Binding of C1q to Galectin-3 Binding Protein on Microvesicles Released by Mononuclear Cells from Patients with Systemic Lupus Erythematosus

Background: Generation of galectin-3 binding protein (G3BP)-expressing microvesicles can be induced in-vitro by Toll-like receptor 9 ligation in mononuclear cells. Microvesicles co-expressing G3BP and double-stranded DNA are associated with active lupus nephritis. However, whether the microvesicular G3BP mainly deposits from circulation or is endogenously derived is unknown. In this study, we aim to delineate the origin of G3BP on in-vitro generated microvesicles by using serum as a source of native G3BP. Methods: G3BP-expressing microvesicles, generated by stimulation of systemic lupus erythematosus patient-derived mononuclear cells with the Toll-like receptor 9-agonist ODN2395, were incubated with normal human serum, heat-inactivated human serum, recombinant human C1q or human albumin. The expression of G3BP by microvesicles was examined by flow cytometry, and the binding of soluble recombinant human C1q to recombinant human G3BP was investigated by ELISA. Results: Approximately half of the microvesicles released from mononuclear cells expressed G3BP. Surprisingly, the staining was abrogated by incubation of the microvesicles with normal human serum, while incubation with heat-inactivated human serum did not have a similar effect. Reasoning that C1q might be the heat-labile factor blocking access of our G3BP antibody detection system, we incubated microvesicles with recombinant human C1q, which on average inhibited the detectable proportion of G3BP-bearing microvesicles by 87%. Soluble recombinant human C1q bound to immobilized recombinant human G3BP in a dose-dependent manner. Conclusion: Our data suggest that soluble C1q binds to G3BP on Toll-like receptor 9-induced microvesicles released from systemic lupus erythematosus patient-derived mononuclear cells. This interaction may exarcerbate inflammation in systemic lupus erythematosus but may also serve as a general mechanism for the appropriate clearance of these potentially pathogenic factors. Keywords: microvesicles, C1q, complement, clearance, Toll-like receptor 9, peripheral blood mononuclear cells, galectin-3 binding protein, systemic lupus erythematosus