Natural variation in the contribution of microbial density to inducible immune dynamics

Immune responses evolve to balance the benefits of microbial killing against the costs of autoimmunity and energetic resource use. Models that explore the evolution of optimal immune responses generally include a term for constitutive immunity, or the level of immunological investment prior to microbial exposure, and for inducible immunity, or investment in immune function after microbial challenge. However, studies rarely consider the functional form of inducible immune responses with respect to microbial density, despite the theoretical dependence of immune system evolution on microbe- versus immune-mediated damage to the host. In this study, we analyze antimicrobial peptide (AMP) gene expression from seven wild-caught flour beetle populations (Tribolium spp.) during acute infection with the virulent bacteria Bacillus thuringiensis (Bt) and Photorhabdus luminescens (P.lum) to demonstrate that inducible immune responses mediated by the humoral IMD pathway exhibit natural variation in both microbe density-dependent and independent temporal dynamics. Beetle populations that exhibited greater AMP expression sensitivity to Bt density were also more likely to die from infection, while populations that exhibited higher microbe density-independent AMP expression were more likely to survive P. luminescens infection. Reduction in pathway signaling efficiency through RNAi-mediated knockdown of the imd gene reduced the magnitude of both microbe-independent and dependent responses and reduced host resistance to Bt growth, but had no net effect on host survival. This study provides a framework for understanding natural variation in the flexibility of investment in inducible immune responses and should inform theory on the contribution of non-equilibrium host-microbe dynamics to immune system evolution.

免疫应答的演化旨在平衡杀灭微生物的收益与自身免疫反应及能量资源消耗带来的成本。探索最优免疫应答演化的模型通常涵盖两个核心组分：一是组成型免疫（constitutive immunity），即微生物暴露前的基础免疫投入水平；二是诱导型免疫（inducible immunity），即遭遇微生物侵染后启动的免疫功能投入。然而，尽管免疫系统的演化理论上取决于微生物与免疫应答对宿主造成的损伤孰轻孰重，但现有研究极少关注诱导型免疫应答随微生物密度变化的函数形式。本研究分析了7个野生捕获的面粉甲虫种群（拟谷盗属，*Tribolium* spp.）在感染强致病菌苏云金芽孢杆菌（*Bacillus thuringiensis*, Bt）与发光杆菌（*Photorhabdus luminescens*, P.lum）的急性感染过程中的抗菌肽（antimicrobial peptide, AMP）基因表达情况，证实由体液IMD通路介导的诱导型免疫应答，在微生物密度依赖型与非依赖型的时间动态特征上均存在自然变异。对苏云金芽孢杆菌密度具有更高抗菌肽表达敏感性的甲虫种群，其感染后的死亡率也更高；而抗菌肽表达不受微生物密度调控的种群，则更易在发光杆菌感染中存活。通过RNA干扰（RNAi）介导的imd基因敲低以降低该通路的信号转导效率，会同时削弱微生物非依赖型与依赖型的免疫应答强度，并降低宿主对苏云金芽孢杆菌增殖的抗性，但对宿主存活率无显著净影响。本研究为理解诱导型免疫应答投入灵活性的自然变异提供了理论框架，也将为探索非平衡宿主-微生物动态对免疫系统演化的贡献提供理论参考。