Michel Gilliet

Human epithelial cells trigger dendritic cell–mediated allergic inflammation by producing TSLP

Whether epithelial cells play a role in triggering the immune cascade leading to T helper 2 (TH2)-type allergic inflammation is not known. We show here that human thymic stromal lymphopoietin (TSLP) potently activated CD11c+ dendritic cells (DCs) and induced production of the TH2-attracting chemokines TARC (thymus and activation-regulated chemokine; also known as CCL17) and MDC (macrophage-derived chemokine; CCL22). TSLP-activated DCs primed naïve TH cells to produce the proallergic cytokines interleukin 4 (IL-4), IL-5, IL-13 and tumor necrosis factor-α, while down-regulating IL-10 and interferon-γ. TSLP was highly expressed by epithelial cells, especially keratinocytes from patients with atopic dermatitis. TSLP expression was associated with Langerhans cell migration and activation in situ. These findings shed new light on the function of human TSLP and the role played by epithelial cells and DCs in initiating allergic inflammation.

Plasmacytoid dendritic cells sense self-DNA coupled with antimicrobial peptide

Plasmacytoid dendritic cells (pDCs) sense viral and microbial DNA through endosomal Toll-like receptors to produce type 1 interferons. pDCs do not normally respond to self-DNA, but this restriction seems to break down in human autoimmune disease by an as yet poorly understood mechanism. Here we identify the antimicrobial peptide LL37 (also known as CAMP) as the key factor that mediates pDC activation in psoriasis, a common autoimmune disease of the skin. LL37 converts inert self-DNA into a potent trigger of interferon production by binding the DNA to form aggregated and condensed structures that are delivered to and retained within early endocytic compartments in pDCs to trigger Toll-like receptor 9. Thus, our data uncover a fundamental role of an endogenous antimicrobial peptide in breaking innate tolerance to self-DNA and suggest that this pathway may drive autoimmunity in psoriasis.

Plasmacytoid dendritic cells: Sensing nucleic acids in viral infection and autoimmune diseases

Plasmacytoid dendritic cells (pDCs) are important mediators of antiviral immunity through their ability to produce large amounts of type I interferons (IFNs) on viral infection. This function of pDCs is linked to their expression of Toll-like receptor 7 (TLR7) and TLR9, which sense viral nucleic acids within the early endosomes. Exclusion of self nucleic acids from TLR-containing early endosomes normally prevents pDC responses to them. However, in some autoimmune diseases, self nucleic acids can be modified by host factors and gain entrance to pDC endosomes, where they activate TLR signalling. Several pDC receptors negatively regulate type I IFN responses by pDCs during viral infection and for normal homeostasis.

Plasmacytoid predendritic cells initiate psoriasis through interferon-α production

Psoriasis is one of the most common T cell–mediated autoimmune diseases in humans. Although a role for the innate immune system in driving the autoimmune T cell cascade has been proposed, its nature remains elusive. We show that plasmacytoid predendritic cells (PDCs), the natural interferon (IFN)-α–producing cells, infiltrate the skin of psoriatic patients and become activated to produce IFN-α early during disease formation. In a xenograft model of human psoriasis, we demonstrate that blocking IFN-α signaling or inhibiting the ability of PDCs to produce IFN-α prevented the T cell–dependent development of psoriasis. Furthermore, IFN-α reconstitution experiments demonstrated that PDC-derived IFN-α is essential to drive the development of psoriasis in vivo. These findings uncover a novel innate immune pathway for triggering a common human autoimmune disease and suggest that PDCs and PDC-derived IFN-α represent potential early targets for the treatment of psoriasis.

Neutrophils activate plasmacytoid dendritic cells by releasing self-DNA-peptide complexes in systemic lupus erythematosus.

