Prevalence of anti-protein S antibodies in patients with systemic lupus erythematosus

OBJECTIVE: To determine the prevalence of circulating anti-protein S (anti-PS) antibodies in association with PS deficiency in patients with systemic lupus erythematosus (SLE). METHODS: Plasma was obtained from blood samples collected from 27 patients with SLE (5 men, 22 women; mean +/- SD age 32 +/- 10 years). Anti-PS antibodies were detected by immunoblotting according to a previously described method. Levels of free and total PS antigen were measured by enzyme-linked immunosorbent assay. PS activity was assayed by a clotting method. Levels of C4b binding protein (C4bBP) were measured by latex immunoassay. RESULTS: Among 27 patients with SLE, 19 (70.4%) had PS activity, and 12 (44.4%) had PS free antigen below the lower limit established for a normal population (mean -2SD). There was good correlation between PS activity and levels of PS free antigen (r = 0.851, P < 0.001), PS total antigen (r = 0.743, P < 0.001), and C4bBP (r = 0.597, P = 0.001). Circulating anti-PS antibodies and antiphospholipid antibodies (aPL) (IgG type) were detected in 7 (25.9%) and 14 (51.9%) of the 27 patients with SLE, respectively. Only 1 patient (3.7%) had both anti-PS antibodies and aPL. PS activities in patients who were positive for anti-PS antibodies (mean 38.7%) tended to be lower than those in patients who were negative for anti-PS antibodies (mean 54.9%), but the difference was not statistically significant. CONCLUSION: Anti-PS antibodies, independently of aPL, may play a role in the occurrence of PS deficiency in some patients with SLE, with possible effects on the function of PS that do not change the levels of PS antigens.