Anti-Sm and Anti-U1-RNP Antibodies: An Update

From their discovery anti-Sm autoantibodies (Ab) have been associated with systemic lupus erythematosus (SLE), while anti-U1-RNP Ab detected alone are predominant in patients with mixed connective disease (MCTD). However, the identification of anti-Sm/U1-RNP Ab in a patient may be challenging, and usually requiring a two-step process including a screening step performed by indirect immunofluorescence (IIF) on HEp-2 cells showing a coarse speckled nuclear staining at an elevated level, followed by a confirmatory assay using specific antigens. The recent development of novel assays and the characterization of the target epitopes have been beneficial to improve the sensitivity for anti-Sm/U1-RNP Ab detection, but, in some cases, the necessity to use a different assay remains mandatory. Another recent and unexpected observation is related to the suspected role played by environmental and epigenetic factors in the induction of anti-Sm/U1-RNP Abs. Altogether, better knowledge regarding anti-Sm/U1- RNP Ab will undoubtedly provide improvements for the management and treatment of these patients.