Analysis of receptor-mediated C1q binding to human peripheral blood mononuclear cells

The binding of C1q to human peripheral blood leukocytes has been investigated. Studies with fluorescein conjugated F(ab')2 anti-C1q show that few (0 to 4%) normal leukocytes isolated in the presence of EDTA have C1q on their surface. However, approximately 26% of the mononuclear cell population is able to bind added C1q. Quantitative binding studies using 125I-C1q show that the binding to mononuclear cells is specific, saturable, and reversible. Scatchard plot analyses indicate an approximate equilibrium constant of 1.2 times 10(7) M-1. C1q binding appears to be mediated via the collagenous portion of the molecule in that 1) type I collagen inhibits this binding; 2) C1 reconstituted from purified C1q, C1r, and C1s does not bind to mononuclear cells, whereas the same amount of free C1q binds avidly; and 3) C1q enhances the binding of aggregated IgG to mononuclear cells.