Anti-U1-RNP positivity remains mandatory for the mixed connective tissue disease (MCTD) diagnosis, reason for which anti-U1-RNP occurrence in patients with lupus clinical features might determine diagnostic issues. Thus, the prevalence of 25-30% for anti-RNP was reported in John Hopkins and LUMINA lupus cohorts and also 13% prevalence for the anti-U1-RNP in Euro-Lupus cohort. Presence of anti-U1-RNP antibodies in patients fulfilling SLE criteria (but not the MCTD ones) was associated with manifestations such as Raynaud phenomenon, musculoskeletal and lung impairment or nail fold capillaroscopy changes, some clinical features frequently encountered in MCTD patients and only rarely described in lupus population. The use of more specific markers such as 70 kDa anti-U1-RNP or anti-Sm-D was proposed for discriminating between SLE and MCTD. In addition, the IgM serotype of anti-U1-RNP seems more frequently expressed in SLE, while the IgG serotype alone in MCTD. Better acknowledgement of possible clinical involvements in lupus subsets, such as the peculiarities related to the anti-U1-RNP positivity, could provide access to early diagnosis of rather rare but possible severe lupus organ impairments (e.g. pulmonary arterial hypertension).
Keywords: Criteria MCTD; Scleroderma; Systemic lupus erythematosus; U1 70 k; U1 RNP.