Background: In systemic sclerosis (SSc), autoantibodies provide the most accurate tool to predict the disease subset and pattern of organ involvement. Scleroderma autoantibodies target nucleic acids or DNA/RNA-binding proteins, thus SSc immune complexes (ICs) can embed nucleic acids. Our working hypothesis envisaged that ICs containing scleroderma-specific autoantibodies might elicit proinflammatory and profibrotic effects in skin fibroblasts.
Methods: Fibroblasts were isolated from skin biopsies obtained from healthy subjects and patients with diffuse cutaneous SSc (dcSSc). ICs were purified by polyethylene-glycol precipitation from sera of SSc patients bearing different autoantibodies. ICs from patients with systemic lupus erythematosus (SLE) and primary anti-phospholipid syndrome (PAPS) and from normal healthy subjects (NHS) were used as controls. After incubation with ICs, fibroblasts were evaluated for ICAM-1 expression, interleukin (IL)-6, IL-8, monocyte chemoattractant protein (MCP)-1, matrix metalloproteinase (MMP)-2, tumor growth factor (TGF)-β1 and Pro-CollagenIα1 secretion, collagen (col)Iα1, mmp-1, toll-like receptor (tlr)2, tlr3, tlr4, tlr7, tlr8, tlr9, interferon (ifn)-α, ifn-β and endothelin-1 mRNA, and NFκB, p38MAPK and SAPK-JNK activation rate. Experiments were also performed after pretreatment with DNase I/RNase and NFκB/p38MAPK inhibitors.
Results: The antigenic reactivity for each SSc-IC mirrored the corresponding serum autoantibody specificity, while no positivity was observed in NHS-ICs or sera. SSc-ICs but not NHS-ICs increased ICAM-1 expression, stimulated IL-6, IL-8, MMP-2, MCP-1, TGF-β1 and Pro-CollagenIα1 secretion, upregulated et-1, ifn-α, ifn-β, tlr2, tlr3 and tlr4, and activated NFκB, p38MAPK and SAPK-JNK. tlr9 was significantly upregulated by ARA-ICs, mmp-1 was significantly induced by ACA-ICs whereas colIα1 was not modulated by any SSc-ICs. SLE-ICs and PAPS-ICs significantly upregulated MMP-2 and activated NFκB, p38MAPK and SAPK-JNK. SLE-ICs and PAPS-ICs did not affect colIα1, mmp-1 and Pro-CollagenIα1. DNase I and RNase treatment significantly reduced the upregulation of study mediators induced by SSc-ICs. Pretreatment with NFκB/p38MAPK inhibitors suggested that response to anti-Th/To-ICs was preferentially mediated by p38MAPK whereas ATA-ICs, ACA-ICs and ARA-ICs engaged both mediators. In dcSSc fibroblasts, stimulation with SSc-ICs and NHS-ICs upregulated IL-6 and IL-8.
Conclusions: These data provide the first demonstration of the proinflammatory and profibrotic effects of SSc-ICs on fibroblasts, suggesting the potential pathogenicity of SSc autoantibodies. These effects might be mediated by Toll-like receptors via the interaction with nucleic acid fragments embedded in SSc-ICs.
Keywords: Autoantibodies; Fibroblasts; Fibrosis; Immune complexes; Inflammation; Systemic sclerosis; Toll-like receptors.