Competitive repopulation of an empty microglial niche gives rise to functionally distinct subsets of microglia-like cells. Competitive repopulation of an empty microglial niche gives rise to functionally distinct subsets of microglia-like cells

Although most tissue macrophages are embryonically derived it is evident that circulating monocytes can compete for virtually any macrophage niche. Monocytes can thus become long-lived replacements of tissue macrophages that are indistinguishable from their embryonic counterparts, but the factors regulating this process are incompletely understood. To study niche competition in the CNS we depleted microglia with >95% efficiency using CX3CR1CreER/+R26DTA/+ mice and monitored long-term repopulation. The microglial niche was repopulated within weeks by a combination of local microglia proliferation giving rise to CX3CR1+F4/80lowClec12a– microglia, as well as infiltration of CX3CR1+F4/80hiClec12a+ macrophages. Adoptive transfer experiments demonstrated that peripherally-derived macrophages arose directly from Ly6Chi monocytes without contribution from hematopoietic progenitors and was independent of BBB breakdown. After repopulation we sorted microglia and monocyte-derived macrophages and performed transcriptional, epigenetic (DNA methylation) and ex vivo functional profiling. This revealed that Ly6Chi monocytes upregulated microglia gene expression and adopted microglia DNA methylation signatures. However, in contrast to proliferating microglia, which rapidly regained their homeostatic gene signature, monocyte-derived macrophages retained a distinct gene signature associated with antigen presentation, interferon signaling and chemotaxis. This translated into functional changes in monocyte-derived macrophages, as demonstrated by differences in surface marker expression, phagocytosis and cytokine production. Our results demonstrate that monocytes are imprinted by the CNS microenvironment but remain transcriptionally, epigenetically and functionally distinct. This may have implications for neuroinflammatory disease states and direct design of novel therapies. Data was processed using the mEPIC tool PMID:29141580 Overall design: 25 sorted samples from mice on C57BL/6 background

尽管绝大多数组织巨噬细胞均为胚胎起源，但已有研究证实循环单核细胞可竞争性占据几乎所有巨噬细胞生态位。单核细胞可作为组织巨噬细胞的长期替代细胞，且与胚胎起源的巨噬细胞难以区分，但调控该过程的分子机制仍未完全阐明。为研究中枢神经系统（CNS）内的生态位竞争，本研究使用CX3CR1CreER/+R26DTA/+小鼠实现了>95%效率的小胶质细胞清除，并对其长期重定植过程进行监测。数周内，小胶质细胞生态位通过两种途径实现重定植：一是局部小胶质细胞增殖，产生CX3CR1+F4/80lowClec12a–表型小胶质细胞；二是CX3CR1+F4/80hiClec12a+巨噬细胞浸润。过继转移实验证实，外周来源巨噬细胞直接由Ly6Chi单核细胞分化而来，无需造血祖细胞参与，且该过程不依赖血脑屏障（Blood-Brain Barrier，BBB）破坏。重定植完成后，我们分选了小胶质细胞与单核细胞来源巨噬细胞，并对其进行转录组、表观基因组（DNA甲基化）以及离体功能谱分析。结果显示，Ly6Chi单核细胞会上调小胶质细胞相关基因表达，并获得小胶质细胞的DNA甲基化特征。然而，与快速恢复自身稳态基因特征的增殖性小胶质细胞不同，单核细胞来源巨噬细胞仍保留与抗原呈递、干扰素信号通路及趋化作用相关的独特基因特征。这一特征差异转化为单核细胞来源巨噬细胞的功能改变，具体表现为表面标志物表达、吞噬功能及细胞因子产生水平的差异。本研究结果表明，单核细胞会被中枢神经系统微环境所印记，但仍在转录组、表观基因组及功能层面保持独特性。该发现可为神经炎症性疾病的研究及新型治疗方案的开发提供参考。本研究数据通过mEPIC工具进行处理，相关文献引用PMID:29141580。实验整体设计：从C57BL/6背景小鼠中获取25个分选样本。