Immunity to Pneumonic Tularemia

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。这是由于潜伏期短(3-5天)、无特定症状和高死亡率(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。 在过去的几年里，我的实验室提供了大量的证据，证明图拉氏杆菌在宿主体内成功感染和复制的主要机制之一是通过积极干预肺部炎症反应的发展。在活体内研究有毒的图拉氏丝虫的一个主要障碍是，动物在感染后几天内需要安乐死。这不允许对细菌与宿主免疫系统的相互作用进行可靠的评估。为了克服这一障碍，我们开发了一个使用抗生素的模型，在该模型中，我们促使图拉氏丝虫缓慢清除，并在受感染的动物中存活。这个模型既确定了图拉热症存活的宿主要求，更重要的是，证明了这些要求中的许多不同于使用减毒株观察到的要求。具体地说，我们已经证明了IL-12p40和IL-12p35在防御图拉氏丝虫感染方面都是必不可少的。我们现在正在利用这个模型来确定导致图拉氏肺吸虫感染加剧的宿主因素。 我们还利用体外系统确定了有助于抑制宿主炎症反应的细菌成分。在与我们的同事Richard Lee(圣裘德儿童医院)的合作中，我们已经证明，从毒力较强的图拉氏杆菌中分离出的脂类，而不是减毒株，无法刺激炎症反应，(重要的)干扰宿主细胞对无关刺激的反应能力。我们目前正在确定体内和体外抑制IL-12产生的脂类调节的特定宿主细胞通路。从这些研究中收集的数据将扩大我们对图拉热症的理解，并使新疗法和疫苗的开发成为可能。此外，这些脂类可用作有效的抗炎剂，用于治疗过敏性哮喘等无关疾病。 最后，我们建立了毒力图拉氏杆菌刺激干扰素-β的快速产生作为选择性抑制人树突状细胞产生IL-12的机制。这一发现最近发表在《免疫学杂志》上。此外，与弱毒株感染不同，干扰素-b不有助于感染的解决，宿主细胞因子的存在也不有助于炎症体的激活。我们将这些观察扩展到我们的小鼠模型，并表明图拉氏F菌逃避炎症体的激活，但不抑制其组装。相反，这种细菌专门针对依赖炎症体激活的促炎症细胞因子的mRNA(通过失稳)。我们目前正在确定图拉氏F菌破坏宿主mRNA稳定以抑制炎性细胞因子产生的具体机制。

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%). 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. In the last several years my laboratory has provided abundant evidence that one of the primary mechanisms by which F. tularensis successfully infects and replicates in the host is via active interference with development of inflammatory responses in the lungs. A major hurdle in studying virulent F. tularensis in vivo is that animals require euthanasia within a few days of infection. This does not allow reliable assessment of the interaction of the bacterium with the host immune system. To overcome this obstacle we developed a model using antibiotics in which we provoke slow clearance of F. tularensis and survival among infected animals. This model has both identified host requirements for survival of tularemia and, importantly, demonstrated that many of these requirements are different than those observed using attenuated strains. Specifically, we have shown that both IL-12p40 and IL-12p35 are essential in defense against F. tularensis infection. We are now utilizing this model to identify host factors that contribute to the exacerbation of F. tularensis infection. We have also identified components of the bacterium that contribute to dampening inflammatory responses in the host using in vitro systems. In collaboration with our colleagues, Richard Lee (St. Jude Childrens Hospital) we have shown that lipids isolated from virulent F. tularensis, but not attenuated strains, fail to stimulate inflammatory responses and (importantly) interfere with the ability of host cells to respond to unrelated stimuli. We are currently identifying the specific host cell pathways modualted by tehse lipids that inhibit production of IL-12 in vivo adn in vitro. Data gathered from these studies will expand our understanding of tularemia and enable development of novel therapeutics and vaccines. Additionally, these lipids may be adapted for use as potent anti-inflammatory agents for use against unrelated conditions such as allergic asthma. Finally, we established that virulent F. tularensis stimulates rapid production of IFN-beta as a mechanism to selectively inhibit production of IL-12 in human dendritic cells. The findings were recently published in the Journal of Immunology. Further, unlike infection with attenuated strains IFN-b did not aid in resolution of infection nor did the presence of this host cytokine contribute to activation of the inflammasome. We are extended these observations to our mouse model and have shown that F. tularensis evades activation of the inflammasome but does not inhibit its assembly. Rather the bacterium specifically targets mRNA for pro-inflammatory cytokines (via destabilization) that depend on the inflammasome for activation. We are currently identifying the specific mechanism by which F. tularensis destabilizes host mRNA to inhibit production of inflamamtory cytokines.