Evasion of apoptosis and immune recognition during host adaptation of Chlamydia and Chlamydia-like bacteria

衣原体是一种人类致病细菌，因此可以在人类宿主细胞内感染和复制。衣原体是从一种早期的内共生菌进化而来的，这种共生菌一度感染了单细胞生物。从相同的早期前体进化而来的另一组今天的细菌被称为衣原体样细菌(例如Parachlamydia)，它们生长在自由生活的阿米巴中。最近发现，副枝原体通过自杀(细胞凋亡)在昆虫细胞中诱导细胞死亡，这是细菌在培养过程中无法在这些细胞中生长的原因。我们发现，感染诱导的细胞凋亡也发生在哺乳动物(小鼠和人类)细胞中。这表明，昆虫和哺乳动物细胞在从单细胞生物体进化的过程中，已经获得了检测副衣原体和经历细胞凋亡的手段，很可能是出于自卫(即“细胞自主免疫”)。从机理上讲，这意味着识别细菌的受体，然后激活一条通往细胞凋亡的信号通路。在进化适应过程中，人类致病衣原体似乎找到了逃避这种识别和细胞死亡诱导的方法，使其能够在人类细胞中生长(衣原体也可以在昆虫细胞中生长到一定程度)。在这个项目中，我们将检验这一假说，并努力得出相关的分子框架。从细胞凋亡诱导的初步观察开始，从细胞凋亡诱导的初步数据出发，我们将确定Parachlamydia诱导细胞凋亡所必需的信号通路的组成部分。我们将进一步测试识别副疟原虫所需的上游信号组件的要求，从而激活核心的细胞凋亡机制。这些实验将在昆虫(果蝇)和小鼠或人类细胞的细胞培养中进行，特别是使用缺乏各种信号蛋白的细胞系。不同细胞类型的比较将进一步缩小可能涉及的识别系统的范围。通过使用衣原体的比较研究，我们将努力找出激活的差异机制，以及允许生长或导致凋亡的差异机制。我们希望这将反过来允许识别对这种形式的细菌免疫逃避至关重要的系统，这些系统适应后生动物宿主，这些宿主配备了比原生动物(如衣原体)更复杂的防御系统。已确定的结果将通过使用来自已知宿主范围的衣原体目的其他候选细菌而得到进一步加强。我们相信，这个项目将为我们提供大量信息，了解针对专性细胞内细菌的细胞自主免疫逃避的机制和重要性，以及识别衣原体时的信号。

Chlamydia is a human-pathogenic bacterium and can accordingly infect and replicate inside human host cells. Chlamydia has evolved from an early endosymbiont that at one stage infected single-celled organisms. Another group of present-day bacteria that have evolved from the same early precursor are known as Chlamydia-like bacteria (e.g. Parachlamydia), which grow in free-living amoebae. It has recently been found that Parachlamydia induces cell death by suicide (apoptosis) in insect cells, and that this is the reason for the inability of the bacteria to grow in these cells in culture. We have found that infection-induced apoptosis also occurs in mammalian (mouse and human) cells. This suggests that insect and mammalian cells have, during evolution from a single-celled organism, acquired means to detect Parachlamydia and to undergo apoptosis, most likely as a matter of self-defence ("cell-autonomous immunity"). Mechanistically, this implies receptors that recognize the bacteria and that then activate a signalling pathway to apoptosis. During evolutionary adaptation, human-pathogenic chlamydiae appear to have found ways of escaping this recognition and cell death induction, enabling growth in human cells (Chlamydia also can grow to a degree in insect cells). In this project we will test this hypothesis and endeavour to work out the relevant molecular framework. Starting from the initial observation of apoptosis induction and from preliminary data on how apoptosis is induced we will identify components of the signalling pathways that are necessary for apoptosis induction by Parachlamydia. We will further test for the requirements of upstream signalling components that are required for the recognition of Parachlamydia, leading to the activation of the core apoptosis machinery. These experiments will be conducted in insect (Drosophila) and in mouse or human cells in cell culture, using in particular cell lines deficient in a variety of signalling proteins. Comparison of different cell types will further narrow down the circle of possible recognition systems that may be involved. By comparative studies using Chlamydia we will endeavour to work out the differential mechanisms that are activated and that either permit growth or lead to apoptosis. We hope that this will in turn permit the identification of systems critical for this form of immune evasion in bacteria that are adapted to metazoan hosts equipped with more sophisticated defence systems than protozoa (such as Chlamydia). Established results will be further strengthened by the use of other candidate bacteria from the order Chlamydiales of known host range. We believe that this project will provide us with substantial information on mechanisms and importance of the evasion of cell-autonomous immunity against obligate intracellular bacteria as well as signalling upon recognition of chlamydiae.