Molecular mechanisms of innate immune host defense mediated by lung-surfactant proteins A and D

肺表面活性物质蛋白A和D(SP-A和SP-D)与甘露糖结合蛋白一样，属于C型凝集素超家族中的胶凝素亚类。除了某些磷脂和糖鞘脂外，该集合体还与细菌成分结合，并与巨噬细胞相互作用。已知这些蛋白质在肺的先天免疫中发挥关键作用。CD14和Toll样受体(TLR)作为病原体受体(模式识别受体)，在多种病原体的识别和信号转导中起着至关重要的作用。本研究旨在探讨肺表面活性蛋白和模式识别受体介导天然免疫的分子机制：(1)肺收集SP-A和SP-D，通过不同机制与CD14结合。它们改变了内毒素-CD14的相互作用。MBP还与CD14结合，提示与CD14的结合是集合素基团的独特性质。(2)金黄色葡萄球菌来源的肽聚糖(PGN)不是SP-A的配体。SP-A显著减弱PGN诱导的U937细胞和大鼠肺泡巨噬细胞的肿瘤坏死因子-α。SP-A抑制PGN诱导的TLR2 CDNA转染HEK293细胞中的NF-kappa活性。此外，SP-A还与库立克病毒免疫细胞系统中产生的胞外TLR2结构域的可溶性形式(STLR2)结合，并改变sTLR2与PGN的结合。这些结果表明，SP-A通过与TLR2的直接相互作用来抑制PGN诱导的肿瘤坏死因子-α的分泌。(3)细胞外TLR2结构域直接与PGN结合，提示TLR2与PGN直接相互作用激活了核因子-kappaB。(4)CD14/TLR2嵌合体分析表明，CD14取代了细胞外TLR2结构域，CD14不能取代细胞外TLR2结构域。对TLR2的几个缺失突变体的研究也表明，在TLR2介导的PGN信号转导中，Ser40-Ile64的胞外区域而不是Cys30-Ser39区域是关键的。

Pulmonary surfactant proteins A and D (SP-A and SP-D) belong to the collectin subgroup of C-type lectin superfamily, along with mannose binding proteins. The collecting bind to bacterial components in addition to certain phospholipids and glyco-sphingolipids and interact with macrophages. The proteins have been known to play critical roles in innate immunity of the lung. CD14 and Toll-like receptors (TLR) function as pathogen receptors (pattern recognition receptors) and are essential for recognition and signal transduction of various pathogen. The purpose of this study was to investigate molecular mechanisms of innate immunity mediated by surfactant proteins and pattern recognition receptors.(1)Lung collecting SP-A arid SP-D, bind CD14 by different mechanisms. They alter LPS-CD14 interactions. MBP also binds CD14, suggesting that binding to CD14 is a unique property of collectin group.(2) Peptidoglycan (PGN) derived from Staphylococcus aureus is not a ligand for SP-A. SP-A significantly attenuates PGN-elicited TNF-alpha in U937 cells and rat alveolar macrophages. SP-A attenuates PGN-induced NF-kappa activation in HEK293 cells transfected with TLR2 CDNA. In addition, SP-A binds the soluble form of the extracellular TLR2 domain (sTLR2) which was produced in culovirus-irisect cell system, and alters the binding of sTLR2 to PGN. These results demonstrate that SP-A inhibits PGN-induced TNF-alpha secretion by direct interaction with TLR2.(3) The extracellular TLR2 domain has been onstrated to bind directlt to PGN, indicating the direct interaction of TLR2 with PGN activate NF-kappa B.(4) Analaysis with CD14/TLR2 chimera in which CD14 was substituted for the extracellular TLR2 domain has demonstrated that CD14 cannot functionally replace with the extracellular TLR2 domain. The studies with- several deletion mutants of TLR2 also demonastrate that the extracellular TLR2 region of ser40-Ile64 but not the region of Cys30-Ser39 is critical for PGN signaling mediated by TLR2.