Josette Fontaine

Peroxisome proliferator-activated receptor γ activators affect the maturation of human monocyte-derived dendritic cells

Peroxisome proliferator‐activated receptor γ  (PPARγ ), a member of the nuclear receptor superfamily, has recently been described as a modulator of macrophage functions and as an inhibitor of T cell proliferation. Here, we investigated the role of PPARγ  in dendritic cells (DC), the most potent antigen‐presenting cells. We showed that PPARγ  is highly expressed in immature human monocyte‐derived DC (MDDC) and that it may affect the immunostimulatory function of MDDC stimulated with lipopolysaccharide (LPS) or via CD40 ligand (CD40L). We found that the synthetic PPARγ  agonist rosiglitazone (as well as pioglitazone and troglitazone) significantly increases on LPS‐ and CD40L‐activated MDDC, the surface expression of CD36 (by 184% and 104%, respectively) and CD86 (by 54% and 48%), whereas it reduces the synthesis of CD80 (by 42% and 42%). Moreover, activation of PPARγ  resulted in a dramatic decreased secretion of the Th1‐promoting factor IL‐12 in LPS‐ and CD40L‐stimulated cells (by 47% and 62%), while the production of IL‐1β , TNF‐α , IL‐6 and IL‐10 was unaffected. Finally, PPARγ  ligands down‐modulate the synthesis of IFN‐γ ‐inducible protein‐10 (recently termed as CXCL10) and RANTES (CCL5), both chemokines involved in the recruitment of Th1 lymphocytes (by 49% and 30%), but not the levels of the Th2 cell‐attracting chemokines,macrophage‐derived chemokine (CCL22) and thymus and activation regulated chemokine (CCL17), in mature MDDC. Taken together, our data suggest that activation of PPARγ  in human DC may have an impact in the orientation of primary and secondary immune responses by favoring type 2 responses.

Peroxisome proliferator-activated receptor γ activators inhibit interleukin-12 production in murine dendritic cells

Peroxisome proliferator‐activated receptors (PPARs) are members of the nuclear receptor superfamily. They are divided into three subtypes (α, β or δ, and γ) and are involved in lipid and glucose homeostasis and in the control of inflammation. In this study, we analyzed the expression of PPARs in murine dendritic cells (DCs), the most potent antigen presenting cells. We find that immature as well as mature spleen‐derived DCs express PPARγ, but not PPARα, mRNA and protein. We also show that the PPARγ activator rosiglitazone does not interfere with the maturation of DCs in vitro nor modifies their ability to activate naive T lymphocytes in vivo. Finally, we present evidence that PPARγ activators down‐modulate the CD40‐induced secretion of interleukin‐12, a potent Th1‐driving factor. These data suggest a possible role for PPARγ in the regulation of immune responses.

Implications for invariant natural killer T cell ligands due to the restricted presence of isoglobotrihexosylceramide in mammals

Development of invariant natural killer T (iNKT) cells requires the presentation of lipid ligand(s) by CD1d molecules in the thymus. The glycosphingolipid (GSL) isoglobotrihexosylceramide (iGb3) has been proposed as the natural iNKT cell-selecting ligand in the thymus and to be involved in peripheral activation of iNKT cells by dendritic cells (DCs). However, there is no direct biochemical evidence for the presence of iGb3 in mouse or human thymus or DCs. Using a highly sensitive HPLC assay, the only tissue where iGb3 could be detected in mouse was the dorsal root ganglion (DRG). iGb3 was not detected in other mouse or any human tissues analyzed, including thymus and DCs. Even in mutant mice that store isoglobo-series GSLs in the DRG, we were still unable to detect these GSLs in the thymus. iGb3 is therefore unlikely to be a physiologically relevant iNKT cell-selecting ligand in mouse and humans. A detailed study is now warranted to better understand the nature of iNKT cell-selecting ligand(s) in vivo.

Interleukin-22 Is Produced by Invariant Natural Killer T Lymphocytes during Influenza A Virus Infection POTENTIAL ROLE IN PROTECTION AGAINST LUNG EPITHELIAL DAMAGES

Background: Invariant natural killer T (iNKT) cells play a beneficial role during experimental influenza A virus (IAV) infection. Results: iNKT cells produce IL-22 during infection, and IL-22 prevents the IAV-triggered cell death of pulmonary epithelium. Conclusion: IL-22 produced by iNKT cells might be important during IAV infection. Significance: Understanding how iNKT cells function during IAV infection might be instrumental to control IAV-associated pathogenesis. Invariant natural killer T (iNKT) cells are non-conventional lipid-reactive αβ T lymphocytes that play a key role in host responses during viral infections, in particular through the swift production of cytokines. Their beneficial role during experimental influenza A virus (IAV) infection has recently been proposed, although the mechanisms involved remain elusive. Here we show that during in vivo IAV infection, mouse pulmonary iNKT cells produce IFN-γ and IL-22, a Th17-related cytokine critical in mucosal immunity. Although permissive to viral replication, IL-22 production by iNKT cells is not due to IAV infection per se of these cells but is indirectly mediated by IAV-infected dendritic cells (DCs). We show that activation of the viral RNA sensors TLR7 and RIG-I in DCs is important for triggering IL-22 secretion by iNKT cells, whereas the NOD-like receptors NOD2 and NLRP3 are dispensable. Invariant NKT cells respond to IL-1β and IL-23 provided by infected DCs independently of the CD1d molecule to release IL-22. In vitro, IL-22 protects IAV-infected airway epithelial cells against mortality but has no role on viral replication. Finally, during early IAV infection, IL-22 plays a positive role in the control of lung epithelial damages. Overall, IAV infection of DCs activates iNKT cells, providing a rapid source of IL-22 that might be beneficial to preserve the lung epithelium integrity.

