The effect of short silica fibers (0.3 μm 3.2 μm) on macrophages

Synthetic silica fibers of different length, diameter and shape: synthesis and interaction with rat (NR8383) and human (THP-1) macrophages in vitro, including chemotaxis and gene expression profile

BACKGROUND

Inhalation of biopersistent fibers like asbestos can cause strong chronic inflammatory effects, often resulting in fibrosis or even cancer. The interplay between fiber shape, fiber size and the resulting biological effects is still poorly understood due to the lack of reference materials.

RESULTS

We investigated how length, diameter, aspect ratio, and shape of synthetic silica fibers influence inflammatory effects at doses up to 250 µg cm-2. Silica nanofibers were prepared with different diameter and shape. Straight (length ca. 6 to 8 µm, thickness ca. 0.25 to 0.35 µm, aspect ratio ca. 17:1 to 32:1) and curly fibers (length ca. 9 µm, thickness ca. 0.13 µm, radius of curvature ca. 0.5 µm, aspect ratio ca. 70:1) were dispersed in water with no apparent change in the fiber shape during up to 28 days. Upon immersion in aqueous saline (DPBS), the fibers released about 5 wt% silica after 7 days irrespectively of their shape. The uptake of the fibers by macrophages (human THP-1 and rat NR8383) was studied by scanning electron microscopy and confocal laser scanning microscopy. Some fibers were completely taken up whereas others were only partially internalized, leading to visual damage of the cell wall. The biological effects were assessed by determining cell toxicity, particle-induced chemotaxis, and the induction of gene expression of inflammatory mediators.

CONCLUSIONS

Straight fibers were only slightly cytotoxic and caused weak cell migration, regardless of their thickness, while the curly fibers were more toxic and caused significantly stronger chemotaxis. Curly fibers also had the strongest effect on the expression of cytokines and chemokines. This may be due to the different aspect ratio or its twisted shape.

Cell-Biological Response and Sub-Toxic Inflammatory Effects of Titanium Dioxide Particles with Defined Polymorphic Phase, Size, and Shape

Six types of titanium dioxide particles with defined size, shape, and crystal structure (polymorphic form) were prepared: nanorods (70 × 25 nm²), rutile sub-microrods (190 × 40 nm²), rutile microspheres (620 nm), anatase nanospheres (100 nm), anatase microspheres (510 nm), and amorphous titania microspheres (620 nm). All particles were characterized by scanning electron microscopy, X-ray powder diffraction, dynamic light scattering, infrared spectroscopy, and UV spectroscopy. The sub-toxic cell-biological response to these particles by NR8383 macrophages was assessed. All particle types were taken up well by the cells. The cytotoxicity and the induction of reactive oxygen species (ROS) were negligible for all particles up to a dose of 100 µg mL^-1, except for rutile microspheres which had a very rough surface in contrast to anatase and amorphous titania microspheres. The particle-induced cell migration assay (PICMA; based on chemotaxis) of all titanium dioxide particles was comparable to the effect of control silica nanoparticles (50 nm, uncoated, agglomerated) but did not show a trend with respect to particle size, shape, or crystal structure. The coating with carboxymethylcellulose (CMC) had no significant biological effect. However, the rough surface of rutile microspheres clearly induced pro-inflammatory cell reactions that were not predictable by the primary particle size alone.

Barium sulphate microparticles are taken up by three different cell types: HeLa, THP-1, and hMSC

Cytotoxicity and cellular uptake of spherical barium sulphate microparticles (diameter 1 µm) were studied with three different cell lines, i.e. THP-1 cells (monocytes; model for a phagocytosing cell line), HeLa cells (epithelial cells; model for a non-phagocytosing cell line), and human mesenchymal stem cells (hMSCs; model for non-phagocytosing primary cells). Barium sulphate is a chemically and biologically inert solid which allows to distinguish two different processes, e.g. the particle uptake and potential adverse biological reactions. Barium sulphate microparticles were surface-coated by carboxymethylcellulose (CMC) which gave the particles a negative charge. Fluorescence was added by conjugating 6-aminofluorescein to CMC. The cytotoxicity of these microparticles was studied by the MTT test and a live/dead assay. The uptake was visualized by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). The particle uptake mechanism was quantified by flow cytometry with different endocytosis inhibitors in THP-1 and HeLa cells. The microparticles were easily taken up by all cell types, mostly by phagocytosis and micropinocytosis, within a few hours. STATEMENT OF SIGNIFICANCE: The interaction of particles and cells is of primary importance in nanomedicine, drug delivery, and nanotoxicology. It is commonly assumed that cells take up only nanoparticles unless they are able to phagocytosis. Here, we demonstrate with chemically and biologically inert microparticles of barium sulphate that even non-phagocytosing cells like HeLa and hMSCs take up microparticles to a considerable degree. This has considerable implication in biomaterials science, e.g. in case of abrasive debris and particulate degradation products from implants like endoprostheses.