Improving Healing Outcomes in Diabetic Wounds by Understanding and Modulating Immune Responses

Abstract

Diabetic Foot Ulcers (DFUs) are chronic wounds that show delayed or absence of healing in individuals with diabetes. Currently, diabetic wounds are managed clinically through regular debridement, pressure off-loading, glycemic control, and patient education. Other treatment strategies used in the clinic include topical antibiotics to prevent infections, growth factor ointments to accelerate tissue regrowth, and negative pressure therapy for drainage of wound fluid. However, such strategies have had limited success in a significant number of individuals with DFU, and the exact reasons remain unclear.

Towards identifying factors associated with poor healing, we designed a single-center cohort study involving 52 DFU patients to evaluate the role of clinical, biochemical, and immunological parameters in foot ulcers. Data from these individuals analyzed using multivariable logistic regression analysis revealed that a combination of low-density lipoprotein (LDL) cholesterol and expression level of CD63 protein on monocytes as strong predictors of non-healing in individuals with early-stage DFUs. Both these parameters are associated with obesity and chronic inflammation, which could be drivers of poor healing.

In parallel, in a mouse model of diabetes (Leptin receptor knockout mouse) used to test therapeutic strategies, we characterized the phenotype and function of myeloid cells driving chronic inflammation in diabetes that remains to be unknown. These diabetic mice showed extensive changes in the numbers and phenotype of monocytes and neutrophils in circulation, suggestive of underlying inflammation. As such alterations in immune cells can drive aberrant wound healing activity, we hypothesized that targeting myeloid cells for immunomodulation at wound site might accelerate wound closure.

To achieve immunomodulation of myeloid cells in chronic wounds, we developed an immuno-modulatory drug-eluting bandage that could be applied on wounds for improving healing outcomes. A chitosan-based hydrogel scaffold system containing rapamycin or tetracycline was used as a bandage and evaluated for accelerated healing using the leptin receptor knockout mouse model. In vivo studies revealed that treatment with low dose rapamycin loaded scaffold significantly accelerated wound healing compared to tetracycline and blank scaffold in diabetic mouse suggesting that rapamycin can effectively modulate immune cells at wound environment. The developed novel strategy show promise for clinical translation upon further validation in higher animal models.

In summary, this thesis explores the role of innate immune cells in DFU pathology and describes a new treatment approach for healing diabetic wounds. We specifically show that neutrophils and monocytes are phenotypically altered in individuals with early-stage diabetic foot ulcers, identify parameters that clinicians may use to stratify the risk of poor healing, demonstrate that a leptin receptor knockout diabetic mouse model has similar changes in myeloid cells, and develop a possible treatment strategy for healing diabetic wounds.