Titanium dioxide exposure induces acute eosinophilic lung inflammation in rabbits.

Titanium dioxide (TiO2) is increasingly widely used in industrial, commercial and home products. TiO2 aggravates respiratory symptoms by induction of pulmonary inflammation although the mechanisms have not been well investigated. We aimed to investigate lung inflammation in rabbits after intratracheal instillation of P25 TiO2. One ml of 10, 50 and 250µg of P25 TiO2 was instilled into one of the lungs of rabbits, chest computed-tomography was performed, and bronchoalveolar lavage (BAL) fluid was collected before, at 1 and 24 h after P25 TiO2 exposure. Changes in inflammatory cells in the BAL fluids were measured. Lung pathological assay was also carried out at 24 h after P25 TiO2 exposure. Ground glass opacities were noted in both lungs 1 h after P25 TiO2 and saline (control) instillation. Although the control lung showed complete resolution at 24 h, the lung exposed to P25 TiO2 showed persistent ground glass opacities at 24 h. The eosinophil counts in BAL fluid were significantly increased after P25 TiO2 exposure. P25 TiO2 induced a dose dependent increase of eosinophils in BAL fluid but no significant differences in neutrophil and lymphocyte cell counts were detected. The present findings suggest that P25 TiO2 induces lung inflammation in rabbits which is associated with eosinophilic inflammation.

Introduction

Over the past decades, advances in nanotechnology have led to their rapid applications in the fields of medicine, pharmaceutics, biotechnology, energy production and environmental sciences1). The increasing use of nanomaterials in various products at workplaces and in the home setting, including many consumer items such as clothing and plastic wares2) therefore pose an obvious risk to humans.

Titanium dioxide (TiO2) nanoparticles (NPs) are one of the most abundantly utilized nanomaterials because of their chemical stability, low toxicity and relatively cheap price3). It is used as a white pigment in paint, food coloring, as an ultraviolet blocker in cosmetics, disinfectant in environment and wastewater, and as a photosensitizer for photodynamic therapy. Oral ingestion and entry through the dermal route are mainly mediated by therapeutic or cosmetic application4, 5). The respiratory route is most important because the intake of NPs into the body is from atmospheric air via the upper respiratory tract6). TiO2 NPs are increasingly being manufactured, leading to increased occupational exposure and release into the atmospheric environment. Nano-sized particles are generally more toxic to the lung than their larger-sized counterparts7) which are why there has recently been increasing concern about the impact of TiO2 NPs in the lung.

Several epidemiological studies have reported that TiO2 NPs exposure at the work place aggravate respiratory symptoms8, 9). Besides, earlier studies indicated that inhalation TiO2 NPs can induce pulmonary response such as inflammation, fibrosis, emphysema-like lung injury, and lung cancer6, 10, 11). However, the pulmonary effects of TiO2 NPs are not fully understood. Previous animal studies have shown that exposure to TiO2 NPs causes oxidative stress, induce lung inflammation in the airways and alveolar spaces12, 13). Moreover, it has been reported that TiO2 NPs are able to induce neutrophilic pulmonary inflammation14, 15). Recent studies found that TiO2 NPs cause lung inflammation by activation of T-helper 2 cells and that the exposure of high concentration of TiO2 NPs in the lung induced an innate immune activation16, 17). Although in vitro and in vivo studies suggest that TiO2 NPs cause various forms of pulmonary inflammation11, 18), to our knowledge, relatively few studies have investigated the pulmonary effect in a rabbit model, and its pathogenic mechanism. The present study investigated the effect of TiO2 NPs on rabbit lungs, evaluated by lung image analysis, bronchoalveolar lavage (BAL) fluids examination, and histopathologic analysis.

