Antinuclear antibodies (ANA) represent valuable biomarkers in the diagnosis of systemic sclerosis (SSc) being present in the vast majority of the patients. Besides anti-topoisomerase I, anti-centromere and anti-RNA polymerase III antibodies as the main specificities, several other autoantibodies can be present in SSc patients including autoantibodies targeting the PM/Scl complex (also known as the exosome), U3-RNP/fibrillarin and the Th/To autoantigens. Anti-Th/To antibodies are one of the specificities that reportedly show homogenous nucleolar staining in conventional indirect immunofluorescence (IIF) ANA tests. Almost all protein components of the mitochondrial RNA processing (MRP) and the evolutionarily related RNase P complex have been reported to be the target of anti-Th/To antibodies in systemic autoimmune rheumatic disease (SARD) patients. However, Rpp25, Rpp38 and hPop1 have been described as the main autoantigen.
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