Anti-C1q autoantibodies are frequently detected in patients with systemic sclerosis associated with pulmonary fibrosis.

Background: Anti-C1q autoantibodies (autoAbs) are associated with systemic lupus erythematosus (SLE), but their presence in other rheumatic diseases has not been adequately investigated.

Objectives: We aimed to assess anti-C1q autoAbs and circulating immune complexes (CICs) in systemic sclerosis (SSc).

Methods: In total 124 patients with SSc were studied; 106 were female and the median age was 59·4 years (range 25-81·4). Overall 75 (60·5%) had limited cutaneous SSc and 49 (39·5%) had diffuse cutaneous SSc. Also included were 25 patients with Sjögren syndrome (SjS), 29 with rheumatoid arthritis (RA), 38 with SLE and 53 healthy controls. Enzyme-linked immunosorbent assays with high- and low-salt buffers were used to measure anti-C1q antibodies and CICs. The former allows only anti-C1q antibody binding to C1q and the latter also allows IgG Fc to bind to C1q.

Results: Anti-C1q antibodies were present in 20 of 124 (16·1%) patients with SSc: five had high levels (> 80 RU mL ) and 10 (50%) had moderate levels (40-80 RU mL ). Anti-C1q antibodies were also present in one of 25 (4%) patients with SjS, one of 29 (3%) with RA (P < 0·05 for both) and three of 53 (6%) healthy controls (P < 0·01). Anti-C1q antibodies were detected in 13 of 38 (34%) patients with SLEs. Anti-C1q antibodies were more frequent in male than female patients with SSc (P = 0·005); this association remained after multivariate regression analysis. Anti-C1q antibody level was the most important factor in predicting the presence of pulmonary fibrosis, and the second most important in predicting pulmonary arterial hypertension. Fourteen patients with SSc (11·3%) had CICs.

Conclusions: Anti-C1q autoAbs were frequently detected in patients with SSc, and their high levels predict the co-occurrence of pulmonary fibrosis or pulmonary arterial hypertension.