Detection of a high-affinity binding site for the globular head regions of the C1q complement protein on a human diploid fibroblast subtype.

A cultured fibroblast subtype with growth and synthetic properties expected of cells residing in healing wounds and in inflammatory lesions binds the Clq component of complement with a functional affinity much higher than that of the remaining cell population. In this study we examined the optimal conditions that favor the interaction between purified 125I-labeled Clq and this cell subtype, following its isolation from the parent culture using a cell sorter, and assessed the biologic consequences of binding. Binding of 125I-Clq to the cell surface is specific, saturable and reversible. It is maximal between pH 5.5 and 8.5 at an ionic strength of mu = 0.10 and decreases as a function of increasing salt concn, with half saturation near physiologic ionic strength. Scatchard analysis of binding data indicates a single class of sites with an average association constant in the order of 1.5 x 10(9)/M and an average number of 2.5 x 10(6) binding sites per cell. Unlabeled globular fragments of Clq inhibit intact 125I-Clq binding by 64%, while unlabeled collagen-like fragments have no effect. Thus it appears that binding of Clq to this high-affinity site is mediated by a region of the globular domain of the molecule. Only the fibroblast subtype with binding sites for the globular domain of Clq appear to have the capacity to induce non-immune activation of the classical complement cascade, as assessed by the generation of C4a and C4d fragments in normal AB serum following exposure to the cells. This activation may generate products that account for a previously reported complement mitogenicity for fibroblasts.