The interplay between platelets and pathogenic bacteria

血小板是由巨核细胞（MK）在骨髓和肺中脱落而产生的无核细胞。它们越来越被认为是宿主免疫系统的重要组成部分，可以被认为是对入侵细菌的第一个反应的先天免疫细胞。从感染部位扩散的致病菌经常穿过宿主屏障进入循环系统，导致菌血症和败血症。与血小板功能异常相关的菌血症并发症包括血小板减少症和急性呼吸窘迫综合征（ARDS）。血小板和细菌之间的相互作用以细菌与血小板的直接或间接结合或通过释放细菌因子为特征。以金黄色葡萄球菌和肺炎链球菌为模型微生物，我们发现血小板和致病菌相互影响，结果不同。两种病原体引起严重的肺炎，然而，机制和与血小板的相互作用是不同的。金黄色葡萄球菌诱导的血小板激活通过血小板释放中的化合物（可能是防御素）杀死病原体，而肺炎球菌通过其成孔毒素溶肺素杀死血小板本身。这使得血小板丧失功能，内皮细胞的封闭功能受损。重要的是，我们在体外证明了药物免疫球蛋白制剂（如IVIG）可以中和毒素，并完全保护血小板表型功能以及溶气素存在下的功能。在我们提出的项目中，我们将把我们之前的体外实验结果转移到体内小鼠模型中，并破译血小板-细菌相互作用的复杂机制以及封闭肺屏障的后果。我们将研究肺炎球菌溶肺素和神经氨酸酶对急性肺炎球菌小鼠肺炎模型呼吸窘迫发展的影响。在感染不同菌株和炎症标志物时，将分析肺屏障上的渗漏，并调查中性粒细胞、巨核细胞（mk）和血小板的浸润情况。利用我们的肺炎模型和免疫球蛋白制剂，我们还旨在确定治疗干预的机制。我们假设血小板在治疗干预后仍将保持功能，并避免毛细血管渗漏。这些体内研究将通过阐明血小板糖蛋白VI （GPVI）在肺炎球菌性肺炎中的作用来扩展，因为GPVI被认为在抑制炎症和感染方面起着至关重要的作用。作为探索性实验，我们将在体外和离体心肺模型中评估金黄色葡萄球菌、肺炎球菌及其蛋白和毒素与人类巨核细胞的相互作用。作为我们体内研究的补充，我们将在体外研究肺炎球菌和细菌蛋白在血小板-中性粒细胞轴上的相互作用。

Platelets are anucleated cells produced by megakaryocyte (MK) shedding in the bone marrow and the lungs. They are increasingly recognized as important components of the host immune system and can be considered as the first responding innate immune cells towards invading bacteria. Pathogenic bacteria spreading from the site of infection often cross host barriers and enter the circulatory system leading to bacteraemia and sepsis. Complications of bacteraemia associated with abnormal platelet functions are e.g., thrombocytopenia and the acute respiratory distress syndrome (ARDS). Interactions between platelets and bacteria are characterized by direct or indirect binding of bacteria to platelets or via released bacterial factors. Using Staphylococcus aureus and Streptococcus pneumoniae as model microorganisms, we have shown that platelets and pathogenic bacteria mutually affect each other with varying outcome. Both pathobionts cause severe pneumonia, however, the mechanisms and interplay with platelets are different. Whereas S. aureus induced platelet activation leads to killing of the pathogen by compounds in the platelet releasate (likely defensins), pneumococci kill platelets themselves via its pore-forming toxin pneumolysin. This renders platelets non-functional and their sealing function of the endothelium is impaired. Importantly, we showed in vitro that pharmaceutical immunoglobulin preparations such as IVIG neutralize the toxin and completely secured the platelet phenotype function as well as the function in the presence of pneumolysin. In our proposed project we will transfer the findings of our previous in vitro experiments into an in vivo mouse model and decipher the complex mechanisms of platelet-bacteria interplay and the consequences for sealing the lung barrier. We will investigate the influence of pneumococcal pneumolysin and neuraminidase on the development of respiratory distress in our acute pneumococcal mouse pneumonia model. Leakage over the pulmonary barrier will be analysed upon infection with the different strains and markers of inflammation as well as the infiltration of neutrophils, megakaryocytes (MKs), and platelets will be investigated. Using our pneumonia model and immunoglobulin preparations, we also aim to identify mechanisms of therapeutic interventions. We hypothesize that platelets will remain functional upon therapeutic intervention and that capillary leakage is avoided. These in vivo studies will be extended by elucidating the role of platelet glycoprotein VI (GPVI) in pneumococcal pneumonia, because GPVI is suggested to play a crucial role in dampening inflammation and infections. As exploratory experiment we will assess the interaction of S. aureus, pneumococci, their proteins and toxins with human megakaryocytes in vitro and in an ex vivo heart-lung model. Complementary to our in vivo studies, the interplay of pneumococci and bacterial proteins on the platelet-neutrophil axis will be investigated in vitro.