Tissue Type Plasminogen Activator Regulates Myeloid-Cell Dependent Neoangiogenesis During Tissue Regeneration.

Abstract 3052

Poster Board II-1028

The fibrinolytic system comprises an inactive proenzyme, plasminogen (Plg), which is converted by activators such as tPA to the active enzyme, plasmin. Treatment with tissue type plasminogen activator (tPA) can dissolve blood clots and can ameliorate the clinical outcome in ischemic diseases (e.g. such as pulmonary embolism, myocardial infarction and stroke). Better survival in tPA-treated vs. placebo patients have been attributed to its effect on thrombus lysis. But the underlying mechanism how tPA improves ischemic tissue regeneration is not well understood. Bone marrow (BM)-derived hematopoietic cells have been shown to promote neoangiogenesis during tissue regeneration or cancer growth. Here we report that a serpin-resistant form of tPA by activating the extracellular proteases matrix metalloproteinase-9 and plasmin mobilizes CD45⁺/CD11b⁺ pro-angiogenic, myeloid cells, a process dependent on vascular endothelial growth factor-A (VEGF-A) and Kit ligand signalling. To study the role of tPA for cell-driven tissue regeneration, we choose a model of hindlimb ischemia. tPA improves cell incorporation of CD11b⁺ cells into ischemic tissues, and increases expression of neoangiogenesis-related genes including VEGF-A. Remarkably, transplantation of BM-derived tPA-mobilized CD11b⁺ cells and VEGFR-1⁺ cells, but not the same number of carrier-mobilized CD11b⁺ cells or CD11b⁻ cells, accelerates neovascularization and ischemic tissue regeneration, showing that tPA administration had qualitatively changed CD11b⁺ cells and made them more angiogenic. Inhibition of VEGF-signalling suppresses tPA-induced neovascularization in a model of hindlimb ischemia. Thus, tPA mobilizes CD11b⁺ cells from the BM, increases systemic and local (cellular) VEGF-A, which can promote angiogenesis locally during ischemic recovery. We provide clinically relevant evidence that administration of the single agent tPA promotes ischemic tissue regeneration by recruiting pro-angiogenic CD11b+ myeloid cells, which qualitatively are more angiogenic then their PBS stimulated counterparts, and which incorporate with high efficiency into peripheral ischemic tissues. These data introduce a new paradigm in cell biology whereby fibrinolytic enzymes mediate systemic and localized effects on hematopoietic cells thereby modulating their tissue integration potential via integrin modulation and cytokine release and thus these data establish a novel role for tPA in the growing field of regenerative medicine.