MPO and acute respiratory distress syndrome: MSC transplantation (M, p < 0.01 vs. E), but not fibroblast transplantation (F, p > 0.05 vs. E), significantly attenuated ALI-induced increases in lung MPO activity (an indication of neutrophil accumulation) (E, p < 0.01 vs. S) at post-injury day 3 (Figure 4).