Hematopoietic stem cells (HSCs) and progenitors (HPCs) are bone-marrow derived cells that give rise to terminally differentiated circulating blood cells. Recent work has implicated these cells in the repair of parenchymal tissue in the setting of inflammation, but the signals that regulate this trafficking are poorly understood. In other NIH funded worked we discovered and cloned CCR2, the chemokine receptor that regulates monocyte migration to MCP-1, and showed that CCR2-/- mice are protected in murine models of atherosclerosis. In preliminary, unpublished results we have now found that CCR2 is expressed on subsets of primitive HSCs as well as some myeloid HPCs. CCR2 mediates the chemotaxis of c-Kit+Lin- bone marrow derived cells to MCP-1 and MCP-3. Following instillation of thioglycollate WT, but not CCR2-/- stem cells were actively recruited to the peritoneum, and to the liver following administration of acetaminophen. Significantly, infusion of CCR2+/+, but not CCR2-/- HSCs/HPCs accelerated the resolution of liver damage, and the recruited cells expressed genes characteristic of the M2 macrophage phenotype. Building on these recent findings, we propose three interrelated specific aims to define the role of CCR2 in HSC/HPC trafficking, and their potential roles in the resolution of inflammation and injury.
In Specific Aim 1 we will quantify expression of CCR2 on early hematopoietic stem cells. We will test the hypothesis that CCR2 is expressed on true, self-replicating stem cells, and mediates their chemotaxis to sites of tissue inflammation and injury. Using novel CCR2/RFP knock-in mice, we will quantify the expression of CCR2 on HSCs/HPCs in all branches of the hematopoietic tree in mice and determine whether activation of CCR2 mobilizes HSCs and HPCs from bone marrow.
In Specific Aim 2 we will determine whether CCR2-mediated recruitment of HSCs or HPCs contributes to the resolution of inflammation and ischemic injury in models of acute (acetaminophen)- and chronic (carbon tetrachloride)-induced hepatotoxicity, will determine the fate of the recruited stem and progenitor cells, and will test the hypothesis that enriching HSCs/HPCs for those that express CCR2 will significanlty enhances tissue repair.
In Specific Aim 3 we will turn our attention to a model of experimental myocardial infraction in mice, and will determine if CCR2+ or CCR2- HSCs/HPCs accelerate recovery of cardiac function, and whether CCR2 antagonists reduce inflammation and enhance functional recovery. Completion of the work described above will identify the signals regulating the homing of bone marrow stem and progenitor cells to injured tissue, and further our mechanistic understanding of their role in tissue repair and regeneration.
Work proposed in this application will help to identify a new and mechanisitic marker for hematopoietic (ie., bone marrow derived) stem cells. Transfusion of such cells has the potential to enhance treatment of important human diseases, such as liver failure and acute myocardial infarction. The preliminary results described in this application identify CCR2, a well-characterized chemokine receptor, as being critical for the mobilization of stem cells to sites of inflammation and injury. Completion of the Specific Aims of this application will greatly accelerate efforts to optimize the therapeutic use of bone marrow stem cells.
