PPARγ ligand, and its nuclear localization in human monocytes. Finally, we demonstrate that CD36 and PPARγ are involved in IL‐13‐mediated phagocytosis of Plasmodium falciparum‐parasitized erythrocytes. These results reveal a novel role for PPARγ in the alternative activation of monocytes by IL‐13, suggesting that endogenous PPARγ ligands, produced by phospholipase A2 activation, could contribute to the biochemical and cellular functions of CD36.

PPARγ controls Dectin-1 expression required for host antifungal defense against Candida albicans.

We recently showed that IL-13 or peroxisome proliferator activated receptor γ (PPARγ) ligands attenuate Candida albicans colonization of the gastrointestinal tract. Here, using a macrophage-specific Dectin-1 deficient mice model, we demonstrate that Dectin-1 is essential to control fungal gastrointestinal infection by PPARγ ligands. We also show that the phagocytosis of yeast and the release of reactive oxygen intermediates in response to Candida albicans challenge are impaired in macrophages from Dectin-1 deficient mice treated with PPARγ ligands or IL-13. Although the Mannose Receptor is not sufficient to trigger antifungal functions during the alternative activation of macrophages, our data establish the involvement of the Mannose Receptor in the initial recognition of non-opsonized Candida albicans by macrophages. We also demonstrate for the first time that the modulation of Dectin-1 expression by IL-13 involves the PPARγ signaling pathway. These findings are consistent with a crucial role for PPARγ in the alternative activation of macrophages by Th2 cytokines. Altogether these data suggest that PPARγ ligands may be of therapeutic value in esophageal and gastrointestinal candidiasis in patients severely immunocompromised or with metabolic diseases in whom the prevalence of candidiasis is considerable.

Systems analysis of a RIG-I agonist inducing broad spectrum inhibition of virus infectivity.

The RIG-I like receptor pathway is stimulated during RNA virus infection by interaction between cytosolic RIG-I and viral RNA structures that contain short hairpin dsRNA and 5′ triphosphate (5′ppp) terminal structure. In the present study, an RNA agonist of RIG-I was synthesized in vitro and shown to stimulate RIG-I-dependent antiviral responses at concentrations in the picomolar range. In human lung epithelial A549 cells, 5′pppRNA specifically stimulated multiple parameters of the innate antiviral response, including IRF3, IRF7 and STAT1 activation, and induction of inflammatory and interferon stimulated genes - hallmarks of a fully functional antiviral response. Evaluation of the magnitude and duration of gene expression by transcriptional profiling identified a robust, sustained and diversified antiviral and inflammatory response characterized by enhanced pathogen recognition and interferon (IFN) signaling. Bioinformatics analysis further identified a transcriptional signature uniquely induced by 5′pppRNA, and not by IFNα-2b, that included a constellation of IRF7 and NF-kB target genes capable of mobilizing multiple arms of the innate and adaptive immune response. Treatment of primary PBMCs or lung epithelial A549 cells with 5′pppRNA provided significant protection against a spectrum of RNA and DNA viruses. In C57Bl/6 mice, intravenous administration of 5′pppRNA protected animals from a lethal challenge with H1N1 Influenza, reduced virus titers in mouse lungs and protected animals from virus-induced pneumonia. Strikingly, the RIG-I-specific transcriptional response afforded partial protection from influenza challenge, even in the absence of type I interferon signaling. This systems approach provides transcriptional, biochemical, and in vivo analysis of the antiviral efficacy of 5′pppRNA and highlights the therapeutic potential associated with the use of RIG-I agonists as broad spectrum antiviral agents.

