(447c) A Microscale, Full-Thickness, Human Skin on a Chip Assay Simulating Neutrophil Responses and Antibiotic Treatment to Skin Infection

Skin and soft tissue infections (SSTIs) are caused by microbial invasion of the skin. In the case of cellulitis, the most common SSTI, Staphylococcus aureus and Streptococcus pyogenes are the most commonly identified causes. Cellulitis accounts for $3.7 billion/year in ambulatory care costs with 14.5 million cases in the United States. However, an incomplete understanding of cellulitis biology results in misdiagnosis for one-third of these patients. Thus, a novel tool is required to advance the current understanding of human SSTIs, which may lead to the development of an objective diagnosis.

Novel skin on a chips (SOCs) have been developed to test the skin response to drugs, chemicals, and UV irradiation. However, current in vitro SOCs have a limitation in mimicking the complex structural and cellular components of real human skin. While ex vivo SOCs use skin biopsy samples to more closely represent the complexity in vivo, current systems require relatively large biopsy sizes (4-5 mm), leading to donor site scarring. Moreover, such a large skin sample limits real-time imaging capabilities and instead requires immunohistochemical staining, which provides only a snapshot in time.

In this study, we present a new ex vivo SOC integrated with full thickness, human, microscopic skin tissue columns (MSTCs). MSTCs are harvested using a novel micro-biopsy needle with >100 times smaller cross-section area than a 4 mm biopsy. Importantly, the harvesting of MTSCs has previously demonstrated rapid healing without donor site scarring. Using our platform, we observe neutrophil migratory behavior towards S. aureus inoculated MSTCs, a model of SSTIs. The microfluidic channel design allows for neutrophil migration directly from a drop of whole blood (< 10 µl) without an additional isolation step. The result shows the positive correlation between the number of migrated neutrophils and the amount of S. aureus on the skin column. Results suggest that neutrophils are a potent biomarker for bacterial skin infection in our SOC model system.

To achieve the detection of bacteria at clinically relevant concentrations, we pre-incubate the device for 4 hours before loading the blood. Such pre-incubation amplifies the neutrophil migration towards bacteria inoculated MSTCs. As a result, compared to control, neutrophils show a significantly higher accumulation near ‘infected’ MSTCs with clinically relevant bacteria concentration (~10⁵ colony forming unit/cm³ of skin tissue, P ≤ 0.05). Our platform provides results within 8 hours while the traditional microbial culture generally requires more than 24 hours.

Further, we also demonstrate that our platform can be used to evaluate the efficiency of antibiotic treatment to SSTIs. In this demonstration, we test the effect of penicillin on the S. aureus inoculated columns. Penicillin treated columns attract a significantly lower number of neutrophils compared to non-treated columns (P ≤ 0.001). In the future, we envision that our platform could facilitate personalized antibiotic screening using patient derived SSTI samples using neutrophils as biomarker. Further, our platform will provide a human model to study SSTIs and innovative diagnostic strategies that may prevent misdiagnosis and the overuse of antibiotics.