Imaging flow cytometry assays for quantifying pigment grade titanium dioxide particle internalization and interactions with immune cells in whole blood

Food grade titanium dioxide (E171) interferes with monocyte-macrophage cell differentiation and their phagocytic capacity

Food grade titanium dioxide E171 has been used in products such as confectionery, doughs and flours to enhance organoleptic properties. The European Union has warned about adverse effects on humans due to oral consumption. After oral exposure, E171 reaches the bloodstream which raises the concern about effects on blood cells such as monocytes. One of the main functions of these cells is the differentiation of macrophages leading to the phagocytosis of foreign particles. The aim of this study was to evaluate the effect of E171 exposure on the phagocytic capacity and differentiation process of monocytes (THP-1) into macrophages. Physicochemical E171 properties were evaluated, and THP-1 monocytes were exposed to 4, 40 and 200 μg/ml. Cell viability, uptake capacity, cytokine release, the differentiation process, cytoskeletal arrangement and E171 internalization were assayed. Results showed that E171 particles had an amorphous shape with a mean of hydrodynamic size of ∼46 nm in cell culture media. Cell viability decreased until the 9th day of exposure, while the uptake capacity decreased up to 62% in a concentration dependent manner in monocytes. Additionally, the E171 exposure increased the proinflammatory cytokines release and decreased the cell differentiation by a 61% in macrophages. E171 induced changes in cytoskeletal arrangement and some of the E171 particles were located inside the nuclei. We conclude that E171 exposure in THP-1 monocytes induced an inflammatory response, impaired the phagocytic capacity, and interfered with cell differentiation from monocytes to macrophages.

Distinct targeting and uptake of platelet and red blood cell-derived extracellular vesicles into immune cells

Blood-derived extracellular vesicles (EVs) hold great therapeutic potential. As blood contains mixed EV populations, it is challenging to study EVs originating from different cells separately. Blood cell concentrates manufactured in blood banks offer an excellent non-invasive source of blood cell-specific EV populations. To study blood cell-specific EVs, we isolated EVs from platelet (TREVs) and red blood cell (EryEVs) concentrates and characterized them using nanoparticle tracking analysis, imaging flow cytometry, electron microscopy and western blot analysis and co-cultured them with peripheral blood mononuclear cells (PBMCs). Our aim was to use imaging flow cytometry to investigate EV interaction with PBMCs as well as study their effects on T-lymphocyte populations to better understand their possible biological functions. As a conclusion, TREVs interacted with PBMCs more than EryEVs. Distinctively, TREVs were uptaken into CD11c+ monocytes rapidly and into CD19+ B-lymphocytes in 24 h. EryEVs were not uptaken into CD11c+ monocytes before the 24-h time point, and they were only seen on the surface of lymphocytes. Neither TREVs nor EryEV were uptaken into CD3+ T-lymphocytes and no effect on T-cell populations was detected. We have previously seen similar differences in targeting PC-3 cancer cells. Further studies are needed to address the functional properties of blood cell concentrate-derived EVs. This study demonstrates that imaging flow cytometry can be used to study the distinctive differences in the interaction and uptake of EVs. Considering our current and previous results, EVs present a new valuable component for the future development of blood-derived therapeutics.

Label-Free Identification of Persistent Particles in Association with Primary Immune Cells by Imaging Flow Cytometry

The frequency of human exposure to persistent particles via consumer products, air pollution, and work environments is a modern-day hazard and an active area of research. Particle density and crystallinity, which often dictate their persistence in biological systems, are associated with strong light absorption and reflectance. These attributes allow several persistent particle types to be identified without the use of additional labels using laser light-based techniques such as microscopy, flow cytometry, and imaging flow cytometry. This form of identification allows the direct analysis of environmental persistent particles in association with biological samples after in vivo studies and real-life exposures. Microscopy and imaging flow cytometry have progressed with computing capabilities and fully quantitative imaging techniques can now plausibly detail the interactions and effects of micron and nano-sized particles with primary cells and tissues. This chapter summarises studies which have utilized the strong light absorption and reflectance characteristics of particles for their detection in biological specimens. This is followed by the description of methods for the analysis of whole blood samples and the use of imaging flow cytometry to identify particles in association with primary peripheral blood phagocytic cells, using brightfield and darkfield parameters.

