Novel Functions for a Fibrinolytic Pathway in Controlling Hematopoiesis.
Authors: Hattori Koichi , Akiyama Haruyo , Lund Leif R. , Werb Zena , Heissig Beate 
Published: November 2019

Abstract
The adult mammalian hematopoietic system maintains a well-regulated supply of mature and immature hematopoietic cells within the circulation throughout life. The system is capable of rapid recovery and compensation following injury, or physiological stress. There is still an incomplete understanding of which factors regulate such a stress response. We show that plasminogen (Plg), a classical fibrinolytic factor, is a key regulator controlling the hematopoietic stress response.The fibrinolytic factors Plg and tPA are found within the BM after myelosuppression, where they could activate another protease cascade, namely matrix metalloproteinases (MMPs). Activation of MMP-9 releases e.g. Kit ligand (KitL), which in turn control hematopoietic cell proliferation and differentiation. This process helps to replenish the hematopoietic cells within the BM by promoting hematopoietic stem and progenitor cell proliferation and differentiation. We were able to demonstrate that deletion of Plg in mice prevented hematopoietic stem cells from entering the cell cycle and undergoing multi-lineage differentiation after myelosuppression, leading to the death of the mice. Activation of Plg by tissue type plasminogen activator (tPA) activated MMPs and stimulated the release of KitL from stromal cells.The tPA-mediated release of KitL from stromal cells took place via MMP activation as addition of a MMP inhibitor inhibited KitL release from stromal cells.tPA administration in vivo augmented the number of BM cells in wild-type controls, but this was attenuated in both Plg deficient and MMP-9 deficient mice. tPA increased the number of immature spleen colony forming units (colony forming units-spleen), and long term culture-initiating cells and the c-Kit+/ Sca-1+/lin− (KSL) cell fraction in BM cells of treated animals. To test if tPA-mediated hematopoietic cell proliferation is dependent on KitL in vivo, we treated KitL deficient Sl/Sld mice and WBB6F1+/+ control mice with tPA. BM cellularity and the number of immature CFU-S progenitor cells increased in WBB6F1+/+ animals by day 2 after tPA injections, but not in Sl/Sld (KitL deficient) mice. To elucidate whether the observed tPA effects on hematopoietic cells were mediated directly by tPA or indirectly, e.g., via BM stromal cells, we examined the effect of tPA on hematopoietic cell expansion in vitro. tPA improved proliferation of lin− cells only in stromal-cell based cultures (MS-5 feeder layer), but not in the absence of a feeder layer. In synergy, tPA and KitL expanded the number of progenitors in stromal-cell based cultures. If c-Kit/KitL signaling following tPA treatment promotes hematopoietic cell proliferation, blocking the signaling should prevent tPA-induced progenitor proliferation. Indeed, addition of neutralizing antibodies against c-Kit and/or KitL prevented the tPA-mediated generation of CFU-Cs from lin− cells in stromal cell-based cultures. Our data indicate that fibrinolytic factors can be found in the regenerative phase after myelosuppression in the BM. Here, fibrinolytic co-assemble and are activated result in an orderly, protease cascade, like MMPs. This proteolytic growth factors bioavailable, which in a fine-tuned hematopoietic cell proliferation and differentiation. The involvement of the fibrinolytic system in the regulation of adult stress hematopoiesis represents a new paradigm with important implications for cancer therapy and regenerative medicine.