Materials and Methods

Titanium dioxide (TiO2)

P25 TiO2 nanoparticles (Brunauer-Emmett-Teller (BET) specific surface area of 53.8 m2/g) were obtained from Degussa. The particles were suspended as follows: 1.5 g of P25 TiO2 powder was suspended in 100 ml of distilled water in a pyrex glass beaker, and sonicated for 15 min by a Branson Digital Cell Disruptor Sonifier 250 (Branson, USA) with a double-stepped microtip (3 mm diameter); this process was repeated 3 times. To stabilize temperature during sonication, the beaker was placed in a bucket of ice throughout the process. The particle suspension was then centrifuged at 3,000 × g for 20 min at 20 °C. The supernatant was carefully collected and filtered through a 1 μm filter to remove the large agglomerates (>1 μm). A given volume of particle suspension was evaporated, after which the weight of the remaining evaporate was measured and P25 TiO2 concentration determined (w/v; in mg/ml). The hydrodynamic size distribution by number of P25 TiO2 particles suspended in water was analyzed using a Dynamic Light Scattering Zetasizer Nano (Malvern Instruments, UK), and the average particle size was calculated to be 61.9 ± 5.1 nm.

Animals and study protocol

Nine Male New Zealand white rabbits (Taesung Laboratory Animal Science, Busan, Republic of Korea) weighing 3.0 to 3.5 kg were used for this experiment. The rabbits were housed at 20–25 °C and 50–70% relative humidity with a 12 h light/dark cycle. They had free access to water and diet and were acclimatized for at least 1 wk before starting the experiments. Radiologic image analysis (computer-tomography (CT)) was performed to ascertain lung inflammation at 1 and 24 h after P25 TiO2 exposure, and also to investigate the pathogenic mechanism, bronchoalveolar lavage (BAL) was performed at before P25 TiO2 exposure, 1 and 24 h after P25 TiO2 exposure. For further histological analysis, all rabbits were euthanized using CO2 gas at 24 h after P25 TiO2 exposure.

Animal experimental procedure was approved by the Animal Research Ethical Committee in Kosin Gospel Hospital, Busan, Republic of Korea.

P25 TiO2 nanoparticles exposure

Rabbits were anesthetized by intramuscular injection of ketamine 5 mg/kg (Huons Co., Korea) and xylazine 0.8 mg/kg (Bayer, Republic of Korea). Oxygen saturation was monitored by pulse oxymeter in the ear. Transbronchial P25 TiO2 instillation was performed using an ultrathin bronchoscope (BF-XP260F, Olympus; Tokyo, Japan). The ultrathin bronchoscope was inserted into the target bronchus as deep as possible under direct vision. The instillation catheter was inserted beyond the visible bronchus through working channel. One ml of 10 µg P25 TiO2 was once instilled into the right lung through the catheter and 1 ml of normal saline (as control) was instilled into the left lung (N=3). One ml of 50 and 250 µg P25 TiO2 were instilled in the same way (N=3 in each group).

Bronchoalveolar Lavage and Cell Counting

Bronchoalveolar lavage (BAL) was performed before P25 TiO2 exposure, at 1 and 24 h after P25 TiO2 exposure through an ultrathin bronchoscope, which was wedged into the 1st branch bronchus of the right lung. Sterile saline solution (2 ml) was instilled through the bronchoscope. The fluid was immediately recovered by gentle suction after each instillation. The measurement of recovered fluids showed an approximately 90% recovery. To maximize cell viability, the harvested BALF was immediately placed on ice and centrifuged at 1,000 × g for 10 min. The supernatants were immediately stored at −80 °C for further analysis. The cell pellet was used to prepare slides, which were stained according to the May-Grunwald and Giemsa procedures to morphologically assess the cells in the fluid. The differential cell counts were then counted by hemocytometer.

Lung Pathologic Examination Assay

The lung was harvested for pathologic examination at 24 h after P25 TiO2 exposure. Tissue pretreatments and preparation of hematoxylin and eosin (H&E) stained slices were carried out as previously described16). They were evaluated by light microscopy.

