Immune phenotypes of microglia in human neurodegenerative disease: challenges to detecting microglial polarization in human brains

Abstract

Inflammatory responses in the brain, which can be demonstrated by changes in properties of microglia, the brain-resident macrophages, are a common feature of human neurodegenerative diseases. Different monocyte/macrophage phenotypes have been defined by changes in expression of cytokines, receptors and other markers as a response to different classes of stimuli. Monocytes, macrophages and microglia can have a range of phenotypes with associated properties depending on their microenvironment. Macrophage/microglia polarization states have been defined as classical activation (M1), alternative activation (M2a), type II alternative activation (M2b) or acquired deactivation (M2c). Available markers for identifying microglial phenotypes in human brains are still limited; those available provide incomplete information on the functions or polarization states of microglia observed in tissues from diseases such as Alzheimer's disease, Parkinson's disease and multiple sclerosis. The most widely used marker to describe activated microglia in human brains, particularly diseased brains, has been HLA-DR, the major histocompatibility complex II protein. HLA-DR-positive microglia can have a wide range of activation morphologies that are affected not only by disease pathology, but also by their differentiation states and brain regions. Two other widely used markers to identify microglia in human brains are ionized calcium binding adaptor molecule-1 and CD68. Although their expression changes in diseased brains, these markers do not show specificity for different phenotypes. Over the years there have been studies with additional markers that attempt to further define microglial properties, particularly in Alzheimer's disease brains. Most studies have employed immunohistochemical techniques to identify microglia in tissue sections, but recent advances in this field have allowed gene expression profiling of microglia upon immediate isolation from brains. We will review which markers might better define different activation phenotypes of microglia in human brains and whether they fit into current microglial polarization schemes.

Fig. 1

Various morphologies of microglia in human brain sections. Progressive changes in morphology of HLA-DR-expressing microglia in a pathology-rich section from an AD case. HLA-DR-expressing microglia can be found with various activation morphologies ranging from a highly ramified to c moderately hypertrophic to e highly activated with enlarged cell body and processes. b, d Intermediate changes in morphology. Sections were stained using antibody LN3 (1:1,000 dilution; Abcam, Cambridge, MA, USA) using nickel-enhanced diaminobenzidine peroxidase immunohistochemistry and counterstained with neutral red

Fig. 2

Possible markers for microglial phenotyping in human brains. Scheme to illustrate different markers that could be used for identifying different microglial phenotypes in human brains. Data show some markers that have been applied from more than one study. We include markers whose function is suggestive of polarity (e.g., CD200R, CD33 and TREM-2) but not proven with published data for human microglia. CCL C–C chemokine ligand, CD200R CD200 receptor, CSF-1R colony-stimulating factor-1 receptor, IBA-1 ionized calcium binding adaptor molecule-1, IFN-γ interferon gamma, IL interleukin, LPS lipopolysaccharide, TGF transforming growth factor, TNF tumor necrosis factor, TREM-2 triggering receptor expressed by myeloid cells-2