In systemic lupus erythematosus, neutrophils release peptide/self-DNA complexes that trigger plasmacytoid dendritic cell activation and autoantibody formation. Lupus Neutrophils Cast a Wide NET Systemic lupus erythematosus, also known as SLE or lupus, is a systemic, chronic autoimmune disease that can affect the skin, joints, kidneys, and other organs. In lupus, the body’s immune system turns against antigens in the body’s own nuclei, with activated B cells producing antibodies against self-DNA and associated proteins. The resulting immune complexes accumulate in the body, causing inflammation and tissue damage. Now, two new studies, by Lande et al. and Garcia-Romo et al., demonstrate a role for neutrophils and the “neutrophil extracellular traps,” a specialized structure they release when activated, in the pathogenesis of the disease. A key characteristic of lupus is the presence of chronically activated plasmacytoid dendritic cells, which secrete type I interferons. Lupus patients also display increased numbers of immature neutrophils in the blood, but the exact role of neutrophils in the disease had been unclear. Lande et al. began with the observation that patient serum contains immunogenic complexes that include the antimicrobial peptide LL37, human neutrophil peptide (HNP), and self-DNA. These complexes are taken up by and activate dendritic cells, and patients carry antibodies directed against LL37, HNP, and self-DNA. What is the origin of these complexes? Activated neutrophils can undergo NETosis, a particular type of cell death in which their nuclear DNA is released in long chromatin filaments that form web-like structures, neutrophil extracellular traps (NETs). NETs contain antimicrobial peptides, and can entrap bacteria, enabling them to be killed. Lande et al. now show that the anti-LL37 and anti-HNP antibodies present in lupus patient serum can activate neutrophils and induce them to release NETs. Patient-derived neutrophils release more NETs upon exposure to antibody than control neutrophils. In a parallel study, Garcia-Romo et al. look in detail at neutrophils in lupus, and show that lupus patient neutrophils undergo accelerated cell death in culture. Anti-ribonucleoprotein antibodies present in patient serum induce NETosis, and the released NETs contain LL37 and another neutrophil protein, HMGB1. Induction of NETosis requires FcRIIa, signaling through the pattern recognition receptor Toll-like receptor 7, and formation of reactive oxygen species. Garcia-Romo et al. also show that these NETs potently activate dendritic cells, leading to secretion of high levels of interferon-α. Together, these findings portray an important role for neutrophils in lupus pathogenesis, whereby neutrophils activated by anti-self antibodies release NETs. These NETs, which contain antimicrobial peptides complexed with self-DNA, activate plasmacytoid dendritic cells, leading to interferon release and furtherment and aggravation of inflammation and disease. Systemic lupus erythematosus (SLE) is a severe and incurable autoimmune disease characterized by chronic activation of plasmacytoid dendritic cells (pDCs) and production of autoantibodies against nuclear self-antigens by hyperreactive B cells. Neutrophils are also implicated in disease pathogenesis; however, the mechanisms involved are unknown. Here, we identified in the sera of SLE patients immunogenic complexes composed of neutrophil-derived antimicrobial peptides and self-DNA. These complexes were produced by activated neutrophils in the form of web-like structures known as neutrophil extracellular traps (NETs) and efficiently triggered innate pDC activation via Toll-like receptor 9 (TLR9). SLE patients were found to develop autoantibodies to both the self-DNA and antimicrobial peptides in NETs, indicating that these complexes could also serve as autoantigens to trigger B cell activation. Circulating neutrophils from SLE patients released more NETs than those from healthy donors; this was further stimulated by the antimicrobial autoantibodies, suggesting a mechanism for the chronic release of immunogenic complexes in SLE. Our data establish a link between neutrophils, pDC activation, and autoimmunity in SLE, providing new potential targets for the treatment of this devastating disease.

Generation of Human CD8 T Regulatory Cells by CD40 Ligand–activated Plasmacytoid Dendritic Cells

Although CD8 T cell–mediated immunosuppression has been a well-known phenomenon during the last three decades, the nature of primary CD8 T suppressor cells and the mechanism underlying their generation remain enigmatic. We demonstrated that naive CD8 T cells primed with allogeneic CD40 ligand–activated plasmacytoid dendritic cells (DC)2 differentiated into CD8 T cells that displayed poor secondary proliferative and cytolytic responses. By contrast, naive CD8 T cells primed with allogeneic CD40 ligand–activated monocyte-derived DCs (DC1) differentiated into CD8 T cells, which proliferated to secondary stimulation and killed allogeneic target cells. Unlike DC1-primed CD8 T cells that produced large amounts of interferon (IFN)-γ upon restimulation, DC2-primed CD8 T cells produced significant amounts of interleukin (IL)-10, low IFN-γ, and no IL-4, IL-5, nor transforming growth factor (TGF)-β. The addition of anti–IL-10–neutralizing monoclonal antibodies during DC2 and CD8 T cell coculture, completely blocked the generation of IL-10–producing anergic CD8 T cells. IL-10–producing CD8 T cells strongly inhibit the allospecific proliferation of naive CD8 T cells to monocytes, and mature and immature DCs. This inhibition was mediated by IL-10, but not by TGF-β. IL-10–producing CD8 T cells could inhibit the bystander proliferation of naive CD8 T cells, provided that they were restimulated nearby to produce IL-10. IL-10–producing CD8 T cells could not inhibit the proliferation of DC1-preactivated effector T cells. This study demonstrates that IL-10–producing CD8 T cells are regulatory T cells, which provides a cellular basis for the phenomenon of CD8 T cell–mediated immunosuppression and suggests a role for plasmacytoid DC2 in immunological tolerance.

Dendritic cell lineage, plasticity and cross-regulation

Dendritic cells (DCs) are professional antigen-presenting cells that have an extraordinary capacity to stimulate naïve T cells and initiate primary immune responses. Here we review progress in understanding the additional functions of DCs in regulating the types of T cell–mediated immune responses and innate immunity to microbes. In addition, evidence for the existence of myeloid and lymphoid DC lineages and their different functions are summarized. We propose that the diverse functions of DCs in immune regulation are dictated by the instructions they received during innate immune responses to different pathogens and from their evolutionary lineage heritage.

