Altered Immunity in Crowded Locust Reduced Fungal (<em>Metarhizium anisopliae</em>) Pathogenesis

The stress of living conditions, similar to infections, alters animal immunity. High population density is empirically considered to induce prophylactic immunity to reduce the infection risk, which was challenged by a model of low connectivity between infectious and susceptible individuals in crowded animals. The migratory locust, which exhibits polyphenism through gregarious and solitary phases in response to population density and displays different resistance to fungal biopesticide (Metarhizium anisopliae), was used to observe the prophylactic immunity of crowded animals. We applied an RNA-sequencing assay to investigate differential expression in fat body samples of gregarious and solitary locusts before and after infection. Solitary locusts devoted at least twice the number of genes for combating M. anisopliae infection than gregarious locusts. The transcription of immune molecules such as pattern recognition proteins, protease inhibitors, and anti-oxidation proteins, was increased in prophylactic immunity of gregarious locusts. The differentially expressed transcripts reducing gregarious locust susceptibility to M. anisopliae were confirmed at the transcriptional and translational level. Further investigation revealed that locust GNBP3 was susceptible to proteolysis while GNBP1, induced by M. anisopliae infection, resisted proteolysis. Silencing of gnbp3 by RNAi significantly shortened the life span of gregarious locusts but not solitary locusts. By contrast, gnbp1 silencing did not affect the life span of both gregarious and solitary locusts after M. anisopliae infection. Thus, the GNBP3-dependent immune responses were involved in the phenotypic resistance of gregarious locusts to fungal infection, but were redundant in solitary locusts. Our results indicated that gregarious locusts prophylactically activated upstream modulators of immune cascades rather than downstream effectors, preferring to quarantine rather than eliminate pathogens to conserve energy meanwhile increasing the “distance” of infectious and target individuals. Our study has obvious implications for bio-pesticides management of crowded pests, and for understanding disease epidemics and adaptiveness of pathogens.与病原感染类似，生存环境胁迫会改变动物的免疫功能。长期以来，学界基于经验认为高密度种群可诱导预防性免疫（prophylactic immunity）以降低感染风险，但该观点受到了一项研究的挑战：该研究构建了拥挤种群中感染个体与易感个体间低接触度的模型。本研究以具有表型多型性（polyphenism）的飞蝗为材料——该虫可因种群密度变化呈现群居相（gregarious phase）与独居相（solitary phase），且对真菌生物农药绿僵菌（Metarhizium anisopliae）的抗性存在显著差异——用以探究拥挤种群的预防性免疫机制。我们通过RNA测序（RNA-sequencing）实验，分析了感染绿僵菌前后，群居型与独居型飞蝗脂肪体样本的基因差异表达情况。相较于群居型飞蝗，独居型飞蝗用于对抗绿僵菌感染的基因数量至少是其两倍。群居型飞蝗的预防性免疫过程中，模式识别蛋白、蛋白酶抑制剂、抗氧化蛋白等免疫分子的转录水平均显著上调。可降低群居型飞蝗对绿僵菌易感性的差异表达转录本，已在转录与翻译水平得到验证。进一步研究发现，飞蝗的革兰氏阴性菌结合蛋白3（GNBP3）易被蛋白水解，而由绿僵菌感染诱导的GNBP1则可抵抗蛋白水解。通过RNA干扰（RNAi）沉默gnbp3基因，会显著缩短群居型飞蝗的寿命，但对独居型飞蝗无显著影响。与之相反，沉默gnbp1基因并不会影响感染绿僵菌后的群居型与独居型飞蝗的寿命。由此可见，依赖GNBP3的免疫应答参与了群居型飞蝗对真菌感染的表型抗性，但在独居型飞蝗中该通路功能冗余。本研究结果表明，群居型飞蝗会预防性激活免疫级联反应的上游调控因子，而非下游效应分子；它们更倾向于隔离而非清除病原体，以此节约能量，同时增加感染个体与靶标个体间的“接触距离”。本研究对拥挤害虫的生物农药防控、以及疾病流行与病原体适应性的研究均具有重要参考价值。