Modulation of Human Cells by Virulent Francisella tularensis

摘要：图拉氏方济氏菌是图拉热病的病原体，可通过多种途径感染人类，包括通过媒介传播。然而，吸入细菌和由此导致的肺炎图拉热症是最危险的疾病形式。这是由于未经治疗的患者的潜伏期短(3-5天)、非特异性症状以及高死亡率(大于80%)。此外，美国和前苏联都已将其武器化，使其成为用作生物武器的可行候选者。尽管世界各地对图拉氏杆菌进行了80多年的研究，但人们对这种细菌与宿主的动态相互作用知之甚少，特别是在气溶胶感染之后。 我的实验室已经证实，与小鼠细胞类似，人类树突状细胞和巨噬细胞对图拉氏杆菌感染非常敏感，但不能产生促炎细胞因子或经历成熟。此外，毒力较强的图拉氏杆菌能有效干扰人DC和巨噬细胞对二次刺激的反应能力。了解图拉氏杆菌主动抑制DC和巨噬细胞功能的机制是我的实验室的中心指示。我们正在以两种不同的方式处理这一指令。 首先，我们正在分析弗朗西塞氏菌脂类在调节抗炎反应中所起的作用。细菌表面的结构是宿主细胞遇到的第一组分。因此，在图拉氏丝虫感染的背景下，这些结构可能有助于早期、快速地抑制人类树突状细胞。细菌脂类就是这样一种结构。我们最近发表了从致病的但不是减毒的图拉氏丝虫中分离出来的脂类，有效地抑制了人树突状细胞和巨噬细胞的炎症反应。此外，我们还鉴定了受弗朗西斯菌脂类调控的特异性信号转导蛋白。我们目前正在确定导致这种抑制的特定脂质(S)，以及它们干扰人类树突状细胞功能的机制。 其次，我们正在探索与图拉氏丝虫外表面相关的碳水化合物在指导人类细胞免疫抑制程序中的作用。土拉藻的主要外表面碳水化合物结构是与内毒素相关的O-抗原(O-Ag)。通常，O-Ag被认为只是掩盖了细菌表面存在的蛋白质，这些蛋白质可能会刺激炎症反应。然而，我们有初步数据表明，O-Ag直接抑制人类细胞的促炎反应。利用O-Ag合成中具有特定缺陷的突变体，我们目前正在鉴定由F·tularensis O-Ag调控的特定受体和宿主信号通路，以启动抗炎计划。

Summary: Francisella tularensis, the causative agent for tularemia, can infect humans by a number of routes, including vector-borne transmission. However, it is inhalation of the bacterium, and the resulting pneumonic tularemia, that represents the most dangerous form of disease. This is due to the short incubation time (3-5 days), non-specific symptoms, and a high mortality rate (greater than 80%) in untreated individuals. Furthermore, F. tularensis has been weaponized by both the United States and the former Soviet Union making it a viable candidate for use as a biological weapon. Despite over 80 years of research on F. tularensis around the world, very little is understood about the dynamic interaction of this bacterium with the host, especially following aerosol infection. My laboratory has established that, similarly to murine cells, human dendritic cells and macrophages are acutely susceptible to infection with F. tularensis, but fail to produce pro-inflammatory cytokines or undergo maturation. Further, virulent F. tularensis actively interferes with the ability of human DC and macrophages to respond to secondary stimuli. Understanding the mechanism by which F. tularensis actively suppresses DC and macrophage function is a central directive of my laboratory. We are tackling this directive in two different ways. First, we are analyzing the role Francisella lipids play in mediating anti-inflammatory responses. Structures present on the surface of bacteria are the first components encountered by the host cell. Thus, it is possible that, in the context of F. tularensis infections, these structures contribute to the early, rapid suppression of human dendritic cells. Bacterial lipids represent one such structure. We recently published that lipids isolated from virulent, but not attenuated F. tularensis, potently suppress inflammatory responses in human dendritic cells and macrophages. Further, we identified the specific signal transduction proteins modulated by Francisella lipids. We are currently identifying the specific lipid(s) responsible for this suppression and the mechanism by which they interfere with human dendritic cell functions. Second, we are exploring the role of carbohydrates associated with the outer surface of F. tularensis in directing immunosuppressive programs in human cells. The major outer surface carbohydrate structure of F. tularensis is the O-Antigen (O-Ag) associated with LPS. Typically O-Ag is thought to simply cover up proteins present on the bacterial surface that could stimulate an inflammatory response. However, we have preliminary data which demonstrates that the O-Ag directly inhibits pro-inflammatory responses in human cells. Utilizing mutants with specific defects in O-Ag synthesis, we are currently identifying the specific receptors and host signaling pathways modulated by F. tularensis O-Ag to initiate an anti-inflammatory program.