Interleukin-1 causes CNS inflammatory cytokine expression via endothelia-microglia bi-cellular signaling

Brain Behav Immun. 2019 Oct:81:292-304. doi: 10.1016/j.bbi.2019.06.026. Epub 2019 Jun 19.

Abstract

As a major producer of the inflammatory cytokine interleukin-1 (IL-1), peripheral macrophages can augment IL-1 expression via type 1 IL-1 receptor (IL-1R1) mediated autocrine self-amplification. In the CNS, microglial cells are the major producers of inflammatory cytokines, but express negligible levels of IL-1R1. In the present study, we showed CNS IL-1 induced microglial proinflammatory cytokine expression was mediated by endothelial, not microglial, IL-1R1. This paracrine mechanism was further dissected in vitro. IL-1 was unable to stimulate inflammatory cytokine expression directly from the microglial cell line BV-2, but it stimulated the brain endothelial cell line bEnd.3 to produce a factor(s) in the culture supernatant, which was capable of inducing inflammatory cytokine expression in BV-2. We termed this factor IL-1-induced microglial activation factors (IMAF). BV-2 cytokine expression was inducible by extracellular ATP, but IL-1 did not stimulate the release of ATP from bEnd.3 cells. Filtration of IMAF by size-exclusion membranes showed IMAF activity resided in molecules larger than 50 kd and incubation of IMAF at 95 °C for 5 min did not alter its activity. Microglial inhibitor minocycline was unable to block IMAF activity, even though it blocked LPS induced cytokine expression in BV-2 cells. Adding NF-κB inhibitor to the bEnd.3 cells abolished IL-1 induced cytokine expression in this bi-cellular system, but adding NF-κB inhibitor after IMAF is already produced failed to abrogate IMAF induced cytokine expression in BV-2 cells. RNA sequencing of IL-1 stimulated endothelial cells revealed increased expression of genes involved in the production and processing of hyaluronic acid (HA), suggesting HA as a candidate of IMAF. Inhibition of hyaluronidase by ascorbyl palmitate (AP) abolished IMAF-induced cytokine expression in BV-2 cells. AP administration in vivo also inhibited ICV IL-1-induced IL-1 expression in the hippocampus and hypothalamus. In vitro, either TLR2 or TLR4 inhibitors blocked IMAF induced BV-2 cytokine expression. In vivo, however, IL-1 induced cytokine expression persisted in either TLR2 or TLR4 knockouts. These results demonstrate IL-1 induced inflammatory cytokine expression in the CNS requires a bi-cellular system and HA could be a candidate for IMAF.

Keywords: Hyaluronic acid; Minocycline; TLR4.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Cell Line
  • Central Nervous System / immunology
  • Central Nervous System / metabolism
  • Cytokines / immunology
  • Cytokines / metabolism*
  • Endothelial Cells / metabolism
  • Gene Expression Regulation / genetics
  • Hyaluronic Acid / metabolism
  • I-kappa B Proteins / metabolism
  • Inflammation / metabolism*
  • Interleukin-1 / metabolism*
  • Interleukin-1 / physiology
  • Lipopolysaccharides / pharmacology
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Microglia / metabolism
  • NF-kappa B / metabolism
  • Signal Transduction / drug effects
  • Tumor Necrosis Factor-alpha / metabolism

Substances

  • Cytokines
  • I-kappa B Proteins
  • Interleukin-1
  • Lipopolysaccharides
  • NF-kappa B
  • Tumor Necrosis Factor-alpha
  • Hyaluronic Acid