Tissue-resident memory CD8+ T cells shape local and systemic secondary T cell responses.

Tissue-resident memory CD8+ T cells (TRM cells) are crucial in protecting against reinvading pathogens, but the impact of reinfection on their tissue confinement and contribution to recall responses is unclear. We developed a unique lineage tracer mouse model exploiting the TRM-defining transcription factor homolog of Blimp-1 in T cells (Hobit) to fate map the TRM progeny in secondary responses. After reinfection, a sizeable fraction of secondary memory T cells in the circulation developed downstream of TRM cells. These tissue-experienced ex-TRM cells shared phenotypic properties with the effector memory T cell population but were transcriptionally and functionally distinct from other secondary effector memory T cell cells. Adoptive transfer experiments of TRM cells corroborated their potential to form circulating effector and memory cells during recall responses. Moreover, specific ablation of primary TRM cell populations substantially impaired the secondary T cell response, both locally and systemically. Thus, TRM cells retain developmental plasticity and shape both local and systemic T cell responses on reinfection.