Macrophage-like rapid uptake and toxicity of tattoo ink in human monocytes

Macrophages play a critical role for the persistence of tattoo ink in human skin. However, a comparison to other skin-resident and blood circulating immune cells and a profound analysis of REACH-compliant tattoo ink are unmet medical needs. We hence characterized the size distribution of ink particles using physicochemical methods. We studied the uptake of tattoo ink by key human skin cells and blood-derived immune cells using optical and electron microscopy as well as flow cytometry. Scanning electron microscopy of ink revealed its crystalline structure, and a tendency towards aggregations was indicated by size changes upon diluting it. Flow cytometric analyses of skin and immune cells after incubation with tattoo ink demonstrated an increase in cellular granularity upon uptake and red ink additionally evoked fluorescent signals. Human macrophages were most potent in internalizing ink in full thickness 3D skin models. Macrophage cultures demonstrated that the ink did not lead to elevated inflammatory mediators, and showed no indications for toxicity, even after nice days. Strikingly, monocytes were most efficient in ink uptake, but displayed reduced viability, whereas granulocytes and lymphocytes showed only temporary ink uptake with flow cytometric signals declining after 1 day. Mechanistic studies on ink retention by corticosteroids or dexpanthenol in macrophage cultures demonstrated that these compounds do not lead to ink excretion, but even slightly increase the ink load in macrophages. The highly motile monocytes, precursors of macrophages, may play an underrated role for tattoo ink translocation from dermal blood vessels into internal organs.