GPX4 and adult-onset Still disease: Given that both xCT and GPX4 are endogenous antioxidants acting against free iron-mediated LPO [37,38], the levels of xCT protein on PBMCs were significantly lower in AOSD patients (median 0.938, IQR 0.31–0.53) than in HC subjects (median 1.18, IQR 1.06–1.57, p < 0.05), but the expression levels of GPX4 were not significantly different between the two groups.