Defining the cellular heterogeneity of healthy human skin using single cell technologies

Abstract

Skin is a complex organ comprising many synergising cell types. It serves a vital role in the immune system as the first line of defence against pathogens. In homeostatic conditions, skin must be primed to respond to an overwhelming diversity of pathogens from the environment while simultaneously avoiding excessive immune responses and autoimmunity. Understanding of the intercellular heterogeneity that is required for this balance during steady state is crucial to understanding how these cells act during dysregulation, and could aid in the understanding of diseases such as eczema and psoriasis. Healthy adult skin, obtained from reconstructive surgery, was enzymatically dissociated and profiled using single cell RNA sequencing and mass cytometry, and further validated using flow cytometry and immunohistochemistry. By sequencing 82,490 single cells from healthy adult human skin of three donors, it was possible to profile the different transcriptomic states of keratinocytes, melanocytes, vascular and lymphatic endothelium, pericytes, fibroblasts, schwann cells, lymphoid cells and antigen presenting cells. 1,959,717 single cells from four donors were analysed using a 37-marker mass cytometry panel, allowing for a comparison between the proteomes and transcriptomes of skin cells. Interrogation of the stromal cells at high resolution revealed previously unreported heterogeneity, including the presence of specialised vascular endothelial structures in healthy skin, which appear to mirror the high endothelial venules found in lymphoid tissues. These structures may be critical to leukocyte infiltration during homeostasis and inflammatory conditions. This study provides a powerful resource, as a repository of healthy skin single cell heterogeneity, for future research into deviations from health, including inflammation, infection and disease.