Introduction: Joint hypervascularity is characteristic to hemophilic arthropathy (HA), which is a hallmark complication in patients with hemophilia. Angiogenesis in the hemophilic joint is triggered by bleeding and results in excessive vascular remodeling with enlarged irregularly shaped vessels that appear fragile, potentially causing re-bleeding. Therefore, targeting angiogenesis may offer novel treatment avenues in HA. Vascular endothelial growth factor A (VEGF-A) is an important angiogenic mediator implicated in the pathogenesis of HA, but a complete blockade may cause vascular instability with more bleeding, or inhibition of anti-angiogenic isoforms, thereby decreasing therapeutic efficacy. An alternative approach may be to target the co-receptor Neuropilin 1 (Nrp1) that facilitates binding of the pro-angiogenic isoform VEGF165 to its signaling receptor, VEGFR2. Here, we determine whether blocking the binding of VEGF165 to Nrp1 can curb the pathological vascular remodeling associated with hemophilic joint bleeding.

Methods and results: Joint bleeding was induced by subpatellar needle puncture in FVIII-/- mice, resulting in pronounced intra- and peri-articular bleeding, evidenced by a drop in hematocrit from ~45% to 28 ± 10% 2 days after injury. The following antibody treatments were initiated 2 days after injury and were administered at 5 mg/kg, twice weekly for 4 weeks: B20-4 targeting murine VEGF-A, and YW107.4.87 blocking the VEGF165 binding site on Nrp1. The CXCR4 antagonist, AMD3100 (300 µg/day by subcutaneous osmotic pump) was included as positive control as it corrects hypervascularity induced by hemophilic joint bleeding (Wyseure et al . ISTH 2017).

Four weeks after injury blood flow in the joint, measured by ultrasound-Power Doppler (PD), was increased 2.8-fold in injured vs uninjured FVIII-/- mice (p<0.001; n=13). Anti-VEGF treatment resulted in a similar 3.2-fold increase in PD signal (n=8), indicating that total inhibition of VEGF had no effect on the aberrant blood flow. Inhibition of Nrp1 had a partial effect (1.9-fold increase over baseline; p=0.16 vs no treatment; n=7), whereas inhibition of CXCL12-mediated vascular cell recruitment by the CXCR4 antagonist significantly blunted aberrant PD signals (p<0.05 vs no treatment; n=10). On histology, the total vessel count after anti-Nrp1 treatment was similar to no treatment and significantly greater than baseline (86 anti-Nrp1 vs 80 no treatment vs 42 baseline; p<0.01). Similarly, anti-Nrp1 treatment had no effect on the number of abnormally enlarged vessels (with a diameter ≥ 20 µm) compared to no treatment (15 anti-Nrp1 vs 13 no treatment vs 3 baseline; p<0.01 vs baseline). In contrast, with CXCR4 antagonist treatment both the total vessel count (64; p=0.3 vs baseline) and the number of abnormally enlarged vessels (7; p=0.1 vs baseline) remained low. However, anti-Nrp1 treatment did prevent the 2.5-fold increased perivascular expression of smooth muscle cell actin (αSMA; p<0.01) that typically accompanies the abnormally enlarged vessels in injured FVIII-/- mice. Thus, inhibition of Nrp1 lowered αSMA expression after hemophilic bleeding, similar to baseline and CXCR4 antagonist treatment, although it had no effects on vessel number and enlargement. This suggests that anti-Nrp1 treatment prevents the maturation during vascular remodeling in HA, potentially leaving the vessels destabilized and prone to vascular permeability. Towards that end, staining with Prussian Blue demonstrated increased hemosiderin deposits in anti-Nrp1 treated mice, similar to untreated injured FVIII-/- mice, whereas no signs of macroscopic joint bleeding or hemosiderin deposits were observed after treatment with the CXCR4 antagonist.

Conclusions: Abrogating the VEGF165-Nrp1 interaction was not sufficient to prevent neo-angiogenesis and vessel lumen expansion in a murine hemophilic joint injury model. However, inhibition of Nrp1 did reduce perivascular remodeling that may allow regression of the aberrant vasculature over time, but also may promote further vessel instability and additional hemorrhaging. Thus, inhibition of the CXCL12/CXCR4 pathway affects neo-angiogenesis, whereas inhibition of the VEGF165/Nrp1 pathway predominantly affects perivascular maturation, but not neo-angiogenesis. These findings provide novel insights on the potential benefits and pitfalls of anti-angiogenic treatments in HA.


von Drygalski: Hematherix LLC: Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria; Novo Nordisk: Honoraria; CSL Behring: Honoraria; Bioverativ: Honoraria; Bayer: Honoraria; Shire: Honoraria. Mosnier: Hematherix LLC: Membership on an entity's Board of Directors or advisory committees.

Author notes


Asterisk with author names denotes non-ASH members.