Inhibition of endotoxin-induced cytokine levels by polymyxin B (PMB) in lung tissue slices at 24 h post-exposure to ENCF-1 and ENCF-4 PM (11 μg per 8 mm diameter slice). (A) IL-6, (B) TNF-α, and (C) MIP-2 levels normalized to negative control levels. Data are means ± SEM (n = 3 in each group). $p < 0.05 compared with the PMB non-treated group from the same time point. Lung slices exposed to 87 ng/mL of LPS served as a positive control.