Prenatal immune stress blunts microglia reactivity which impairs neurocircuitry [Adult_Rescue]

Recent studies suggested that microglia, the primary brain immune cells, can affect circuit connectivity and neuronal function. Microglia infiltrate the neuroepithelium early in embryonic development and are maintained in the brain throughout adulthood. Several maternal environmental factors, such as aberrant microbiome, immune activation, and poor nutrition, can influence prenatal brain development. Nevertheless, it is unknown how changes in the prenatal environment instruct the developmental trajectory of infiltrating microglia, which in turn affect brain development and function. Here we show that after maternal immune activation (MIA) microglia from the offspring have a long-lived decrease in immune reactivity (blunting) across the developmental trajectory. The blunted immune response was concomitant with changes in the chromatin accessibility and reduced transcription factor occupancy of the open chromatin. Single-cell RNA sequencing revealed that MIA does not induce a distinct subpopulation but rather decreases the contribution to inflammatory microglia states. Prenatal replacement of MIA microglia with physiological infiltration of naïve microglia ameliorated the immune blunting and restored a decrease in presynaptic vesicle release probability onto dopamine receptor type-two medium spiny neurons, indicating that aberrantly formed microglia due to an adverse prenatal environment impacts the long-term microglia reactivity and proper striatal circuit development. Overall design: RNA-seq of FACS-sorted microglia (Cd11b+, Cd45-low, P2ry12+) from the striatum of control and maternal immune activation mice of 8- to 10-week-old mice after saline or LPS treatment.

近期研究表明，作为大脑主要免疫细胞的小胶质细胞（microglia）可影响神经环路连接与神经元功能。小胶质细胞会在胚胎发育早期浸润神经上皮，并在成年后持续存在于大脑之中。多种母体环境因素，如菌群异常、免疫激活及营养不足，均可影响产前大脑发育。然而目前尚不清楚，产前环境的改变如何调控浸润性小胶质细胞的发育轨迹，进而影响大脑发育与功能。
本研究发现，母体免疫激活（maternal immune activation, MIA）后，子代小鼠的小胶质细胞在整个发育进程中呈现出免疫反应性的长期降低（即免疫钝化）。这种免疫钝化响应伴随染色质开放性的改变，以及开放染色质区域内转录因子结合占有率的下降。单细胞RNA测序（single-cell RNA sequencing）结果显示，MIA并未诱导产生独特的小胶质细胞亚群，反而降低了其向炎症性小胶质细胞状态的贡献比例。通过将未致敏小胶质细胞（naïve microglia）的生理性浸润替代MIA模型中的异常小胶质细胞，可改善免疫钝化现象，并恢复多巴胺2型受体中型多棘神经元的突触前囊泡释放概率降低的表型，这表明不良产前环境诱导形成的异常小胶质细胞，会损害小胶质细胞的长期免疫反应性，并干扰纹状体环路的正常发育。
实验总体设计：对8至10周龄对照组与MIA模型小鼠的纹状体组织，在经生理盐水或脂多糖（lipopolysaccharide, LPS）处理后，通过荧光激活细胞分选（FACS）纯化得到标记为Cd11b+、Cd45-low、P2ry12+的小胶质细胞，并对其进行RNA测序。