Department of Infectious Diseases and Immunology, Utrecht Centre for Tick-borne Diseases, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584CL Utrecht, The Netherlands.
BMC Immunol. 2010 Feb 19;11:7. doi: 10.1186/1471-2172-11-7.
Ticks (Acari: Ixodidae) are vectors of pathogens worldwide that cause diseases in humans and animals. Ticks and pathogens have co-evolved molecular mechanisms that contribute to their mutual development and survival. Subolesin was discovered as a tick protective antigen and was subsequently shown to be similar in structure and function to akirins, an evolutionarily conserved group of proteins in insects and vertebrates that controls NF-kB-dependent and independent expression of innate immune response genes. The objective of this study was to investigate subolesin expression in several tick species infected with a variety of pathogens and to determine the effect of subolesin gene knockdown on pathogen infection. In the first experiment, subolesin expression was characterized in ticks experimentally infected with the cattle pathogen, Anaplasma marginale. Subolesin expression was then characterized in questing or feeding adult ticks confirmed to be infected with Anaplasma, Ehrlichia, Rickettsia, Babesia or Theileria spp. Finally, the effect of subolesin knockdown by RNA interference (RNAi) on tick infection was analyzed in Dermacentor variabilis males exposed to various pathogens by capillary feeding (CF).
Subolesin expression increased with pathogen infection in the salivary glands but not in the guts of tick vector species infected with A. marginale. When analyzed in whole ticks, subolesin expression varied between tick species and in response to different pathogens. As reported previously, subolesin knockdown in D. variabilis infected with A. marginale and other tick-borne pathogens resulted in lower infection levels, while infection with Francisella tularensis increased in ticks after RNAi. When non-tick-borne pathogens were fed to ticks by CF, subolesin RNAi did not affect or resulted in lower infection levels in ticks. However, subolesin expression was upregulated in D. variabilis exposed to Escherichia coli, suggesting that although this pathogen may induce subolesin expression in ticks, silencing of this molecule reduced bacterial multiplication by a presently unknown mechanism.
Subolesin expression in infected ticks suggested that subolesin may be functionally important for tick innate immunity to pathogens, as has been reported for the akirins. However, subolesin expression and consequently subolesin-mediated innate immunity varied with the pathogen and tick tissue. Subolesin may plays a role in tick innate immunity in the salivary glands by limiting pathogen infection levels, but activates innate immunity only for some pathogen in the guts and other tissues. In addition, these results provided additional support for the role of subolesin in other molecular pathways including those required for tissue development and function and for pathogen infection and multiplication in ticks. Consequently, RNAi experiments demonstrated that subolesin knockdown in ticks may affect pathogen infection directly by reducing tick innate immunity that results in higher infection levels and indirectly by affecting tissue structure and function and the expression of genes that interfere with pathogen infection and multiplication. The impact of the direct or indirect effects of subolesin knockdown on pathogen infection may depend on several factors including specific tick-pathogen molecular interactions, pathogen life cycle in the tick and unknown mechanisms affected by subolesin function in the control of global gene expression in ticks.
蜱（蜱螨目：硬蜱科）是全球病原体的传播媒介，可导致人类和动物患病。蜱和病原体共同进化出了有助于它们相互发育和生存的分子机制。亚油酸是一种蜱的保护性抗原，随后被发现与 akirins 结构和功能相似，akirins 是昆虫和脊椎动物中进化上保守的一组蛋白质，可控制 NF-kB 依赖性和非依赖性固有免疫反应基因的表达。本研究的目的是研究几种感染了多种病原体的蜱中亚油酸的表达，并确定亚油酸基因敲低对病原体感染的影响。在第一个实验中，我们研究了经实验感染牛病原体安氏巴尔通体的蜱中亚油酸的表达。然后，我们在确认感染了无形体、埃立克体、立克次体、巴贝西虫或泰勒虫的刺吸或吸血成年蜱中对亚油酸的表达进行了研究。最后，我们通过毛细血管喂养（CF）分析了 Dermacentor variabilis 雄性在暴露于各种病原体时亚油酸敲低对 tick 感染的影响。
在感染 A. marginale 的唾液腺中，随着病原体感染，亚油酸的表达增加，但在感染的 tick 载体物种的肠道中没有增加。当在整个 tick 中进行分析时，亚油酸的表达因 tick 物种而异，也因不同的病原体而异。如前所述，在感染了 A. marginale 和其他 tick 传播病原体的 D. variabilis 中敲低亚油酸，导致感染水平降低，而在 RNAi 后， tick 中感染弗朗西斯菌的水平增加。当非 tick 传播病原体通过 CF 喂食给 tick 时，亚油酸 RNAi 对 tick 的感染没有影响或导致感染水平降低。然而，在暴露于大肠杆菌的 D. variabilis 中，亚油酸的表达上调，这表明尽管这种病原体可能在 tick 中诱导亚油酸的表达，但沉默该分子通过目前未知的机制减少了细菌的增殖。
感染 tick 中的亚油酸表达表明，亚油酸可能在 tick 固有免疫中对病原体具有功能性重要性，就像 akirins 一样。然而，亚油酸的表达和因此亚油酸介导的固有免疫因病原体和 tick 组织而异。亚油酸可能通过限制病原体感染水平在唾液腺中发挥 tick 固有免疫的作用，但仅在肠道和其他组织中针对某些病原体激活固有免疫。此外，这些结果为亚油酸在包括组织发育和功能以及病原体感染和增殖所需的其他分子途径中的作用提供了额外的支持。因此，RNAi 实验表明，在 tick 中敲低亚油酸可能通过降低导致感染水平升高的 tick 固有免疫直接影响病原体感染，也可能通过影响组织结构和功能以及干扰病原体感染和增殖的基因表达间接影响病原体感染。亚油酸敲低对病原体感染的直接或间接影响的影响可能取决于几个因素，包括特定的 tick-病原体分子相互作用、tick 中的病原体生命周期以及亚油酸功能对 tick 中控制全局基因表达的未知机制的影响。
