Prevalence and Clinical Associations of Systemic Sclerosis-Related Autoantibodies: A Nationwide Reuma.pt Cohort Study

Background: Autoantibodies are central to the diagnosis and risk stratification of systemic sclerosis (SSc), but their prevalence and clinical associations may vary across populations. This study aimed to evaluate the prevalence and immuno-clinical associations of different autoantibodies in the Rheumatic Diseases Portuguese Register systemic sclerosis cohort. Methods: This was a multicentre observational registry-based study that used data from the Reuma.pt SSc module. Patients were divided according to autoantibody status, and associations between autoantibody expression and clinical data were assessed using appropriate statistical tests, with Bonferroni correction applied for multiple comparisons. Multivariable binary logistic regression was performed for clinically relevant outcomes, with adjustment for sex, age at diagnosis and disease duration. Results: A total of 1080 patients were included, 87.5% female, with a mean age at last evaluation of 60.2 ± 14.6 years and a mean disease duration of 12.4 ± 10.0 years. Limited cutaneous SSc was the most frequent clinical category (57.4%), followed by diffuse cutaneous SSc (17.7%), very early diagnosis of systemic sclerosis (VEDOSS; 12.3%), overlap syndromes (9.8%) and SSc sine scleroderma (2.8%). Antinuclear antibodies were present in 93.4% of patients. Anti-centromere antibodies (ACAs) were the most frequent SSc-specific autoantibodies (54.6%), followed by topoisomerase I antibodies (ATAs; 21.8%). ACA positivity was associated with limited cutaneous disease; older age at diagnosis; and lower frequency of interstitial lung disease (ILD), myositis and flexion contractures. ATA positivity was associated with diffuse cutaneous disease, male sex, higher modified Rodnan skin score (mRSS), digital ulcers, flexion contractures, oesophageal involvement and ILD. Among less frequent autoantibodies, anti-Pm/Scl antibodies were associated with myositis, joint involvement, calcinosis and ILD; anti-U1RNP antibodies with younger age, MCTD overlap, myositis and joint involvement; anti-RNA polymerase III antibodies (ARAs) with scleroderma renal crisis and higher mRSS; anti-U3RNP antibodies with diffuse cutaneous disease and renal involvement; and anti-Ku antibodies with overlap syndromes. Conclusions: In this large real-world SSc cohort, autoantibody status was strongly associated with distinct clinical phenotypes, confirming its value for disease stratification. While most established immuno-clinical associations were reproduced, some differences from international cohorts were observed, supporting the need for population-specific validation of autoantibody associations in SSc.