At the most basic level, a memory CD8 T cell can be defined as a CD8 T cell that has responded to cognate antigen (Ag) and persists long-term.

Tissue-resident memory CD8 + T cells (T RM) are a distinct memory population that is generated and persists at the site of infection or vaccination (1 – 3). Upon exposure to the same or similar diseases, T RM cells provide a first line of adaptive cellular defense and are indispensible in lethal challenge models (2, 4).

After antigen exposure, CD8+ T cells rapidly pro-liferate to give rise to a large pool of effector T cells, which then migrate to peripheral tissues and destroy infected cells. After pathogen clear-ance, most effector T cells undergo apoptosis, leaving behind a smaller pool of ‘memory’ T cells that provide long-term immunity.

Memory T cells persist for long term to mediate robust recall response upon rechallenging with previous encountered pathogens. The memory T cell pool is highly heterogeneous based on distinct phenotypic, functional, and locational properties, and contains discrete subsets, which contribute to diverse immune responses.