INDEPENDENT MECHANISMS FOR MACROPHAGE BINDING AND MACROPHAGE PHAGOCYTOSIS OF DAMAGED ERYTHROCYTES - EVIDENCE OF RECEPTOR COOPERATIVITY

The binding and phagocytosis of oxidatively damaged red blood cells (O xRBCs) by mouse peritoneal macrophages can be inhibited by oxidatively modified LDL (OxLDL), implying some commonality at their receptor-bin ding domains. Studies from many different laboratories support the vie w that OxRBC binding is due to the disruption of plasma membrane phosp holipid asymmetry and the subsequent exposure of phosphatidylserine (P S) on the outer membrane leaflet. Presumably, oxidation of LDL creates a surface structure on it in some way homologous to the PS-rich domai n on OxRBCs. Apoptotic cells in some instances are also recognized bec ause of PS exposure on the outer leaflet of the membrane, and apoptoti c cells are a common feature of atherosclerotic lesions. In the presen t studies, the mechanisms of binding and internalization of cells reco gnized by virtue of their membrane PS were studied using OxRBCs or van adate-treated erythrocytes (VaRBCs) as models. Disruption of phospholi pid asymmetry with vanadate produced cells that were bound by macropha ges in the same divalent cation-dependent manner as OxRBCs. However. w hereas OxRBCs were rapidly phagocytosed, VaRBCs were not. Stimulation of mouse macrophages with phorbol myristate acetate resulted in a conc entration-dependent induction of phagocytosis of bound VaRBCs, on effe ct that could be prevented by the protein kinase C inhibitor staurospo rine. Because phagocytosis of OxRBCs occurred unassisted, we speculate d that there must be additional membrane changes induced by oxidation (over and above the disruption of phospholipid asymmetry) that contrib ute to phagocytosis of OxRBCs, possibly resulting in the ligation of a distinct receptor chat does not necessarily contribute to adherence. This proposal is supported by the finding that ligation of macrophage Fc gamma receptors by the anti-Fc gamma RII/RIII antibody 2.4G2 trigge rs the phagocytosis. of bound VaRBCs. Phagocytosis is also triggered b y subthreshold opsonization of VaRBC, ie, by antibody concentrations t hat do not by themselves cause binding and phagocytosis of native RBCs . Finally, treatment: with low concentrations of glutaraldehyde, which causes membrane protein cross-linking, promotes the phagocytosis of V aRBCs, but, at the low concentration used, has little or no effect on binding and phagocytosis of native RBCs. We that the internalization o f damaged cells, bound because of PS exposure, requires the cooperatio n of a PS-binding receptor with at lease one additional receptor to tr igger an intracellular signaling pathway to initiate phagocytosis.