Thymus-resident memory CD8+ T cells mediate local immunity

The thymus is a primary lymphoid organ responsible for production and selection of Tcells. Nonetheless, mature Tcells and in particular activated Tcells can reenter the thymus. Here, we identified memory CD8(+) Tcells specific for lymphocytic choriomeningitis virus or vaccinia virus in the thymus of mice long-time after the infection. CD8(+) Tcells were mainly located in the thymic medulla, but also in the cortical areas. Interestingly, virus-specific memory CD8(+) Tcells in the thymus expressed the cell surface markers CD69 and CD103 that are characteristic of tissue-resident memory Tcells in a time-dependent manner. Kinetic analyses and selective depletion of peripheral CD8(+) Tcells by antibodies further revealed that thymic virus-specific memory CD8(+) Tcells did not belong to the circulating pool of lymphocytes. Finally, we demonstrate that these thymus-resident virus-specific memory CD8(+) Tcells efficiently mounted a secondary proliferative response, exhibited immediate effector functions and were able to protect the thymus from lymphocytic choriomeningitis virus reinfection. In conclusion, the present study not only describes for the first time virus-specific memory CD8(+) Tcells with characteristics of tissue-resident memory T (T-RM) cells in a primary lymphoid organ but also extends our knowledge about local T-cell immunity in the thymus.