Contemporary issues in fiber toxicology

Toxicity and bio-accumulation of inhaled cerium oxide nanoparticles in CD1 mice

Male CD1 mice were subjected to nose-inhalation exposure of CeO2 nanoparticles (NPs) for 0, 7, 14 or 28 days with 14 or 28 days of recovery time at an aerosol concentration of 2 mg/m(3). Markers of lung injury and pro-inflammatory cytokines (interleukin-1beta, tumour necrosis factor-alpha, interleukin-6 and macrophage inflammatory protein-2) in bronchoalveolar lavage fluid (BALF), oxidative stress in lungs, bio-accumulation, and histopathology of pulmonary and extrapulmonary tissues were assessed. BALF analysis revealed the induction of pulmonary inflammation, as evident by an increase in the influx of neutrophils with a significant secretion of pro-inflammatory cytokines that lead to generation of oxidative stress and cytotoxicity, as is evident by induction of lipid peroxidation, depletion of glutathione and increased BALF lactate dehydrogenase and protein. The histopathological examination revealed that these inhaled CeO2 NPs were located all over the pulmonary parenchyma, inducing a severe, chronic, active inflammatory response characterised by necrosis, proteinosis, fibrosis and well-formed discrete granulomas in the pulmonary tissue and tubular degeneration leading to coagulative necrosis in kidneys. Inductively coupled plasma optical emission spectrometer results showed a significant bio-accumulation of these particles in the pulmonary and extrapulmonary tissues, even after one month of post-inhalation exposure. Together, these findings suggest that inhalation exposure of CeO2 NPs can induce pulmonary and extrapulmonary toxicity.

Use of back-scatter electron signals to visualise cell/nanowires interactions in vitro and in vivo; frustrated phagocytosis of long fibres in macrophages and compartmentalisation in mesothelial cells in vivo

BACKGROUND

Frustrated phagocytosis has been stated as an important factor in the initiation of an inflammatory response after fibre exposure. The length of fibrous structures has been linked to the potential of fibres to induce adverse health effects for at least 40 years. However, we only recently reported for the first time the threshold length for fibre-induced inflammation in the pleural space and we implicated frustrated phagocytosis in the pro-inflammatory effects of long fibres. This study extends the examination of the threshold value for frustrated phagocytosis using well-defined length classes of silver nanowires (AgNW) ranging from 3-28 μm and describes in detail the morphology of frustrated phagocytosis using a novel technique and also describes compartmentalisation of fibres in the pleural space.

METHODS

A novel technique, backscatter scanning electron microscopy (BSE) was used to study frustrated phagocytosis since it provides high-contrast detection of nanowires, allowing clear discrimination between the nanofibres and other cellular features. A human monocyte-derived macrophage cell line THP-1 was used to investigate cell-nanowire interaction in vitro and the parietal pleura, the site of fibre retention after inhalation exposure was chosen to visualise the cell- fibre interaction in vivo after direct pleural installation of AgNWs.

RESULTS

The length cut-off value for frustrated phagocytosis differs in vitro and in vivo. While in vitro frustrated phagocytosis could be observed with fibres≥14 μm, in vivo studies showed incomplete uptake at a fibre length of ≥10 μm. Recently we showed that inflammation in the pleural space after intrapleural injection of the same nanofibre panel occurs at a length of ≥5 μm. This onset of inflammation does not correlate with the onset of frustrated phagocytosis as shown in this study, leading to the conclusion that intermediate length fibres fully enclosed within macrophages as well as frustrated phagocytosis are associated with a pro-inflammatory state in the pleural space. We further showed that fibres compartmentalise in the mesothelial cells at the parietal pleura as well as in inflammatory cells in the pleural space.

CONCLUSION

BSE is a useful way to clearly distinguish between fibres that are, or are not, membrane-bounded. Using this method we were able to show differences in the threshold length at which frustrated phagocytosis occurred between in vitro and in vivo models. Visualising nanowires in the pleura demonstrated at least 2 compartments--in leukocyte aggregations and in the mesothelium--which may have consequences for long term pathology in the pleural space including mesothelioma.

