C1Q, A SUBUNIT OF THE 1ST 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, macrop hages, polymorphonuclear cells, fibroblasts, platelets, lymphocytes, a nd endothelial cells. The present study demonstrates a functional C1Q- R on rat renal mesangial cells (MC). Incubation of MC with increasing concentrations of [I-125]C1Q resulted in a dose-dependent binding of [ I-125]C1Q to MC; the binding of [I-125]C1Q was inhibitable by excess u nlabeled C1Q or C1Q stalks whereas BSA and C1Q globular heads had no e ffect. Scatchard analysis of the data revealed the presence of 6.2 X 1 0(7) binding sites/cell with an affinity of 4.9 X 10(6) M-1 for C1Q. I mmunoprecipitation of I-125-labeled MC membrane proteins with C1Q or m Ab directed against human C1Q-R revealed a single 66- to 68-kDa band u nder reducing conditions. We have shown previously that soluble stable aggregates of IgG bind to rat MC in a dose-dependent fashion. In addi tion 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 AIgG in binding of [I-125]AIgG to MC, we incubated [I-125]AIgG in the presence of increasing concentrations of C1Q, and showed a 5- to 15-fold enhancement of binding of [I-125]AIgG to MC. Neither heat-inac tivated C1Q nor C1Q stalks were able to enhance the binding of [I-125] AIgG 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 C 1Q. These studies indicate that there is a cooperative effect between FcgammaR and C1Q-R on MC in the recognition of immune complexes.