Changes in SDF-1 regulate MK spatial location and platelet production following 4 Gy TBI. (A) Analysis of MK lineage kinetics (right panel) and Gp1Bβ+ MKs physically associated with MECA32+ vasculature (left panel) 24 hours after treatment with Diprotin A (red) or vehicle (gray) at day 2 post-4 Gy TBI. Diprotin A-mediated stabilization of SDF-1 at day 2 following TBI decreases platelets and MKs in the vascular niche. MKP (colony assay) and MK (imaging flow cytometry) numbers are normalized to per femur values and all compartments are expressed as percent of unirradiated control. Mean absolute numbers for 0 Gy controls: 12 420 MKP/femur, 53 946 MK/femur, 470 × 10³ platelets/μL. (B) Analysis of MK lineage kinetics (right panel) and Gp1Bβ+ MKs physically associated with MECA32+ vasculature (left panel) 24 hours after treatment with Diprotin A (red) or vehicle (gray) at day 3 post-4 Gy TBI. Diprotin A-mediated stabilization of SDF-1 at day 3 increases platelets and MKs in the vascular niche. MKP (colony assay) and MK (imaging flow cytometry) numbers are normalized to per femur values and all compartments expressed as percent of unirradiated control. Mean absolute numbers for 0 Gy controls: 13 272 MKP/femur, 52 414 MK/femur, 491 × 10³ platelets/μL. (C) Model depicting the regulation of MK niche occupancy in the marrow by SDF-1 at steady state (top panel) and days 2 (middle panel) and 3 (bottom panel) after 4 Gy TBI. At a steady state, Diprotin A-mediated stabilization of SDF-1 increases MKs in the vascular niche. At day 2 following TBI, when our studies reveal a gradient of SDF-1 toward the endosteum, stabilization of SDF-1 decreases both MK association with vasculature and the number of circulating platelets. In contrast, at day 3 post-TBI, when the endosteal SDF-1 gradient is lost, stabilization of SDF-1 resembles the steady-state condition with increased MKs in the vascular niche. (D) Analysis of MK lineage kinetics (right panel) and Gp1Bβ+ MKs physically associated with MECA32+ vasculature (left panel) 24 hours after IV SDF-1 administration (blue) or vehicle (gray) at day 2 post-4 Gy TBI. At this time point, elevation of vascular SDF-1 with IV administration counteracts the endogenous endosteal SDF-1 gradient and leads to enhanced MK-vasculature association and increased circulating platelets. Mean absolute numbers for 0 Gy controls: 10 996 MKP/femur, 52 615 MK/femur, 459 × 10³ platelets/μL. Error bars represent standard error of the mean of ≥3 independent experiments (n = 5-9 total mice per group). Statistical analyses comparing treated mice to vehicle controls were performed using a 2-tailed Student’s t test. *P ≤ .04; **P < .006.