Smooth muscle and epithelial cells express specific binding sites for the C1q component of complement

In injury and inflammation, interactions of complement C1q with C1q receptors may provide attachment sites for cell localization and tissue regeneration. Cultured smooth muscle cells (58%), epithelial cells (26%), and endothelial cells (25%) attach to C1q-coated surfaces, while only 6% of cultured B cells (Raji) attach. Endothelial and Raji cells express C1q receptors, but C1q receptors (C1qR) on smooth muscle cells and epithelial cells have not previously been demonstrated. Evidence is provided that smooth muscle cells express an average of 1.5 × 10⁶ C1qR/cell (K_a = 10⁸ M^-1) and that epithelial cells express and average of 0.7 × 10⁶ C1qR/cell (K_a = 1.4 × 10⁸ M^-1). Binding properties of C1qR, and immunoreactivity to anti-C1qR antibodies, are characterized. The antibodiesspecifically recognize a 67-kDa component of smooth muscle cell lysates and inhibit cell attachment to C1q substrates. We conclude that distribution of C1qR may be ubiquitous; binding properties, size, and antigenicity of various C1qR may be related, but adhesive function may be tissue specific.