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Cutaneous lupus erythematosus (CLE) is a photosensitive autoimmune disease characterized by a strong type I IFN-associated inflammation. Keratinocytes are known to determine the interface dermatitis pattern in CLE by production of proinflammatory cytokines in the lower epidermis. These cytokines drive a cytotoxic anti-epithelial immune response resulting in keratinocytic cell death and release of endogenous nucleic acids. We hypothesized that these endogenous nucleic acids (RNA and DNA motifs) have the capacity to activate innate immune pathways in keratinocytes via pathogen recognition receptors. Gene expression analyses showed an excessive activation of innate immune response pathways with strong expression of IFN-regulated cytokines in CLE skin lesions. Cultured keratinocytes produce large amounts of these cytokines in response to stimulation of PRR with endogenous nucleic acids. UV stimulation enhances the immunogenicity of endogenous nucleic acids and induces CLE-like skin lesions in knockout mice lacking the cytosolic DNase TREX1. Our results provide evidence for a pathogenetic role of endogenous nucleic acids in CLE. They are released within the cytotoxic inflammation along the dermo-epidermal junction and have the capacity to drive the CLE-typical inflammation. UV irradiation supports this inflammation by generation of highly immunostimulatory DNA motifs (8-hydroxyguanosine). These findings explain the photosensitivity of patients with lupus and identify pathways of the innate immune system as targets for future therapies., (Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.)

Mammalian cells possess mechanisms to detect and defend themselves from invading viruses. In the cytosol, the RIG-I-like receptors (RLRs), RIG-I (retinoic acid-inducible gene I; encoded by DDX58) and MDA5 (melanoma differentiation-associated gene 5; encoded by IFIH1) sense atypical RNAs associated with virus infection. Detection triggers a signalling cascade via the adaptor MAVS that culminates in the production of type I interferons (IFN-α and β; hereafter IFN), which are key antiviral cytokines. RIG-I and MDA5 are activated by distinct viral RNA structures and much evidence indicates that RIG-I responds to RNAs bearing a triphosphate (ppp) moiety in conjunction with a blunt-ended, base-paired region at the 5'-end (reviewed in refs 1, 2, 3). Here we show that RIG-I also mediates antiviral responses to RNAs bearing 5'-diphosphates (5'pp). Genomes from mammalian reoviruses with 5'pp termini, 5'pp-RNA isolated from yeast L-A virus, and base-paired 5'pp-RNAs made by in vitro transcription or chemical synthesis, all bind to RIG-I and serve as RIG-I agonists. Furthermore, a RIG-I-dependent response to 5'pp-RNA is essential for controlling reovirus infection in cultured cells and in mice. Thus, the minimal determinant for RIG-I recognition is a base-paired RNA with 5'pp. Such RNAs are found in some viruses but not in uninfected cells, indicating that recognition of 5'pp-RNA, like that of 5'ppp-RNA, acts as a powerful means of self/non-self discrimination by the innate immune system.

Before they infect red blood cells and cause malaria, Plasmodium parasites undergo an obligate and clinically silent expansion phase in the liver that is supposedly undetected by the host. Here, we demonstrate the engagement of a type I interferon (IFN) response during Plasmodium replication in the liver. We identified Plasmodium RNA as a previously unrecognized pathogen-associated molecular pattern (PAMP) capable of activating a type I IFN response via the cytosolic pattern recognition receptor Mda5. This response, initiated by liver-resident cells through the adaptor molecule for cytosolic RNA sensors, Mavs, and the transcription factors Irf3 and Irf7, is propagated by hepatocytes in an interferon-α/β receptor-dependent manner. This signaling pathway is critical for immune cell-mediated host resistance to liver-stage Plasmodium infection, which we find can be primed with other PAMPs, including hepatitis C virus RNA. Together, our results show that the liver has sensor mechanisms for Plasmodium that mediate a functional antiparasite response driven by type I IFN.

Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe acute respiratory infection with as yet unclear epidemiology. We previously showed that MERS-CoV counteracts parts of the innate immune response in human bronchiolar cells. Here we analyzed accessory proteins 3, 4a, 4b, and 5 for their abilities to inhibit the type I interferon response. Accessory protein 4a was found to block interferon induction at the level of melanoma differentiation-associated protein 5 (MDA5) activation presumably by direct interaction with double-stranded RNA.

Plasmacytoid dendritic cells (pDC) have emerged as a principal subset of dendritic cells in both human and mouse. PDC morphology, surface markers, their migration in vivo and the ability to rapidly produce large amounts of type I interferons (IFN-alpha/beta) in response to toll like receptor (TLR) triggering sets them apart from other dendritic cell subsets. This review highlights the features that make pDC uniquely able to sense and respond to viral infection.