Age‐Related Changes in Macrophage Proliferative Capacity in Response to Chronic Inflammation and Inhaled Ozone

Rationale Resident pulmonary macrophages originate from embryonic progenitor cells. In response to injury resident macrophages can be depleted and are replaced from the finite progenitor pool or from blood monocytes. We previously reported that mice lacking surfactant protein‐D (SftpD−/−) display chronic inflammation and are hypersensitive to ozone‐induced lung injury. Objective In these studies we examined whether constitutive inflammation in SftpD−/− mice results in a loss of the resident progenitor pool, leading to increased recruitment from the vasculature in response to ozone‐induced injury. Methods C57BL/6J (WT) and SftpD −/− mice (8, 27, and 80 wk) were exposed to air or ozone (0.8 ppm, 3 h)r. After 3 days, lung sections were prepared and stained for Ki‐67 (a marker of proliferative capacity), CD11b and Cd11c (surface markers of pulmonary residency or recruitment) and scored on a scale of 0 to 3. Results Reduced expression of Ki‐67 was observed in lungs of 80‐week mice relative to 27 and 8 week mice (p<0.05); this effect was greater in SftpD −/− mice which showed upregulation of Ki‐67 at 8 weeks of age. Across all ages Ki‐67 staining was greater in SftpD −/− mice than in WT, although these differences were lowest at 27 weeks. These results correlate with previously observed increases in inflammatory staining in Sftpd −/− mice. A significant interaction between age and ozone responsiveness was noted in SftpD −/− mice. At 8 weeks of age ozone exposure increased Ki‐67 staining, while at 80 weeks of age this response was reversed. Consistently for both Ki‐67 staining and leukocyte infiltration, the 27‐week old mice were the least responsive. Qualitatively, leukocyte recruitment in the SftpD −/− mice resulted in congregation into granuloma‐like structures. The granuloma structures stained positively for Ki‐67 in their center within 8‐week‐old mice, but this expression was lost with age. Spearman correlation for CD11b and CD11c confirmed that proliferating cells within the lung are of pulmonary origin. Of note Ki‐67 staining was not observed in the large foamy cells previously identified in this model as expressing the antioxidant enzyme HO‐1 Discussion These results support the hypothesis that with age there is a loss of proliferative capacity within the macrophages of the lung in both WT and SPD −/− mice. In the background of chronic inflammation, ozone exposure leads to increased proliferation and activation of resident macrophages in young mice. However, in 80‐week‐old mice ozone exposure results in loss of resident proliferative inflammatory cells and recruitment of acutely activated myeloid cells. This indicates that in the context of inflammation/injury there is a greater reliance on recruited cells. This may represent a significant mechanism within senescence related alterations in pulmonary inflammatory response. Support or Funding Information (NIH HL086621, HL096426, ES004738, AR055073, ES005022)