Food grade titanium dioxide accumulation leads to cellular alterations in colon cells after removal of a 24-hour exposure

Titanium dioxide food grade (E171) is one of the most used food additives containing nanoparticles. Recently, the European Food Safety Authority indicated that E171 could no longer be considered safe as a food additive due to the possibility of it being genotoxic and there is evidence that E171 administration exacerbates colon tumor formation in murine models. However, less is known about the effects of E171 accumulation once the exposure stopped, then we hypothesized that toxic effects could be detected even after E171 removal. Therefore, we investigated the effects of E171 exposure after being removed from colon cell cultures. Human colon cancer cell line (HCT116) was exposed to 0, 1, 10 and 50 μg/cm² of E171. Our results showed that in the absence of cytotoxicity, E171 was accumulated in the cells after 24 of exposure, increasing granularity and reactive oxygen species, inducing alterations in the molecular pattern of nucleic acids and lipids, and causing nuclei enlargement, DNA damage and tubulin depolymerization. After the removal of E171, colon cells were cultured for 48 h more hours to analyze the ability to restore the previously detected alterations. As we hypothesized, the removal of E171 was unable to revert the alterations found after 24 h of exposure in colon cells. In conclusion, exposure to E171 causes alterations that cannot be reverted after 48 h if E171 is removed from colon cells.

Phagocytic Uptake of Polymeric Particles by Immune Cells under Flow Conditions

Particles injected intravenously are thought to be cleared by macrophages residing in the liver and spleen, but they also encounter circulating immune cells. It remains to be established if the circulating cells can take up particles while flowing and if the uptake capacity is similar under static and flow conditions. Here, we use an in vitro peristaltic pump setup that mimics pulsatile blood flow to determine if immune cells take up particles under constant fluidic flow. We use polystyrene particles of varying sizes as the model of a polymeric particle for these studies. Our results show that the immune cells do phagocytose under flow conditions. We demonstrate that cell lines representing myeloid cells, primary human neutrophils, and monocytes take up submicrometer-sized particles at similar or better rates under flow compared to static conditions. Experiments with whole human blood show that, even under the crowding effects of red blood cells, neutrophils and monocytes take up particles while flowing. Together, these data suggest that circulating immune cells are likely to phagocytose intravenously injected particulates, which has implications for the design of particles to evade or target these cells.

Imaging flow cytometry methods for quantitative analysis of label-free crystalline silica particle interactions with immune cells

Exposure to respirable fractions of crystalline silica quartz dust particles is associated with silicosis, cancer and the development of autoimmune conditions. Early cellular interactions are not well understood, partly due to a lack of suitable technological methods. Improved techniques are needed to better quantify and study high-level respirable crystalline silica exposure in human populations. Techniques that can be applied to complex biological matrices are pivotal to understanding particle-cell interactions and the impact of particles within real, biologically complex environments. In this study, we investigated whether imaging flow cytometry could be used to assess the interactions between cells and crystalline silica when present within complex biological matrices. Using the respirable-size fine quartz crystalline silica dust Min-u-sil® 5, we first validated previous reports that, whilst associating with cells, crystalline silica particles can be detected solely through their differential light scattering profile using conventional flow cytometry. This same property reliably identified crystalline silica in association with primary monocytic cells in vitro using an imaging flow cytometry assay, where darkfield intensity measurements were able to detect crystalline silica concentrations as low as 2.5 μg/mL. Finally, we ultilised fresh whole blood as an exemplary complex biological matrix to test the technique. Even after the increased sample processing required to analyse cells within whole blood, imaging flow cytometry was capable of detecting and assessing silica-association to cells. As expected, in fresh whole blood exposed to crystalline silica, neutrophils and cells of the monocyte/macrophage lineage phagocytosed the particles. In addition to the use of this technique in in vitro exposure models, this method has the potential to be applied directly to ex vivo diagnostic studies and research models, where the identification of crystalline silica association with cells in complex biological matrices such as bronchial lavage fluids, alongside additional functional and phenotypic cellular readouts, is required.

Gastrointestinal Absorption and Toxicity of Nanoparticles and Microparticles: Myth, Reality and Pitfalls explored through Titanium Dioxide

Daily oral exposure to vast numbers (>1013/adult/day) of micron or nano-sized persistent particles has become the norm for many populations. Significant airborne particle exposure is deleterious, so what about ingestion? Titanium dioxide in food grade form (fgTiO2) , which is an additive to some foods, capsules, tablets and toothpaste, may provide clues. Certainly, exposed human populations accumulate these particles in specialised intestinal cells at the base of large lymphoid follicles (Peyer's patches) and it's likely that a degree of absorption goes beyond this- i.e. lymphatics to blood circulation to tissues. We critically review the evidence and pathways. Regarding potential adverse effects, our primary message, for today's state-of-art, is that in vivo models have not been good enough and at times woeful. We provide a 'caveats list' to improve approaches and experimentation and illustrate why studies on biomarkers of particle uptake, and lower gut/mesenteric lymph nodes as targets, should be prioritized.

