Delineation of a molecularly distinct terminally differentiated memory CD8 T cell population

Memory CD8 T cells provide durable protection against diverse intracellular pathogens and can be broadly segregated into distinct circulating and tissue-resident populations. Paradigmatic studies have demonstrated circulating memory cells can be further divided into effector memory (TO_SCPLOWEMC_SCPLOW) and central memory (TO_SCPLOWCMC_SCPLOW) populations based on discrete functional characteristics. Following resolution of infection, we identified a persisting antigen-specific CD8 T cell population that was simultaneously terminally-fated with potent effector function but maintained memory T cell qualities and conferred robust protection against reinfection. Notably, this terminally-differentiated effector memory CD8 T cell population (terminal-TO_SCPLOWEMC_SCPLOW) was conflated within the conventional TO_SCPLOWEMC_SCPLOW population, prompting redefinition of the classical characteristics of TO_SCPLOWEMC_SCPLOW cells. Murine terminal-TO_SCPLOWEMC_SCPLOW were transcriptionally, functionally, and developmentally unique compared to TO_SCPLOWEMC_SCPLOW cells. Through mass cytometry and single-cell RNAseq analyses of human peripheral blood from healthy individuals, we also identified an analogous terminal-TO_SCPLOWEMC_SCPLOW population of CD8 T cells that was transcriptionally distinct from TO_SCPLOWEMC_SCPLOW and TO_SCPLOWCMC_SCPLOW. A key finding of this study was that parsing of terminal-TO_SCPLOWEMC_SCPLOW from conventionally defined TO_SCPLOWEMC_SCPLOW challenges classical characteristics of TO_SCPLOWEMC_SCPLOW biology, including enhanced presence in lymphoid tissues, robust IL-2 production and recall potential, greater than expected homeostatic fitness, refined transcription factor dependencies, and a distinct molecular phenotype. Classification of terminal-TO_SCPLOWEMC_SCPLOW and clarification of TO_SCPLOWEMC_SCPLOW biology hold broad implications for understanding the molecular regulation of memory cell states and harnessing immunological memory to improve immunotherapies.