U1 and systemic lupus erythematosus: Previous studies have supported a possible association of antiphospholipid antibody (aPL), particularly the lupus anticoagulant (LA) [25] and anti-cardiolipin (aCL) [5], as well the anti-U1 RNP, and elevated concentrations of endothelin-1 (ET-1) [25] with the occurrence and/or pathogenesis of PH in SLE [40].