Department of Biological Sciences, Old Dominion University, Norfolk, Virginia, USA.
Department of Veterinary Medicine, University of Maryland, College Park, Maryland, USA.
Cell Microbiol. 2020 Oct;22(10):e13237. doi: 10.1111/cmi.13237. Epub 2020 Jul 27.
Reactive oxygen species (ROS) that are induced upon pathogen infection plays an important role in host defence. The rickettsial pathogen Anaplasma phagocytophilum, which is primarily transmitted by Ixodes scapularis ticks in the United States, has evolved many strategies to escape ROS and survive in mammalian cells. However, little is known on the role of ROS in A. phagocytophilum infection in ticks. Our results show that A. phagocytophilum and hemin induce activation of l-tryptophan pathway in tick cells. Xanthurenic acid (XA), a tryptophan metabolite, supports A. phagocytophilum growth in tick cells through inhibition of tryptophan dioxygenase (TDO) activity leading to reduced l-kynurenine levels that subsequently affects build-up of ROS. However, hemin supports A. phagocytophilum growth in tick cells by inducing TDO activity leading to increased l-kynurenine levels and ROS production. Our data reveal that XA and kynurenic acid (KA) chelate hemin. Furthermore, treatment of tick cells with 3-hydroxyl l-kynurenine limits A. phagocytophilum growth in tick cells. RNAi-mediated knockdown of kynurenine aminotransferase expression results in increased ROS production and reduced A. phagocytophilum burden in tick cells. Collectively, these results suggest that l-tryptophan pathway metabolites influence A. phagocytophilum survival by affecting build up of ROS levels in tick cells.
病原体感染诱导的活性氧（ROS）在宿主防御中发挥重要作用。在美国主要通过肩突硬蜱传播的立克次氏体病原体嗜吞噬细胞无形体，已经进化出许多逃避 ROS 并在哺乳动物细胞中存活的策略。然而，对于 ROS 在蜱中的嗜吞噬细胞无形体感染中的作用知之甚少。我们的结果表明，嗜吞噬细胞无形体和血红素诱导蜱细胞中 l-色氨酸途径的激活。黄尿酸（XA），一种色氨酸代谢物，通过抑制色氨酸双加氧酶（TDO）活性来支持蜱细胞中的嗜吞噬细胞无形体生长，导致 l-犬尿氨酸水平降低，进而影响 ROS 的积累。然而，血红素通过诱导 TDO 活性来支持蜱细胞中的嗜吞噬细胞无形体生长，导致 l-犬尿氨酸水平升高和 ROS 产生。我们的数据表明，XA 和犬尿氨酸（KA）螯合血红素。此外，用 3-羟基 l-犬尿氨酸处理蜱细胞可限制蜱细胞中的嗜吞噬细胞无形体生长。通过 RNAi 介导的犬尿氨酸氨基转移酶表达敲低导致 ROS 产生增加和蜱细胞中嗜吞噬细胞无形体负担减少。总的来说，这些结果表明，l-色氨酸途径代谢物通过影响蜱细胞中 ROS 水平的积累来影响嗜吞噬细胞无形体的存活。
