Differences in protein S and C4b-binding protein levels in different groups of patients with antiphospholipid antibodies.

Total protein S (tPS), free protein S (fPS) and C4b-binding protein (C4b-BP) were measured by immunological assays in 73 patients with antiphospholipid (aPL) antibodies, in order to determine whether the previously reported abnormalities in PS levels in this group of patients could be related to the presence of lupus anticoagulant (LA) or anticardiolipin (aCL) antibodies. As compared with the normal controls (n = 44), the authors found a significant decrease of tPS, fPS and C4b-BP in 45 LA(+)aCL(+) patients (P < 0.001), a decrease of tPS (P < 0.001), fPS and C4b-BP (P < 0.01) in eight LA(-)aCL(+) patients and a decrease of only fPS (P < 0.05) in 20 LA(+)aCL(-) patients. There was no difference in the levels of tPS, fPS and C4b-BP between LA(+)aCL(+) and LA(-)aCL(+) patients. In contrast, the LA(+)aCL(+) patients had lower values of tPS, fPS and C4b-BP than LA(+)aCL(-) patients (P < 0.05). In some patients, protein S activity (PSact) was also measured and a high correlation was observed between fPS antigen and PSact (r = 0.93, P < 0.001). The data show that the presence of aCL antibodies is associated with a probably acquired deficiency of PS and C4b-BP. On the other hand, in LA patients without a CL antibodies, the fPS deficiency is unrelated to an increase in C4b-BP levels and may be due to abnormal binding of PS to C4b-BP.