Schistosome N‐glycans containing core α3‐fucose and core β2‐xylose epitopes are strong inducers of Th2 responses in mice

During murine schistosomiasis, egg‐derived glycoconjugates play a key role in skewing the immune response towards a Th2 phenotype. Among the candidates responsible for this effect, complex‐type N‐glycans containing the core α3‐fucose and core β2‐xylose determinants, two glycan epitopes found in some invertebrate‐ and plant‐derived allergens, may be important. Here, we show that core α3‐fucose and core β2‐xylose determinants are expressed in the different developmental stages of Schistosoma mansoni, particularly in the excretory‐secretory systems of schistosomula and adult worms and in eggs deposited in the liver. Glycosyltransferase assays confirmed the presence of core α3‐fucosyltransferase and core β2‐xylosyltransferase activities in egg extracts. Using a model of immunization with pulsed dendritic cells, we show that egg‐derived glycoproteins containing the core α3‐fucose and core β2‐xylose determinants generate a strong Th2‐biased cellular response in mice and that the glycan moieties of this extract are important in this effect. During murine infection, these complex‐type N‐glycans induce a glycan‐specific Th2 cellular response and elicit T‐dependent anti‐core α3‐fucose and anti‐core β2‐xylose IgG1 (a Th2‐associated isotype), but not IgG2b (a Th1‐associated isotype) Ab. Taken together, our results point out theimportance of core fucosylated/xylosylated N‐glycans in the Th2 immune response during murine schistosomiasis.

Antigen Presentation by CD1d Contributes to the Amplification of Th2 Responses to Schistosoma mansoni Glycoconjugates in Mice

During murine schistosomiasis, there is a gradual switch from a predominant Th1 cytokine response to a Th2-dominated response after egg laying, an event that favors the formation of granuloma around viable eggs. Egg-derived glycoconjugates, including glycolipids, may play a crucial role in this phenomenon. In this study, we used a model of dendritic cell sensitization to study the role of egg glycoconjugates in the induction of specific immune response to soluble egg Ag (SEA) and to investigate the possibility that CD1d, a molecule implicated in glycolipid presentation, may be involved in such a phenomenon. We show that, when captured, processed, and presented to naive T lymphocytes by dendritic cells, egg, but not larval, Ag skew the immune response toward a Th2 response. Periodate treatment reversed this effect, indicating that the sugar moiety of SEA is important in this phenomenon. Using DC treated ex vivo with a neutralizing anti-CD1d Ab or isolated from CD1d knockout mice, we show that CD1d is crucial in the priming of SEA-specific Th2 lymphocytes. We then evaluated the contribution of CD1d on the development of the SEA-specific immune response and on the formation of the egg-induced liver granuloma during murine schistosomiasis. We find that CD1d knockout mice have a reduced Th2 response after egg laying and develop a less marked fibrotic pathology compared with wild-type mice. Altogether, our results suggest that Ag presentation of parasite glycoconjugates to CD1d-restricted T cells may be important in the early events leading to the induction of Th2 responses and to egg-induced pathology during murine schistosomiasis.

Galectin-3 Modulates Immune and Inflammatory Responses during Helminthic Infection: Impact of Galectin-3 Deficiency on the Functions of Dendritic Cells

ABSTRACT Galectin-3 (Gal-3) is a multifunctional β-galactoside-binding lectin that senses self-derived and microbial glycoconjugates. Although Gal-3 is important in immune reactions and host defense in some experimental models, the function of Gal-3 during helminthic diseases (e.g., schistosomiasis) is still elusive. We show that, compared to wild-type Schistosoma mansoni-infected mice, infected Gal-3−/− mice have a reduced number of T and B lymphocytes in the spleen, develop reduced liver granulomas at 7 weeks (acute phase) and 14 weeks (chronic phase) postinfection, and mount a biased cellular and humoral Th1 response. In an attempt to understand this latter phenomenon, we studied the role of endogenous Gal-3 in dendritic cells (DCs), the most potent antigen-presenting cells, both in vitro and in vivo. Although Gal-3 deficiency in DCs does not impact their differentiation and maturation processes, it greatly influences the strength (but not the nature) of the adaptive immune response that they trigger, suggesting that Gal-3 deficiency in some other cell types may be important during murine schistosomiasis. As a whole, this study implies that Gal-3 is a modulator of the immune/inflammatory responses during helminthic infection and reveals for the first time that Gal-3 expression in DCs is pivotal to control the magnitude of T-lymphocyte priming.

