The development of a direct co-culture-based model for diabetic foot ulcer mimicking inflammation and impaired phagocytosis.

PURPOSE: Diabetic foot ulcers (DFU) are characterized by delayed healing and high infection rates. DFU affect approximately 25% of individuals with diabetes. Secondary to hyperglycaemia, both chronic inflammation and defective phagocytosis have been identified as contributing factors to the non-healing status of DFU. Both inflammation and defective phagocytosis in DFU were sought to be modelled in vitro using pHRODO bioparticles for the first time. The pHRODO bioparticles, popularly used as phagocytic cargos, are chemically killed microorganisms conjugated to the pH-sensitive pHRODO dye that solely fluoresces within the acidic lysosomes where phagocytosis occurs. METHODS: The in vitro DFU model was developed by identifying which ratio of diabetic fibroblasts to THP-1-derived Mɸ, choice of pHRODO bioparticles, FBS concentration, and oxygen level exhibited both significant inflammation and reduced phagocytic ability. Inflammation was confirmed via simultaneous TNF-α and MCP-1 release by direct co-cultures of diabetic fibroblasts and THP-1-derived macrophages (Mɸ) following pHRODO bioparticle exposure using ELISA. Phagocytic activity, derived from the emitted fluorescence of ingested pHRODO bioparticles within acidic lysosomes, was quantified using an automated, whole-well, fluorescent imaging system. The kinase Bay 11-7085, shown to stimulate phagocytosis previously, was used to verify the usefulness of the developed in vitro DFU model. RESULTS: Inflammation and reduced phagocytic activity were observed maximally for a 1:4 ratio of diabetic dermal fibroblasts to THP-1-derived Mɸ upon 4-h incubation with 200 µg/ml pHRODO green Staphylococcus aureus bioparticles under hypoxia (2% oxygen) and low nutrient level (2% fetal bovine serum)-compared with the in vitro healthy wound model. When co-delivered with Bay 11-7085, significant increased uptake of pHRODO green S. aureus bioparticles was observed in the in vitro DFU model. CONCLUSION: Optimized parameters for modeling inflammation and reduced phagocytic activity in DFU in vitro were identified. Modulating inflammation could be useful in stimulating phagocytosis in DFU based on the positive effect of Bay 11-7085 on the in vitro DFU model. This finding paves the way for screening and re-purposing immunomodulatory drugs to stimulate phagocytosis in DFU. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s44164-024-00080-5.