Hypocrealean 昆虫病原真菌 (EPF) (Sordariomycetes, Ascomycota) 是陆地环境中昆虫种群的天然调节剂。它们强制杀死的生命周期意味着可能有很强的选择压力，使它们能够逃避宿主免疫系统的影响。在这项研究中，我们量化了虫草素（3'-脱氧腺苷），一种由蛹虫草（Hypocreales，Cordycipitaceae）产生的次生代谢物，对昆虫对 EPF 感染的易感性和昆虫免疫基因表达的影响。免疫刺激剂凝胶多糖（20 µg ml -1，线性β-1,3-葡聚糖，真菌细胞壁的一种成分）在黑腹果蝇中的应用与仅使用 DMSO 的对照相比，S2r+ 细胞导致免疫效应基因mechnikowin的表达显着增加，但当凝胶多糖与溶解在 DMSO 中的25 µg ml -1虫草素共同应用时，没有显着增加。将虫草素注射到大蜡螟（鳞翅目：螟虫科）的幼虫中导致剂量依赖性死亡率（虫草素的LC 50 = 处理后 6 天每只昆虫 2.1 毫克）。用 3.0 mg ml -1虫草素孵育C. militaris和白僵菌（Hypocreales，Cordycipitaceae；一种不合成虫草素的 EPF）的分生孢子对萌发的分生孢子数没有影响体外。与注射 EPF 相比，将G. mellonella与低浓度虫草素 (3.0 mg ml -1 ) 加 10 或 100 个分生孢子与C. militaris或B. bassiana 的每只昆虫共同注射导致昆虫中位存活时间显着减少他们自己。预测死亡率与观察死亡率的分析表明虫草素和 EPF 之间存在协同相互作用。将蛹虫草和球孢菌注射到大蜡螟中导致昆虫免疫效应基因溶菌酶、IMPI和gallerimycin的表达增加。在注射后 72 小时，但当 EPF 与 3.0 mg ml -1虫草素共同注射时不会发生这种情况。此外，我们观察到IMPI和溶菌酶在注射蛹虫草、球孢菌和假注射后 48 小时表达增加（表明产生了创伤反应），但这也被虫草素的应用所阻止。这些结果表明，虫草素有可能在昆虫宿主的真菌感染过程中起到抑制免疫反应的作用。
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Cordycepin, a metabolite of Cordyceps militaris, reduces immune-related gene expression in insects
Hypocrealean entomopathogenic fungi (EPF) (Sordariomycetes, Ascomycota) are natural regulators of insect populations in terrestrial environments. Their obligately-killing life-cycle means that there is likely to be strong selection pressure for traits that allow them to evade the effects of the host immune system. In this study, we quantified the effects of cordycepin (3’-deoxyadenosine), a secondary metabolite produced by Cordyceps militaris (Hypocreales, Cordycipitaceae), on insect susceptibility to EPF infection and on insect immune gene expression. Application of the immune stimulant curdlan (20 µg ml-1, linear beta-1,3-glucan, a constituent of fungal cell walls) to Drosophila melanogaster S2r+ cells resulted in a significant increase in the expression of the immune effector gene metchnikowin compared to a DMSO-only control, but there was no significant increase when curdlan was co-applied with 25 µg ml-1 cordycepin dissolved in DMSO. Injection of cordycepin into larvae of Galleria mellonella (Lepidoptera: Pyralidae) resulted in dose-dependent mortality (LC50 of cordycepin = 2.1 mg per insect 6 days after treatment). Incubating conidia of C. militaris and Beauveria bassiana (Hypocreales, Cordycipitaceae; an EPF that does not synthesize cordycepin) with 3.0 mg ml-1 cordycepin had no effect on the numbers of conidia germinating in vitro. Co-injection of G. mellonella with a low concentration of cordycepin (3.0 mg ml-1) plus 10 or 100 conidia per insect of C. militaris or B. bassiana caused a significant decrease in insect median survival time compared to injection with the EPF on their own. Analysis of predicted vs. observed mortalities indicated a synergistic interaction between cordycepin and the EPF. The injection of C. militaris and B. bassiana into G. mellonella resulted in increased expression of the insect immune effector genes lysozyme, IMPI and gallerimycin at 72 h post injection, but this did not occur when the EPF were co-injected with 3.0 mg ml-1 cordycepin. In addition, we observed increased expression of IMPI and lysozyme at 48 h after injection with C. militaris, B. bassiana and sham injection (indicating a wounding response), but this was also prevented by application of cordycepin. These results suggest that cordycepin has potential to act as a suppressor of the immune response during fungal infection of insect hosts.
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