Bone marrow is a hematopoietic site harboring multiple populations of myeloid cells in different stages of differentiation. Murine bone marrow eosinophils are traditionally identified by Siglec-F(+) staining using flow cytometry, whereas neutrophils are characterized by Ly6G(+) expression. However, using flow cytometry to characterize bone marrow hematopoietic cells in wild-type mice, we found substantial gray areas in identification of these cells. Siglec-F(+) mature eosinophil population constituted only a minority of bone marrow Lin(+)CD45(+) pool (5%). A substantial population of Siglec-F(−) cells was double positive for neutrophil marker Ly6G and eosinophil lineage marker, IL-5Rα. This granulocyte population with mixed neutrophil and eosinophil characteristics is typically attributable to neutrophil pool based on neutral granule staining and expression of Ly6G and myeloid peroxidase. It is distinct from Lineage(−) myeloid progenitors or Siglec-F(+)Ly6G(+) maturing eosinophil precursors, and can be accurately identified by Lineage(+) staining and positive expression of markers IL-5Rα and Ly6G. At 15–50% of all CD45(+) hematopoietic cells in adult mice (percentage varies by sex and age), this is a surprisingly dominant population, which increases with age in both male and female mice. RNA-seq characterization of these cells revealed a complex immune profile and the capacity to secrete constituents of the extracellular matrix. When sorted from bone marrow, these resident cells had neutrophilic phenotype but readily acquired all characteristics of eosinophils when cultured with G-CSF or IL-5, including expression of Siglec-F and granular proteins (Epx, Mbp). Surprisingly, these cells were also able to differentiate into Ly6C(+) monocytes when cultured with M-CSF. Herein described is the discovery of an unexpected hematopoietic flexibility of a dominant population of multipotent myeloid cells, typically categorized as neutrophils, but with the previously unknown plasticity to contribute to mature pools of eosinophils and monocytes.

中文翻译：

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不仅仅是嗜中性粒细胞：Lin(+)Ly6G(+)IL-5Rα(+) 多能骨髓细胞 (MMC) 在正常小鼠骨髓中占主导地位，并保留分化为嗜酸性粒细胞和单核细胞的能力

骨髓是一个造血部位，含有多个处于不同分化阶段的骨髓细胞群。小鼠骨髓嗜酸性粒细胞传统上使用流式细胞术通过 Siglec-F(+) 染色进行鉴定，而嗜中性粒细胞的特征在于 Ly6G(+) 表达。然而，使用流式细胞术来表征野生型小鼠的骨髓造血细胞，我们在这些细胞的识别中发现了大量的灰色区域。Siglec-F(+) 成熟嗜酸性粒细胞群仅构成少数骨髓 Lin(+)CD45(+) 池 (5%)。大量的 Siglec-F(−) 细胞对中性粒细胞标记物 Ly6G 和嗜酸性粒细胞谱系标记物 IL-5Rα 呈双阳性。这种具有中性粒细胞和嗜酸性粒细胞混合特征的粒细胞群通常归因于基于中性颗粒染色和 Ly6G 和髓样过氧化物酶表达的中性粒细胞池。它不同于谱系 (−) 骨髓祖细胞或 Siglec-F(+)Ly6G(+) 成熟嗜酸性粒细胞前体，可以通过谱系 (+) 染色和标记物 IL-5Rα 和 Ly6G 的阳性表达来准确识别。在成年小鼠的所有 CD45(+) 造血细胞中占 15-50%（百分比因性别和年龄而异），这是一个令人惊讶的优势群体，在雄性和雌性小鼠中随着年龄的增长而增加。这些细胞的 RNA-seq 表征揭示了复杂的免疫特征和分泌细胞外基质成分的能力。当从骨髓中分离出来时，这些驻留细胞具有中性粒细胞表型，但在与 G-CSF 或 IL-5 一起培养时很容易获得嗜酸性粒细胞的所有特征，包括 Siglec-F 和颗粒蛋白（Epx、Mbp）的表达。令人惊讶的是，这些细胞在与 M-CSF 一起培养时也能够分化为 Ly6C(+) 单核细胞。此处描述的是发现多能骨髓细胞的主要群体具有意想不到的造血灵活性，通常归类为嗜中性粒细胞，但具有以前未知的可塑性，有助于成熟的嗜酸性粒细胞和单核细胞库。