Abstract
The effect of different murine monoclonal antibodies (Mab) specific for the glycoprotein complement receptor type 1 (CR1), type 2 (CR2), and type 3 (CR3) on the adhesion to and on the phagocytosis of human senescent red blood cells (S-RBC) by monocytes or by monocyte-derived macrophages (M phi) was investigated. Murine Mab anti-CR3 (anti-Leu 15 and OKM1) were found to inhibit, in the same order of magnitude, on one hand, the Fc receptors (FcR)-dependent rosetting and phagocytosis, and, on the other hand, the S-RBC rosetting and phagocytosis by adherent monocytes. Thus, the specific involvement of the CR3 epitopes recognized by Mab anti-Leu 15 or by OKM1 in the interactions between S-RBC and monocyte/macrophage could not be demonstrated. Murine Mab anti-CR1 was found to be a significant inhibitor of binding to and of phagocytosis of S-RBC (but not of young [Y] RBC) by monocytes or M phi, whereas Mab OKM5 carrying the same isotype as Mab anti-CR1, but a different specificity, was devoid of any significant inhibitory effect. Furthermore, Y-RBC or S-RBC opsonized with Mab anti-CR1 did not form FcR-dependent rosettes and were not internalized by monocytes; in addition, preincubation of phagocytes with Mab anti-CR1 did not inhibit FcR-dependent rosetting and phagocytosis. These results suggest that the effect of anti-CR1 is mediated through a specific binding to CR1 and not through an FcR blockade. As the role of specifically bound IgG on phagocytosis of human S-RBC by macrophages has previously been demonstrated by several authors, the present study suggests that monocyte-macrophage complement receptor type 1 may act in synergy with Fc receptors in the recognition of S-RBC by macrophages. It is shown in addition that the tripeptide Arg-Gly-Asp, identical to the region of iC3b recognized by CR3 and by several adhesion-promoting receptors that are structurally similar to CR3, such as fibronectin or vitronectin, is a significant inhibitor of the binding to and the phagocytosis of S-RBC by monocytic-macrophagic cells.
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