Rachael A. Clark, MD, PhD, of the Department of Dermatology, and her colleagues have investigated the different populations of T cells that protect human skin, identifying four distinct populations of resident and recirculating memory T cells. They report their findings in the March 18 issue of Science Translational Medicine.

The skin of an adult human is home to 20 billion memory T cells, which are critical in protecting the skin and other barrier tissues against infection. Recent studies in mice have demonstrated that there are both resident populations of T cells, which remain long-term in one place in tissues, and recirculating T cells, which move between the blood lymph nodes and peripheral tissues. But the relative importance and behavior of these cells in peripheral tissues has not been fully examined, and has never been studied in human skin.

Using mice grafted with human skin, healthy human skin samples and biopsies from patients with cutaneous T cell lymphoma, Clark et al. studied the T cells that protect human skin.

The research team found that there were four distinct populations of T cells in human skin, two that remained resident and two that recirculated. The resident cells were powerfully protective, producing high levels of inflammatory cytokines, whereas the recirculating cells produced fewer cytokines but had the ability to move in and out of skin.

“These studies demonstrate the elegant way in which the immune system protects skin by providing both very powerful soldiers that locally guard against infection, and highly mobile messengers that can move from one place to another and respond to infections in different sites in the skin and other tissues such as the gut and lung,” said Clark.