Selective proliferative response of microglia to alternative polarization signals

Giovanna Pepe,Adriana Maggi,Lucia Minoli,Elisabetta Vegeto,Marcella De Maglie,Alessandro Villa

doi:10.1186/s12974-017-1011-6

Giovanna Pepe, Adriana Maggi + Show 4 more

Open Access

https://doi.org/10.1186/s12974-017-1011-6

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Abstract

BackgroundMicroglia are resident myeloid cells of the central nervous system (CNS) that are maintained by self-renewal and actively participate in tissue homeostasis and immune defense. Under the influence of endogenous or pathological signals, microglia undertake biochemical transformations that are schematically classified as the pro-inflammatory M1 phenotype and the alternatively activated M2 state. Dysregulated proliferation of M1-activated microglia has detrimental effects, while an increased number of microglia with the alternative, pro-resolving phenotype might be beneficial in brain pathologies; however, the proliferative response of microglia to M2 signals is not yet known. We thus evaluated the ability of interleukin-4 (IL-4), a typical M2 and proliferative signal for peripheral macrophages, to induce microglia proliferation and compared it with other proliferative and M2 polarizing stimuli for macrophages, namely colony-stimulating factor-1 (CSF-1) and the estrogen hormone, 17β-estradiol (E2).MethodsRecombinant IL-4 was delivered to the brain of adult mice by intracerebroventricular (i.c.v.) injection; whole brain areas or ex vivo-sorted microglia were analyzed by real-time PCR for assessing the mRNA levels of genes related with cell proliferation (Ki67, CDK-1, and CcnB2) and M2 polarization (Arg1, Fizz1, Ym-1) or by FACS analyses of in vivo BrdU incorporation in microglia. Primary cultures of microglia and astrocytes were also tested for proliferative effects.ResultsOur results show that IL-4 only slightly modified the expression of cell cycle-related genes in some brain areas but not in microglia, where it strongly enhanced M2 gene expression; on the contrary, brain delivery of CSF-1 triggered proliferation as well as M2 polarization of microglia both in vivo and in vitro. Similar to IL-4, the systemic E2 administration failed to induce microglia proliferation while it increased M2 gene expression.ConclusionsOur data show that, in contrast to the wider responsiveness of peripheral macrophages, microglia proliferation is stimulated by selected M2 polarizing stimuli suggesting a role for the local microenvironment and developmental origin of tissue macrophages in regulating self-renewal following alternative activating stimuli.

Highlights

Microglia are resident myeloid cells of the central nervous system (CNS) that are maintained by self-renewal and actively participate in tissue homeostasis and immune defense
Peritoneal macrophage proliferation following the local delivery of IL-4 In order to analyze the proliferative activity of IL-4 in the brain, we first set up in vivo proliferative assays on macrophages in the peritoneum, a peripheral tissue where resident macrophages were shown to increase in number in response to this immune signal [17]
Peritoneal macrophages were collected from the peritoneal lavage of mice 24 h after an i.p. injection of IL-4 and further purified by immunosorting; a control group of mice were injected with vehicle alone

Summary

Introduction

Microglia are resident myeloid cells of the central nervous system (CNS) that are maintained by self-renewal and actively participate in tissue homeostasis and immune defense. Dysregulated proliferation of M1-activated microglia has detrimental effects, while an increased number of microglia with the alternative, pro-resolving phenotype might be beneficial in brain pathologies; the proliferative response of microglia to M2 signals is not yet known. IL-4 has been recently shown to induce peripheral macrophage proliferation [17]; brain IL-4 levels may increase during pathological events, while administration of this molecule associates with beneficial effects on neuronal survival [18,19,20,21]; promoting proliferation of microglia endowed with pro-resolving properties may have beneficial effects on disease progression and offer new therapeutic strategies to selectively modulate disease outcome. Endocrine signals such as the sex steroid hormone 17βestradiol (E2) were shown to modulate the phenotypic activation and to induce proliferation of peritoneal macrophages in vivo, exerting anti-inflammatory effects in diverse macrophage populations including microglia [22, 23]

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