Endogenous tPA is required for PαS-MSC expansion following 5-FU–induced myelosuppression. (A) The absolute numbers of PαS-MSCs per femur from WT B6, tPA^−/−, and PAI-1^−/− mice 2 days after injection of a single dose of 5-FU was determined by FACS analysis (n = 5/group). (B) The absolute numbers of PαS-MSCs per femur from tPA^−/− mice treated with vehicle or 5-FU with or without tPA was determined by FACS analysis (n = 4/group). (C) Cell cycle analysis of PαS-MSCs isolated from WT B6 and tPA^−/− mice treated with vehicle or 5-FU was determined by FACS analysis (n = 4/group) with (left) 1 representative FACS plot and (right) the percentage of PαS-MSCs in S/G2M phase. Mean ± SEM. *P < .05; **P < .01; ***P < .001 by Student t test or ANOVA with Tukey HSD tests for multigroup analysis. Data are representative of 2 independent experiments. (D) Proposed model describing tPA as a key regulator of a crosstalk between ECs and PαS-MSCs. By activating various proteases (eg, plasmin and MMPs), tPA creates a proteolytic niche that enhances the release of KitL from PαS-MSCs. In turn, KitL promotes the release of PDGF-BB and FGF2 from c-Kit⁺ ECs. In synergy FGF2 and PDGF-BB augment PDGFRα expression on PαS-MSCs. Therefore, tPA expands PαS-MSCs within the BM niche by amplifying PDGFR signaling in PαS-MSCs.