CR1-receptor recycling in phorbol ester-activated polymorphonuclear leucocytes

Complement-receptor type 1, CR1, which recognizes the C3b cleavage fragment of C3, is present on the membranes of human phagocytic cells, but does not mediate phagocytosis or undergo internalization unless activated by one of a variety of stimuli. Among these stimuli low doses of phorbol esters have been shown to induce a consistently increased expression of CR1, despite apparently continuous receptor internalization. We have studied the fate of internalized receptor-ligand complexes in neutrophils activated with low concentrations of phorbol dibutyrate. In our studies, we followed CR1 with either 125I-C3b, the physiologic ligand, or with 125I-Fab fragments of a monoclonal anti-CR1 antibody. We observed rapid internalization of CR1-C3b complexes by PMN treated with 10 ng/ml (1.98 x 10(-8)M) PDBu, consistent in rate and extent with previously reported results using monoclonal antibodies. The fate of the internalized ligand was studied after elution of cell-surface C3b at 0 degrees. Intracellular ligand was externalized in a time- and temperature-dependent fashion, reaching a plateau at 10-15 min. Released C3b was totally TCA precipitable and structurally unaltered, as determined by SDS-PAGE, suggesting that recycling occurs via a prelysosomal predegradative compartment. Loading the cells with chloroquine did not affect this process. A monoclonal anti-CR1 Fab probe behaved in exactly the same manner, suggesting that the recycling of intact ligand-receptor complexes takes place. The possible physiological consequences of this finding are discussed.