Flexibility of Mouse Classical and Plasmacytoid-derived Dendritic Cells in Directing T Helper Type 1 and 2 Cell Development Dependency on Antigen Dose and Differential Toll-like Receptor Ligation

Distinct dendritic cell (DC) subsets have been suggested to be preprogrammed to direct either T helper cell (Th) type 1 or Th2 development, although more recently different pathogen products or stimuli have been shown to render these DCs more flexible. It is still unclear how distinct mouse DC subsets cultured from bone marrow precursors, blood, or their lymphoid tissue counterparts direct Th differentiation. We show that mouse myeloid and plasmacytoid precursor DCs (pDCs) cultured from bone marrow precursors and ex vivo splenic DC subsets can induce the development of both Th1 and Th2 effector cells depending on the dose of antigen. In general, high antigen doses induced Th1 cell development whereas low antigen doses induced Th2 cell development. Both cultured and ex vivo splenic plasmacytoid-derived DCs enhanced CD4+ T cell proliferation and induced strong Th1 cell development when activated with the Toll-like receptor (TLR)9 ligand CpG, and not with the TLR4 ligand lipopolysaccharide (LPS). The responsiveness of plasmacytoid pDCs to CpG correlated with high TLR9 expression similarly to human plasmacytoid pDCs. Conversely, myeloid DCs generated with granulocyte/macrophage colony-stimulating factor enhanced Th1 cell development when stimulated with LPS as a result of their high level of TLR4 expression. Polarized Th1 responses resulting from high antigen dose were not additionally enhanced by stimulation of DCs by TLR ligands. Thus, the net effect of antigen dose, the state of maturation of the DCs together with the stimulation of DCs by pathogen-derived products, will determine whether a Th1 or Th2 response develops.

The Development of Murine Plasmacytoid Dendritic Cell Precursors Is Differentially Regulated by FLT3-ligand and Granulocyte/Macrophage Colony-Stimulating Factor

Plasmacytoid predendritic cells or type 1 interferon (IFN)-producing cells (IPCs) have recently been identified in mice. Although culture systems giving rise to different murine dendritic cell subsets have been established, the developmental regulation of murine plasmacytoid IPCs and the culture conditions leading to their generation remain unknown. Here we show that large numbers of over 40% pure CD11c+CD11b−B220+Gr-1+ IPCs can be generated from mouse bone marrow cultures with FLT3-ligand. By contrast GM-CSF or TNF-α, which promote the generation of CD11c+CD11b+B220− myeloid DCs, block completely the development of IPCs. IPCs generated display similar features to human IPCs, such as the plasmacytoid morphology, the ability to produce large amounts of IFN-α in responses to herpes simplex virus, and the capacity to respond to ligands for Toll-like receptor 9 (TLR-9; CpG ODN 1668), but not to ligands for TLR-4 (lipopolysaccharide [LPS]). Unlike human IPCs which produce little IL-12p70, mouse IPCs produce IL-12p70 in response to CpG ODN 1668 and herpes simplex virus. This study demonstrates that the development of murine CD11c+CD11b−B220+Gr-1+ IPCs and CD11c+CD11b+B220− myeloid DCs is differentially regulated by FLT3-ligand and granulocyte/macrophage colony-stimulating factor. Human IPCs and mouse IPCs display different ability to produce IL-12p70. Large numbers of mouse IPCs can now be obtained from total bone marrow culture.

Plasmacytoid dendritic cells prime IL-10–producing T regulatory cells by inducible costimulator ligand

Although there is evidence for distinct roles of myeloid dendritic cells (DCs [mDCs]) and plasmacytoid pre-DCs (pDCs) in regulating T cell–mediated adaptive immunity, the concept of functional DC subsets has been questioned because of the lack of a molecular mechanism to explain these differences. In this study, we provide direct evidence that maturing mDCs and pDCs express different sets of molecules for T cell priming. Although both maturing mDCs and pDCs upregulate the expression of CD80 and CD86, only pDCs upregulate the expression of inducible costimulator ligand (ICOS-L) and maintain high expression levels upon differentiation into mature DCs. High ICOS-L expression endows maturing pDCs with the ability to induce the differentiation of naive CD4 T cells to produce interleukin-10 (IL-10) but not the T helper (Th)2 cytokines IL-4, -5, and -13. These IL-10–producing T cells are T regulatory cells, and their generation by ICOS-L is independent of pDC-driven Th1 and Th2 differentiation, although, in the later condition, some contribution from endogenous IL-4 cannot be completely ruled out. Thus, in contrast to mDCs, pDCs are poised to express ICOS-L upon maturation, which leads to the generation of IL-10–producing T regulatory cells. Our findings demonstrate that mDC and pDCs are intrinsically different in the expression of costimulatory molecules that drive distinct types of T cell responses.