Ilia D. Vainchtein

In acute experimental autoimmune encephalomyelitis, infiltrating macrophages are immune activated, whereas microglia remain immune suppressed.

Multiple sclerosis (MS) is an autoimmune demyelinating disorder of the central nervous system (CNS) characterized by loss of myelin accompanied by infiltration of T‐lymphocytes and monocytes. Although it has been shown that these infiltrates are important for the progression of MS, the role of microglia, the resident macrophages of the CNS, remains ambiguous. Therefore, we have compared the phenotypes of microglia and macrophages in a mouse model for MS, experimental autoimmune encephalomyelitis (EAE). In order to properly discriminate between these two cell types, microglia were defined as CD11bpos CD45int Ly‐6Cneg, and infiltrated macrophages as CD11bpos CD45high Ly‐6Cpos. During clinical EAE, microglia displayed a weakly immune‐activated phenotype, based on the expression of MHCII, co‐stimulatory molecules (CD80, CD86, and CD40) and proinflammatory genes [interleukin‐1β (IL‐1β) and tumour necrosis factor‐ α (TNF‐α)]. In contrast, CD11bpos CD45high Ly‐6Cpos infiltrated macrophages were strongly activated and could be divided into two populations Ly‐6Cint and Ly‐6Chigh, respectively. Ly‐6Chigh macrophages contained less myelin than Ly‐6Cint macrophages and expression levels of the proinflammatory cytokines IL‐1β and TNF‐α were higher in Ly‐6Cint macrophages. Together, our data show that during clinical EAE, microglia are only weakly activated whereas infiltrated macrophages are highly immune reactive. GLIA 2014;62:1724–1735

Transcriptomic analysis of purified human cortical microglia reveals age-associated changes

Microglia are essential for CNS homeostasis and innate neuroimmune function, and play important roles in neurodegeneration and brain aging. Here we present gene expression profiles of purified microglia isolated at autopsy from the parietal cortex of 39 human subjects with intact cognition. Overall, genes expressed by human microglia were similar to those in mouse, including established microglial genes CX3CR1, P2RY12 and ITGAM (CD11B). However, a number of immune genes, not identified as part of the mouse microglial signature, were abundantly expressed in human microglia, including TLR, Fcγ and SIGLEC receptors, as well as TAL1 and IFI16, regulators of proliferation and cell cycle. Age-associated changes in human microglia were enriched for genes involved in cell adhesion, axonal guidance, cell surface receptor expression and actin (dis)assembly. Limited overlap was observed in microglial genes regulated during aging between mice and humans, indicating that human and mouse microglia age differently.

Glia Open Access Database (GOAD): A comprehensive gene expression encyclopedia of glia cells in health and disease

Recently, the number of genome‐wide transcriptome profiles of pure populations of glia cells has drastically increased, resulting in an unprecedented amount of data that offer opportunities to study glia phenotypes and functions in health and disease. To make genome‐wide transcriptome data easily accessible, we developed the Glia Open Access Database (GOAD), available via www.goad.education. GOAD contains a collection of previously published and unpublished transcriptome data, including datasets from isolated microglia, astrocytes and oligodendrocytes both at homeostatic and pathological conditions. It contains an intuitive web‐based interface that consists of three features that enable searching, browsing, analyzing, and downloading of the data. The first feature is differential gene expression (DE) analysis that provides genes that are significantly up and down‐regulated with the associated fold changes and p‐values between two conditions of interest. In addition, an interactive Venn diagram is generated to illustrate the overlap and differences between several DE gene lists. The second feature is quantitative gene expression (QE) analysis, to investigate which genes are expressed in a particular glial cell type and to what degree. The third feature is a search utility, which can be used to find a gene of interest and depict its expression in all available expression data sets by generating a gene card. In addition, quality guidelines and relevant concepts for transcriptome analysis are discussed. Finally, GOAD is discussed in relation to several online transcriptome tools developed in neuroscience and immunology. In conclusion, GOAD is a unique platform to facilitate integration of bioinformatics in glia biology. GLIA 2015;63:1495–1506

Survival and Functionality of Human Induced Pluripotent Stem Cell-Derived Oligodendrocytes in a Nonhuman Primate Model for Multiple Sclerosis

Fast remyelination by endogenous oligodendrocyte precursor cells (OPCs) is essential to prevent axonal and subsequent retrograde neuronal degeneration in demyelinating lesions in multiple sclerosis (MS). In chronic lesions, however, the remyelination capacity of OPCs becomes insufficient. Cell therapy with exogenous remyelinating cells may be a strategy to replace the failing endogenous OPCs. Here, we differentiated human induced pluripotent stem cells (hiPSCs) into OPCs and validated their proper functionality in vitro as well as in vivo in mouse models for MS. Next, we intracerebrally injected hiPSC‐derived OPCs in a nonhuman primate (marmoset) model for progressive MS; the grafted OPCs specifically migrated toward the MS‐like lesions in the corpus callosum where they myelinated denuded axons. hiPSC‐derived OPCs may become the first therapeutic tool to address demyelination and neurodegeneration in the progressive forms of MS.

