In vivo imaging of immune cell dynamics in skin in response to zinc-oxide nanoparticle exposure.

Zinc oxide (ZnO) nanoparticles (NPs) are widely used in cosmetic and sunscreen products which are applied topically to the skin. Despite their widespread use, the safety and biological response of these particles remains an active area of investigation. In this paper we present methods based on in vivo multiphoton microscopy (MPM) in skin to address relevant questions about the potential toxicity and immunological response of ZnO NPs. Registration of time-lapse volumetric MPM images allows the same skin site to be tracked across multiple days for visualizing and quantifying cellular and structural changes in response to NP exposure. Making use of the unique optical properties of ZnO enables high contrast detection of the NPs in the presence of strong autofluorescence and second harmonic generation (SHG) background from the skin. A green fluorescent protein (GFP) bone marrow (BM) transplanted mouse model is used to visualize and assess the dynamic response of BM-derived immune cells. These cells are visualized to assess the potential for ZnO NPs to interact with immune cells and elicit an immune reaction in skin. We investigate both topical and dermal exposure of the ZnO NPs. The methods and findings presented in this paper demonstrate a novel approach for tracking ZnO NPs in vivo and for visualizing the cellular response of the exposed tissue to assess the immunological response and potential toxicity of these particles.