Type I interferons (alpha/beta) in immunity and autoimmunity

Spatiotemporal regulation of MyD88-IRF-7 signalling for robust type-I interferon induction

Robust type-I interferon (IFN-alpha/beta) induction in plasmacytoid dendritic cells, through the activation of Toll-like receptor 9 (TLR9), constitutes a critical aspect of immunity. It is absolutely dependent on the transcription factor IRF-7, which interacts with and is activated by the adaptor MyD88. How plasmacytoid dendritic cells, but not other cell types (such as conventional dendritic cells), are able to activate the MyD88-IRF-7-dependent IFN induction pathway remains unknown. Here we show that the spatiotemporal regulation of MyD88-IRF-7 signalling is critical for a high-level IFN induction in response to TLR9 activation. The IFN-inducing TLR9 ligand, A/D-type CpG oligodeoxynucleotide (CpG-A), is retained for long periods in the endosomal vesicles of plasmacytoid dendritic cells, together with the MyD88-IRF-7 complex. However, in conventional dendritic cells, CpG-A is rapidly transferred to lysosomal vesicles. We further show that conventional dendritic cells can also mount a robust IFN induction if CpG-A is manipulated for endosomal retention using a cationic lipid. This strategy also allows us to demonstrate endosomal activation of the IFN pathway by the otherwise inactive TLR9 ligand B/K-type oligodeoxynucleotide (CpG-B). Thus, our study offers insights into the regulation of TLR9 signalling in space, potentially suggesting a new avenue for therapeutic intervention.

Requirement of type I interferon signaling for arthritis triggered by double-stranded RNA

OBJECTIVE

Arthralgias and overt arthritides are often associated with viral infections. Viral infections expose the infected host to proinflammatory double-stranded RNA (dsRNA), which can cause joint inflammation and is a potent activator of interferon-alpha (IFNalpha). The aim of this study was to determine the role of IFNalpha and dsRNA-related signaling molecules in the onset of joint inflammation induced by viral dsRNA.

METHODS

IFNalpha and different forms of RNA were injected into the knee joints of wild-type mice, mice lacking the type I interferon receptor (IFNAR(-/-)), and mice deficient in dsRNA-dependent protein kinase (PKR(-/-)). Histologic evidence of joint damage and the ability of splenocytes to produce cytokines in response to dsRNA or IFNalpha were assessed.

RESULTS

Viral dsRNA, but not short single-stranded RNA, induced arthritis. The arthritis was aggravated by intracellular delivery of dsRNA. The expression of PKR was not mandatory for dsRNA-induced joint inflammation. In contrast, IFNalpha/beta signaling was important for dsRNA-induced joint inflammation because IFNAR(-/-) mice did not develop arthritis. Furthermore, intraarticular deposition of IFNalpha induced arthritis in PKR(-/-) and control mice, whereas IFNAR(-/-) mice were protected. The arthritogenic effect of IFNalpha was attenuated by in vivo depletion of monocyte/macrophages.

CONCLUSION

Arthritis triggered by dsRNA is not dependent on the expression of the dsRNA-signaling molecule PKR (or Toll-like receptor 3, as previously shown), but is associated with the ability to produce type I IFN and is critically dependent on type I IFN receptor signaling. The intrinsic arthritogenic properties of IFNalpha implicate a role of this cytokine in joint manifestations triggered by various interferogenic stimuli.

RNA released from necrotic synovial fluid cells activates rheumatoid arthritis synovial fibroblasts via toll-like receptor 3

OBJECTIVE

To assess the expression of Toll-like receptor 3 (TLR-3) protein in synovial tissues and cultured synovial fibroblasts obtained from patients with rheumatoid arthritis (RA) and osteoarthritis (OA) and to investigate the consequences of stimulation of cultured synovial fibroblasts with TLR-3 ligands.

METHODS

TLR-3 expression in synovial tissues was determined by immunohistochemistry and immunofluorescence, and expression in cultured RA synovial fibroblasts (RASFs) was determined by fluorescence-activated cell sorting and real-time polymerase chain reaction techniques. TLR-3 signaling was assessed by incubating RASFs with poly(I-C), lipopolysaccharide, palmitoyl-3-cysteine-serine-lysine-4, or necrotic synovial fluid cells from RA patients in the presence or absence of hydroxychloroquine or Benzonase. Subsequent determination of interferon-beta (IFNbeta), CXCL10, CCL5, and interleukin-6 (IL-6) protein production in the culture supernatants was performed by enzyme-linked immunosorbent assays.

RESULTS

TLR-3 protein expression was found to be higher in RA synovial tissues than in OA synovial tissues. TLR-3 expression was localized predominantly in the synovial lining, with a majority of the TLR-3-expressing cells coexpressing fibroblast markers. Stimulation of cultured RASFs with the TLR-3 ligand poly(I-C) resulted in the production of high levels of IFNbeta, CXCL10, CCL5, and IL-6 protein. Similarly, coincubation of RASFs with necrotic synovial fluid cells from patients with RA resulted in up-regulation of these cytokines and chemokines in a TLR-3-dependent manner.

CONCLUSION

Our findings demonstrate the expression of TLR-3 in RA synovial tissue and the activation of RASFs in vitro by the TLR-3 ligand poly(I-C) as well as by necrotic RA synovial fluid cells, and indicate that RNA released from necrotic cells might act as an endogenous TLR-3 ligand for the stimulation of proinflammatory gene expression in RASFs.