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Expression and Localization of Lung Surfactant Protein A in Human Tissues

Department of Immunology and Microbiology, Institute of Medical Biology, University of Southern Denmark, Odense; Department of Pathology, Odense University Hospital, Odense; and Statens Serum Institut, Copenhagen, Denmark; and Altana Pharma, Konstanz, Germany

Address correspondence to: Uffe Holmskov, Department of Immunology and Microbiology, Institute of Medical Biology, University of Southern Denmark, Winsløwparken 21.1, DK-5000 Odense C, Denmark. E-mail: holmskov{at}health.sdu.dk

Lung surfactant protein A (SP-A) is a collectin produced by alveolar type II cells and Clara cells. It binds to carbohydrate structures on microorganisms, initiating effector mechanisms of innate immunity and modulating the inflammatory response in the lung. Reverse transcriptase–polymerase chain reaction was performed on a panel of RNAs from human tissues for SP-A mRNA expression. The lung was the main site of synthesis, but transcripts were readily amplified from the trachea, prostate, pancreas, and thymus. Weak expression was observed in the colon and salivary gland. SP-A sequences derived from lung and thymus mRNA revealed the presence of both SP-A1 and SP-A2, whereas only SP-A2 expression was found in the trachea and prostate. Monoclonal antibodies were raised against SP-A and characterized. One of these (HYB 238-4) reacted in Western blotting with both reduced and unreduced SP-A, with N-deglycosylated and collagenase-treated SP-A, and with both recombinant SP-A1 and SP-A2. This antibody was used to demonstrate SP-A in immunohistochemistry of human tissues. Strong SP-A immunoreactivity was seen in alveolar type-II cells, Clara cells, and on and within alveolar macrophages, but no extrapulmonary SP-A immunoreactivity was observed. In contrast to lung surfactant protein D (SP-D), which is generally expressed on mucosal surfaces, SP-A seems to be restricted to the respiratory system.

Abbreviations: bronchoalveolar lavage, BAL • carbohydrate-recognition domain, CRD • mannose-binding lectin, MBL • reverse transcription-polymerase chain reaction, RT-PCR • sodium dodecyl sulfate-polyacrylamide gel electrophoresis, SDS-PAGE • lung surfactant protein A, SP-A • lung surfactant protein D, SP-D • Tris-buffered saline, TBS

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