Myeloperoxidase Clearance of Carbon Nanotubes Results and Discussion Pulmonary inflammatory responses in w/t and MPO k/o mice exposed to SWCNT Upon pharyngeal aspiration, SWCNT elicited typical inflam- matory responses in w/t and MPO k/o mice documented by increased production of pro-inflammatory cytokines (TNF-a, IL-6 and MCP-1) 1 day after the exposure (Fig. 1a, b, c). BAL cytology indicated a robust and early (day 1) accumulation of neutrophils - slightly weaker in MPO k/o mice vs w/t animals (Fig. 1d), followed by a sequential appearance of macrophages in both groups of animals (with a peak at day 7) (data not shown) [4]. At day 28 post exposure, the amounts of PMN in BAL fluid from exposed MPO k/o mice and w/t mice are not different from those in the respective control groups of animals (Fig. 1d). In BAL, phagocytized SWCNT were detected inside PMNs and macro- phages (Fig. 1e). Because of the enrichment of PMNs with MPO, we compared the SWCNT content in these cells. An ,10 times greater number of PMNs from MPO k/o mice had SWCNT inclusions than those from w/t animals (Fig. 1f), in agreement with the lack of MPO-driven biodegradation in MPO k/o mice. Assessments of fibrosis by measurements of collagen deposition on day 28 post-exposure revealed its significantly higher amounts in the lungs of MPO k/o mice compared to w/t animals (Fig. 2a). A stronger fibrogenic response in k/o vs w/t mice was also evident from quantitative morphometry of the thickness of fibrous collagen in the alveolar connective tissue (Fig. 2b). An increase of the thickness was observed already at day 1 post-exposure and progressed further by day 28 compared with air control groups. A markedly higher collagen content of the alveolar wall was detected Figure 1. Characterization of pulmonary inflammatory responses to SWCNT in w/t and MPO k/o mice at day 1 after pharyngeal aspiration exposure. a–c. Levels of pro-inflammatory cytokines (a - TNF-a; b - IL-6; c – MCP-1) in BAL fluid of w/t and MPO k/o mice. Mean 6 SEM (n = 6 mice/group). *p,0.05, vs control PBS-exposed mice. d. Content of PMNs in BAL fluid of w/t and MPO k/o mice. Mean 6 SEM (n = 6 mice/group). *p,0.05, vs control PBS-exposed mice. e. Typical microscopic images of inflammatory cells in BAL fluid with SWCNT inclusions (red arrows). f. Content of PMNs with engulfed SWCNT in BAL fluid of w/t and MPO k/o mice. Mean 6 SEM (n = 6 mice/group). *p,0.05, vs w/t mice. doi:10.1371/journal.pone.0030923.g001 PLoS ONE | www.plosone.org 2 March 2012 | Volume 7 | Issue 3 | e30923