The Phagocytosis of Crystalline Silica Particles by Macrophages

Abstract

Silicosis is a chronic lung disease induced by the inhalation of crystalline silica. Exposure of cultured macrophages to crystalline silica leads to cell death; however, the mechanism of cell-particle interaction, the fate of particles, and the cause of death are unknown. Time-lapse imaging shows that mouse macrophages avidly bind particles that settle onto the cell surface and that cells also extend protrusions to capture distant particles. Using confocal optical sectioning, silica particles were shown to be present within the cytoplasmic volume of live cells. In addition, electron microscopy and elemental analysis showed silica in internal cellular sections. To further examine the phagocytosis process, the kinetics of particle uptake was quantified using an assay in which cells were exposed to ovalbumin (OVA)-coated particles, and an anti-OVA antibody was used to distinguish surface-bound from internalized particles. Fc receptor-mediated uptake of antibody-coated silica particles was nearly complete within 5 minutes. In contrast, no OVA-coated particles were internalized at this time. After 30 minutes, 30% of bound silica was internalized and uptake continued slowly thereafter. OVA-coated latex beads, regardless of surface charge, were internalized at a similarly slow rate. These results demonstrate that macrophages internalize silica and that nonopsonized phagocytosis occurs by a temporally, and possibly mechanistically, distinct pathway from Fc receptor-mediated phagocytosis. Eighty percent of macrophages die within 12 hours of silica exposure. Neither OVA coating nor tetramethylrhodamine isothiocyanate labeling has any effect on cell death. Interestingly, antibody coating dramatically reduces silica toxicity. We hypothesize that the route of particle entry and subsequent phagosome trafficking affects the toxicity of internalized particles.

Phagolysosome acidification is required for silica and engineered nanoparticle-induced lysosome membrane permeabilization and resultant NLRP3 inflammasome activity

“…Within AM, the phagosome containing particle(s) fuses with lysosomes in order to attempt to degrade the particle. Consistent with this notion, others and our group have observed silica and ENM, including Multi-walled Carbon Nanotubes (MWCNT) and Titanium Nanobelts (TNB), within phagolysosomal compartments in AM following exposure (Gilberti et al , 2008; Hamilton et al , 2009; Hamilton et al , 2013; Hamilton et al , 2014). Silica, MWCNT, TNB and numerous other nanomaterials are resistant to lysosomal degradation, and consequently, downstream consequences are not fully understood.…”

Catastrophic Inflammatory Death of Monocytes and Macrophages by Overtaking of A Critical Dose of Endocytosed Synthetic Amorphous Silica Nanoparticles/Serum Protein Complexes

“…Among such nanosystems, synthetic amorphous silica (SiO 2 )based nanosystems are the most interesting for two reasons: they are emerging as candidates for nanomedical-biotechnological applications (including the use of SiO 2 as a coat ing agent to protect biofluids and cells from the very toxic action of other NPs, such as quantum dots); and they are widely used in industry as clarifying agents for beverages or additives for paper, rubber and plastics. Although crystalline SiO 2 microparticles are well known to be toxic to cells and strongly proinflammatory in vitro, in animal models and in humans [4,5], a grow ing amount of evidence suggests that amorphous SiO 2 NPs also exert a significant degree of cell damage and toxicological effects [6,7]. Xie et al demonstrated that SiO 2 NPs (20 and 80nm diameter) injected in murine models are preva lently found in lungs, liver and spleen macro phages, but are rarely found in other nonphago cytic tissue cells [8].…”

Can We Translate Our Understanding of the Pathogenic Mechanisms of Silicosis Into a Therapeutic Plan?

“…Eighty percent of these macrophages die within 12 hours. 9 It may be that the phospholipidosis in the lung of silica exposed animals is the result of an interaction between stimulated AMs and fibroblasts to induce phospholipid production by type II cells in an attempt to coat particles and lessen toxicity. 10 In vitro studies show that coating of quartz particles with polyvinylpyridine-N-oxide (PVNO) or aluminum lactate (AL) impairs the ability of the particle to elicit inflammation or generate ROS by neutrophils or AMs.…”