Binding of C1q and complement activation by vascular endothelium.

Exposure of intermediate filaments of cultured cells to serum leads to antibody-independent binding of complement (C) component C1q, C4, and C3. The apparent C-activating property of cytoskeletal intermediate filaments was examined on a tissue level in The present study by using frozen sections of human kidney and term placentas as targets. The assay used fresh human serum or isolated C1q as sources of C and the bound C components were demonstrated by fluorescent conjugates against C1q, C4, and C3. A consistent finding was that the endothelium of vessel walls had the capacity to bind both C1q and C components C4 and C3. Less intense binding of C to vascular smooth muscle was noted. Binding of C1q was very rapid, taking place within 10 sec. Subsequently, accumulation of C4 and C3 was seen. Subsequently, accumulation of C4 and C3 was seen. Upon prolonged incubation, a decrease in demonstrable C1q and C4 was observed. Unfixed vascular endothelium bound both isolated C1q and C1q of EDTA- or enzyme inhibitor-treated serum. However, if unfixed, the binding structures appeared to be destroyed in the process of C activation. They were also readily destroyed by trypsin. The described observations seem to suggest a possible mechanism whereby C components may become deposited in vessel walls independently of antibodies.