Activation of Invariant NKT Cells by the Helminth Parasite Schistosoma mansoni

Mouse CD1d-restricted NKT cells, including invariant (i)NKT cells, are innate cells activated by glycolipid Ags and play important roles in the initiation and regulation of immune responses. Through their ability to promptly produce large amounts of Th1 and/or Th2 cytokines upon TCR engagement, iNKT cells exert crucial functions in the immune/inflammatory system during bacterial, protozoan, fungal, and viral infections. However, their roles during metazoan parasite infection, which are generally associated with strong Th2 responses, still remain elusive. In this study, we show that during the course of murine schistosomiasis, iNKT cells exhibit an activated phenotype and that following schistosome egg encounter in the liver, hepatic iNKT cells produce both IFN-γ and IL-4 in vivo. We also report that schistosome egg-sensitized dendritic cells (DCs) activate, in a CD1d-dependent manner, iNKT cells to secrete IFN-γ and IL-4 in vitro. Interestingly, transfer of egg-sensitized DCs promotes a strong Th2 response in recipient wild-type mice, but not in mice that lack iNKT cells. Engagement of TLRs in DCs is not necessary for iNKT cell stimulation in response to egg-sensitized DCs, suggesting an alternative pathway of activation. Finally, we propose that self, rather than parasite-derived, CD1d-restricted ligands are implicated in iNKT cell stimulation. Taken together, our data show for the first time that helminths can activate iNKT cells to produce immunoregulatory cytokines in vivo, enabling them to influence the adaptive immune response.

Interleukin-22 reduces lung inflammation during influenza A virus infection and protects against secondary bacterial infection

ABSTRACT Interleukin-22 (IL-22) has redundant, protective, or pathogenic functions during autoimmune, inflammatory, and infectious diseases. Here, we addressed the potential role of IL-22 in host defense and pathogenesis during lethal and sublethal respiratory H3N2 influenza A virus (IAV) infection. We show that IL-22, as well as factors associated with its production, are expressed in the lung tissue during the early phases of IAV infection. Our data indicate that retinoic acid receptor-related orphan receptor-γt (RORγt)-positive αβ and γδ T cells, as well as innate lymphoid cells, expressed enhanced Il22 transcripts as early as 2 days postinfection. During lethal or sublethal IAV infections, endogenous IL-22 played no role in the control of IAV replication and in the development of the IAV-specific CD8+ T cell response. During lethal infection, where wild-type (WT) mice succumbed to severe pneumonia, the lack of IL-22 did not accelerate or delay IAV-associated pathogenesis and animal death. In stark contrast, during sublethal IAV infection, IL-22-deficient animals had enhanced lung injuries and showed a lower airway epithelial integrity relative to WT littermates. Of importance, the protective effect of endogenous IL-22 in pulmonary damages was associated with a more controlled secondary bacterial infection. Indeed, after challenge with Streptococcus pneumoniae, IAV-experienced Il22 −/− animals were more susceptible than WT controls in terms of survival rate and bacterial burden in the lungs. Together, IL-22 plays no major role during lethal influenza but is beneficial during sublethal H3N2 IAV infection, where it limits lung inflammation and subsequent bacterial superinfections.

Invariant and Noninvariant Natural Killer T Cells Exert Opposite Regulatory Functions on the Immune Response during Murine Schistosomiasis

ABSTRACT CD1d-restricted natural killer T (NKT) cells represent a heterogeneous population of innate memory immune cells expressing both NK and T-cell markers distributed into two major subsets, i.e., invariant NKT (iNKT) cells, which express exclusively an invariant T-cell receptor (TCR) α chain (Vα14Jα18 in mice), and non-iNKT cells, which express more diverse TCRs. NKT cells quickly produce Th1- and/or Th2-type cytokines following stimulation with glycolipid antigen (Ag) and, through this property, play potent immunoregulatory roles in autoimmune diseases, cancer, and infection. No study has addressed the role of NKT cells in metazoan parasite infections so far. We show that during murine schistosomiasis, the apparent frequency of both iNKT cells and non-iNKT cells decreased in the spleen as early as 3 weeks postinfection (p.i.) and that both populations expressed a greater amount of the activation marker CD69 at 6 weeks p.i., suggesting an activated phenotype. Two different NKT-cell-deficient mouse models, namely, TCR Jα18−/− (exclusively deficient in iNKT cells) and CD1d−/− (deficient in both iNKT and non-iNKT cells) mice, were used to explore the implication of these subsets in infection. We show that whereas both iNKT and non-iNKT cells do not have a major impact on the immune response during the early phase (1 and 4 weeks) of infection, they exert important, although opposite, effects on the immune response during the acute phase of the disease (7 and 12 weeks), after schistosome egg production. Indeed, iNKT cells contribute to Th1 cell differentiation whereas non-iNKT cells might be mostly implicated in Th2 cell differentiation in response to parasite Ag. Our findings suggest, for the first time, that helminths activate both iNKT and non-iNKT cells in vivo, enabling them to differentially influence the Th1/Th2 balance of the immune response.