Ultrasmall silica nanoparticles directly ligate the T cell receptor complex

The impact of ultrasmall nanoparticles (<10-nm diameter) on the immune system is poorly understood. Recently, ultrasmall silica nanoparticles (USSN), which have gained increasing attention for therapeutic applications, were shown to stimulate T lymphocytes directly and at relatively low-exposure doses. Delineating underlying mechanisms and associated cell signaling will hasten therapeutic translation and is reported herein. Using competitive binding assays and molecular modeling, we established that the T cell receptor (TCR):CD3 complex is required for USSN-induced T cell activation, and that direct receptor complex-particle interactions are permitted both sterically and electrostatically. Activation is not limited to αβ TCR-bearing T cells since those with γδ TCR showed similar responses, implying that USSN mediate their effect by binding to extracellular domains of the flanking CD3 regions of the TCR complex. We confirmed that USSN initiated the signaling pathway immediately downstream of the TCR with rapid phosphorylation of both ζ-chain-associated protein 70 and linker for activation of T cells protein. However, T cell proliferation or IL-2 secretion were only triggered by USSN when costimulatory anti-CD28 or phorbate esters were present, demonstrating that the specific impact of USSN is in initiation of the primary, nuclear factor of activated T cells-pathway signaling from the TCR complex. Hence, we have established that USSN are partial agonists for the TCR complex because of induction of the primary T cell activation signal. Their ability to bind the TCR complex rapidly, and then to dissolve into benign orthosilicic acid, makes them an appealing option for therapies targeted at transient TCR:CD3 receptor binding.

Precise Quantitative Analysis of Cell Targeting by Particle-Based Agents Using Imaging Flow Cytometry and Convolutional Neural Network

Understanding the intricacies of particle-cell interactions is essential for many applications such as imaging, phototherapy, and drug/gene delivery, because it is the key to accurate control of the particle properties for the improvement of their therapeutic and diagnostic efficiency. Recently, high-throughput methods have emerged for the detailed investigation of these interactions. For example, imaging flow cytometry (IFC) collects up to 60,000 images of cells per second (in 12 optical channels) and provides information about morphology and organelle localization in combination with fluorescence and side scatter intensity data. However, analysis of IFC data is extremely difficult to perform using conventional methods that calculate integral parameters or use mask-based object recognition. Here, we show application of a convolutional neural network (CNN) for precise quantitative analysis of particle targeting of cells using IFC data. CNN provides high-throughput object detection with almost human precision but avoids the subjective choice of image processing parameters that often leads to incorrect data interpretation. The method allows accurate counting of cell-bound particles with reliable discrimination from the nonbound particles in the field of view. The proposed method expands capabilities of spot counting applications (such as organelle counting, quantification of cell-cell and cell-bacteria interactions) and is going to be useful not only for high-throughput analysis of IFC data but also for other imaging techniques. © 2019 International Society for Advancement of Cytometry.

In Vitro Methods for Assessing Nanoparticle Toxicity

As a consequence of their increase in annual production and widespread distribution in the environment, nanoparticles potentially pose a significant public health risk. The sought-after catalytic activity granted by their physiochemical properties doubles as a hazard to physiological processes following exposure through inhalation, oral, transdermal, subcutaneous, and intravenous uptake. Upon uptake into the body, their size, morphology, surface charge, coating, and chemical composition augment the response of biological systems to the materials and enhance their toxicity. Identification of each property is necessary to predict the harm imposed by foreign nanomaterials in the body. Assay methods ranging from endotoxin and lactate dehydrogenase (LDH) signaling to apoptosis and oxidative stress detection supply valuable techniques for exposing biomarkers of nanoparticle-induced cellular damage. Spectroscopic investigation of epithelial barrier permeation and distribution within living cells reveals the proclivity of nanoparticles to penetrate the body's natural defensive boundaries and deposit themselves in cytotoxic locations. Combination of the various characterization methodologies and assays is required for every new nanoparticulate system despite preexisting data for similar systems due to the lack of deterministic trends among investigated nanoparticles. The propensity of nanomaterials to denature proteins and oxidize substrates in their local environment generates significant concern for the applicability of several traditional in vitro assays, and the modification of susceptible approaches into novel methods suitable for the evaluation of nanoparticles comprises the focus of future work centered on nanoparticle toxicity analysis.

