Additive effect of diesel exhaust particulates and ozone on airway hyperresponsiveness and inflammation in a mouse model of asthma

Abstract

Allergic airway diseases are related to exposure to atmospheric pollutants, which have been suggested to be one factor in the increasing prevalence of asthma. Little is known about the effect of ozone and diesel exhaust particulates (DEP) on the development or aggravation of asthma. We have used a mouse asthma model to determine the effect of ozone and DEP on airway hyperresponsiveness and inflammation. Methacholine enhanced pause (P(enh)) was measured. Levels of IL-4 and IFN-gamma were quantified in bronchoalveolar lavage fluids by enzyme immunoassays. The OVA-sensitized-challenged and ozone and DEP exposure group had higher P(enh) than the OVA-sensitized-challenged group and the OVA-sensitized-challenged and DEP exposure group, and the OVA-sensitized-challenged and ozone exposure group. Levels of IFN-gamma were decreased in the OVA-sensitized-challenged and DEP exposure group and the OVA-sensitized-challenged and ozone and DEP exposure group compared to the OVA-sensitized-challenged and ozone exposure group. Levels of IL-4 were increased in the OVA-sensitized-challenged and ozone exposure group and the OVA-sensitized-challenged and DEP exposure group, and the OVA-sensitized-challenged and ozone and DEP exposure group compared to OVA-sensitized-challenged group. Co-exposure of ozone and DEP has additive effect on airway hyperresponsiveness by modulation of IL-4 and IFN-gamma suggesting that DEP amplify Th2 immune response.

Fig. 1

Schematic diagram of the experimental protocol. Mice were sensitized on days 1 and 14 by intraperitoneal injection of OVA emulsified in 1 mg aluminum hydroxide. On days 21, 22, and 23 after the initial sensitization, the mice were challenged for 30 min with an aerosol of 1% (wt/vol) OVA in saline (or with saline as a control) using an ultrasonic nebulizer. The mice housed in whole-body exposure chambers were exposed to ozone concentrations of 2 ppm for 3 hr (n=6/group) and DEP 2,000 µg/µL for 1 hr after 1% (wt/vol) OVA challenge on days 21 to 23.

Fig. 2

The effect of ozone and DEP on airway responsiveness in OVA-sensitized and challenged mice. Airway responsiveness was measured 24 hr after the last challenge in mice. Airway responsiveness to aerosolized methacholine was measured in unrestrained, conscious mice. Mice were placed in the main chamber of a barometric plethysmograph and nebulized first with saline and then with increasing doses (from 2.5 to 50 mg/mL) of methacholine for 3 min for each nebulization. Readings of breathing parameters were taken for 3 min after each nebulization, during which time P_enh values were determined. Data represent mean±SEM from six independent experiments. ^*p<0.05 versus sham; ^†p<0.01 vs. sham group and the OVA-sensitized/OVA-challenged group, ^‡p<0.01 vs. sham group and the OVA-sensitized/OVA-challenged group and the OVA-sensitized/ozone-exposure group, and the OVA-sensitized/DEP-exposure group.

Fig. 3

The effect of ozone and DEP on IL-4, IFN-γ levels in BAL fluids of OVA-sensitized and challenged mice. Enzyme immunoassay of IL-4 (A), IFN-γ (B). Sampling was performed 72 hr after the last challenge in mice treated as described in Fig. 1. Bars represent mean±SEM from six independent experiments. ^*p<0.05 versus sham group. ^†p<0.05 versus the OVA-sensitized/ozone exposure group.