Localized CCR2 Activation in the Bone Marrow Niche Mobilizes Monocytes by Desensitizing CXCR4

PLoS One. 2015 Jun 1;10(6):e0128387. doi: 10.1371/journal.pone.0128387. eCollection 2015.

Abstract

Inflammatory (classical) monocytes residing in the bone marrow must enter the bloodstream in order to combat microbe infection. These monocytes express high levels of CCR2, a chemokine receptor whose activation is required for them to exit the bone marrow. How CCR2 is locally activated in the bone marrow and how their activation promotes monocyte egress is not understood. Here, we have used double transgenic lines that can visualize CCR2 activation in vivo and show that its chemokine ligand CCL2 is acutely released by stromal cells in the bone marrow, which make direct contact with CCR2-expressing monocytes. These monocytes also express CXCR4, whose activation immobilizes cells in the bone marrow, and are in contact with stromal cells expressing CXCL12, the CXCR4 ligand. During the inflammatory response, CCL2 is released and activates the CCR2 on neighboring monocytes. We demonstrate that acutely isolated bone marrow cells co-express CCR2 and CXCR4, and CCR2 activation desensitizes CXCR4. Inhibiting CXCR4 by a specific receptor antagonist in mice causes CCR2-expressing cells to exit the bone marrow in absence of inflammatory insults. Taken together, these results suggest a novel mechanism whereby the local activation of CCR2 on monocytes in the bone marrow attenuates an anchoring signalling provided by CXCR4 expressed by the same cell and mobilizes the bone marrow monocyte to the blood stream. Our results also provide a generalizable model that cross-desensitization of chemokine receptors fine-tunes cell mobility by integrating multiple chemokine signals.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Bone Marrow Cells / cytology*
  • Mice
  • Mice, Transgenic
  • Monocytes / cytology*
  • Receptors, CCR2 / metabolism*
  • Receptors, CXCR4 / metabolism*
  • Signal Transduction

Substances

  • CXCR4 protein, mouse
  • Ccr2 protein, mouse
  • Receptors, CCR2
  • Receptors, CXCR4

Grants and funding

This work was supported by Basic Science Research Program (2013R1A1A1009625) and Bio and Medical Technology Development Program (2014M3A9B4043638), funded through the National Research Foundation of Korea (NRF). It was also supported by the Ministry of Education, Science and Technology (MSTP) of the Korean government, and a faculty research grant from the Yonsei University College of Medicine (6-2013-0064-2-1). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.