Repression of inflammasome by Francisella tularensis

描述(申请人提供)：图拉方济氏菌(Ft)是一种革兰氏阴性细胞内病原体，可导致一种称为图拉热症的致命疾病。基于它的高毒性，过去用于生物武器计划，以及可能用于生物恐怖袭击，疾病控制中心(CDC)已将Ft列为A类选择剂。Ft的极端毒力是由于它能够颠覆宿主的先天免疫反应。为了支持这一观点，我们的初步研究表明，强毒株Ft SchuS4和活疫苗株(LVS)对先天性免疫信号级联反应具有抑制作用，导致极低水平的促炎细胞因子与显著延迟感染细胞的程序性死亡有关。然而，调节这些效应的机制和弗朗西塞拉因子还不清楚。Toll样受体(TLRs)和胞质Nod样受体(NLRs)是两类主要的天然免疫感受器，它们对特定的微生物成分做出反应而被激活。这些感受器的激活导致了促炎细胞因子的诱导、先天免疫细胞向感染部位的招募、适应性免疫反应的产生和感染的解决。胞质NLR对病原体的识别激活了称为炎症体的多分子复合体的组装。炎症体复合体激活半胱氨酸蛋白酶caspase1，半胱氨酸蛋白酶caspase1进而激活促炎细胞因子IL-1和IL-18的前体，使其成为成熟的活性形式。炎性小体组装和caspase-1激活也会导致一种快速的促炎细胞死亡，称为下垂。我们的初步研究表明，图拉夫酵母能够抑制TLR和NLR介导的反应和嗜热性细胞死亡。本研究旨在探讨Ft强毒株抑制NLR依赖反应的机制。我们的假设是“炎性小体的主动抑制是Ft细胞内生存所必需的”。我们认为，通过抑制或延迟这种保护性宿主的先天免疫反应，Ft能够在胞浆中创建一个复制利基，导致宿主巨噬细胞内的不受控制的增殖。 这一假说将通过特定目标1来验证，该目标将建立Ft LVS和SchuS4抑制炎症体的关键部位，而在特定目标2中，我们将确定Ft介导的抑制炎症体的机制。FT作为一种独特的模型来理解炎症小体介导的反应的复杂机制。这些研究的结果将确定Ft用来抑制炎症小体的机制(S)，这是一种关键的宿主先天性免疫防御机制。这些研究将促进我们对图拉热病发病机制的了解，并将使我们更接近于实现确定有效治疗和预防药物开发目标的最终目标。这一建议在理解图拉热症的免疫发病机制方面向前迈进了一步。

DESCRIPTION (provided by applicant): Francisella tularensis (Ft) is a gram-negative intracellular pathogen that causes a fatal disease known as tularemia. Based on its high virulence, use in bioweapon programs in the past, and potential to be used in bioterror attacks, the Centers for Disease Control (CDC) have classified Ft as a Category A select agent. The extreme virulence of Ft is due to its ability to subvert host's innate immune response. In support of this notion, our preliminary studies demonstrate that the virulent strain Ft SchuS4 and Live Vaccine Strain (LVS), a derivative of F. holarctica exert suppressive effects on innate immune signaling cascades resulting in very low levels of pro-inflammatory cytokines associated with significant delay in the programmed death of the infected cells. However, the mechanisms and the Francisella factors mediating these effects are ill defined. Two major classes of innate immune sensors, Toll-like receptors (TLRs) and cytosolic Nod-like receptors (NLRs) are activated in response to specific microbial components. Activation of these sensors leads to induction of pro- inflammatory cytokines, recruitment of innate immune cells to the sites of infection, generation of an adaptive immune response and resolution of infection. Recognition of pathogens by cytosolic NLRs activates assembly of a multi-molecular complex termed as inflammasome. The inflammasome complex activates cysteine protease caspase1 which in-turn activates pro-forms of pro-inflammatory cytokines IL-1¿ and IL-18 to their mature active forms. Inflammasome assembly and caspase-1 activation also induces a rapid form of pro- inflammatory cell death, termed pyroptosis. Our preliminary studies demonstrate that F. tularensis is capable of suppressing both TLR and NLR-mediated responses and pyroptotic cell death. This proposal is aimed at investigating the mechanism by which virulent strains of Ft suppress NLR-dependent responses. Our hypothesis is that "active repression of the inflammasome is essentially required for intracellular survival of Ft". We propose that by repressing or delaying this protective host's innate immune response, Ft is able to create a replicative niche in the cytosol leading to uncontrolled proliferation within the host macrophages. This hypothesis will be tested via Specific aim 1 which will establish the key sites of repression of inflammasome by Ft LVS and SchuS4 while in Specific aim 2, we will identify the mechanisms of Ft mediated repression of inflammasome. Ft serves as a unique model to understand the intricate mechanisms of inflammasome mediated responses. The outcome from these studies will identify mechanism(s) employed by Ft to repress the inflammasome, a critical host innate immune defense mechanism. These studies will advance our knowledge of tularemia pathogenesis and will bring us closer to achieving the ultimate goal of identifying the targets for development of effective therapeutics and prophylactics. This proposal is a step ahead in understanding the immunopathogenesis of tularemia.