Activation of the complement system in systemic sclerosis. Relationship to clinical severity

Using newly developed techniques, we investigated the complement pathways and the extent of their activation in patients with systemic sclerosis (SSc) of both the diffuse cutaneous and limited cutaneous types. Plasma levels of the fragments C3d, C4d, and Ba were measured in patients with SSc and in matched control subjects. All fragments and ratios were higher in SSc patients than in controls (P < 0.05), demonstrating that complement activation occurs in SSc. Levels of C3d, C3d:C3, Ba, and Ba:factor B were higher in patients with diffuse cutaneous SSc than in those with limited SSc (P < 0.01). All fragments and ratios were higher in diffuse cutaneous SSc patients than in controls (P < 0.01). C3d, C3d:C3, C4d, and C4d:C4 levels were also higher in patients with limited cutaneous SSc than in controls (P < 0.05). These results show that complement activation occurs in SSc patients and that it reflects clinical severity. Complement activation may therefore have a pathogenetic role in SSc, and its measurement may prove useful in monitoring the disease.