In Vitro and In Vivo Properties of Ellagic Acid in Malaria Treatment

ABSTRACT Malaria is one of the most significant causes of infectious disease in the world. The search for new antimalarial chemotherapies has become increasingly urgent due to the parasites’ resistance to current drugs. Ellagic acid is a polyphenol found in various plant products. In this study, antimalarial properties of ellagic acid were explored. The results obtained have shown high activity in vitro against all Plasmodium falciparum strains whatever their levels of chloroquine and mefloquine resistance (50% inhibitory concentrations ranging from 105 to 330 nM). Ellagic acid was also active in vivo against Plamodium vinckei petteri in suppressive, curative, and prophylactic murine tests, without any toxicity (50% effective dose by the intraperitoneal route inferior to 1 mg/kg/day). The study of the point of action of its antimalarial activity in the erythrocytic cycle of Plasmodium falciparum demonstrated that it occurred at the mature trophozoite and young schizont stages. Moreover, ellagic acid has been shown to potentiate the activity of current antimalarial drugs such as chloroquine, mefloquine, artesunate, and atovaquone. This study also proved the antioxidant activity of ellagic acid and, in contrast, the inhibitory effect of the antioxidant compound N-acetyl-l-cysteine on its antimalarial efficacy. The possible mechanisms of action of ellagic acid on P. falciparum are discussed in light of the results. Ellagic acid has in vivo activity against plasmodia, but modification of the compound could lead to improved pharmacological properties, principally for the oral route.

Inhibition of Dengue and Chikungunya Virus Infections by RIG-I-Mediated Type I Interferon-Independent Stimulation of the Innate Antiviral Response

ABSTRACT RIG-I is a cytosolic sensor critically involved in the activation of the innate immune response to RNA virus infection. In the present study, we evaluated the inhibitory effect of a RIG-I agonist on the replication of two emerging arthropod-borne viral pathogens, dengue virus (DENV) and chikungunya virus (CHIKV), for which no therapeutic options currently exist. We demonstrate that when a low, noncytotoxic dose of an optimized 5′triphosphorylated RNA (5′pppRNA) molecule was administered, RIG-I stimulation generated a robust antiviral response against these two viruses. Strikingly, 5′pppRNA treatment before or after challenge with DENV or CHIKV provided protection against infection. In primary human monocytes and monocyte-derived dendritic cells, the RIG-I agonist blocked both primary infection and antibody-dependent enhancement of DENV infection. The protective response against DENV and CHIKV induced by 5′pppRNA was dependent on an intact RIG-I/MAVS/TBK1/IRF3 axis and was largely independent of the type I IFN response. Altogether, this in vitro analysis of the antiviral efficacy of 5′pppRNA highlights the therapeutic potential of RIG-I agonists against emerging viruses such as DENV and CHIKV. IMPORTANCE DENV and CHIKV are two reemerging mosquito-borne viruses for which no therapeutic options currently exist. Both viruses overlap geographically in tropical regions of the world, produce similar fever-like symptoms, and are difficult to diagnose. This study investigated the inhibitory effect of a RIG-I agonist on the replication of these two viruses. RIG-I stimulation using 5′pppRNA before or after DENV or CHIKV infection generated a protective antiviral response against both pathogens in immune and nonimmune cells; interestingly, the protective response against the viruses was largely independent of the classical type I interferon response. The antiviral efficacy of 5′pppRNA highlights the therapeutic potential of RIG-I agonists against emerging viruses such as DENV and CHIKV.

PPARγ Ligands Switched High Fat Diet-Induced Macrophage M2b Polarization toward M2a Thereby Improving Intestinal Candida Elimination

