Role of Spleen-Derived Macrophages in Ozone-Induced Lung Inflammation and Injury

Macrophages and inflammatory mediators have been implicated in ozone toxicity. In these studies, we used splenectomized (SPX) mice to assess the contribution of splenic monocytes to pulmonary inflammation and injury induced by ozone. Cells and tissue were collected 24-72h after exposure of mice to air or ozone (0.8 ppm, 3 h). Following ozone exposure, increased numbers of pro-inflammatory CD11b(+)Ly6C(Hi) and anti-inflammatory CD11b(+) Ly6C(Lo) monocytes were observed in spleens of control (CTL) mice. CD11b(+) Ly6C(Hi) and MMP-9(+)pro-inflammatory macrophages were also observed in lungs of CTL mice after ozone, along with CD11b(+) Ly6C(Lo) and mannose receptor (MR)(+) anti-inflammatory macrophages. This was accompanied by increased lung expression of proteins involved in monocyte/macrophage trafficking including CCL3, CCL4, CCR1, and AT1R. Splenectomy resulted in decreases in pro-inflammatory macrophages in the lung and down regulation of CCR2, CCL2, and CCL4, but increases in CD11b(+) Ly6C(Lo) anti-inflammatory macrophages. CD11b(+)Ly6G(+)Ly6C(+) granulocytic (G)- and monocytic (M)-myeloid derived suppressor cells (MDSC) s were also detected in the lungs and spleens of CTL mice; these increased after ozone exposure. Splenectomy was associated with a decrease in G-MDSCs in the lung, with no effect on M-MDSCs. Changes in lung macrophage subpopulations and MDSCs in SPX mice were correlated with reduced ozone toxicity, as measured by decreases in bronchoalveolar lavage protein content and reduced 4-hydroxynonenal expression in the lung. These data suggest that the spleen is a source of pro-inflammatory/cytotoxic macrophages that contribute to ozone-induced lung injury, inflammation, and oxidative stress.