The third component of complement is transcribed and secreted by cultured human endothelial cells.

Monotypic cultures of passaged human umbilical vein endothelial cells secrete C3 protein, which is specifically immunoprecipitable from cell lysates and conditioned medium. A transcript migrating at 5.3 kilobases hybridizes with a mouse-derived C3 DNA probe in parallel cultures of endothelial cells, hepatoma cells (HepG2), and freshly isolated human monocytes. Steady-state transcript levels were as follows: HepG2 cells, 1.0; monocytes, 0.05; endothelial cells, 0.01. These data suggest that C3, an indispensible component for activation of complement by both the classical and alternative pathways, is locally synthesized in the vascular bed.