Surfactant protein D (SP-D) is a member of the collectin family of the innate host defense proteins. In the lung, SP-D is expressed primarily by type II cells. Gene-targeted SP-D-deficient [SP-D(−/−)] mice have three- to fivefold higher surfactant lipid pool sizes. However, surfactant synthesis and secretion by type II cells and catabolism by alveolar macrophages are normal in SP-D(−/−) mice. Therefore, we hypothesized that SP-D might regulate surfactant homeostasis by influencing surfactant structure, thereby altering its uptake by type II cells. Large (LA) and small aggregate (SA) surfactant were isolated from bronchoalveolar lavage fluid (BALF) from SP-D(−/−), wild-type [SP-D(+/+)], and transgenic mice in which SP-D was expressed under conditional control of doxycycline in alveolar type II cells. Uptake of both LA and SA isolated from SP-D(-/-) mice by normal type II cells was decreased. Abnormally dense lipid forms were observed by electron microscopy of LA from SP-D(−/−) mice. SA from SP-D(−/−) mice consisted of atypical multilamellated small vesicles. Abnormalities in surfactant uptake by type II cells and in surfactant ultrastructure were corrected by conditional expression of SP-D in vivo. Preincubation of BALF from SP-D(−/−) mice with SP-D changed surfactant ultrastructure to be similar to that of SP-D(+/+) mice in vitro. The rapid changes in surfactant structure, increased uptake by type II cells, and decreased pool sizes normally occurring in the postnatal period were not seen in SP-D(−/−) mice. SP-D regulates uptake and catabolism by type II cells and influences the ultrastructure of surfactant in the alveolus.
Surfactant protein d (SP-D) is a 43-kDa member of the collectin family of calcium-dependent lectins whose host defense function has been well recognized (4). The critical role of SP-D in pulmonary homeostasis was not apparent until the SP-D gene-targeted mouse was developed (2, 17). Targeted inactivation of the SP-D gene in mice [SP-D(−/−)] caused two major unexpected abnormalities in the lung: 1) marked accumulation of tissue and alveolar surfactant phospholipids (13) and 2) inflammation and emphysema associated with increased numbers of activated alveolar macrophages (30, 35). Genetic experiments in which SP-D was replaced with mutant SP-D protein demonstrated that phospholipid abnormalities were modulated independently of lung inflammation and emphysema (22, 36, 37). In SP-D(−/−) mice, the normal reduction in alveolar and lung saturated phosphatidylcholine (Sat PC) pools that occurs from newborns to adults was not observed. Lung Sat PC concentrations were increased three- to fourfold in SP-D(−/−) mice at all ages (13). Surfactant phospholipids and proteins are synthesized, stored, secreted, and recycled in type II cells (33). Alveolar type II epithelial cells and alveolar macrophages contribute equally to surfactant phospholipid catabolism (6). In healthy individuals, alveolar and lung tissue surfactant pool sizes are tightly regulated. We previously demonstrated that Sat PC synthesis, secretion by alveolar type II cells, and catabolism by alveolar macrophages were normal in SP-D(−/−) mice (9). Therefore, lipid abnormalities in SP-D(−/−) mice might relate primarily to changes in surfactant uptake, catabolism, and/or recycling by type II cells. The ultrastructure of surfactant from SP-D(−/−) mice contained abnormal dense large aggregate (LA) forms (17). We hypothesized that SP-D might regulate surfactant homeostasis in the lung, at least in part by influencing the physical structure of the phospholipid-rich aggregates that, in turn, might influence their uptake and catabolism by type II epithelial cells (31). In the present study, effects of SP-D on surfactant ultrastructure and uptake by type II cells were assessed. Because surfactant pool sizes change during normal development, we also determined the role of SP-D in the modulation of surfactant ultrastructure and pool sizes during the postnatal period.
Ikegami, M, et al.
American Journal of Physiology: Lung Cellular and Molecular Physiology 2005