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.

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.

Dendritic cell (DC) responses to extracellular self-DNA and self-RNA are prevented by the endosomal seclusion of nucleic acid–recognizing Toll-like receptors (TLRs). In psoriasis, however, plasmacytoid DCs (pDCs) sense self-DNA that is transported to endosomal TLR9 upon forming a complex with the antimicrobial peptide LL37. Whether LL37 also interacts with extracellular self-RNA and how this may contribute to DC activation in psoriasis is not known. Here, we report that LL37 can bind self-RNA released by dying cells, protect it from extracellular degradation, and transport it into endosomal compartments of DCs. In pDC, self-RNA–LL37 complexes activate TLR7 and, like self-DNA–LL37 complexes, trigger the secretion of IFN-α without inducing maturation or the production of IL-6 and TNF-α. In contrast to self-DNA–LL37 complexes, self-RNA–LL37 complexes also trigger the activation of classical myeloid DCs (mDCs). This occurs through TLR8 and leads to the production of TNF-α and IL-6, and the differentiation of mDCs into mature DCs. We also found that self-RNA–LL37 complexes are present in psoriatic skin lesions and are associated with mature mDCs in vivo. Our results demonstrate that the cationic antimicrobial peptide LL37 converts self-RNA into a trigger of TLR7 and TLR8 in human DCs, and provide new insights into the mechanism that drives the auto-inflammatory responses in psoriasis.

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.

Psoriasis is a common T-cell-mediated skin disease with 2-3% prevalence worldwide. Psoriasis is considered to be an autoimmune disease, but the precise nature of the autoantigens triggering T-cell activation remains poorly understood. Here we find that two-thirds of patients with moderate-to-severe plaque psoriasis harbour CD4 þ and/or CD8 þ T cells specific for LL37, an antimicrobial peptide (AMP) overexpressed in psoriatic skin and reported to trigger activation of innate immune cells. LL37-specific T cells produce IFN-g, and CD4 þ T cells also produce Th17 cytokines. LL37-specific T cells can infiltrate lesional skin and may be tracked in patients blood by tetramers staining. Presence of circulating LL37-specific T cells correlates significantly with disease activity, suggesting a contribution to disease pathogenesis. Thus, we uncover a role of LL37 as a T-cell autoantigen in psoriasis and provide evidence for a role of AMPs in both innate and adaptive immune cell activation.

In humans, the type I interferon (IFN) family consists of 13 IFN- § subtypes, IFN-g and IFN-J ; the newly discovered IFN-like family consists of IFN-Q 1, -Q 2 and -Q 3. We have investigated the expression of type I and Q IFN genes following virus infections or Toll-like receptor (TLR) triggering in monocyte-derived DC (MDDC) and plasmacytoid DC (pDC). We found that all IFN- § , -g , -J and -Q subtypes are expressed in influenza-virus-infected MDDC or pDC. Conversely, differential type I IFN gene transcription was induced in MDDC and pDC stimulated by specific TLR agonists. TLR-9 stimulation by CpG DNA induced the expression of all IFN- § , -g , -J and -Q subtypes in pDC, whereas TLR-4 stimulation by LPS, or TLR-3 stimulation by poly I:C, induced only IFN-g and IFN-Q gene expression in MDDC. The expression pattern of IFN regulatory factor (IRF)-5 and IRF-7 in MDDC and pDC was also determined. IRF-5 was constitutively expressed in the two DC subsets whereas IRF-7 was constitutive in pDC but its expression was induced along MDDC maturation. Overall, our data indicate that the coordinated expression of IFN-Q with IFN-g would be of crucial importance for the maturation of DC.

Plasmacytoid dendritic cells (pDCs) are a rare population of circulating cells that selectively express Toll-like receptors (TLR)7 and TLR9 and have the capacity to produce large amounts of type I interferons (IFNs) in response to viruses or host-derived nucleic acid-containing complexes. Through the production of type I IFNs, pDCs initiate protective immunity by activating classical DCs, T cells, natural killer cells, and B cells. Upon activation, pDCs also differentiate into mature DCs and may contribute to the contraction of T-cell responses. In this review, we describe how pDCs are key players in the initiation and regulation of immune responses and discuss their emerging role in the pathogenesis of human diseases.

The intracellular location of nucleic acid sensors prevents recognition of extracellular self-DNA released by dying cells. However, on forming a complex with the endogenous antimicrobial peptide LL37, extracellular DNA is transported into endosomal compartments of plasmacytoid dendritic cells, leading to activation of Toll-like receptor-9 and induction of type I IFNs. Whether LL37 also transports self-DNA into nonplasmacytoid dendritic cells, leading to type I IFN production via other intracellular DNA receptors is unknown. Here we found that LL37 very efficiently transports self-DNA into monocytes, leading the production of type I IFNs in a Toll-like receptor-independent manner. This type I IFN induction was mediated by double-stranded B form DNA, regardless of its sequence, CpG content, or methylation status, and required signaling through the adaptor protein STING and TBK1 kinase, indicating the involvement of cytosolic DNA sensors. Thus, our study identifies a novel link between the antimicrobial peptides and type I IFN responses involving DNA-dependent activation of cytosolic sensors in monocytes. IntroductionType I IFNs, such as IFN-␣ and IFN-␤, are key cytokines in the antiviral host defense because of their ability to limit viral replication in infected cells. 1,2 In addition, type I IFNs shape antiviral immune responses by promoting maturation of conventional DCs (cDCs), survival and proliferation of T cells, activation of NK cells, and differentiation of B cells into antibody-secreting plasma cells. [1][2][3][4] Type I IFN induction is mediated by innate immune recognition of viral nucleic acids in endosomal compartments (during viral entry), or in the cytosol (during viral replication). 5 In the endosome, viral DNA is recognized by Toll-like receptor-9 (TLR9), which is primarily expressed in plasmacytoid dendritic cells (pDCs). 6 TLR9 recognizes hypomethylated CpG islets and signals via the adaptor molecule MyD88, leading to the activation of IRF7 in pDCs. 6 In the cytosol, several DNA receptors have been identified and shown to be responsible for TLR9-independent sensing of viral DNA and type I IFN production by non-pDCs. Cytosolic DNA receptors include DNA-dependent activator of IFN-regulatory factors, 7,8 RNA polymerase III, 9 IFI16, a protein that belongs to the PYHIN protein family, 10 and the recently described helicase DDX41 expressed by dendritic cells. 11 All these receptors recognize B form double-stranded DNA regardless of its sequence 5,12,13 and signal via the adaptor protein STING (stimulator of interferon genes) and Tank-binding kinase 1 (TBK1), which activates and phosphorylates IRF3. 5,14,15 Another cytosolic DNA receptor, which induces type III IFNs and signals via IRF7 and not IRF3, is Ku70, a protein also involved in DNA repair. 16 DNA is abundantly released into the extracellular environment by dying cells in the context of tissue injury. This self-DNA is normally nonimmunogenic because of its rapid extracellular degradation and because of the intracellular seclusion of DNA recog...