Understanding cell-to-cell heterogeneity in African trypanosomes field isolates

细胞间的异质性是病原体成功建立感染的关键。它可以使耐药，允许免疫逃避，或触发分化到能够在不同宿主中生存的生命周期阶段。细胞间异质性的最佳研究例子之一是抗原变异——生物体周期性地改变向宿主免疫系统显示的蛋白质身份的能力。尽管在感染过程中病原体群体中细胞间异质性的重要性，但长期以来的研究主要集中在适应细胞培养条件的病原体分离物上，这些分离物通常比该领域的亲属表现出更少的异质性。因此，很难阐明导致细胞变异的机制。原生寄生虫布鲁氏锥虫是人类昏睡病和牲畜非洲锥虫病的病原体，它包含迄今为止所描述的最大的变异表面抗原家族，并具有许多属性，使其成为研究抗原变异的最重要模型之一。主要的布鲁氏体表面抗原是变异表面糖蛋白（VSG），虽然寄生虫的基因组编码超过2000个VSG，但在任何时候只有一个以互斥的方式表达。VSG异构体表达的周期性转换允许寄生虫逃避宿主免疫反应并建立慢性感染。然而，大多数布鲁氏杆菌研究使用适应细胞培养的分离株，其转换频率低得多，并且在形态上比最近的野外分离株更均匀。为了解决这一问题，本文提出的研究的中心目标是揭示在细胞培养适应过程中导致布鲁氏杆菌分离株细胞间异质性降低的遗传或分子事件。更具体地说，我们打算将单细胞组学方法与实地研究相结合，研究锥虫细胞培养适应过程中发生的事件。一个特定的重点将放在异质性的表达的寄生虫的表面抗原。更好地了解影响病原体种群异质性水平的机制将使我们能够更好地预测病原体如何适应环境挑战，包括其哺乳动物宿主的免疫反应。

Cell-to-cell heterogeneity is key to the ability of pathogens to establish successful infections. It can enable drug resistance, allow immune evasion, or trigger differentiation to life cycle stages capable of surviving in different hosts. One of the best studied examples of cell-to-cell heterogeneity is antigenic variation – the ability of an organism to periodically change the identity of the proteins displayed to the host immune system. Despite the importance of cell-to-cell heterogeneity within pathogen populations during infections, research has long focused on pathogen isolates adapted to cell-culture conditions that typically exhibit much less heterogeneity than its relatives in the field. As a consequence, it has been difficult to elucidate the mechanism contributing to cellular variation.The protozoan parasite Trypanosoma brucei, the causative agent of sleeping sickness in humans and animal African trypanosomiasis in livestock, contains the largest family of variant surface antigens described to date and possesses many attributes that have made it one of the most important models to study antigenic variation. The major T. brucei surface antigen is the variant surface glycoprotein (VSG), and whilst the parasite’s genome encodes for more than 2,000 VSGs, only one is expressed in a mutually exclusive manner at any one time. Periodic switching in VSG isoform expression allows the parasite to evade the host immune response and to establish chronic infections. Yet, most T. brucei research uses isolates adapted to cell culture that switch at much lower frequency and are more homogenous in morphology than recent field isolates.To address this problem, the central goal of the research proposed here is to uncover the genetic or molecular events that lead to a decrease in cell-to-cell heterogeneity during cell-culture adaption of T. brucei field isolates. More specifically, we intend to merge single-cell omics approaches with field research to study the events occurring during cell-culture adaptation of trypanosomes. A specific focus will be placed on the heterogeneity in expression of the parasite’s surface antigens.A better understanding of the mechanisms affecting the level of heterogeneity within a pathogen population will enable us to better predict how pathogens adapt to environmental challenges, including the immune response of their mammalian hosts.