近日，國際著名雜志PNAS在線刊登了北京生命科學研究所邵峰博士實驗室的最新研究成果“Preventing bacterial DNA release and absent in melanoma 2 inflammasome activation by a Legionella effector functioning in membrane trafficking,，”,，該篇文章首次報道了肺炎軍團菌可以通過其四型分泌系統(tǒng)向宿主細胞注入一個效應(yīng)蛋白分子，該效應(yīng)蛋白作用的結(jié)果可以避免軍團菌被宿主AIM2炎癥小體天然免疫系統(tǒng)所識別,。
肺炎軍團菌（Legionella pneumophila）是軍團菌肺炎（Legionaires’ disease）的致病菌,。肺炎軍團菌通常感染肺泡巨噬細胞，在胞內(nèi)形成一個膜泡結(jié)構(gòu)并大量繁殖,，最終引發(fā)嚴重的肺炎,。肺炎軍團菌可以通過其四型分泌系統(tǒng)分泌數(shù)百個毒力效應(yīng)蛋白分子進入宿主細胞內(nèi)，它們可以阻斷或調(diào)節(jié)宿主免疫防御相關(guān)的信號通路,，從而幫助肺炎軍團菌完成其在宿主細胞中的存活和繁殖,。另一方面，宿主可以通過天然免疫系統(tǒng)來識別外來細菌,，限制抵抗它們的侵染,。AIM2（absent in melanoma 2）便是天然免疫系統(tǒng)中的一個重要的受體。AIM2主要負責識別由外來侵染所帶來的,、進入巨噬細胞細胞質(zhì)的雙鏈DNA分子,，進而激活下游的蛋白酶caspase-1，促進白細胞介素1和白細胞介素18的分泌,，最終誘導(dǎo)巨噬細胞炎癥反應(yīng),，幫助徹底清除外來微生物的感染,。
通過對細菌和宿主兩方面的遺傳學研究，該篇文章證實,，肺炎軍團菌如果缺失其一個四型分泌系統(tǒng)效應(yīng)蛋白SdhA，細菌的DNA將更多的進入到巨噬細胞的細胞質(zhì)中,，進而激活A(yù)IM2炎癥小體,，最終導(dǎo)致caspase-1的活化和白細胞介素1的分泌，以及巨噬細胞炎癥性壞死,。所以,，SdhA效應(yīng)蛋白的缺失的結(jié)果最終導(dǎo)致肺炎軍團菌無法逃避被宿主AIM2炎癥小體介導(dǎo)的天然免疫系統(tǒng)所識別。作者同時發(fā)現(xiàn),，SdhA具有一個功能必須的GRIP結(jié)構(gòu)域（和膜泡運輸功能相關(guān)）,，并且在宿主細胞中呈現(xiàn)出一系列的直徑在1微米左右的特征環(huán)形膜結(jié)構(gòu)。進一步的細胞生物學研究結(jié)果提示SdhA很可能對維持肺炎軍團菌存在的膜泡的結(jié)構(gòu)完整性必不可少,，SdhA功能的缺失使得更多的細菌DNA有機會泄漏到巨噬細胞的細胞質(zhì)中,，從而被AIM2炎癥小體所識別。
此項研究的意義在于首次證明了肺炎軍團菌可以激活A(yù)IM2炎癥小體介導(dǎo)的天然免疫反應(yīng),，并且可以通過自身的四型分泌系統(tǒng)效應(yīng)蛋白SdhA的功能來逃逸被宿主細胞的這種天然免疫反應(yīng)所識別,。這補充了前人認為的肺炎軍團菌只被NAIP5/NLRC4炎癥小體通路所識別的觀點。
葛建寧博士和鞏乙南博士為本文共同第一作者,；其他參與此項工作的還有,，徐穎（技術(shù)員）等；邵峰博士為本文通訊作者,。此項研究為科技部973和北京市科委資助課題,，完全在北京生命科學研究所完成。（生物谷Bioon.com）
doi:10.1073/pnas.1117490109
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Preventing bacterial DNA release and absent in melanoma 2 inflammasome activation by a Legionella effector functioning in membrane trafficking
Jianning Ge1, Yi-Nan Gong1, Ying Xu, and Feng Shao2
Legionella pneumophila, the causative agent of Legionnaires’ pneumonia, resides in a distinct vacuole structure called Legionella-containing vacuole (LCV). The LCV resists fusion with the lysosome and permits efficient bacterial replication in host macrophages, which requires a Dot/Icm type IVB secretion system. Dot/Icm-translocated effector SdhA is critical for L. pneumophila intracellular growth and functions to prevent host cell death. Here, we show that the absence of SdhA resulted in elevated caspase-1 activation and IL-1β secretion as well as macrophage pyroptosis during Legionella infection. These inflammasome activation phenotypes were independent of the established flagellin-NAIP5-NLRC4 axis, but relied on the DNA-sensing AIM2 inflammasome. We further demonstrate that Legionella DNA was released into macrophage cytosol, and this effect was significantly exaggerated by the absence of SdhA. SdhA bears a functional Golgi-targeting GRIP domain that is required for preventing AIM2 inflammasome activation. Ectopically expressed SdhA formed a unique ring-shape membrane structure, further indicating a role in membrane trafficking and maintaining LCV membrane integrity. Our data together suggest a possible link, mediated by the function of SdhA, between LCV trafficking/maturation and suppression of host innate immune detection.