Non-Functionalized Ultrasmall Silica Nanoparticles Directly and Size-Selectively Activate T Cells

Sub-micron-sized silica nanoparticles, even as small as 10-20 nm in diameter, are well-known for their activation of mononuclear phagocytes. In contrast, the cellular impact of those <10 nm [ i.e., ultrasmall silica nanoparticles (USSN)] is not well-established for any cell type despite anticipated human exposure. Here, we synthesized discrete populations of USSN with volume median diameters between 1.8 to 16 nm and investigated their impact on the mixed cell population of human primary peripheral mononuclear cells. USSN 1.8-7.6 nm in diameter, optimally 3.6-5.1 nm in diameter, induced dose-dependent CD4 and CD8 T-cell activation in terms of cell surface CD25 and CD69 up-regulation at concentrations above 150 μM Si_total (∼500 nM particles). Induced activation with only ∼2.4 μM particles was (a) equivalent to that observed with typical positive control levels of Staphylococcal enterotoxin B (SEB) and (b) evident in antigen presenting cell-deplete cultures as well as in a pure T-cell line (Jurkat) culture. In the primary mixed-cell population, USSN induced IFN-γ secretion but failed to induce T-cell proliferation or the secretion of IL-2, IL-10, or IL-4. Collectively, these data indicate that USSN initiate activation, with Th₁ polarization, of T cells via direct particle-cell interaction. Finally, similarly sized iron hydroxide particles did not induce the expression of T-cell activation markers, indicating some selectivity of the ultrasmall particle type. Given that humans may be exposed to ultrasmall particles and that these materials have emerging bioclinical applications, their off-target immunomodulatory effects via direct T-cell activation should be carefully considered.

Analysis of soluble or titanium dioxide derived titanium levels in human whole blood: consensus from an inter-laboratory comparison

Exposure to titanium (Ti),viathe ingestion of pigment grade Ti dioxide (TiO₂), is commonplace for westernised populations.

Pro-inflammatory adjuvant properties of pigment-grade titanium dioxide particles are augmented by a genotype that potentiates interleukin 1β processing

Background

Pigment-grade titanium dioxide (TiO₂) particles are an additive to some foods (E171 on ingredients lists), toothpastes, and pharma−/nutraceuticals and are absorbed, to some extent, in the human intestinal tract. TiO₂ can act as a modest adjuvant in the secretion of the pro-inflammatory cytokine interleukin 1β (IL-1β) when triggered by common intestinal bacterial fragments, such as lipopolysaccharide (LPS) and/or peptidoglycan.

Given the variance in human genotypes, which includes variance in genes related to IL-1β secretion, we investigated whether TiO₂ particles might, in fact, be more potent pro-inflammatory adjuvants in cells that are genetically susceptible to IL-1β-related inflammation.

Methods

We studied bone marrow-derived macrophages from mice with a mutation in the nucleotide-binding oligomerisation domain-containing 2 gene (Nod2^m/m), which exhibit heightened secretion of IL-1β in response to the peptidoglycan fragment muramyl dipeptide (MDP). To ensure relevance to human exposure, TiO₂ was food-grade anatase (119 ± 45 nm mean diameter ± standard deviation). We used a short ‘pulse and chase’ format: pulsing with LPS and chasing with TiO₂ +/− MDP or peptidoglycan.

Results

IL-1β secretion was not stimulated in LPS-pulsed bone marrow-derived macrophages, or by chasing with MDP, and only very modestly so by chasing with peptidoglycan. In all cases, however, IL-1β secretion was augmented by chasing with TiO₂ in a dose-dependent fashion (5–100 μg/mL). When co-administered with MDP or peptidoglycan, IL-1β secretion was further enhanced for the Nod2^m/m genotype. Tumour necrosis factor α was triggered by LPS priming, and more so for the Nod2^m/m genotype. This was enhanced by chasing with TiO₂, MDP, or peptidoglycan, but there was no additive effect between the bacterial fragments and TiO₂.

Conclusion

Here, the doses of TiO₂ that augmented bacterial fragment-induced IL-1β secretion were relatively high. In vivo, however, selected intestinal cells appear to be loaded with TiO₂, so such high concentrations may be ‘exposure-relevant’ for localised regions of the intestine where both TiO₂ and bacterial fragment uptake occurs. Moreover, this effect is enhanced in cells from Nod2^m/m mice indicating that genotype can dictate inflammatory signalling in response to (nano)particle exposure. In vivo studies are now merited.

Electronic supplementary material

The online version of this article (10.1186/s12989-017-0232-2) contains supplementary material, which is available to authorized users.