Antibody-Mediated Phagocytosis of the Amyloid β-Peptide in Microglia Is Differentially Modulated by C1q

Abstract

Microglial ingestion of the amyloid β-peptide (Aβ) has been viewed as a therapeutic target in Alzheimer’s disease, in that approaches that enhance clearance of Aβ relative to its production are predicted to result in decreased senile plaque formation, a proposed contributor to neuropathology. In vitro, scavenger receptors mediate ingestion of fibrillar Aβ (fAβ) by microglia. However, the finding that cerebral amyloid deposition in a transgenic mouse model of Alzheimer’s disease was diminished by inoculation with synthetic Aβ has suggested a possible therapeutic role for anti-Aβ Ab-mediated phagocytosis. Microglia also express C1qRP, a receptor for complement protein C1q, ligation of which in vitro enhances phagocytosis of immune complexes formed with IgG levels below that required for optimal FcR-mediated phagocytosis. The data presented here demonstrate FcR-dependent ingestion of Aβ-anti-Aβ complexes (IgG-fAβ) by microglia that is a function of the amount of Ab used to form immune complexes. In addition, C1q incorporated into IgG-fAβ enhanced microglial uptake of these complexes when they contained suboptimal levels of anti-Aβ Ab. Mannose binding lectin and lung surfactant protein A, other ligands of C1qRP, also enhanced ingestion of suboptimally opsonized IgG-fAβ, whereas control proteins did not. Our data suggest that C1qRP-mediated events may promote efficient ingestion of Aβ at low Ab titers, and this may be beneficial in paradigms that seek to clear amyloid via FcR-mediated mechanisms by minimizing the potential for destructive Ab-induced complement-mediated processes.

Accelerated Phagocytosis of Amyloid-β by Mouse and Human Microglia Overexpressing the Macrophage Colony-stimulating Factor Receptor

“…We found that cytochalasin D treatment resulted in an ϳ70% reduction in A␤ ingestion by BV-2 cells overexpressing M-CSFR. This level of A␤ phagocytosis inhibition is similar to that observed with cytochalasin D for primary rodent microglia (26). These results, along with those showing low cell surface binding and confocal images showing Fluo-A␤ internalization, demonstrate that M-CSFRinduced internalization is phagocytic.…”

“…Uptake of A␤ not blocked by fucoidan may have been because of interactions between A␤ and other microglial receptors. Although we used serum-free medium or media containing heat-inactivated serum, microglia express C1q and its receptor (31), which could also have been involved in uptake of A␤ (26). These results suggest that increased M-CSFR expression activates multiple pathways involved in the internalization of A␤.…”

“…MSR-A are important in microglial internalization of A␤ (4,26). Pretreatment of BV-2 microglia with the MSR ligand fucoidan resulted in a decrease in the uptake of Fluo-A␤ by nontransfected cells, and a proportionately similar decrease in uptake by cells overexpressing M-CSFR.…”

“…Another A␤ preparation, similar to that described by Webster et al (26), was utilized to study phagocytosis at higher A␤ concentrations. This preparation consisted of preaggregated Fluo-A␤ 1-40 added to nonlabeled A␤ 1-40 (Bachem, Torrance, CA) that had been aggregated by 60 h incubation at 4°C, followed by 8 h incubation with gentle shaking at 37°C as previously described (24).…”

The adaptive immune system restrains Alzheimer’s disease pathogenesis by modulating microglial function

“…Phagocytosis was assessed following previous methods with some modifications (36,62). First, BV2 cells (63) (3.0 × 10 5 cells per well) were exposed to either proinflammatory cytokine IL-1β (20 ng/mL; Thermo Fisher Scientific) or 0.1% BSA in PBS for 10-12 h. After IL-1β or PBS preexposure, the cells were changed to serum-free DMEM and then exposed to fAβ (10 μg/mL) with murine IgG (0.5 mg/mL; Jackson ImmunoResearch Laboratories) or PBS for 1 h at 37°C.…”