Resident T lymphocytes (T_RM) protect tissues during pathogen reexposure. Although T_RM phenotype and restricted migratory pattern are established, we have a limited understanding of their response kinetics, stability, and turnover during reinfections. Such characterizations have been restricted by the absence of in vivo fate-mapping systems. We generated two mouse models, one to stably mark CD103⁺ T cells (a marker of T_RM cells) and the other to specifically deplete CD103^- T cells. Using these models, we observed that intestinal CD103⁺ T cells became activated during viral or bacterial reinfection, remained organ-confined, and retained their original phenotype but failed to reexpand. Instead, the population was largely rejuvenated by CD103⁺ T cells formed de novo during reinfections. This pattern remained unchanged upon deletion of antigen-specific circulating T cells, indicating that the lack of expansion was not due to competition with circulating subsets. Thus, although intestinal CD103⁺ resident T cells survived long term without antigen, they lacked the ability of classical memory T cells to reexpand. This indicated that CD103⁺ T cell populations could not autonomously maintain themselves. Instead, their numbers were sustained during reinfection via de novo formation from CD103^- precursors. Moreover, in contrast to CD103^- cells, which require antigen plus inflammation for their activation, CD103⁺ T_RM became fully activated follwing exposure to inflammation alone. Together, our data indicate that primary CD103⁺ resident memory T cells lack secondary expansion potential and require CD103^- precursors for their long-term maintenance.

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