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Surfactant Protein A (SP-A)

Surfactant protein A (SP-A; surfactant- associated protein A – SFTPA; pulmonary surfactant- associated protein A – PSPA) is the major protein component of the lung surfactant in mammals. It belongs, together with SP-D, to the collectins’ family, due to its C-type lectin domain and the collagen-like N-terminal triple helix. The lectin domain mediates the interaction between a collectin and a wide variety of pathogens. The oligomeric structure of SP-A comprises two types of polypeptide chains: SP-A1 and SP-A2 transcribed from sftpa1 and sftpa2 genes. Six trimers of SP-A form its complete octadecameric structure. Human SP-A is expressed primarily in the lung, mostly in alveolar type II cells and also in non-ciliated bronchial epithelial cells. SP-A mRNA can be detected in fetal lung cells after only 20 weeks of gestation. SP-A has been found in various biological fluids, such as bronchoalveolar lavage fluid, sputum, serum, amniotic fluid and vaginal lavage fluid. It has also been found to be expressed in cells of the small and large intestine. SP-A is considered to be an important host defence components against respiratory allergens and pathogens. SP-A takes part in modulating the function of immune system cells – especially dendritic cells and T-cells. Many studies have shown that inflammatory mediators (e.g. TNF) are controlled by SP-A, both positively and negatively. Changes in SP-A production in the lung or its baseline concentrations in serum might be associated with progression or mortality at idiopatic pulmonary fibrosis (IPF). Reduced levels of SP-A in alveolar fluid were also reported in other pulmonary diseases, e.g. acute respiratory distress syndrome (ARDS). Furthermore, elevated levels of SP-A in lungs have been observed in pulmonary alveolar proteinosis (PAP).

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