Surfactant Protein D (SP-D) is a collagenous carbohydrate binding protein that is secreted into the airspaces of the lung by alveolar type II and nonciliated bronchiolar epithelial cells. SP-D molecules consist of four trimeric subunits, each with about 100 nm triple-helical arm and a carboxy-terminal cluster of carbohydrate binding domains (CRD). The trimeric subunits are crosslinked at their amino-termini, and SP-D molecules can further associate to form stellate multimers with peripheral arrays of up to 32 trimeric CRDs. SP-D shows lectin-dependent interactions with leukocytes and a variety of microorganisms in vitro, suggesting that it plays important roles in the pulmonary response to microbial challenge. Furthermore, multivalency and the capacity to participate in interactions with spatially separated glycoconjugates appears to be important for SP-D function. For these reasons, it is important to understand the molecular and cellular mechanisms that regulate the complex molecular assembly of SP-D and its subsequent accumulation in the airspace. In this application, we propose two specific aims. First, we will employ biochemical, molecular, and cell culture techniques to elucidate the intracellular events required for the production of SP-D molecules and multimers. For these studies we will use a model system consisting of mammalian cells (CHO-K1) transfected with wild-type or mutant SP-Ds in order to study the pathway of secretion and the structural requirements for molecular assembly. Second, we will investigate the transcriptional regulation of the human SP-D gene, focusing on the modulation of SP-D gene expression by glucocorticoids and the control of tissue-restricted expression. These studies will provide important new information related to the regulation of SP-D production and increase our understanding of potential cellular mechanisms regulating non-immune pulmonary host defense. Such mechanisms are important during the body's initial encounter with inhaled pathogens, and may assume a critical role in the settings of immature or impaired immune function.