Statistical Analysis

Results were expressed as mean ± standard error (SE). Mann-Whitney U test was used in the case of two independent samples. All analyses were carried out using SPSS 16.0 (SPSS Inc., Chicago, IL, USA). A p<0.05 was considered statistically significant.

Results

Lung image analysis

To ascertain lung inflammation by P25 TiO2 exposure in the rabbit, chest CT was performed after P25 TiO2 instillation. Both lungs were clear before the experiment (Fig. 1A), but at 1 h ground glass opacities (GGO) were noted in each lung instilled with P25 TiO2 (10 µg/ml) and normal saline in Fig. 1B. At 24 h after exposure, persistent lung inflammation with GGO was noted in the right lung instilled with P25 TiO2, while lung inflammation disappeared in the control lung, instilled with normal saline in Fig. 1C. Similar results were obtained from the experiments using 50 and 250 µg/ml of P25 TiO2 (data not shown).

Fig. 1.

Lung inflammatory change by chest CT image analysis. Both lungs were instilled with 10 µg/ml of P25 TiO2 in the right lung and normal saline in the left lung. (A) normal lung at baseline; (B) lung inflammation with ground glass opacity in both lung fields 1 h after instillation of P25 TiO2 NP; (C) after 24 h of P25 TiO2 NP exposure, ground glass opacities in the left lung (control) disappeared but they remained in the right lower lung field.

Inflammatory cell changes in BAL fluids

After one single instillation of 10 µg/ml P25 TiO2 in the rabbit lung, the total cell count in BAL fluids was increased at 1 h (p=0.24) and 24 h (p=0.42) (Fig. 2). At 50 and 250 µg/ml of P25 TiO2 the total cell count in BAL fluids was high at 1 h and 24 h compared to the baseline.

Fig. 2.

Total cell count change in bronchoalveolar lavage fluid (BALF). Total cell count in BALF was significantly increased at 1 and 24 h after P25 TiO2 NP challenge. The bars represent mean ± SE; *p<0.05 significantly different from baseline; Mann-Whitney U test.

There was an especially marked increase in eosinophil percentage at 1 h (4.0 ± 0.4%) and 24 h (17.5 ± 5.5%) after 10 µg/ml of P25 TiO2 (p<0.05, Fig. 3A). When exposed to higher concentrations of P25 TiO2 (50 and 250 µg/ml), a dose-dependent increase in eosinophil percentage was detected at 1 h and 24 h (Fig. 3A). The eosinophil count on BAL fluids increased at 1 and 24 h after 50 µg/ml P25 TiO2 exposure when compared to baseline (Fig. 3B).

Fig. 3.

Eosinophil percentage change in bronchoalveolar lavage fluid (BALF). The increase in eosinophil percentage was noted at 1 and 24 h after P25 TiO2 NP challenge with dose-dependent responses. (A) the eosinophil count in BALF increased at 1 and 24 h after P25 TiO2 NP challenges; (B) the photos of eosinophil count in BALF at the dose of 50 µg/ml compared to baseline. The bars represent mean ± SE; *p<0.05 significantly different from baseline; Mann-Whitney U test.

Pathogenic changes after P25 TiO2 exposure

Analysis of BAL fluids confirmed that P25 TiO2 induced inflammation in lung tissues. After 24 h of P25 TiO2 instillation (50 µg/ml), severe eosinophilic inflammation was noted in the alveolar, peribronchial and perivascular regions, with moderate hemorrhage (Fig. 4A). Mild eosinophilic inflammation was noted in lung tissues exposed to 10 µg/ml P25 TiO2 (Fig. 4B). On the other hand, in the control lung (not exposed to P25 TiO2), there was no eosinophilic inflammation (Fig. 4C).

Fig. 4.

Pathological examination of the lungs following P25 TiO2 NP challenge. After 24 h of P25 TiO2 instillation, (A) severe eosinophilic inflammation was noted in the lung that was instilled with 50 g/ml; (B) mild eosinophil inflammation was noted in the lung exposed to the dose of 10 g/ml; (C) normal baseline. Left pane at magnification of 100× and right pane at a magnification of 1,000×.