Obesity is associated with a chronic low-grade inflammation that predisposes to insulin resistance and the development of type 2 diabetes. In this metabolic context, gastrointestinal (GI) candidiasis is common. We recently demonstrated that the PPARγ ligand rosiglitazone promotes the clearance of Candida albicans through the activation of alternative M2 macrophage polarization. Here, we evaluated the impact of high fat diet (HFD)-induced obesity and the effect of rosiglitazone (PPARγ ligand) or WY14643 (PPARα ligand) both on the phenotypic M1/M2 polarization of peritoneal and cecal tissue macrophages and on the outcome of GI candidiasis. We demonstrated that the peritoneal macrophages and the cell types present in the cecal tissue from HF fed mice present a M2b polarization (TNF-αhigh, IL-10high, MR, Dectin-1). Interestingly, rosiglitazone induces a phenotypic M2b-to-M2a (TNF-αlow, IL-10low, MRhigh, Dectin-1high) switch of peritoneal macrophages and of the cells present in the cecal tissue. The incapacity of WY14643 to switch this polarization toward M2a state, strongly suggests the specific involvement of PPARγ in this mechanism. We showed that in insulin resistant mice, M2b polarization of macrophages present on the site of infection is associated with an increased susceptibility to GI candidiasis, whereas M2a polarization after rosiglitazone treatment favours the GI fungal elimination independently of reduced blood glucose. In conclusion, our data demonstrate a dual benefit of PPARγ ligands because they promote mucosal defence mechanisms against GI candidiasis through M2a macrophage polarization while regulating blood glucose level.

Global analyses revealed age-related alterations in innate immune responses after stimulation of pathogen recognition receptors

Aging leads to dysregulation of multiple components of the immune system that results in increased susceptibility to infections and poor response to vaccines in the aging population. The dysfunctions of adaptive B and T cells are well documented, but the effect of aging on innate immunity remains incompletely understood. Using a heterogeneous population of peripheral blood mononuclear cells (PBMCs), we first undertook transcriptional profiling and found that PBMCs isolated from old individuals (≥ 65 years) exhibited a delayed and altered response to stimulation with TLR4, TLR7/8, and RIG‐I agonists compared to cells obtained from adults (≤ 40 years). This delayed response to innate immune agonists resulted in the reduced production of pro‐inflammatory and antiviral cytokines and chemokines including TNFα, IL‐6, IL‐1β, IFNα, IFNγ, CCL2, and CCL7. While the major monocyte and dendritic cell subsets did not change numerically with aging, activation of specific cell types was altered. PBMCs from old subjects also had a lower frequency of CD40+ monocytes, impaired up‐regulation of PD‐L1 on monocytes and T cells, and increased expression of PD‐L2 and B7‐H4 on B cells. The defective immune response to innate agonists adversely affected adaptive immunity as TLR‐stimulated PBMCs (minus CD3 T cells) from old subjects elicited significantly lower levels of adult T‐cell proliferation than those from adult subjects in an allogeneic mixed lymphocyte reaction (MLR). Collectively, these age‐associated changes in cytokine, chemokine and interferon production, as well as co‐stimulatory protein expression could contribute to the blunted memory B‐ and T‐cell immune responses to vaccines and infections.

Nrf2, a PPARγ Alternative Pathway to Promote CD36 Expression on Inflammatory Macrophages: Implication for Malaria

CD36 is the major receptor mediating nonopsonic phagocytosis of Plasmodium falciparum-parasitized erythrocytes by macrophages. Its expression on macrophages is mainly controlled by the nuclear receptor PPARγ. Here, we demonstrate that inflammatory processes negatively regulate CD36 expression on human and murine macrophages, and hence decrease Plasmodium clearance directly favoring the worsening of malaria infection. This CD36 downregulation in inflammatory conditions is associated with a failure in the expression and activation of PPARγ. Interestingly, using siRNA mediating knock down of Nrf2 in macrophages or Nrf2- and PPARγ-deficient macrophages, we establish that in inflammatory conditions, the Nrf2 transcription factor controls CD36 expression independently of PPARγ. In these conditions, Nrf2 activators, but not PPARγ ligands, enhance CD36 expression and CD36-mediated Plasmodium phagocytosis. These results were confirmed in human macrophages and in vivo where only Nrf2 activators improve the outcome of severe malaria. Collectively, this report highlights that the Nrf2 transcription factor could be an alternative target to PPARγ in the control of severe malaria through parasite clearance.