show Abstracthide AbstractMemory T cells are important for protective immunity against infectious microorganisms. Such protection is achieved by cooperative action of memory T cell populations that differ in their tissue localization and functionality. We report on the identification of the fractalkine receptor CX3CR1 as marker for stratification of memory T cells with cytotoxic effector function from those with proliferative function in both, mice and man. Based on CX3CR1 and CD62L expression levels four distinct memory T cell populations can be distinguished based on their functional properties. Transcriptome and proteome profiling revealed that CX3CR1 expression was superior to CD62L to resolve memory T cell functionality and allowed determination of a core signature of memory T cells with cytotoxic effector function. This identifies a CD62Lhi CX3CR1+ memory T cell population with an identical gene signature to CD62LlowCX3CR1+ effector memory T cells. In lymph nodes, this so far unrecognized CD62LhiCX3CR1+ T cell population shows a distinct migration pattern and anatomic positioning compared to CD62LhiCX3CR1neg TCM. Furthermore, CX3CR1+ memory T cells were scarce or absent during chronic HBV, HCV and HIV infection in man and chronic LCMV infection in mice confirming the value of CX3CR1+ in understanding principles of protective immune memory. Overall design: CD8+ T cells were isolated and directly assessed. After harvesting, cells were immediately lysed in Trizol (Invitrogen) before storage at -80°C for RNA isolation.