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菌，因此可以在人类宿主细胞内感染和复制。衣原体是从早期的内共生体进化而来，在一个阶段感染单细胞生物。从相同的早期前体进化而来的另一组现代细菌被称为衣原体样细菌（如副衣原体），它们生长在自由生活的变形虫体内。最近发现副原体在昆虫细胞中诱导细胞自杀（凋亡），这是细菌在培养中不能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进化的过程中，获得了检测副原体和进行细胞凋亡的手段，这很可能是一种自我防御（“细胞自主免疫”）。从机制上讲，这意味着受体可以识别细菌，然后激活细胞凋亡的信号通路。在进化适应过程中，人类致病性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，我们将测试这一假设，并努力找出相关的分子框架。从细胞凋亡诱导的初步观察和细胞凋亡诱导的初步数据出发，我们将确定副原体诱导细胞凋亡所必需的信号通路成分。我们将进一步测试识别副原体所需的上游信号成分，从而激活核心凋亡机制。这些实验将在昆虫（果蝇）和小鼠或人类细胞培养中进行，使用缺乏各种信号蛋白的特定细胞系。对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。通过对衣原体的比较研究，我们将努力找出被激活的不同机制，这些机制要么允许生长，要么导致细胞凋亡。我们希望这将反过来允许识别细菌中对这种免疫逃避形式至关重要的系统，这些系统适应具有比原生动物（如衣原体）更复杂防御系统的后生动物宿主。利用已知宿主范围衣原体目的其他候选细菌将进一步加强已建立的结果。我们相信这个项目将为我们提供大量的信息，了解细胞对专性细胞内细菌的自主免疫逃避的机制和重要性，以及识别衣原体的信号。

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.