BACKGROUND: Complement plays a major role in inflammation and thrombotic complications associated with SLE and the antiphospholipid syndrome (APS). Recently, direct activation of both classical and alternative pathways of complement was described on activated platelets. Moreover, platelet associated C4d was reported to be highly specific for SLE, and the presence of complement fixing antiphospholipid antibodies was correlated with thrombotic manifestations in patients with aPL.

METHODS: A cross-sectional retrospective analysis was performed to test the hypothesis that enhanced in situ classical pathway complement activation on platelets is associated with thrombotic complications in patients with SLE and/or aPL. Banked sera and clinical data were obtained from the Hospital for Special Surgery Autoimmune Disease Registry for patients with SLE (S, n=91) or SLE with aPL (SA, n=78), and from the national Antiphospholipid Syndrome Collaborative Registry (APSCORE) for patients with primary APS (n=96) or primary aPL without APS (n=57). Complement fixation on immobilized platelets was evaluated by measuring C4 deposition using an ELISA approach. C4 activation on platelets was correlated with laboratory and clinical findings including presence of antibodies against beta-2 glycoprotein I and incidence of venous/arterial thrombosis and fetal loss using Chi2 analysis. The serotonin release assay (SRA) was performed to assess platelet activation.

RESULTS: An increase in C4 activation on platelets greater than 2 fold over baseline, defined by an internal assay standard, and designated complement positive, was seen in 14% of SLE patients with aPL (p<0.001), 0% of patients with SLE only, 50% of patients with primary APS, and 59% of patients with primary aPL without APS. C4 activation was associated with the presence of positive serum beta-2 glycoprotein I IgG (p< 0.05). Complement positive sera induced platelet activation, resulting in 41± 23% serotonin release (n=28) compared to 16 ± 5% secretion effected by selected complement negative sera (n=5). Moreover, enhanced C4 activation on platelets was associated with a history of arterial thrombosis in 70% of SLE patients with aPL (p <0.05). Interestingly, no statistically significant association was noted between C4 activation on platelets and incidence of arterial thrombosis in patients with primary APS or aPL without APS. Consistent with reports of anti-complement effects of heparin therapy, C4 activation on platelets could be inhibited by unfractionated (0.5 U/ml) and low molecular weight (1 U/ml) heparin, but not by direct thrombin inhibition (50 μM PPACK).

CONCLUSION: Sera from patients with aPL demonstrate an enhanced capacity for classical pathway complement activation on platelets. In aPL patients with underlying SLE, this is associated with an increased incidence of arterial thrombosis. Prospective longitudinal studies are warranted to determine the predictive value of enhanced complement fixation on platelets as a marker for arterial thrombosis risk in patients with SLE and aPL.

Author notes

Disclosure:Research Funding: This work was supported by grants HL67211(EIBP) and AI060866 (BG) from the National Institutes of Health, an American Heart Association Heritage Affiliate post doctoral award 0625900T (WY), and the Mary Kirkland Center for Lupus Research (DRA, JES).