Distinct recirculation potential of CD69⁺CD103^- and CD103⁺ thymic memory CD8⁺ T cells

Tissue-resident memory T (T_RM) cells occupy peripheral and lymphoid tissues where they confer protection against local infection. While epithelial CD8⁺ T_RM cells coexpress CD69 and CD103, CD103^- memory cells have been described in various organs and are often presumed non-recirculating based on their expression of CD69. We found that both CD69⁺CD103⁺ and CD69⁺CD103^- memory cells populated the thymus upon transfer of CD8⁺ effector T cells into uninfected recipients. Transcriptionally and phenotypically, CD103⁺ thymic cells resembled non-lymphoid T_RM cells, whereas CD69⁺CD103^- cells displayed a profile that was more closely related to recirculating cells. Although CD69 was required for optimal CD103⁺ T_RM formation, its expression alone did not identify permanently resident cells, as CD69⁺CD103^- cells disappeared from the thymus following antibody-mediated depletion of recirculating cells. Our findings highlight a distinct migration potential of phenotypically divergent thymic CD8⁺ memory T cells and emphasise the inadequacy of CD69 as a marker of tissue residency.