USP18 and systemic sclerosis: The ubiquitin and IFN pathways are linked through ISG15, a ubiquitin-like protein, and USP18, a deubiquitinating enzyme specific for ISG15, both upregulated in SFRP2/DPP4 and MYOC/CCL19 SSc fibroblast populations and downregulated in tofacitinib-treated SFRP2+ (but not CCL19+) fibroblasts (52).