Neuroinvasive Listeria monocytogenes Infection Triggers IFN-Activation of Microglia and Upregulates Microglial miR-155

Front Immunol. 2018 Nov 27:9:2751. doi: 10.3389/fimmu.2018.02751. eCollection 2018.

Abstract

MicroRNA (miR) miR-155 modulates microglial activation and polarization, but its role in activation of microglia during bacterial brain infection is unclear. We studied miR-155 expression in brains of C57BL/6 (B6.WT) mice infected i.p. with the neuro-invasive bacterial pathogen Listeria monocytogenes (L. monocytogenes). Infected mice were treated with ampicillin starting 2 days (d) post-infection (p.i.) and analyzed 3d, 7d, and 14d p.i. Virulent L. monocytogenes strains EGD and 10403s upregulated miR-155 in whole brain 7 d p.i. whereas infection with avirulent, non-neurotropic Δhly or ΔactA L. monocytogenes mutants did not. Similarly, infection with virulent but not mutated bacteria upregulated IFN-γ mRNA in the brain at 7 d p.i. Upregulation of miR-155 in microglia was confirmed by qPCR of flow cytometry-sorted CD45intCD11bpos brain cells. Subsequently, brain leukocyte influxes and gene expression in sorted microglia were compared in L. monocytogenes-infected B6.WT and B6.Cg-Mir155tm1.1Rsky/J (B6.miR-155-/-) mice. Brain influxes of Ly-6Chigh monocytes and upregulation of IFN-related genes in microglia were similar to B6.WT mice at 3 d p.i. In contrast, by d 7 p.i. expressions of microglial IFN-related genes, including markers of M1 polarization, were significantly lower in B6.miR-155-/- mice and by 14 d p.i., influxes of activated T-lymphocytes were markedly reduced. Notably, CD45highCD11bpos brain cells from B6.miR-155-/- mice isolated at 7 d p.i. expressed 2-fold fewer IFN-γ transcripts than did cells from B6.WT mice suggesting reduced IFN-γ stimulation contributed to dampened gene expression in B6.miR-155-/- microglia. Lastly, in vitro stimulation of 7 d p.i. brain cells with heat-killed L. monocytogenes induced greater production of TNF in B6.miR-155-/- microglia than in B6.WT microglia. Thus, miR-155 affects brain inflammation by multiple mechanisms during neuroinvasive L. monocytogenes infection. Peripheral miR-155 promotes brain inflammation through its required role in optimal development of IFN-γ-secreting lymphocytes that enter the brain and activate microglia. Microglial miR-155 promotes M1 polarization, and also inhibits inflammatory responses to stimulation by heat-killed L. monocytogenes, perhaps by targeting Tab2.

Keywords: Listeria; brain inflammation; interferon; meningitis; microRNA; microglia.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Communication / immunology
  • Encephalitis / genetics
  • Encephalitis / immunology
  • Female
  • Gene Expression / genetics
  • Gene Expression / immunology
  • Interferon-gamma / genetics
  • Interferon-gamma / immunology*
  • Listeria monocytogenes / immunology*
  • Listeriosis / genetics
  • Listeriosis / immunology
  • Listeriosis / microbiology
  • Macrophages / immunology
  • Mice
  • Mice, Inbred C57BL
  • MicroRNAs / genetics
  • MicroRNAs / immunology*
  • Microglia / immunology*
  • Microglia / microbiology
  • Monocytes / immunology
  • RNA, Messenger / genetics
  • RNA, Messenger / immunology
  • T-Lymphocytes / immunology
  • Up-Regulation / genetics
  • Up-Regulation / immunology*

Substances

  • MicroRNAs
  • Mirn155 microRNA, mouse
  • RNA, Messenger
  • Interferon-gamma