Abstract
Several types of myeloid cell are resident in the CNS. In the steady state, microglia are present in the CNS parenchyma, whereas macrophages reside in boundary regions of the CNS, such as perivascular spaces, the meninges and choroid plexus. In addition, monocytes infiltrate into the CNS parenchyma from circulation upon blood-brain barrier breakdown after CNS injury and inflammation. Although several markers, such as CD11b and ionized calcium-binding adapter molecule 1 (Iba1), are frequently used as microglial markers, they are also expressed by other types of myeloid cell and microglia-specific markers were not defined until recently. Previous transcriptome analyses of isolated microglia identified a transmembrane lectin, sialic acid-binding immunoglobulin-like lectin H (Siglec-H), as a molecular signature for microglia; however, this was not confirmed by histological studies in the nervous system and the reliability of Siglec-H as a microglial marker remained unclear. Here, we demonstrate that Siglec-H is an authentic marker for microglia in mice by immunohistochemistry using a Siglec-H-specific antibody. Siglec-H was expressed by parenchymal microglia from developmental stages to adulthood, and the expression was maintained in activated microglia under injury or inflammatory condition. However, Siglec-H expression was absent from CNS-associated macrophages and CNS-infiltrating monocytes, except for a minor subset of cells. We also show that the Siglech gene locus is a feasible site for specific targeting of microglia in the nervous system. In conclusion, Siglec-H is a reliable marker for microglia that will allow histological identification of microglia and microglia-specific gene manipulation in the nervous system.
Highlights
Microglia are mononuclear phagocytes in the CNS parenchyma
Because Siglec-E was shown as a microglial signature gene among Siglec family members in mice (Bennett et al, 2016; Claude et al, 2013; Hickman et al, 2013), we confirmed that the Siglec-H antibody did not cross-react with
In Siglechdtr/dtr mice, an Ires and the gene encoding the DT receptor were knocked into the 3′ untranslated region of the Siglech gene (Takagi et al, 2016; Takagi et al, 2011). This genetic modification was not expected to affect the expression of Siglech; our previous study found that Siglec-H expression was knocked-down in pDCs in the immune system of Siglechdtr/dtr mice. quantitative real-time PCR (qPCR) showed that levels of Siglech mRNA were knocked-down in the cerebral cortex of Siglechdtr/dtr mice
Summary
Microglia are mononuclear phagocytes in the CNS parenchyma. 2017; Prinz & Priller, 2014), including perivascular macrophages (MΦ, pvMΦ) in the perivascular space around medium- or large-sized vessels, meningeal MΦ (mMΦ) in the meninges, and choroid plexus MΦ (cpMΦ) in the choroid plexus. In addition to these “CNS-associated MΦ”, monocytes infiltrate into the CNS parenchyma from the blood circulation upon blood–brain barrier breakdown under injury or inflammatory conditions (King, Dickendesher, & Segal, 2009; Mildner et al, 2009; Saederup et al, 2010; Varvel et al, 2016).
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