Skin-Resident T Cells Exhibit Site-Specific Morphology and Activation States.

Skin-resident T cells play an important role in maintaining the immune barrier at the epithelial surface. Their roles in wound healing, regulation of immune response to injury, and reepithelialization have been characterized extensively in the mouse, though their function in human skin remains largely unknown. Human skin-resident T cells sparsely populate the skin and are often small and rounded in appearance. Those in the mouse ear and back, which line the dermal barrier, are highly arborized cells with many processes extending from the cell body. To date, these cells have been studied primarily in the mouse ear and back; however, it is important to further identify and characterize T cells in other body sites to better understand their function and study their contribution to injury and disease. We developed a novel method to visualize these cells in the skin (whole-mount and cryosections) that when combined with flow cytometry allowed us to assess differences in skin-resident T cell numbers, morphology, and activation state in the ear, back, and footpad (chosen for their importance in immunological and pain research). In comparing cell length, number of dendritic processes, and expression of the activation marker CD69, we found that T cell morphology and activation states vary significantly among the three tissue environments. Specifically, T cells in the footpad are smaller, have fewer processes, and show the highest levels of activation compared to back- and ear-resident cells. Our observations suggest that our understanding of skin-resident T cell functionality, drawn from the experiments performed in the ear and back tissue, may not be applicable to all skin environments. The footpad-resident cells also more closely resemble T cells in human skin, suggesting that cells in this tissue environment may serve as a better translational model when studying T cell function/activity.