Tissue-resident memory T cells: local guards of the thymus.

T-cell surveillance of nonlymphoid tissues has traditionally been ascribed to recirculating memory T cells that continuously patrol the body. Extending this concept, recent evidence suggests that T cells also exist as nonmigratory memory cells that provide local immune protection in a broad range of peripheral tissues, including barrier locations such as skin and mucosa. In this issue of the European Journal of Immunology, Pircher and colleagues [Eur. J. Immunol. 2013. 43: 2295-2304] demonstrate, for the first time, the existence of such permanently tissue-resident CD8(+) memory T (TRM) cells in a primary lymphoid organ, the thymus. TRM cells in this location provide potent local immunity, which may help to preserve thymic integrity and normal T-cell development in the face of infection with thymus-invading pathogens.