MEMBRANE-ASSOCIATED C-REACTIVE PROTEIN ON RAT-LIVER MACROPHAGES IS SYNTHESIZED WITHIN THE MACROPHAGES, EXPRESSED AS NEO-C-REACTIVE PROTEIN AND BOUND THROUGH A C-REACTIVE PROTEIN-SPECIFIC MEMBRANE-RECEPTOR

We could recently show that rat liver macrophages (Kupffer cells) expr ess a membrane-bound form of C-reactive protein on their surface. Beca use it is removed by washing the cells in buffers containing Ca++-chel ators, membrane-bound C-reactive protein is a peripheral protein rathe r than an integral part of the Kupffer cell membrane. This Kupffer cel l membrane-bound C-reactive protein is identical to the galactose-spec ific particle receptor previously characterized. We now present eviden ce that Kupffer cells do not acquire soluble serum C-reactive protein but synthesize their own membrane-bound C-reactive protein. By RNA-RNA in situ hybridization, it was found that hepatocytes are not the only sort of liver cells synthesizing C-reactive protein, but C-reactive p rotein-specific mRNA is present also in Kupffer cells. During acute-ph ase response C-reactive protein mRNA is found in increased amounts wit hin liver macrophages too. Furthermore, by labeling experiments with a ntisera against native, pentameric soluble serum C-reactive protein an d monoclonal antibodies against a neoepitope present on C-reactive pro tein subunits only, we could establish that the membrane-bound C-react ive protein expressed on the liver macrophage is not the pentameric mo lecule of soluble serum C-reactive protein, but rather consists of C-r eactive protein subunits. Finally, we present evidence that liver macr ophages contain a binding protein in their plasma membrane, with an ap parent molecular weight of 59 to 61 kD, specific for C-reactive protei n and similar to the one previously isolated from macrophage cell line s.