Coordinate regulation of neutrophil homeostasis by liver X receptors in mice

J Clin Invest. 2012 Jan;122(1):337-47. doi: 10.1172/JCI58393. Epub 2011 Dec 12.

Abstract

The most abundant immune cell type is the neutrophil, a key first responder after pathogen invasion. Neutrophil numbers in the periphery are tightly regulated to prevent opportunistic infections and aberrant inflammation. In healthy individuals, more than 1 × 10⁹ neutrophils per kilogram body weight are released from the bone marrow every 24 hours. To maintain homeostatic levels, an equivalent number of senescent cells must be cleared from circulation. Recent studies indicate that clearance of senescent neutrophils by resident tissue macrophages and DCs helps to set homeostatic levels of neutrophils via effects on the IL-23/IL-17/G-CSF cytokine axis, which stimulates neutrophil production in the bone marrow. However, the molecular events in phagocytes underlying this feedback loop have remained indeterminate. Liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate both lipid metabolic and inflammatory gene expression. Here, we demonstrate that LXRs contribute to the control of neutrophil homeostasis. Using gain- and loss-of-function models, we found that LXR signaling regulated the efficient clearance of senescent neutrophils by peripheral tissue APCs in a Mer-dependent manner. Furthermore, activation of LXR by engulfed neutrophils directly repressed the IL-23/IL-17/G-CSF granulopoietic cytokine cascade. These results provide mechanistic insight into the molecular events orchestrating neutrophil homeostasis and advance our understanding of LXRs as integrators of phagocyte function, lipid metabolism, and cytokine gene expression.

Publication types

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

MeSH terms

  • Animals
  • Apoptosis
  • Cellular Senescence
  • Cytokines / genetics
  • Feedback, Physiological
  • Gene Expression
  • Homeostasis
  • Interleukin-17 / genetics
  • Interleukin-17 / metabolism
  • Interleukin-23 Subunit p19 / genetics
  • Interleukin-23 Subunit p19 / metabolism
  • Lipid Metabolism
  • Liver / cytology
  • Liver / immunology
  • Liver / metabolism
  • Liver X Receptors
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Mice, Transgenic
  • Neutrophils / immunology*
  • Neutrophils / metabolism*
  • Orphan Nuclear Receptors / deficiency
  • Orphan Nuclear Receptors / genetics
  • Orphan Nuclear Receptors / metabolism*
  • Phagocytosis
  • Signal Transduction
  • Spleen / cytology
  • Spleen / immunology
  • Spleen / metabolism

Substances

  • Cytokines
  • Il17a protein, mouse
  • Il23a protein, mouse
  • Interleukin-17
  • Interleukin-23 Subunit p19
  • Liver X Receptors
  • Orphan Nuclear Receptors