C1Q, a subunit of the first component of complement, enhances the binding of aggregated IgG to rat renal mesangial cells.
Previous reports have shown the presence of C1Q-R on monocytes, macrophages, polymorphonuclear cells, fibroblasts, platelets, lymphocytes, and endothelial cells. The present study demonstrates a functional C1Q-R on rat renal mesangial cells (MC). Incubation of MC with increasing concentrations of [125I]C1Q resulted in a dose-dependent binding of [125I]C1Q to MC; the binding of [125I]C1Q was inhibitable by excess unlabeled C1Q or C1Q stalks whereas BSA and C1Q globular heads had no effect. Scatchard analysis of the data revealed the presence of 6.2 x 10(7) binding sites/cell with an affinity of 4.9 x 10(6) M-1 for C1Q. Immunoprecipitation of 125I-labeled MC membrane proteins with C1Q or mAb directed against human C1Q-R revealed a single 66- to 68-kDa band under reducing conditions. We have shown previously that soluble stable aggregates of IgG bind to rat MC in a dose-dependent fashion. In addition the presence of a receptor for IgG has been described on rat MC. In order to find out whether there is a cooperative effect between C1Q and AlgG in binding of [125I]AlgG to MC, we incubated [125I]AlgG in the presence of increasing concentrations of C1Q, and showed a 5- to 15-fold enhancement of binding of [125I]AlgG to MC. Neither heat-inactivated C1Q nor C1Q stalks were able to enhance the binding of [125I]AlgG to MC. Enhanced binding by C1Q was only observed when aggregated IgG was used; the binding of monomeric IgG to MC was not affected by C1Q. These studies indicate that there is a cooperative effect between Fc gamma R and C1Q-R on MC in the recognition of immune complexes.
van den Dobbelsteen
M E
ME
University Hospital Leiden, Dept. of Nephrology, The Netherlands.
van der Woude
F J
FJ
Schroeijers
W E
WE
Klar-Mohamad
N
N
van Es
L A
LA
Daha
M R
MR
D000818:Animals; D002352:Carrier Proteins; D002478:Cells, Cultured; D015922:Complement C1q / Q000502:physiology*; D005920:Glomerular Mesangium / Q000276:immunology*; D006801:Humans; D018960:Hyaluronan Receptors; D007074:Immunoglobulin G / Q000378:metabolism*; D008562:Membrane Glycoproteins; D024101:Mitochondrial Proteins; D008970:Molecular Weight; D051381:Rats; D017207:Rats, Sprague-Dawley; D011951:Receptors, Complement / Q000502:physiology; D017452:Receptors, IgG / Q000502:physiology
C087299:C1QBP protein, human; C490195:C1qbp protein, rat; D002352:Carrier Proteins; D018960:Hyaluronan Receptors; D007074:Immunoglobulin G; D008562:Membrane Glycoproteins; D024101:Mitochondrial Proteins; D011951:Receptors, Complement; D017452:Receptors, IgG; C032402:complement 1q receptor; C044411:polymeric IgG; D015922:Complement C1q