The Differential Role of Stromal Derived Factor-1 (SDF-1) and Monocyte Chemoattractant Protein-1 (MCP-1) in the Recruitment of Angiogenic Cells to Ischemic Tissue.

There is compelling evidence that circulating angiogenic cells exist in humans that are able to home to sites of vascular injury and stimulate angiogenesis. This observation has led to tremendous interest in the translational potential of these cells to mediate therapeutic angiogenesis. However, the number of angiogenic cells in the blood at baseline is low, limiting their delivery to sites of ischemia. To circumvent this limitation angiogenic cells can be mobilized into the blood. Using a murine model of acute hindlimb ischemia, we previously showed that treatment with G-CSF and/or AMD3100 potently enhanced revascularization. Adoptive transfer studies showed that mobilized monocytes mediate this response. Finally, the monocytes, once recruited to the ischemic tissue, stimulate angiogenesis in a paracrine fashion. The signals that recruit monocytes to sites of ischemia remain poorly defined. A better understanding of these signals may provide important insights into strategies to enhance or inhibit angiogenic cell recruitment. Herein, we summarize our recent findings suggesting that MCP-1 and SDF-1 play key but distinct roles in this process.

SDF-1, through its major receptor CXCR4, is thought to play an important role in angiogenic cell recruitment and/or retention at sites of ischemia. To test this hypothesis, we studied the angiogenic capacity of CXCR4^−/− cells. Since CXCR4 deficiency is embryonic lethal, we first generated bone marrow chimeras by transplanting CXCR4^−/− fetal liver cells into irradiated syngeneic mice. Hematopoietic reconstitution with CXCR4^−/− cells was confirmed by flow cytometry. Endogenous revascularization following the induction of hindlimb ischemia in these CXCR4^−/− chimeric mice was comparable to wild type chimeric mice. To further assess the angiogenic capacity of CXCR4^−/− cells, adoptive transfer experiments into wild type recipient mice were performed. As reported previously, adoptive transfer of wild type bone marrow cells markedly improved blood flow following induction of hindlimb ischemia [ratio of ischemic to nonischemic limb perfusion on day 14 after surgery: 0.54 ± 0.04 (saline control) vs. 0.81 ± 0.06 (WT bone marrow); p<0.001]. Surprisingly, adoptive transfer of CXCR4^−/− bone marrow cells resulted in a comparable revascularization (0.81 ± 0.16; p<0.001 compared with saline treated controls). Since there is data that T lymphocytes may modulate the angiogenic response to ischemia, we repeated these experiments using RAG-1 deficient recipient mice, which lack B and T lymphocytes. Adoptive transfer of CXCR4^−/− bone marrow cells into RAG-1^−/− mice failed to stimulate angiogenesis [ratio of ischemic to nonischemic limb perfusion on day 14 after surgery: 0.61 ± 0.04 (saline control) vs. 0.68 ± 0.07(CXCR4^−/− bone marrow) vs. 0.82 ± 0.07 (WT bone marrow)]. Collectively, these data show that the contribution of CXCR4 signals to angiogenesis in this vascular injury model is dependant upon the presence of inflammatory cells at the site of injury.

MCP-1 is a major chemokine regulating monocyte chemotaxis and activation. Thus, we repeated the adoptive transfer experiments using MCP-1 deficient recipient mice. Adoptive transfer of wild type bone marrow cells into these mice had no effect on revascularization. Moreover, treatment of MCP-1^−/− mice with G-CSF and AMD3100, though inducing the expected level of monocyte mobilization, failed to stimulate angiogenesis. These data highlight the importance of MCP-1 to the recruitment, retention, and/or activation of angiogenic cells at sites of ischemia.