Pulmonary toxicity of indium-tin oxide production facility particles in rats

Abstract

Indium-tin oxide (ITO) is used to make transparent conductive coatings for touch-screen and liquid crystal display electronics. Occupational exposures to potentially toxic particles generated during ITO production have increased in recent years as the demand for consumer electronics continues to rise. Previous studies have demonstrated cytotoxicity in vitro and animal models have shown pulmonary inflammation and injury in response to various indium-containing particles. In humans, pulmonary alveolar proteinosis (PAP) and fibrotic interstitial lung disease have been observed in ITO facility workers. However, which indium materials or specific processes in the workplace may be the most toxic to workers is unknown. Here we examined the pulmonary toxicity of three different particle samples that represent real-life worker exposures, as they were collected at various production stages throughout an ITO facility. Indium oxide (In2O3), sintered ITO (SITO) and ventilation dust (VD) particles each caused pulmonary inflammation and damage in rats over a time course (1, 7 and 90 days post-intratracheal instillation), but SITO and VD appeared to induce greater toxicity in rat lungs than In2O3 at a dose of 1 mg per rat. Downstream pathological changes such as PAP and fibrosis were observed in response to all three particles 90 days after treatment, with a trend towards greatest severity in animals exposed to VD when comparing animals that received the same dose. These findings may inform workplace exposure reduction efforts and provide a better understanding of the pathogenesis of an emerging occupational health issue.

Keywords: cytokines; indium-tin oxide; occupational exposure; phagocytosis; pulmonary toxicity.

Figure 1

Lactate dehydrogenase (LDH) levels and inflammatory cell counts in rat bronchoalvelolar lavage fluid (BALF) over a time course of 90 days post-exposure. (A) LDH concentration in BALF from days 1, 7, and 90 post-IT of particle suspensions. In₂O₃ (circles), SITO (squares), and VD (triangles) symbols represent the mean ± SEM (n = 6–8 rats). From the same BALF, (B) total cells, (C) macrophages and (D) PMNs were counted. (Note: PMNs were not counted on day 1 for SITO). *P < 0.05 compared with phosphate-buffered saline (PBS) vehicle controls (0 mg) on that day.

Figure 2

Cell differentials from bronchoalvelolar lavage fluid (BALF) over a time course of exposure. Cells from (A) day 1, (B) day 7 and (C) day 90 rats were designated as macrophages, PMNs, lymphocytes, or eosinophils (200 cells rat⁻¹). The percentage of cells scored as each cell type was calculated, and bars represent the mean percentage ± SEM (n = 6–8 rats).

Figure 3

Cytokine concentrations in bronchoalvelolar lavage fluid (BALF). Concentrations of (A) TNF-α, (B) IL-6 and (C) IL-1β were measured in BALF from In₂O₃, SITO and VD-treated rats. *P < 0.05 compared with the 0-mg controls on that day.

Figure 4

Phagocytosis of Esherichia coli by bronchoalvelolar lavage fluid (BALF) phagocytes. Cells from the BALF of (A) day 7 or (B) day 90 rats were plated at 2 x 10⁵ macrophages/well, washed to remove unattached cells, and treated with pHrodo™ Red E. coli BioParticles® for 2 h to allow phagocytosis. A separate set of plated control cells (0 mg) received Cytochalasin D (Cyto D) to prevent phagocytosis. Plates were read to measure changes in fluorescence, and values were normalized to the phosphate-buffered saline (PBS) controls (0 mg) to represent maximal phagocytosis. Error bars represent the mean ± SEM (n = 4–5 rats). *P < 0.05 compared with 0-mg controls.

Figure 5

Plasma indium concentrations over a time-course. Whole blood was collected from rats, and plasma indium content measured using inductively coupled plasma mass spectrometry (ICP-MS). Error bars represent the mean ± SEM (n = 4 rats). *P< 0.05 compared with 0-mg controls.

Figure 6

Histopathological findings in rat lungs 90 days post-IT. Lung sections from day 90 rats were stained with Hematoxylin and eosin (H&E) and Picrosirius Red and evaluated for inflammation, fibrosis and alveolar proteinosis. (A) Photomicrograph of H&E stained section of lung from a control animal (200 x magnification). (B) Lung from an animal exposed to 0.5-mg VD showing alveoli filled with foamy macrophages (asterisk) and alveolar walls lined by hyperplastic type II pneumocytes (arrows) (200 x magnification). (C) Alveoli containing abundant eosinophilic material consistent with lipoprotein (asterisk) from animal exposed to 5-mg SITO (200 x magnification). (D) Photomicrograph of Picrosirius red stained section of lung from a control animal showing normal pattern of collagen staining in alveolar walls (400 x magnification). (E) Increased interstitial collagen observed in alveolar walls, associated with deposition of particles (arrows) from an animal treated with 5 mg of In₂O₃ (400 x magnification). (F) Lung from an animal exposed to 1 mg of VD. Alveolar walls are thickened by fibrosis (arrows) (400 x magnification).