Immunocytochemical localization of surfactant protein D (SP-D) in type II cells, Clara cells, and alveolar macrophages of rat lung.

Abstract

We investigated the cellular and subcellular distribution of surfactant protein D (SPD) by immunogold l a w g in lungs of adult rats that had been given bovine s e " albumin coupled to 5-nm gold (BSAG) for 2 hr to visualize the endocytotic pathway. Specific gold labeling for SPD was found in alveolar Type II cells, Clara cells, and alveolar macrophages. In Type II cells abundant labeling was observed in the endoplasmic reticulum, whereas the Golgi complex and multivesicular bodies were labeled to a limited extent only. Lamellar bodies did not seem to contain SPD. Gold labeling in alveolar macrophages was restricted to structures containing endocytosed BSAG. In Clara ceh labeling was found in the endoplasmic reticulum, the Golgi complex, and was most ,ntroductionPulmonary surfactant is a complex mixture of lipids and proteins (1,2) which prevents the alveoli from collapsing at end expiration (3). Surfactant is synthesized by alveolar Type I1 cells, in which it is stored in lamellar bodies. On fusion of the limiting membrane of lamellar bodies with the apical plasma membrane, pulmonary surfactant is secreted into the alveolar space, where it is transformed into tubular myelin (4). Some surfactant constituents are not synthesized solely by the alveolar Type I1 cell but also by non-ciliated broncheolar epithelial (Clara) cells (5,6).Surfactant protein D (SP-D) is a collagenous surfactant-associated glycoprotein which is synthesized and secreted by freshly isolated rat Type I1 cells and has been isolated from rat bronchoalveolar lavage fluids (7-9). In rat lung it is synthesized as a 39.3 KD primary translation product that undergoes post-translational N-linked glycosylation, as well as hydroxylation and glycosylation of lysine residues within the collagenous domain (10). SP-D is secreted as a 43 KD protein and is isolated from bronchoalveolar lavage as a prominent in granules present in the apical domain of the cell. Double labeling experiments with anti-surfactant protein A (SPA) showed that both SPA and SPD were present in the same granules. However, SPA was distributed throughout the granule contents, whereas SPD was confined to the periphery of the granule. The Clara cell granules are considered seaetory granules and not lysosomes, because they were not labeled for the lysosomal markers cathepsin D and LGP120, and they did not contain endocytosed BSAG. multimeric complex of disulfide-bonded trimers composed of apparently identical subunits. SP-D is a calcium-dependent carbohydrate binding protein (9) which is structurally related to surfactant protein A (SP-A), conglutinin, and other collagenous C-type lectins (11,12). The structural similarities of SP-D and SP-A suggest that these two proteins have similar functions. Both SP-A (13) and SP-D (14) have been found immunohistochemically in alveolar Type I1 cells and Clara cells of the rat lung at the light microscopic (LM) level, and recently SP-D has been localized in Type I1 cells and in the airspace of silica-treated rats at the electron microscopic (EM) level (...

Maintenance of surfactant protein A and D secretion by rat alveolar type II cells in vitro

“…In our experiments, we demonstrated agonist-induced phospholipid secretion without a concomitant increase in SP-A secretion. SP-D has always been thought to be secreted independently of lamellar bodies (7,50). There are no agonists known to induce SP-D secretion in culture.…”

KL-6 and Surfactant Proteins A and D in Serum and Bronchoalveolar Lavage Fluid in Patients with Acute Eosinophilic Pneumonia

“…They are very specific and are produced only in the lung by two types of epithelial cells, clara and alveolar type II cells (10)(11)(12). These surfactant proteins function in the local immune system of the lung (13).…”

A non-glycosylated form of pulmonary surfactant protein A appears in rat amniotic fluid

“…The pulmonary surfactant isolated from rat bronchoalveolar lavage fluid contains a triplet of SP-A, 26, 32 and 36 kDa, under reducing conditions, which is attributed to varying levels of glycosylation of the 26 kDa form, and each of the three forms of SP-A has the same amino acid sequence [11]. SP-A is produced and secreted into the airway lumen by alveolar epithelial type II cells and bronchial and bronchiolar epithelial Clara cells [8,[21][22][23].…”

“…Tracheal lavage fluid contained only non-glycosylated SP-A. Among respiratory cells it has been reported that 92 SP-A is synthesized and secreted by nonciliated epithelial Clara cells, but not by the other cells, including ciliated epithelial, goblet and subepithelial cells [8,[21][22][23]. Furthermore, there has been no report of cells that synthesize SP-A in the trachea and the other organs and tissues, except for the lung.…”