The skin is a important organ with distinct layers and a multitude of skin-associated immune cells. The aim of this study was to map and characterize the proteins of the human skin with respect to their
spatial location. We performed state-of-the-art mass spectrometry based proteomics for an in-depth proteome analysis of the healthy human skin (N=5), cultivated keratinocytes and fibroblasts (N=5) and fluorescence
activated cell sorted (FACS) immune cells, endothelial cells and melanocytes (N=12). Using basic reversephase peptide fractionation followed by liquid chromatography tandem mass spectrometry (LC-MS/MS) 173,228 sequence-unique peptides were identified, accounting for 11,468 protein groups (8,987 proteins in stratum corneum, 9,140 proteins in the inner epidermis, 8,340 proteins in the dermis and 6,334 proteins in the subcutis). We quantified the relative abundance of 46 keratins and 21 collagens in the skin. In addition, lowabundant proteins such as interleukin (IL) 1 family members, associated with regulation in inflammatory skin disease, were identified including IL-1α, IL-1β, IL-1Ra, IL-18, IL-33, IL-36α, IL-36β, IL-36γ, IL-36Ra and IL-37.
In cell subsets including keratinocytes, fibroblasts, endothelial cells, melanocytes, dendritic cells, mast cells and macrophages we identified between 8,727 and 10,337 proteins. In conclusion, this study increases our understanding of human skin biology by the identification of its global proteomic composition. The data is available to serve as a resource for future studies, providing a stepping-stone to relate proteomic changes to skin diseases (https://skin.science).

A spatially resolved proteomic atlas of human skin and its associated immune system