Objective: Vasculogenesis relies on the recruitment of bone marrow-derived endothelial progenitor cells (BMD EPCs) and is stimulated by tissue-level ischemia. We hypothesized that the BMD EPC response is impaired in ischemic wounds and studied the relationship between BMD EPCs and wound healing. Methods: We used transgenic Tie-2/LacZ mice, which carry the β-galactosidase (β-gal) reporter gene under Tie-2 promoter control. Wild-type mice were lethally irradiated and reconstituted with Tie-2/LacZ bone marrow. Four weeks later, the mice underwent unilateral femoral artery ligation/excision and bilateral wounding of the hindlimbs. Ischemia was confirmed and monitored with laser Doppler imaging. A subset of mice received incisional vs excisional nonischemic bilateral hindlimb wounds, without femoral ligation. Excisional wound closure was measured by using daily digital imaging and software-assisted calculation of surface area. Results: Ischemia resulted in significantly delayed wound healing and differentially affected the number of BMD EPCs recruited to wound granulation tissue and muscle underlying the wounds. At 3 days postwounding, the granulation tissue of the wound base contained significantly fewer numbers of BMD EPCs in ischemic wounds compared with the nonischemic wounds (P < .05). In contrast, significantly more BMD EPCs were present in the muscle underlying the ischemic wounds at this same time point compared with the muscle under the nonischemic wounds (P < .05). In ischemic wounds, eventual wound closure significantly correlated with a delayed rise in BMD EPCs within the wound granulation tissue (Kendall's correlation, -.811, P = .0005) and was significantly associated with a gradual recovery of hindlimb perfusion (P < .0001). By 7 days postwounding, BMD EPCs were incorporated into the neovessels in the granulation tissue. At 14 days and 75 days, BMD EPCs were rarely observed within the wounds. Conclusions: Granulation tissue of excisional ischemic wounds showed significantly less BMD EPCs 3 days postwounding, in association with significantly delayed wound closure. However, the number of BMD EPCs were increased in ischemic hindlimb skeletal muscle, consistent with the notion that ischemia is a powerful signal for vasculogenesis. To our knowledge, this is the first report identifying a deficit in BMD EPCs in the granulation tissue of ischemic skin wounds and reporting the key role for these cells in both ischemic and nonischemic wound healing. Clinical Relevance: The reported novel findings offer new insight into the pathophysiology of nonhealing ischemic skin ulcers and point to a potential new target for therapeutic intervention. Further work in this area is needed to better understand the local biology and cytokine milieu of the local environment in the ischemic wound. Ultimately, approaches such as repopulating the local wound pool of bone-marrow derived endothelial progenitor cells (BMD EPCs) in ischemic wounds through direct injection or increasing stimulatory cytokines within the wound's granulation tissue that may increase recruitment of BMD EPCs (or decreasing cytokines inhibitory of this process) may have potential therapeutic benefit. Currently the only way to salvage an ischemic limb with tissue loss is to restore blood flow with bypass surgery or, at times, balloon angioplasty and stenting. These are invasive procedures that often fail to salvage the limb because of poor skin-level tissue perfusion. Thus far, few clinical trials have looked at BMD EPCs as a therapeutic option. A better understanding of the biology of BMD EPCs and their role in ischemic wound healing is paramount before a strong rationale can be built for successful therapeutic interventions.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine