C5a and formyl peptide receptor regulation on human monocytes

Abstract: Regulation of C5a and formyl-methionine-leucine-phenylalanine-lysine (fMLPL) receptors on hu-man monocytes has been studied using fluorescein-conjugated derivatives and flow cytometry. Monocytes have receptors for each of these ligands, as evidenced by their ability to bind specifically biologically active fluorescein derivatives of these ligands. Quenching ex-periments showed that bound fluoresceinated C5a and fMLPL are rapidly internalized at 37#{176}C. Once internal-ized, monocytes are able to reexpress these receptors, returning to control levels within approximately 90 mm. This contrasts with rate differences seen in polymor-phonuclear neutrophils (PMN5), where fMLPL receptors return more rapidly (-30 mm) than do C5a receptors (-100 mm). Monensin inhibited the reexpression of C5a but not fMLPL receptors, suggesting that a receptor recy-cling process is necessary to replenish C5a receptors on the monocyte surface. Similar although less efficient inhi-bition of C5a receptor reexpression was observed with NH4C1 treatment. Reexpression of both C5a and fMLPL receptors was independent of extracellular Ca2. Treat-ment with various agents known to stimulate monocytes and PMNs increased the expression of fMLPL receptors in both cell types but either had no effect on or reduced the level of C5a receptor expression. This would indicate that monocytes, like PMNs, have intracellular pools of preformed fMLPL receptors, available for reexpression. These studies show that, like PMNs, monocytes modulate C5a and fMLPL receptors through different mechanisms. Furthermore, monocytes are capable of reexpressing these receptors following exposure to ligand, a theoretical re-quirement for chemotaxis. J. Leukoc. Biol. 51: 393-399