Mesenchymal stem cell-laden anti-inflammatory hydrogel enhances diabetic wound healing

Diabetes has become a global public health issue and burdens our health care system1. Diabetic ulcers are a severe, persistent complication of diabetes and, in the most extreme cases, can lead to amputation. The complications of these poor-healing wounds include sustained chronic inflammation, decreased secretion of growth factors and disrupted vascularization2,3,4. Current clinical treatments such as wound dressings, hydrogels or scaffolds used individually have not achieved the desired results; therefore, more effective therapeutic approaches are urgently needed. Bone marrow mesenchymal stem cells (BMSCs) are reported to regulate wound healing through a series of paracrine growth factors (e.g., TGF-β, FGF)5, and differentiate into effector cells involved in wound healing, such as keratinocytes, fibroblasts, and endothelial cells6, thereby accelerating wound closure7 and enhancing vascularization8, granulation tissue formation9 and re-epithelialization10. Considering the aforementioned mechanism, the active role of BMSCs in wound healing establishes the foundation for their use in treating diabetic ulcers. The differentiation and secretion of growth factors by stem cells are regulated by a microenvironment known as the stem cell niche11,12. Hydrogels are an ideal physicochemical mimetic of natural extracellular matrix (ECM); currently, many researchers are focused on BMSC-laden hydrogels to treat skin wounds. For example, Rustad et al. reported that collagen-pullulan hydrogels provide a suitable microenvironment for the delivery of MSCs, which accelerates normal wound healing and promotes neovascularization8. Xu et al. reported that MSC-laden gelatin/PEG hydrogels also accelerate wound closure and re-epithelialization and enhance epidermal maturity, neovascularization, and granulation tissue formation10. Although these BMSC-laden hydrogels have achieved success in the wound healing of healthy skin7,13,14, they may not be suitable for treating diabetic ulcers. High concentrations of inflammatory cytokines are present in the inflammatory microenvironment of the ulcer, which leads to increased protease secretion, thereby resulting in the degradation or loss of growth factors secreted by the BMSCs or other effector cells15,16; moreover, this chronic inflammatory microenvironment also can impair the activity of BMSCs. We believe that if BMSC delivery strategies are to reach their full potential, the ideal hydrogel for diabetic ulcers, in addition to having good biocompatibility, should suppress the inflammatory response or inhibit protease activity. To this end, we developed a biodegradable, multifunctional crosslinker and an n-isopropylacrylamide (NIPAM)-based, thermosensitive hydrogel to carry BMSCs to treat diabetic ulcers. The crosslinker contains an RGD-like motif that promotes cell attachment and differentiation of BMSCs17. We hypothesize that the hydrogel can prohibit chronic inflammation to provide a suitable environment for BMSC function and that BMSCs laden within this hydrogel can promote wound healing. In this study, we evaluated the therapeutic effects of topical administration of BMSCs laden with this thermosensitive hydrogel on chronic inflammation, wound contraction, ECM secretion, angiogenesis, re-epithelialization, hair follicle and sebaceous gland regeneration and scar formation in a diabetic ulcer model (Fig. 1). Figure 1 The primary hypothesis of this study.