Imaging Flow Cytometry Assays for Quantifying Pigment Grade Titanium Dioxide Particle Internalisation and Interactions with Immune Cells in Whole Blood.

29 Sep 2017

Pigment grade titanium dioxide is composed of sub-micron sized particles, including a nano fraction, and is widely utilised in food, cosmetic, pharmaceutical and biomedical industries. Oral exposure to pigment grade titanium dioxide results in at least some material entering the circulation in humans, although subsequent interactions with blood immune cells are unknown. Pigment grade titanium dioxide is employed for its strong light scattering properties and this work exploited that attribute to determine whether single cell-particle associations could be determined in immune cells of human whole blood at ‘real life’ concentrations. In vitro assays, initially using isolated peripheral blood mononuclear cells, identified titanium dioxide associated with the surface of, and within, individual immune cells by darkfield reflectance in imaging flow cytometry. This was confirmed at the population level by side scatter measurements using conventional flow cytometry. Next it was demonstrated that imaging flow cytometry could quantify titanium dioxide particle-bearing cells, within the immune cell populations of fresh whole blood, down to titanium dioxide levels of 10 parts per billion, which is in the range anticipated for human blood following titanium dioxide ingestion. Moreover, surface association and internal localisation of titanium dioxide particles could be discriminated in the assays. Overall, results showed that in addition to the anticipated activity of blood monocytes internalising titanium dioxide particles, neutrophil internalisation and cell membrane adhesion also occurred, the latter for both phagocytic and non-phagocytic cell types. What happens in vivo and whether this contributes to activation of one or more of these different cells types in blood merits further attention.