Inthawong et al 2024 Scrub typhus NHP model intradermal inoculation dose evaluation

Scrub typhus, endemic to the Asia-Pacific region is caused by Orientia tsutsugamushi and transmitted to humans through bites of infected larval Leptotrombidium mites (chiggers). Currently, no protective vaccines exist. Animal models have substantially advanced our understanding of immune response dynamics, pathophysiology and pathogenesis of scrub typhus. Our group previously validated a non-human primate (NHP) model using an intradermal (ID) challenge approach. To further refine this model, we evaluated the effects of progressively increasing doses of human pathogenic O. tsutsugamushi on disease progression, clinical outcomes, bacterial kinetics and immune responses in rhesus macaques (Macaca mulatta). Macaques were inoculated ID with O. tsutsugamushi strain Karp at doses of 106, 107 and 107.8 based on murine median Lethal Dose (muLD50). All NHPs demonstrated characteristic signs of human scrub typhus, including eschar formation, regional lymphadenopathy, elevated body temperature, and bacteremia. Dose-dependent responses were observed, with higher doses (107 and 107.8 muLD50), showing more severe clinical signs and pronounced adaptive immune responses. Increased production of Th1 and Th2 cytokines, with a significant rise surrogate markers, were observed in the 107 muLD50 group. This study demonstrates the critical role of differential serum IFN- levels in shaping immunity, and suggests that the 107 muLD50 dose is optimal for reliably inducing surrogate markers and immune responses resembling human infection dynamics. Our study highlights the importance of thorough inoculum dose characterisation in animal models to ensure accurate and reproducible outcome measures. The fundamental findings will serve emerging research on scrub typhus, particularly in establishing experimental studies.

恙虫病，一种流行于亚太地区的疾病，由东方立克次体（Orientia tsutsugamushi）引起，通过感染性幼虫蜱（Leptotrombidium mites，俗称蜱虫）叮咬传播给人类。目前尚无有效的预防疫苗。动物模型在深化我们对免疫反应动力学、恙虫病的病理生理学和发病机制的理解方面取得了显著进展。本研究团队先前已采用皮内挑战法验证了一种非人类灵长类动物（NHP）模型。为进一步完善此模型，我们评估了递增剂量的人类致病性东方立克次体（O. tsutsugamushi）对疾病进展、临床结果、细菌动力学和猕猴（Macaca mulatta）免疫反应的影响。猕猴被皮内接种以O. tsutsugamushi菌株Karp，剂量分别为10^6、10^7和10^7.8，基于小鼠半数致死量（muLD50）。所有非人类灵长类动物均表现出人类恙虫病的典型症状，包括焦痂形成、区域性淋巴结肿大、体温升高和菌血症。观察到剂量依赖性反应，较高剂量（10^7和10^7.8 muLD50）显示更严重的临床症状和明显的适应性免疫反应。在10^7 muLD50组观察到Th1和Th2细胞因子产量的增加，以及替代标志物的显著升高。本研究证明了不同血清IFN-γ水平在塑造免疫力中的关键作用，并建议10^7 muLD50剂量是可靠地诱导类似人类感染动态的替代标志物和免疫反应的最佳选择。本研究突出了在动物模型中彻底进行接种剂量特征化的重要性，以确保准确和可重复的结果测量。基本发现将为恙虫病的最新研究提供支持，特别是在建立实验研究方面。