Discussion

Exposure to TiO2 NPs during production or use is most likely to occur via different routes such as skin penetration, ingestion, or inhalation, but, it is believed that the lung is the most important target organ19). Although there have been a few studies on the pulmonary effects of TiO2 exposure, to the best of our knowledge, the current study is the first investigation in a rabbit model evaluating acute lung changes after P25 TiO2 exposure. Generally, mice or rat models are widely used in research due to ease of handling and their being relatively cost-effective compared to other models. Despite these advantages, mice and humans have considerable differences in lung structure and function which limits their suitability to lung disease studies20, 21). The rabbit is known to have a very similarity to human in terms of airway anatomy and responses to inflammatory mediators, although it has not been widely used probably due to limitation of cost and reagent availability. Differences in the pulmonary effects of TiO2 NPs in mouse and rat species have previously been reported22, 26). Considering rabbit is phylogenetically closer to human than rodents, our study may further provide important knowledge to understanding the acute lung impacts of TiO2 exposure in human.

TiO2 NPs was previously reported to induce pulmonary response11), which has been mainly evaluated subacute and chronic change by histopathological analysis. There were limitations in that the sequential acute changes following TiO2 exposure were not investigated. This is why image analysis was utilized in our study to evaluate acute lung inflammation following TiO2 NPs intratracheal instillation. We observed ground glass opacities of acute pneumonitis at 1 h after single P25 TiO2 NPs exposure. Furthermore we observed persistent pneumonitis in the P25 TiO2 exposed lung, as well as newly developed pneumonitis in the P25 TiO2 unexposed opposite lung at 24 h. These results indicate that single instillation of P25 TiO2 can induce severe acute pulmonary inflammation. Moreover, previous studies reported that high dose TiO2 NPs cause more severe lung inflammation compared with that of low dose of TiO2, as well as induce persistent pulmonary inflammation23,24,25). This information may have clinical implications regarding safety in handling of TiO2 NPs.

To understand the pathogenic mechanism of this acute lung inflammation by P25 TiO2 exposure, in the present study, BAL fluids and histopathology of lung sections were examined. We found that eosinophils were significantly increased during acute response (1 h) after P25 TiO2 NPs exposure, which persisted at 24 h in BAL fluids. Furthermore, eosinophil increases showed a dose-dependent pattern. This finding is consistent with those from the study in rat16). Although we did not observe any significant changes of other inflammatory cells, some studies in mouse models have shown increase of neutrophils and macrophages in the lung, as well as epithelial change after challenge with TiO210, 22, 26,27,28). However, these inflammatory changes were associated with different properties of TiO2 NPs, like crystal structure, surface chemistry, and surface area12, 29,30,31). Therefore, it may be difficult to compare results between NP studies. TiO2 NP exposure in rats induced innate immune activation of eosinophils in the acute and long-lasting lymphocyte responses12). Recently, it has been reported that lung challenge with TiO2 NP in mice cause inflammation by activation of T-helper 2 cells32). It has also been shown that lung exposure to TiO2 NP aggravates an asthmatic response and also promotes allergic sensitization and lung inflammation in a mouse model17, 33). In addition, our histopathologic analysis showed severe eosinophil inflammation in the lung after P25 TiO2 challenge, compared with those of the control lung that was not exposed to P25 TiO2. Considering that eosinophils are the main effector cells in an allergic inflammation such as asthma34, 35), we speculate that P25 TiO2 induce allergic lung inflammation by eosinophil activation. Additional investigations are needed to elucidate how eosinophil activation happens following TiO2 exposure.

In conclusion, this is the first study that investigated lung inflammation after P25 TiO2 exposure in a rabbit model and found the particles to induce eosinophilic lung inflammation. Further research is necessary to investigate the mechanism and implications of this eosinophil activation induced by TiO2 NP.