Objective: Previous studies have demonstrated that the systemic sclerosis (SSc)-associated autoantigen DNA topoisomerase I (topo I) binds specifically to the surface of fibroblasts when released in the extracellular environment and recruits anti-topo I autoantibodies, which subsequently leads to the adhesion and activation of monocytes. This study aimed to characterize the molecular interactions of topo I with fibroblast surfaces in order to elucidate the pathogenic role of topo I/anti-topo I immune complexes (ICs) in SSc.
Methods: Topo I directly coupled to fluorochromes was used to follow its binding to fibroblast surfaces by flow cytometry and fluorescence microscopy. Purified IgG from normal subjects or SSc patients was added with topo I to the cells; unfractionated heparin (UFH) and low molecular weight heparin (LMWH) were used to determine their effects on the binding of topo I and topo I/anti-topo I IC to fibroblast surfaces.
Results: Heparan sulfate (HS) proteoglycans on fibroblast surfaces were found to act as coreceptors for topo I binding. The addition of anti-topo I autoantibodies from SSc sera led to the amplification of topo I binding to HS chains. UFH and LMWH were shown to inhibit topo I and topo I/anti-topo I IC binding to HS chains.
Conclusion: This study is the first to show that topo I binds specifically to HS proteoglycans on fibroblast surfaces and that anti-topo I autoantibodies from SSc patients amplify topo I binding to HS chains. The accumulation of topo I on cell surfaces by anti-topo I autoantibodies could contribute to the initiation of an inflammatory cascade stimulating the fibrosis. UFH and LMWH inhibited the binding of topo I/anti-topo I IC to fibroblasts, suggesting a potential therapeutic role in SSc-associated fibrosis.
Copyright © 2012 by the American College of Rheumatology.