CCR2 Mediates Hematopoietic Stem/Progenitor Cell Trafficking to Peripheral Tissues after Inflammation

Hematopoietic stem cells (HSCs) are bone-marrow derived, self-renewing pluripotent cells that give rise to terminally differentiated circulating blood cells. HSCs have been implicated in parenchymal tissue repair in the setting of inflammation. In response to the antagonist of the chemokine receptor CXCR4, HSCs and their progenitors migrate from bone marrow to the blood. However, little is known about the signals that mediate their trafficking from the blood into peripheral tissues. Recently, we showed that mice genetically deficient in chemokine receptor CCR2 (CCR2−/− mice) have a marked decrease in the number of circulating “inflammatory” (7/4+, Ly6c+) monocytes, but no decrease in myeloid progenitor cells in the bone marrow (Tsou et al, J Clin Invest, 2007, 902). These data indicated that although CCR2 is not necessary for HSCs to differentiate into mature monocytes, it does play a role in monocyte egress from bone marrow to blood. In the current study, we extend this work and investigate the expression of CCR2 on HSCs, and tested the hypothesis that CCR2 is critical for the recruitment of circulating HSCs to sites of inflammation. We found that CCR2 was expressed on subsets of primitive HSCs and myeloid progenitors and mediated HSC movement in response to inflammation. Using traditional transwell chambers, we found that c-Kit+Lin− cells derived from bone marrow underwent chemotaxis in response to the CCR2 ligands MCP-1 (CCL2) and MCP- 3 (CCL7). To determine whether CCR2 mediates HSC movement in vivo, we treated wildtype mice with thioglycollate to induce aseptic inflammation. HSCs were actively recruited to the peritoneum, as shown by fluorescence-activated cell sorting and functional colony formation assays. In contrast, this response was profoundly impaired in CCR2−/− mice. To determine whether the clonogenic cells recruited to peritoneum were true HSCs, we performed competitive transplantation assays. Thioglycollate was instilled into wildtype CD45.2+ mice, and peritoneal Lin− cells were collected, purified, and infused, together with CD45.1+ bone marrow cells, into lethally irradiated CD45.1+ mice. Four months later, up to 12% of the leukocytes in the peripheral blood of these primary recipient mice were CD45.2+. At the time of sacrifice, bone marrow cells were collected from these mice and injected into lethally irradiated secondary CD45.1+ recipient mice. Two months following the transplantation, up to 9% of the blood leukocytes in these secondary recipient mice were CD45.2+, confirming that long-term repopulating HSCs were recruited to the inflamed peritoneum of the donor mice. These findings suggest a novel role for CCR2 in the recruitment of long-term repopulating HSCs to sites of inflammation and injury. We are currently investigating whether recruited HSCs and their progenitors hasten the resolution of the inflammatory response or promote the repair of injured tissue.