Distinct lactation-associated macrophages exist in murine mammary tissue and human milk [RNA-seq: mouse]

Macrophages are involved in immune defense, organogenesis and tissue homeostasis. They also contribute to the different phases of mammary gland remodeling during development, pregnancy and involution post-lactation. Yet, less is known about the dynamics of mammary gland macrophages in the lactation stage. Here, we describe a macrophage population present during lactation in mice. By multi-parameter flow cytometry and single-cell RNA sequencing we reveal this population as distinct from the two resident macrophage subsets present pregestationally. These lactation-induced macrophages (LiMacs) are predominantly monocyte-derived and expand by proliferation in situ concomitant with nursing. LiMacs develop independently of IL-34 but require CSF-1 signaling and are partly microbiota-dependent. Locally, they reside adjacent to the basal cells of the alveoli and extravasate into the milk. Moreover, we also found several macrophage subsets in human milk, resembling LiMacs. Collectively, these findings reveal the emergence of unique macrophages in the mammary gland and milk during lactation. Overall design: 10 000 cells (for 7 dpp, n=1 for virgin mammary gland and n=2 for lactating mammary gland) and 18 000 cells (for 8 dpp, 11 dpp, 14 dpp, n=3 for each time point, time points barcoded) were included in the analysis.

巨噬细胞（macrophage）参与免疫防御、器官发生与组织稳态维持。其还参与发育、妊娠及泌乳后退化阶段的乳腺重塑过程。然而，目前学界对泌乳阶段乳腺巨噬细胞的动态变化仍知之甚少。本研究针对小鼠泌乳阶段存在的一类巨噬细胞群体展开了系统描述。通过多参数流式细胞术与单细胞RNA测序，我们发现该群体与妊娠前存在的两类驻留巨噬细胞亚群存在显著差异。这类泌乳诱导巨噬细胞（lactation-induced macrophages, LiMacs）主要源自单核细胞，并伴随哺乳过程通过原位增殖实现扩增。LiMacs的发育不依赖于IL-34，但需CSF-1信号通路的调控，且一定程度上依赖于机体微生物群。在局部组织中，它们定植于乳腺腺泡的基底细胞旁，并可外渗进入乳汁。此外，我们在人类乳汁中也检测到若干类与LiMacs表型相似的巨噬细胞亚群。综上，本研究揭示了泌乳阶段乳腺与乳汁中独特巨噬细胞群体的存在与特征。实验整体设计：本分析共纳入两类细胞样本：产后7天组（未妊娠乳腺样本n=1，泌乳乳腺样本n=2）的10000个细胞，以及产后8、11、14天组（每个时间点n=3，且各时间点样本均经条码标记）的18000个细胞。