- •Serum C3 and C4 levels were low in primary antiphospholipid syndrome (APS).
- •Serum factor H levels were low and correlated with C3 levels in primary APS.
- •Low factor H may contribute to thrombophilia of APS via complement activation.
Although complement activation has been proposed as a possible thrombophilic mechanism in antiphospholipid syndrome (APS), the origin of complement activation in APS remains unclear. Here, we focused on complement regulatory factors (CRF), which control the complement system to prevent damage to host tissue. We evaluated the function of two major CRF, membrane cofactor protein (MCP) and factor H (FH), in APS patients.
Materials and methods
In this study, we analyzed preserved serum samples from 27 patients with primary APS (PAPS), 20 with APS complicated with SLE (APS + SLE), 24 with SLE (SLE), and 25 with other connective tissue diseases (Other CTD). Serum MCP and FH levels were tested by ELISA. Autoantibodies against FH were determined by both ELISA and western-blotting.
Serum complement levels of PAPS were lower than those of other CTD (median C3: 82 vs 112 mg/dL, p < 0.01, C4: 15 vs 22 mg/dL, p < 0.05). Serum MCP levels did not significantly differ among the groups. Serum FH levels were significantly lower in PAPS patients compared with SLE or other CTD (median 204, 1275, and 1220 μg/mL, respectively, p < 0.01). In PAPS patients, serum FH levels were positively correlated with serum C3 levels (p < 0.01, R = 0.55), but no correlation was found with serum C4 levels (p = 0.22, R = 0.33). Autoantibodies against FH were not detected in any of our patients.
Activation of the alternative complement pathway due to low level of FH is one of the possible thrombophilic mechanisms in PAPS.
Abbreviations:aHUS (atypical hemolytic uremic syndrome), aPL (antiphospholipid antibodies), APS (antiphospholipid syndrome), CRF (complement regulatory factors), CTD (connective tissue diseases), ELISA (enzyme-linked immunosorbent assays), FH (factor H), HRP (hydrogen peroxidase), MASP (mannose-associated serine protease), MCP (membrane cofactor protein), OD (optical density), PAPS (primary antiphospholipid syndrome), SLE (systemic lupus erythematosus), TMA (thrombotic microangiopathy), β2GPI (beta-2-glycoprotein I)
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Published online: February 20, 2018
Accepted: February 19, 2018
Received in revised form: January 25, 2018
Received: November 11, 2017
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