Single- and multi-wall carbon nanotubes versus asbestos: are the carbon nanotubes a new health risk to humans?

Carbon nanotubes (CNT), since their discovery, have become one of the most promising nanomaterials in many industrial and biomedical applications. Due to their unique physicochemical properties, interest is growing in the manufacture of CNT-based products and their subsequent marketing. Since their discovery, the prospect of possible undesirable human health effects has been a focus of many scientific studies. Although CNT possess unique physical properties that include (1) nanoscale diameter, (2) a wide length distribution ranging from tens of nanometers to several micrometers, and (3) high aspect ratio, the fibrous-like shape and durability suggest that their toxic properties may be analogous to those observed with other fibrous particles, such as asbestos. The present study provides a summary of published findings on CNT bioactivity, such as the potential of CNT, especially of multi-wall carbon nanotubes (MWCNT), to activate signaling pathways modulating transcription factor activity, induce apoptosis, induce DNA damage, and initiate biological responses. Assessment of risks to human health and adoption of appropriate exposure controls is critical for the safe and successful introduction of CNT -based products for future applications.

Chronic inhalation of short asbestos: lung fiber burdens and histopathology for monkeys maintained for 11.5 years after exposure

In an earlier report, Platek et al. (1985) presented the results of an 18-month inhalation exposure of rats and monkeys to short chrysotile asbestos. The mean chamber exposure level was 1.0 mg/m(3) with an average of 0.79 fibers/ml > 5 microm in length. Gross and histopathological examination of exposed and control rats indicated no treatment-related lesions. Asbestos bodies adjacent to the terminal bronchioles, but no fibrosis, were found in lung biopsy tissue taken from the exposed monkeys at 10 months post-exposure. Fifteen monkeys (9 exposed and 6 controls) from this study were maintained for 11.5 years following exposure. Lung fiber burdens were determined by transmission electron microscopy. The mean lung burden (+/- standard deviation) for 59 samples from exposed monkeys was 63 +/- 30 x 10(6) fibers/g dry lung (range, 18-139 x 10(6)). The geometric mean fiber length was 3.5 microm with 35% of the fibers being > 5 microm in length. These data indicate some chrysotile fibers are durable in vivo for a significant period of time. Lungs were examined grossly and microscopically. No lesions attributable to the inhalation exposure were noted. Asbestos bodies were seen in the lungs of treated monkeys, primarily in the interstitium near bronchioles or small pulmonary blood vessels (which also may have been near to bronchioles just out of the plane of section).

Respiratory Tract

The chapter describes different aspects of the respiratory tract. In preclinical safety studies, pathologies of the respiratory system can be a result of an intercurrent disease or can be induced by systemically administered drugs. Intranasal or inhalation modes of therapy pose particular challenges in terms of the formulations and technologies required to administer a drug. A complex technology is developed to support the assessment of adverse effects of inhaled substances in rodent and nonrodent species, and the extrapolation of experimental findings to humans. The nasal chambers are the structures that are first to be subjected to the effects of inhaled substances, whether microorganisms or chemical substances. In rodents, the relatively small size of the nose and nasal sinuses facilitates a histological examination. Findings show that infectious agents cause inflammation in the nose and nasal sinuses, and this may be associated with inflammation in the conjunctiva, the middle ear, and the oral cavity. It has been observed that a particular response of the rodent nasal mucosa to some irritant substances, including pharmaceutical agents, is the formation of rounded eosinophilic inclusions in the cytoplasm of sustentacular cells of the olfactory epithelium, and to a lesser extent in respiratory and glandular epithelial cells.

Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety

Carbon nanotubes (CNT) are an important new class of technological materials that have numerous novel and useful properties. The forecast increase in manufacture makes it likely that increasing human exposure will occur, and as a result, CNT are beginning to come under toxicological scrutiny. This review seeks to set out the toxicological paradigms applicable to the toxicity of inhaled CNT, building on the toxicological database on nanoparticles (NP) and fibers. Relevant workplace regulation regarding exposure is also considered in the light of our knowledge of CNT. CNT could have features of both NP and conventional fibers, and so the current paradigm for fiber toxicology, which is based on mineral fibers and synthetic vitreous fibers, is discussed. The NP toxicology paradigm is also discussed in relation to CNT. The available peer-reviewed literature suggests that CNT may have unusual toxicity properties. In particular, CNT seem to have a special ability to stimulate mesenchymal cell growth and to cause granuloma formation and fibrogenesis. In several studies, CNT have more adverse effects than the same mass of NP carbon and quartz, the latter a commonly used benchmark of particle toxicity. There is, however, no definitive inhalation study available that would avoid the potential for artifactual effects due to large mats and aggregates forming during instillation exposure procedures. Studies also show that CNT may exhibit some of their effects through oxidative stress and inflammation. CNT represent a group of particles that are growing in production and use, and therefore, research into their toxicology and safe use is warranted.

Effects of crocidolite asbestos on human bronchoepithelial-dependent fibroblast stimulation in coculture: the role of IL-6 and GM-CSF

Cocultures of human pulmonary epithelial cells (BEAS-2B) and lung fibroblasts (WISTAR-38), representing two cell types of central regulatory potential in (chronic) lung disease, were used as an in vitro model to study the role of interleukin 6 (IL-6) and of granulocyte macrophage-colony stimulating factor (GM-CSF) in early fibrogenesis. For this purpose, epithelial cells were pre-exposed to UICC crocidolite asbestos fibers or titanium dioxide (TiO2) particles for 96 h and subsequently cocultured with fibroblasts for additional 72 h. Gene expression of IL-6 or GM-CSF in both cell types as well as of alpha1 procollagens types I and III in fibroblasts was determined by RT-PCR. Synthesis of IL-6, GM-CSF or collagen I was quantified using IL-6 bioassay or ELISA tests, respectively. Both mediators were directly induced in bronchoepithelial cells by crocidolite but not by TiO2. Likewise, steady-state mRNA levels of procollagens as well as collagen synthesis were upregulated in cocultured fibroblasts. As a result of coculture, cytokine concentrations were synergistically enhanced and further increased by crocidolite in a dose-dependent manner. Suppression of cytokine induction by corresponding neutralizing antibodies consistently abrogated collagen enhancement. Direct stimulation of fibroblast monocultures with recombinant human IL-6 or GM-CSF significantly increased collagen synthesis and transcription in a dose-dependent manner. Thus, our results demonstrate that crocidolite selectively stimulated production of IL-6 and GM-CSF in bronchoepithelial cells. In epithelial-fibroblast interactions, these mediators appear to play a key role in regulating fibroblast activity, indicating a close correlation between these cytokines and the fibrogenic potential of particulates.

Production of reactive oxygen species by man-made vitreous fibres in human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMNL) or erythrocytes, isolated from human blood, were exposed to graded doses of asbestos (chrysotile), quartz, or man-made vitreous fibres (MMVF), i.e. refractory ceramic fibres (RCF), glasswool, or rockwool fibres. None of the MMVF affected either the viability of PMNL, as measured by trypan blue exclusion test, or induced haemolysis, whereas the positive controls, quartz and chrysotile, dose-dependently induced haemolysis in PMNL. MMVF did not increase the release of lactate dehydrogenase (LDH) from the PMNL, whereas the positive controls, chrysotile and quartz, induced a marked and dose-dependent release of LDH. When PMNL were exposed to MMVF, some of the fibre types slightly increased the levels of free intracellular calcium ([Ca2+]i) within the cells in a manner similar to that induced by chrysotile or quartz. All MMVF induced a dose-dependent production of reactive oxygen species (ROS) in PMNL, with RCF-induced production of ROS being the most marked. Production of ROS by MMVF seemed to depend on the availability of extracellular calcium because it could be attenuated with a Ca2+ channel blocker, verapamil, or a Ca2+ chelating agent, EGTA. Production of ROS may be a common pathway through which PMNL respond to MMVF-induced cell activation, but alterations of levels of free intracellular Ca2+ do not seem to be an absolute prerequisite for this effect. Fibre length seemed not to be an important factor in affecting the ability of MMVF to induce ROS production in PMNL. However, the balance between different elements in the fibre seemed importantly to affect the biological activity of a fibre.