Pulmonary toxicity following acute coexposures to diesel particulate matter and α-quartz crystalline silica in the Sprague-Dawley rat

Abstract

The effects of acute pulmonary coexposures to silica and diesel particulate matter (DPM), which may occur in various mining operations, were investigated in vivo. Rats were exposed by intratracheal instillation (IT) to silica (50 or 233 µg), DPM (7.89 or 50 µg) or silica and DPM combined in phosphate-buffered saline (PBS) or to PBS alone (control). At one day, one week, one month, two months and three months postexposure bronchoalveolar lavage and histopathology were performed to assess lung injury, inflammation and immune response. While higher doses of silica caused inflammation and injury at all time points, DPM exposure alone did not. DPM (50 µg) combined with silica (233 µg) increased inflammation at one week and one-month postexposure and caused an increase in the incidence of fibrosis at one month compared with exposure to silica alone. To assess susceptibility to lung infection following coexposure, rats were exposed by IT to 233 µg silica, 50 µg DPM, a combination of the two or PBS control one week before intratracheal inoculation with 5 × 10⁵ Listeria monocytogenes. At 1, 3, 5, 7 and 14 days following infection, pulmonary immune response and bacterial clearance from the lung were evaluated. Coexposure to DPM and silica did not alter bacterial clearance from the lung compared to control. Although DPM and silica coexposure did not alter pulmonary susceptibility to infection in this model, the study showed that noninflammatory doses of DPM had the capacity to increase silica-induced lung injury, inflammation and onset/incidence of fibrosis.

Keywords: Crystalline silica; diesel exhaust particulate matter; immune response; occupational exposure; pulmonary fibrosis.

Figure 1.

FESEM images of particles. (A) 233 mg SIL and 7.89 mg DPM in combination prepared in PBS solution, diluted 1:10 in PBS, then dried onto a filter under 20,000x magnification (scale bar=2 mm with 10 segments of 0.2 mm each) (B) 233 mg SIL and 50 mg DPM prepared in PBS solution, diluted 1:10 in PBS, then dried onto a filter in PBS solution under 20,000x magnification (scale bar =2 mm with 10 segments of 0.2 mm each). White arrows indicate SIL and black arrows indicate DPM. (C) EDX of DPM particles pictured in the FESEM above at 20 keV. Labeled spectral peaks for elements primarily present in the samples. All peaks besides carbon are elements commonly found in DPM but present only in trace amounts in this sample (chromium, copper, iron, nickel, oxygen, silicon and zinc). (D) EDX of SIL particles pictured in FESEM above at 20 keV.

Figure 2.

LDH activity in BALF after exposure to DPM, SIL or a combination of DPM and SIL (DS) at indicated doses (μg). Data are shown as mean fold change over control (y= 0). Neither DPM +7.89 mg nor DS 7.89/233 μg were evaluated at 2 months.^aDifferent from control, DPM 7.89 μg, DPM +50 mg, SIL 50 μg, DS 50/50 μg groups;^bdifferent from all other groups;^cdifferent from control, DPM+50 μg, SIL 50 μg and DS 50/50 μg groups. Statistical significance is p=05.

Figure 3.

Total AMs (A) and neutrophils (B) in the BALF following exposure to DPM,SIL or DS at doses indicated. Production of oxidants by total phagocytes stimulated with PMA (C) or by AMs only stimulated by zymosan (D) measured by chemiluminescence following exposure to DPM, SIL or DS at doses indicated. DPM7.89 mg was not evaluated at 2 months. All data are shown as a mean fold change over control (y = 0).^aDifferent from control, DPM 7.89 μg, DPM+50 μg, SIL 50 μg, DS 50/50 μg groups;^bdifferent from all other groups;^cdifferent from control, DPM+50 μg, SIL 50 μg, and DS 50/50 μg groups;^ddifferent from control and DPM+50 +μg only;^edifferent from control, DPM +50 μg, and SIL 50 μg groups;^fdifferent from control, DPM +7.89 μg, DPM 50 μg, SIL 50 μg, SIL 233, and DS 50/50 μg;^gdifferent from control, DPM 7.89 μg, DPM+50 μg and SIL 50 μg groups. Statistical significance measured as p≥05.

Figure 4.

(A) Bacterial burden in the left lung over the time course. Data are shown on a log10 scale. Total cells (B), total AMs (C) and total neutrophils (D) recovered by lavage following exposure to DPM, SIL or DS at doses indicated. (B–D) Data are shown as a mean fold change over control (y=0). _Different from control and DPM 50 μg; @different from control. Statistical significance is p=05.