Binding of complement component C1q to anti-β2 glycoprotein I antibodies from patients with antiphospholipid syndrome

Objective: To determine if anti-beta(2) GPI reactive with surface-bound beta(2) GPI can bind Clq, i.e. to determine whether surface-bound beta(2) GPI-anti-beta(2) GPI immune complexes can initiate the classical pathway of complement activation. Methods: beta(2) GPI was bound to chemically-activated microtiter plates which had previously been shown to promote antip, GPI reactivity with bound beta(2) GPI. Wells with surface-bound beta(2) GPI (capped with bovine serum albumin) were then reacted with complement-inactivated sera from antiphospholipid syndrome patients (APS) or with control sera. Following removal of unbound serum components, the wells were incubated with biotinylated Clq and probed with peroxidase-conjugated avidin D. Bound Clq was detected at 450 nm using tetramethyl benzidine/peroxidase as a substrate system and expressed as absorbance units (Abs). Results. The identified 20 APS with elevated anti-beta(2) GPI: 4 with IgG only, 4 with IgM only, 1 with IgA only, 1 with IgG and IgA, 6 with IgG and IgM and 4 with IgG, IgA and IgM. Clq binding from 20 healthy controls was 0.039 +/- 0.029 (SD). Of the APS, 17/20 (85%) had Abs >5 SD above controls. The 3 APS with Clq Abs within normal limits had, respectively, IgM only (1), IgA only (1), and both IgG and IgM (1). Statistical analyses (Kruskal-Wallis followed by Dunn's post test) suggest differences in IgG and IgG + IgM groups compared to con (Kruskal-Wallis: p = 0.0002; Dunn's: con vs. IgG, p < 0.05; con vs. IgG + IgM, p < 0.01). Conclusions: Anti- beta(2) GPI from APS appear to have a variable degree of Clq affinity. Those patients with strong Clq binding responses are likely to have an inflammatory component to their disease processes.