Microglia are less pro‐inflammatory than myeloid infiltrates in the hippocampus of mice exposed to status epilepticus

Activated microglia, astrogliosis, expression of pro‐inflammatory cytokines, blood brain barrier (BBB) leakage and peripheral immune cell infiltration are features of mesial temporal lobe epilepsy. Numerous studies correlated the expression of pro‐inflammatory cytokines with the activated morphology of microglia, attributing them a pro‐epileptogenic role. However, microglia and myeloid cells such as macrophages have always been difficult to distinguish due to an overlap in expressed cell surface molecules. Thus, the detrimental role in epilepsy that is attributed to microglia might be shared with myeloid infiltrates. Here, we used a FACS‐based approach to discriminate between microglia and myeloid infiltrates isolated from the hippocampus 24 h and 96 h after status epilepticus (SE) in pilocarpine‐treated CD1 mice. We observed that microglia do not express MHCII whereas myeloid infiltrates express high levels of MHCII and CD40 96 h after SE. This antigen‐presenting cell phenotype correlated with the presence of CD4pos T cells. Moreover, microglia only expressed TNFα 24 h after SE while myeloid infiltrates expressed high levels of IL‐1β and TNFα. Immunofluorescence showed that astrocytes but not microglia expressed IL‐1β. Myeloid infiltrates also expressed matrix metalloproteinase (MMP)−9 and 12 while microglia only expressed MMP‐12, suggesting the involvement of both cell types in the BBB leakage that follows SE. Finally, both cell types expressed the phagocytosis receptor Axl, pointing to phagocytosis of apoptotic cells as one of the main functions of microglia. Our data suggests that, during early epileptogenesis, microglia from the hippocampus remain rather immune supressed whereas myeloid infiltrates display a strong inflammatory profile. GLIA 2016 GLIA 2016;64:1350–1362

Astrocyte-derived interleukin-33 promotes microglial synapse engulfment and neural circuit development

Call to action The developing brain initially makes more synapses than it needs. With further development, excess synapses are pruned away, leaving mature circuits. Synapses can be eliminated by microglia, which engulf and destroy them. Vainchtein et al. found that the microglia are called into action by astrocytes, supportive cells on which neurons rely. Astrocytes near a redundant synapse release the cytokine interleukin-33 (IL-33), which recruits microglia to the site. In mice, disruptions in this process, as caused by deficiency in IL-33, led to too many excitatory synapses and overactive brain circuitry. Science, this issue p. 1269 Astrocytes use microglia to prune redundant neuronal synapses. Neuronal synapse formation and remodeling are essential to central nervous system (CNS) development and are dysfunctional in neurodevelopmental diseases. Innate immune signals regulate tissue remodeling in the periphery, but how this affects CNS synapses is largely unknown. Here, we show that the interleukin-1 family cytokine interleukin-33 (IL-33) is produced by developing astrocytes and is developmentally required for normal synapse numbers and neural circuit function in the spinal cord and thalamus. We find that IL-33 signals primarily to microglia under physiologic conditions, that it promotes microglial synapse engulfment, and that it can drive microglial-dependent synapse depletion in vivo. These data reveal a cytokine-mediated mechanism required to maintain synapse homeostasis during CNS development.

Isolation of Microglia and Immune Infiltrates from Mouse and Primate Central Nervous System

Microglia are the innate immune cells of the central nervous system (CNS) and play an important role in the maintenance of tissue homeostasis, providing neural support and neuroprotection. Microglia constantly survey their environment and quickly respond to homeostatic perturbations. Microglia are increasingly implicated in neuropathological and neurodegenerative conditions, such as Alzheimers disease, Parkinsons disease, and glioma progression. Here, we describe a detailed isolation protocol for microglia and immune infiltrates, optimized for large amounts of post mortem tissue from human and rhesus macaque, as well as smaller tissue amounts from mouse brain and spinal cord, that yield a highly purified microglia population (up to 98 % purity). This acute isolation protocol is based on mechanical dissociation and a two-step density gradient purification, followed by fluorescence-activated cell sorting (FACS) to obtain pure microglia and immune infiltrate populations.