血紅素結合蛋白在A型鏈球菌的致病機轉中所扮演的角色

Abstract

細菌藉助表面的一些特殊成分和結構，吸附於人體皮膚黏膜的表面，以不同的方式引起疾病。A型鏈球菌 (group A Streptococcus, GAS)，是人類中常見的傳染性致病菌，可引起各種疾病，如急性咽炎、蜂窩性組織炎、猩紅熱、壞死性筋膜炎，也可能導致肺部感染或是全身性感染等等。大部分A型鏈球菌會表現hyaluronic acid莢膜，藉此逃避宿主的免疫攻擊，先前研究指出，A型鏈球菌serotype M4 (GAS M4) 沒有hyaluronic acid莢膜的存在，卻依然會造成許多的侵入性的疾病感染，因此我們想研究其中的作用機制和病原菌與宿主間的交互作用，以了解GAS M4造成侵入性疾病的原因。在宿主發炎及感染時，急性期蛋白會被大量合成、釋放，並且增加蛋白表面的醣基化程度，其中一個急性期蛋白—血紅素結合蛋白(haptoglobin)之前已被報導過會抑制細胞激素的表現以及發炎反應。先前亦有研究指出，GAS M4可能與血紅素結合蛋白有特定的鍵結；另一方面，血紅素結合蛋白會鍵結表現在免疫細胞上的Immunoglobulin-like lectins (Siglecs)。Siglecs中Siglec-9與血紅素結合蛋白的親和力最明顯，並且具有Immunoreceptor tyrosine-based inhibitory motif (ITIM)，主要表現在中性粒細胞以及巨噬細胞表面。因此，我們假設GAS M4可能藉由結合血紅素結合蛋白，經免疫細胞表面具有ITIM的Siglec-9受體傳遞訊號，進而影響細胞的的先天免疫反應。為了研究這個假說，實驗室建構一株無法與血紅素結合蛋白鍵結的 GAS M4基因剔除突變株M4ΔSpy0116以及建構表現Siglec-9受體的細胞株，藉此探討GAS M4藉由血紅素結合蛋白與Siglec-9受體鍵結來抑制細胞的免疫反應的可能性。目前實驗結果顯示，血紅素結合蛋白可以幫助GAS M4抑制細胞的殺菌能力以及降低細胞激素的表現，並且增加小鼠體內GAS M4細菌的存活率，導致小鼠死亡率上升。儘管在觀察表現Siglec-9受體的細胞株後，可以發現明顯的細菌存活數增加以及細胞激素表現下降，但是在沒有表現Siglec-9受體的細胞中還是可以看到一些差異。目前還無法確定Siglec-9受體的重要程度，所以未來將進一步探討GAS M4、血紅素結合蛋白與Siglec-9受體之間的關聯性，或是尋找其他受體如Mac-1的可能性。

Bacteria can adhere to the mucous lining on top of the surface of human skin by its own specialized surface components and structures to cause diseases in different levels. Group A Streptococcus (GAS) is a common human pathogen, and infection of GAS can either result in relatively mild diseases, such as acute pharyngitis, cellulitis, scarlet fever or in very severe, life-threatening diseases, including pneumonia, bacteremia, necrotizing fasciitis and streptococcal toxic shock syndrome. Most GAS isolates have a hyaluronic acid capsule and use it to evade the host immune response. Previous reports have demonstrated that GAS serotype M4 (GAS M4) is lacking this important hyaluronic acid capsule, but still causes many invasive infections, therefore we would like to further understand how GAS M4 interacts with host to achieve a successful infection and underlying mechanisms. During infection and inflammation, many acute-phase proteins (APPs) are massively synthesized, accompanying with increased sialylation and haptoglobin one of the APPs has been shown to have immune-suppressive effects on various in vivo and in vitro models. Previous reports have shown haptoglobin can bind to GAS M4, and we further found that haptoglobin can interact with multiple sialic acid-binding immunoglobulin-like lectins (Siglecs) expressed on various immune cells. Among the examined Siglecs, Siglec-9, an immunereceptor tyrosine-based inhibitory motif (ITIM) containing Siglec mainly expressed on neutrophils and macrophages, exhibits most robust interaction with haptoglobin. We are curious to examine whether GAS M4 can impair host innate immune responses via targeting the inhibitory ITIM-containing Siglecs-9 through haptoglobin. To address this hypothesis, we generated a GAS M4 isogenic strain lacking the haptoglobin binding ability (M4ΔSpy0116) as well as the Siglec-9 overexpressing cells to investigate the impacts of haptoglobin on the GAS M4 pathogenicity. Based on our preliminary data, GAS M4 can inhibit the bactericidal activity and cytokine release of innate immune cells via acquiring haptoglobin on its surface. The critical role of Spy0116 on the GAS M4 pathogenicity has also been demonstrated on the in vivo mice intraperitoneal infection model . The enhanced bacteria survival and decreased inflammatory cytokine expression can be observed in the presence or absence of Siglec-9, albeit it is more pronounced in the Siglec-9 expressing cells. It suggests that Siglec-9 may not be the only haptoglobin target on the innate immune cells and the role of Sigelc-9 needs to be further addressed.