Identification of the Raji cell membrane-derived C1q inhibitor as a receptor for human C1q. Purification and immunochemical characterization.

Abstract

We have shown previously that an activity which is capable of precipitating purified C1q and inhibiting some of the C1q-dependent biologic reactions could be solubilized from the membranes of both normal human peripheral B lymphocytes and a B cell-derived lymphoblastoid cell line (Raji), both of which are known to possess receptors for human C1q. In this report we present evidence that this membrane-associated C1q inhibitor is a chondroitinase-insensitive macromolecule and is the receptor for human C1q. The receptor was solubilized from membranes of Raji cells with Nonidet P-40 and purified to homogeneity using C1q-Sepharose 4B affinity chromatography. Equilibrium density gradient centrifugation analysis revealed that the complex could be resolved into a protein-rich, low density fraction and a carbohydrate-rich, high density fraction. The large hydrodynamic size, coupled with the high buoyant density, suggests that a proteoglycan is a constituent of the complex and indicates that the receptor might be a macromolecular complex of a proteoglycan portion noncovalently linked to a 60-70 kD glycoprotein. The glycoprotein moiety, in turn, consists of two or more identical (70,000 mol wt) polypeptide chains held together by disulfide bonds and constitutes the C1q receptor (C1qR). Sucrose density ultracentrifugation analysis showed that the isolated receptor sediments with an apparent rate of 4.2 S. Immunochemical analyses demonstrated that a typical preparation of the C1qR complex consists of approximately 23% uronic acid and approximately 21% galactosamine with a galactosamine-to-glucosamine ratio of 3.2. Binding of C1q to the receptor was found to be optimal at low ionic strength and neutral or near-neutral pH (7-7.4). The isolated receptor was found to inhibit C1q hemolytic function, abrogate C1q-dependent rosette formation, and block the C1q-dependent, cell-mediated cytotoxicity, all of which are activities mediated by the receptor.

Binding of HLA-G to ITIM-Bearing Ig-like Transcript 2 Receptor Suppresses B Cell Responses

“…cell line was used as a B cell model as previously described (31). Indeed, some features of Raji cells may resemble the state of activated B cells, and proliferation of Raji cells was inhibited by HLA-G in a dose-and time-dependent manner similar to normal B cells (32).…”

Identification of a gC1q-binding protein (gC1q-R) on the surface of human neutrophils. Subcellular localization and binding properties in comparison with the cC1q-R.

“…The presence of receptors for Cl q on leukocytes was first suggested in 1972 (3). Many cell types have been described in which Clq binds to their cell surface, including platelets (4), neutrophils, monocytes (5), eosinophils (6), fibroblasts (7), endothelial cells (8), Raji cells (9), smooth muscle, and epithelial cells (10). The binding of Clq to these cells is thought to be via the collagen-like domain of the protein.…”

“…The binding of Clq to these cells is thought to be via the collagen-like domain of the protein. A collagen Clq-receptor (cClq-R)' was purified from Raji cells in 1984 (9) and was found to be a single chain, acidic glycoprotein with a molecular mass of -60,000-68,000 D and present on the cell types above. Recently, a heavily glycosylated surface membrane protein of -126,000 Mr present on monocytes, neutrophils, and on the monocyte-like cell line U937 has also been demonstrated to interact with the collagenlike region of Clq ( 11 ).…”

Novel collectin/C1q receptor mediates mast cell activation and innate immunity

“…37,38 Several different cell surface receptors for C1q and other collectin family members have been reported, including the C1q receptor for phagocytosis enhancement (C1qRp), CR1, calreticulin (CRT), and binding protein for the globular head of C1q (gC1qbp). [39][40][41][42][43][44][45][46][47][48][49][50][51][52] The precise role of each receptor remains an area of active investigation.…”

“…37,38 Several different cell surface receptors for C1q and other collectin family members have been reported, including the C1q receptor for phagocytosis enhancement (C1qRp), CR1, calreticulin (CRT), and binding protein for the globular head of C1q (gC1qbp). [39][40][41][42][43][44][45][46][47][48][49][50][51][52] The precise role of each receptor remains an area of active investigation.In conclusion, we now define the ␣ 2 ␤ 1 integrin as a novel receptor for C1q and several collectin family members, and we demonstrate not only the structural requirements for C1q and collectin binding but also the biologic relevance of the ␣ 2 ␤ 1 integrin/C1q interaction in mast cell activation. The ␣ 2 ␤ 1 integrin is a well-characterized receptor with a large extracellular domain that binds to a variety of ligands and a transmembrane domain and a cytoplasmic domain that activate a number of signaling pathways.…”