Inhibition of Semaphorin 3A Signaling Promotes Regeneration of Hematopoietic Stem Cells and Their Bone Marrow Vascular Niche

Radiation injury damages both the bone marrow hematopoietic stem cell and its supportive endothelial niche. In response to radiation stress, bone marrow endothelial cells (BM ECs) upregulate expression of a gene, Sema3A, which encodes for the secreted protein, Semaphorin 3A (SEMA3A), and its receptor, Neuropilin 1 (Nrp1). Commensurate with this, we observed a substantial increase in SEMA3A protein levels in the BM of mice and in NRP1 expression on VE-cadherin+ BM ECs following 500 cGy total body irradiation (TBI) (p<0.001, p<0.0001). We also found that treatment of irradiated primary murine BM ECs with SEMA3A significantly increased BM EC apoptosis (p<0.01). Based on these results, we hypothesized that SEMA3A may be a negative regulator of BM vascular recovery following injury and that inhibition of SEMA3A/NRP1 signaling might accelerate BM vascular niche recovery following irradiation. In keeping with this hypothesis, we found that a single systemic administration of 2 µg SEMA3A (IV) to C57BL/6 mice following 500 cGy TBI significantly increased BM EC apoptosis in vivo at 24 hours and also markedly decreased BM vascular integrity at 24 hours as measured by Evans Blue Dye extravasation in the BM (p<0.01, p<0.01). Interestingly, SEMA3A administration (IV, every other day) also significantly suppressed BM hematopoietic regeneration with decreased BM cell counts and BM ckit⁺sca-1⁺lin^- (KSL) progenitor cell recovery at day +10 compared to irradiated, control mice (p=0.04, p=0.001).

In contrast, systemic administration of anti-NRP1 antibody (10 µg, IV every other day), which blocks SEMA3A binding to NRP1 on BM ECs, accelerated regeneration of the sinusoidal BM vasculature at day +10 following 500 cGy TBI compared to irradiated controls. Concordant with these findings, anti-NRP1 treatment increased the recovery of peripheral blood WBCs, neutrophils, and BM KSL cells, compared to irradiated controls (p=0.002, p=0.04, p=0.04). Competitive repopulation assays confirmed that anti-NRP1 treatment increased the recovery of long-term HSCs capable of 20-week multilineage repopulation in congenic mice (p=0.03 for % donor CD45.1⁺ cell engraftment at 20 weeks). Importantly, anti-NRP1 treatment also markedly improved 60-day survival of irradiated mice from 17% (2/12 controls) to 83% (10/12 anti-NRP1) following 800 cGy TBI.

In order to confirm the role of SEMA3A/NRP1 signaling in BM ECs in regulating the BM vascular and hematopoietic response to injury, we generated mice with tamoxifen-inducible, EC - specific deletion of Nrp1 (VE-Cad-Cre-ERT2;Nrp1 ^fl/fl mice). Adult VE-Cad-Cre-ERT2;Nrp1 ^fl/fl mice were viable and had no baseline hematologic phenotype. However, following 500 cGy TBI, VE-Cad-Cre-ERT2;Nrp1 ^fl/fl mice displayed accelerated BM vascular regeneration at day +10 compared to irradiated control mice. Furthermore, VE-Cad-Cre-ERT2;Nrp1 ^fl/fl mice demonstrated significantly increased peripheral blood WBCs, neutrophils, and BM KSL cells at day +10 following radiation injury (p<0.001, p=0.002, p=0.002). These data suggest that the SEMA3A-NRP1 pathway is an autocrine signaling mechanism that negatively regulates BM vascular niche recovery following myelosuppressive irradiation. Targeted inhibition of SEMA3A-NRP1 signaling in BM ECs has therapeutic potential to accelerate both BM vascular niche regeneration and hematopoietic reconstitution following myelosuppression.