Human Umbilical Vein Endothelial Cells Possess Binding Sites for the Globular Domain of C1q

Binding sites for both the collagen-like and globular domains of C1q have been described on a variety of cell types. HUVEC were previously shown to express the 60- to 67-kDa receptor recognizing the collagen-like domain of C1q. This study demonstrates the presence of a distinct 28- to 33-kDa HUVEC protein (gC1qR) that interacts with the globular head domain of C1q. Polyclonal Abs raised against the Raji cell gC1qR partially inhibited HUVEC interaction with immobilized C1q and recognized a 28- to 33-kDa protein on Western blots. The Ab also reacted strongly with poly-L-lysine-immobilized, glutaraldehyde-fixed, intact HUVEC in ELISA assays. No significant difference in reactivity was noted if HUVEC were permeabilized with 0.2% Triton X-100. However, unfixed HUVEC grown on gelatin-coated microtiter wells to 80% confluence failed to express significant amounts of gC1qR Ag. Quantitation of HUVEC gC1qR by gel scanning suggested the presence of 5.7 ± 3.8 × 10⁶ molecules/cell (mean ± SD; n = 4). A quantitative sandwich ELISA procedure, however, detected only 3.7 ± 0.6 × 10⁵ gC1qR molecules/cell (mean ± SD; n = 4), consistent with previously described gC1qR multimerization. The capacity of endothelial cells to recognize both the collagen-like and globular domains of C1q via distinct binding sites may have implications for the role of C1q in vascular inflammatory and thrombotic lesions.