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Naive T lymphocytes travel to T-cell areas of secondary lymphoid organs in search of antigen presented by dendritic cells^,. Once activated, they proliferate vigorously, generating effector cells that can migrate to B-cell areas or to inflamed tissues^,,,. A fraction of primed T lymphocytes persists as circulating memory cells that can confer protection and give, upon secondary challenge, a qualitatively different and quantitatively enhanced response^,,. The nature of the cells that mediate the different facets of immunological memory remains unresolved. Here we show that expression of CCR7, a chemokine receptor that controls homing to secondary lymphoid organs, divides human memory T cells into two functionally distinct subsets. CCR7^- memory cells express receptors for migration to inflamed tissues and display immediate effector function. In contrast, CCR7⁺ memory cells express lymph-node homing receptors and …