轉糖鏈球菌之自體溶體素相似蛋白在感染性心內膜炎中所扮演的角色

Abstract

感染性心膜炎(Infective endocarditis, IE)為一種具有高復發率以及死亡率的心血管感染疾病。轉糖鏈球菌(Streptococcus mutans)屬於草綠色鏈球菌(viridans streptococci)，是口腔內的正常菌叢，也是引起感染性心內膜炎的主要致病菌之一。逃避宿主免疫監控以及生物膜形成是細菌引起感染性心內膜炎的兩大毒性因子。我們先前的研究顯示，S. mutans所產生的一種蛋白AtlA，可以透過結合血漿中的纖維蛋白（fibronectin）來增強細菌對於嗜中性白血球殺菌的抵抗能力，同時AtlA也能夠藉由調控細胞外 DNA（extracellular DNA, eDNA)的釋放，以促進細菌在心臟瓣膜上形成生物膜的能力。有趣的是，我們透過針對AtlA的多株抗體辨認到了一種AtlA的相似蛋白，並將之命名為Ahp。因此，我們假設Ahp也可能在S. mutans中發揮與AtlA類似的作用，在生物膜形成與免疫逃避中扮演重要的角色。利用實驗性心內膜炎的老鼠模型，我們證明了Ahp在感染性心內膜炎的致病機制中具有重要的作用，並且在體外實驗中，顯示Ahp主要的功能在於幫助細菌逃避嗜中性白血球的胞殺作用。更有趣的是，我們發現S. mutans可根據Ahp的完整性分為兩類，當原本表現截短形式Ahp的菌株UA159表達完整的形式時，此菌株引起感染性心內膜炎的致病能力將顯著增加。從臨床血液檢體分離出的S. mutans菌株中，具有完整形式Ahp的臨床菌株也表現出更高的逃避免疫攻擊及引起感染性心內膜炎的能力。這些結果證明Ahp能夠幫助S. mutans逃避嗜中性白血球的胞殺作用，有助於細菌引發感染性心內膜炎的致病能力。

Infective endocarditis (IE) is an infectious disease of the cardiovascular system, and carries a high recurrence and mortality rate. Streptococcus mutans, a member of viridans streptococci, is a commensal in the oral cavity and also one of major opportunistic pathogen for causing IE. Escaping immune surveillance and forming biofilm are two determining virulent factors for bacteria to cause IE. Our previous reports demonstrated that one S. mutans protein, AtlA, not only plays the role in enhancing bacterial resistance against neutrophil killing through binding fibronectin in the plasma, but also contributes to bacterial biofilm formation on the heart valve through mediating extracellular DNA release. Interestingly, we identified a AtlA homologous protein (named Ahp) by using polyclonal antibodies against AtlA. Therefore, we hypothesized Ahp may also play similar roles in modulating S. mutans virulence in biofilm formation and immune evasion. By using in vivo rat experimental IE model, we demonstrated the role of Ahp in the pathogenesis of IE. In vitro assays showed that Ahp majorly plays role in mediating bacterial ability to escape neutrophil killing. More interestingly, we found that S. mutans strains can be grouped into two types according to the intactness of Ahp. When the strain UA159, which originally exhibits the truncated form of Ahp, expresses the intact Ahp, its virulence for causing IE will be dramatically increased. The clinical blood isolates of S. mutans with the intact form of Ahp also exhibit higher abilities to escape immune surveillance and cause IE. These data suggested that Ahp mediates S. mutans to escape neutrophil killing, which contributes to the pathogenesis of IE.