肺炎鏈球菌NanA對於宿主細胞發炎及死亡之影響

Abstract

肺炎鏈球菌是一種格蘭氏陽性胞外菌，具有極高的致病率及致死率，尤其在孩 童及老人影響甚巨。因此，讓我們更迫切需要研究肺炎鏈球菌致病機轉及其與宿主 間的交互作用去找到預防及治療的方法。所有的肺炎鏈球菌臨床菌株皆有表現神 經氨酸酶，先前研究大多發現神經氨酸酶可以移除宿主細胞表面的唾液酸，並且 NanA 可幫助肺炎鏈球菌定殖在宿主組織上。我們先前的研究結果顯示肺炎鏈球菌 的神經氨酸酶移除宿主細胞表面的唾液酸後，可能會影響 TLR-2 與 Siglec-5 間的 交互作用，因而促使此細胞的 TNF-α表現量增加，並且會活化 MAPK, PI3k/Akt 和 NF-κB 等路徑。然而，神經氨酸酶如何調控宿主發炎的詳細機制目前仍不清楚，因 此於本篇論文中，我們將進一步探討神經氨酸酶影響發炎相關路徑的詳細機制，而 加劇宿主的發炎反應。實驗結果發現，具有神經氨酸酶的肺炎鏈球菌透過影響 TLR-2 下游分子 IKK complex 的活化，因而造成受感染的 THP-1 細胞中多種促發炎激 素表現量增加，包含 IL-1β , IL-8 和 TNF-α，此劇烈發炎的現象可能來自於神經氨 酸酶影響 Siglec-5 和 SHP-1 間的交互作用。另外，神經氨酸酶主要透過活化 NLRP3 inflammasome, Caspase-1,GSDMD 和部分影響 Caspase-8，而促進 IL-1β的產生，並 且造成受細胞的存活率降低。

Streptococcus pneumoniae (SPN), a Gram-positive extracellular pathogen, causes high morbidity and mortality worldwide, especially in children and elderly. Therefore, understanding the molecular mechanism attributed to pneumococcal pathogenesis and bacteria-host interaction is crucial to develop novel therapeutics to combat pneumococcal infections. All SPN clinical isolates express neuraminidase A (NanA) which has been shown to target sialic acids expressing on the host cell surface and to promote SPN mucosal colonization. We recently demonstrated that NanA-mediated host cell desialylation is able to exacerbate TNF-α production through overactivation of the MAPK, PI3k/Akt and NF-kB pathways, which possibly resulted from dysregulation of the interaction between TLR-2 and Siglec-5. Given that the detailed mechanism regarding how NanA regulates host inflammation remains largely unknown, we aim to identify signaling pathways possibily contributed to this NanA-mediated immune exacerbation. In this study, we showed that WT SPN promotes the expression of multiple pro- inflammatory cytokines including IL-1β, IL-8, and TNF-α through activation of IκB kinase (IKK) in the infected THP-1 cells. This pneumococcal NanA-mediated cell activation may result from disrupting the interaction between Siglec-5 and SHP-1 phosphatase. Moreover, we also found that SPN NanA provokes the activation of NLRP3 inflammasome, Caspase-1, GSDMD, and part of Caspase-8, which together leading tomore IL-1β production and lower survival rate of infected cells.