Ligand binding specificity of a macrophage surface receptor utilized by the fluorescein hapten for uptake into the endocytic pathway.

A series of compounds were tested as inhibitors of receptor-binding and uptake of fluorescein-derivatized antigens in primary peritoneal and J774 murine macrophage. Results were analysed with regard to the inhibitory potency of the tested ligands and revealed that the putative cell surface receptor preferentially recognized and bound aromatic structures containing phenyl rings. L-phenylalanine was found to be the most potent inhibitor among the ligands tested. Significant inhibition of FITC10BSA binding to macrophage was observed even at 10(-9) M concentration of specific monovalent ligands tested indicating that these ligands were bound by the putative macrophage receptor with high apparent affinity. Structural comparisons of the various inhibitors employed, demonstrated that accessible, unconjugated phenyl rings were bound by the putative receptor with high apparent affinity whereas non-phenyl derivatives were bound with either low apparent affinity or via non-specific interactions. Therefore, the fluorescein hapten appeared to utilize a receptor with specificity for an essential aromatic amino acid for gaining entry into the endocytic pathway of murine macrophage. Finally, the binding of the hapten was enhanced when polyvalent fluorescein-derivatized antigens were used as a result of receptor crosslinkage on the cell surface.