Most of our knowledge about the recirculation and distribution of cutaneous tissue-resident memory T cells (TRM) is based on observations from murine studies, while the fate of TRM in human skin is still poorly understood. We followed patients who underwent conditioning regimens (myeloablative therapy, total body irradiation) and subsequent allogenic hematopoietic stem cell transplantation (HSCT) to track repopulation dynamics of cutaneous T cells over time. 
Skin biopsies and peripheral blood from 25 patients were taken at four time points before and after conditioning regimens/HSCT. T cells were characterized using moleculesfor migratory T cells (CCR7, CD62L) and markers defining cutaneous TRM (CD69, CD103). In sex-mismatched transplanted patients, X- and Y-chromosomes on T cells were fluorescently labelled to precisely define their host/donor origin for up to one year after HSCT. All stainings were automatically quantified using imaging analysis software. 
In concordance with data of peripheral blood, the overall number of T cells in the skin declined upon HSCT. While small populations of central and migratory T cells (CD3+CCR7+CD62L+/-) were present in the dermis before HSCT, they disappeared from skin biopsies upon leukocyte depletion by conditioning regimens and repopulated the dermis between week 2 and 14 after HSCT. Notably, a subset of dermal CD69+ T cells with various expression levels of CD103 remained stable throughout all time points. Consistently, XY-chromosome analysis revealed that host T cells survived in the dermis for at least 100 days post HSCT. 
These data present new insights into resilience of skin-resident memory T cells as radiation-resistant and self-contained cells and open a wide range of fundamental questions on T cell biology in peripheral tissues. In addition, host-derived T cells might play a so far unappreciated role in the initiation and propagation of graft-versus-host disease, a  common and potentially fatal complication of HSCT.

Resistant Residents: Turnover of Skin-Resident Memory T cells