Serum complement activation on heterologous platelets is associated with arterial thrombosis in patients with systemic lupus erythematosus and antiphospholipid antibodies

Abstract

Complement plays a major role in inflammation and thrombosis associated with systemic lupus erythematosus (SLE) and the antiphospholipid syndrome (APS). A cross-sectional retrospective analysis was performed to evaluate serum complement fixation on platelets and thrombotic incidence using banked sera and clinical data from patients with SLE (n = 91), SLE with antiphospholipid antibodies (aPL) or APS (n = 78) and primary aPL (n = 57) or APS (n = 96). In-situ complement fixation was measured as C1q and C4d deposition on heterologous platelets using an enzyme-linked immunosorbent assay approach. Platelet activation by patient serum in the fluid phase was assessed via serotonin release assay. Enhanced in-situ complement fixation was associated with the presence of IgG aPL and IgG anti-beta2 glycoprotein 1 antibodies (P < 0.05) and increased platelet activation (P < 0.005). Moreover, enhanced complement fixation, especially C4d deposition on heterologous platelets, was positively associated with arterial thrombotic events in patients with SLE and aPL (P = 0.039). Sera from patients with aPL possess an enhanced capacity for in-situ complement fixation on platelets. This capacity may influence arterial thrombosis risk in patients with SLE.

Figure 1

Complement fixation on heterologous platelets by serum from healthy volunteers (normal, n = 50), patients with SLE without aPL (n = 91) and patients with SLE and aPL or APS (n = 78). Classical pathway C1q (Panel A) and C4d (Panel B) deposition are shown. Each data point represents results for a single patient. The horizontal line indicates mean complement component deposition for the group. Complement deposition is expressed as a ratio relative to a normal human serum pool (NHS), used as an internal assay standard, as described in Methods.

Figure 2

Comparison of complement fixation on heterologous platelets by serum samples from patients with primary aPL, SLE with aPL. The data depict mean (horizontal line), median (box) and standard deviation for C1q (Panel A) and C4d (Panel B) deposition for serum samples from patients with primary aPL with APS (APS, n = 96), primary aPL only (aPL, n = 57), SLE with aPL or APS (SLE +aPL/APS, n = 78) and SLE without aPL (SLE, n = 91). Complement fixation by serum from healthy volunteers (n = 50, normal) is shown for reference. Complement deposition is expressed as a ratio relative to an internal normal human serum standard, as described in Methods. The asterix (*) denotes statistically significant differences in complement deposition compared with normal (P < 0.05).