Complement receptor 3 mediates both sinking phagocytosis and phagocytic cup formation via distinct mechanisms

A long-standing hypothesis is that complement receptors (CRs), especially CR3, mediate sinking phagocytosis, but evidence is lacking. Alternatively, CRs have been reported to induce membrane ruffles or phagocytic cups, akin to those induced by Fc. receptors (Fc.Rs), but the details of these events are unclear. Here we used real-time 3D imaging and KO mouse models to clarify how particles (human red blood cells) are internalized by resident peritoneal F4/80+ cells (macrophages) via CRs and/or Fc gamma Rs. We first show that Fc.Rs mediate highly efficient, rapid (2-3 min) phagocytic cup formation, which is completely abolished by deletion or mutation of the FcR gamma chain or conditional deletion of the signal transducer Syk. Fc gamma R-mediated phagocytic cups robustly arise from any point of cell-particle contact, including filopodia. In the absence of CR3, Fc.R-mediated phagocytic cups exhibit delayed closure and become aberrantly elongated. Independent of Fc gamma Rs, CR3 mediates sporadic ingestion of complement-opsonized particles by rapid phagocytic cup-like structures, typically emanating from membrane ruffles and largely prevented by deletion of the immunoreceptor tyrosine-based activation motif (ITAM) adaptors FcR. chain and DAP12 or Syk. Deletion of ITAM adaptors or Syk clearly revealed that there is a slow (10-25 min) sinking mode of phagocytosis via a restricted orifice. In summary, we show that (1) CR3 indeed mediates a slow sinking mode of phagocytosis, which is accentuated by deletion of ITAM adaptors or Syk, (2) CR3 induces phagocytic cup-like structures, driven by ITAM adaptors and Syk, and (3) CR3 is involved in forming and closing Fc gamma R-mediated phagocytic cups.

Automated summary

Studies indicate that in macrophages, Fc receptors mediate rapid and efficient phagocytic cup formation, while CR3 mediates a slow sinking mode of phagocytosis. Additionally, CR3 is involved in inducing phagocytic cup-like structures and closing Fc gamma receptor-mediated phagocytic cups.