INTRACELLULAR-LOCALIZATION OF THE HUMAN RECEPTOR FOR THE GLOBULAR DOMAINS OF C1Q

This study was performed to determine the localization of the recently described receptor for the globular domain of C1q, gC1qR. In contrast to previous reports, we were not able to detect significant surface e xpression of gC1qR on Raji cells, monocytes, neutrophils, human or rat mesangial cells, the endothelial cell line EA.hy 926, or HUVEC using FACS analysis. Only by using digoxigenin-conjugated Abs could some sur face staining of gC1qR be observed on rat mesangial cells and neutroph ils. However, after permeabilizing these cells with saponin, a strong positive intracellular staining for gC1qR was observed by FACS, fluore scence microscopy on coverslips, and confocal laser scanning microscop ic analysis. By reflection contrast microscopy and electron microscopy on ultrathin sections of permeabilized Raji cells, it was shown that gC1qR is present in double membranous cytoplasmic vesicles located in the proximity of the plasma membrane. To determine whether certain con ditions could induce surface expression of gC1qR, Raji cells were eith er stimulated with T cell growth factor, LPS, or driven to apoptosis b y incubation with fenretinide or by serum depletion. None of the condi tions resulted in significant surface expression of gC1qR. Our hypothe sis that gC1qR is not a surface molecule but a soluble molecule that i s secreted by cells is supported by the observation that gC1qR is foun d in significant concentrations in supernatants of several cultured ce lls and in normal human and rat sera. Our results suggest that the rec ently described gC1qR is not a cell surface receptor, but a soluble bi nding protein with affinity for the globular heads of C1q. Excreted gC 1qR might act as a potential fluid phase regulator of complement activ ation.