Toll-like receptors in mediating pathogenesis in systemic sclerosis

Abstract

Toll-like receptors (TLRs) are evolutionarily conserved receptors essential for the host defence against pathogens. Both immune and non-immune cells can express TLRs, although at different levels. Systemic sclerosis (SSc) is a chronic disease in which autoimmunity, dysregulated profibrotic mediator release and activation of fibroblasts lead to dysregulated collagen deposition and fibrosis. There is now increasing knowledge that the innate immune system and, in particular, TLRs take a part in SSc pathogenesis. The list of endogenous ligands that can stimulate TLRs in SSc is growing: these ligands represent specific danger-associated molecular patterns (DAMPs), involved either in the initiation or the perpetuation of inflammation, and in the release of factors that sustain the fibrotic process or directly stimulate the cells that produce collagen and the endothelial cells. This review reports evidences concerning TLR signalling involvement in SSc. We report the new DAMPs, as well as the TLR-linked pathways involved in disease, with emphasis on type I interferon signature in SSc, the role of plasmacytoid dendritic cells (pDCs) and platelets. The dissection of the contribution of all these pathways to disease, and their correlation with the disease status, as well as their values as prognostic tools, can help to plan timely intervention and design new drugs for more appropriate therapeutic strategies.

Keywords: Toll-like receptor; fibrosis; immune cells; interferon signature; systemic sclerosis.

Figure 1

Endogenous Toll‐like receptor (TLR) ligands identified in systemic sclerosis (SSc). Binding of danger‐associated molecular patterns (DAMPs) (released by injured tissues) to TLR‐4 and TLR‐1 and ‐2 triggers the production of inflammatory cytokines [interleukin (IL)‐6, IL‐8, tumour necrosis factor (TNF)‐α, transforming growth factor (TGF)‐β], as well as factors involved in the extracellular matrix (ECM) deposition, such as the tissue inhibitor of metalloproteinases (TIMP‐1) and collagen 1. Whether TLR‐5 is triggered by endogenous ligand in SSc is unknown (possible candidates may be high‐mobility group protein 1 (HMGB1) or heat shock proteins (HSPs), according to studies in other settings (see text). SSc–immune complexes (SSc–IC), which include autoantibodies anti‐topoisomerase antibodies (ATA) and anti‐centromere antibodies (ACA), etc. as well as nanocristalline particles of platelet‐ and platelet dendritic cell (pDC)‐derived chemokine C‐X‐C ligand motif (CXCL4) bound to nucleic acids (DNA and RNA) can stimulate endosomal nucleic‐acid sensing TLRs TLR‐7, TLR‐8, TLR‐9 and TLR‐3, once internalized. SSc–IC also induces IL‐6, IL‐8, matrix metalloproteinase (MMP)‐2, monocyte chemoattractant protein 1 (MCP‐1), TGF‐β1 and pro‐collagen α1 by fibroblasts.

Figure 2

Toll‐like receptor (TLR)‐stimulation ability depends on the polycation–DNA complex structure, which influences the packaging of the DNA and the inter‐DNA spacing of contiguous DNA molecules. (a) Schematic representation of a structural type of DNA–polycation complex: cationic molecules bind and organize DNA chains (blue cylinders) into a columnar structure with a short‐ranged order. (b) Hypothetical structure of TLR‐9 and its interaction with dsDNA. (c) The optimal geometric spacing (inter‐DNA distance) between ordered dsDNA molecules bound to chemokine C‐X‐C ligand motif (CXCL4) [or other polycations, such as LL‐37 or human β‐defensin (HBD3)] is in the range between 3 and 4 nm (d). This amplitude almost matches the steric size of TLR‐9 and allows activate multiple TLR‐9 at the same time leading to the optimal interferon (IFN)‐α production. Outside this range (d smaller or larger than 3–4 nm), stimulation results in a modest or no IFN‐α production.

Figure 3

Chemokine C‐X‐C ligand 4 motif (CXCL4) in complex with human nucleic acids of various origin stimulates interferon (IFN)‐α release by platelet dendritic cells (pDCs). (a) Purified pDCs (175 × 10³/ml) from five different healthy donor (HD) peripheral blood mononuclear cells (PBMCs) were treated with CXCL4 alone (1 μM), human RNA alone (huRNA, 20 μg/ml) or CXCL4 pre‐complexed with huRNA (at the same concentrations), overnight. The release of IFN‐α in the culture supernatants was tested by enzyme‐linked immunosorbent assay (ELISA), as described 29. (b) PDCs (175 × 10³/ml) from seven different HD PBMCs were stimulated overnight with CXCL4 alone (1 μM), human DNA alone (huDNA, 10 μg/ml), mitochondrial DNA (mitoDNA, 10 μg/ml) or CXCL4 precomplexed with the two DNA types. IFN‐α in the culture supernatants was tested by ELISA. Horizontal bars are the means, vertical bars are standard errors of the mean, P‐values by Wilcoxon’s signed‐rank test.