Chemokines enhance immunity by guiding naive CD8+ T cells to sites of CD4+ T cell–dendritic cell interaction

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Abstract
CD8+ T cells have a crucial role in resistance to pathogens and can kill malignant cells; however, some critical functions of these lymphocytes depend on helper activity provided by a distinct population of CD4+ T cells. Cooperation between these lymphocyte subsets involves recognition of antigens co-presented by the same dendritic cell, but the frequencies of such antigen-bearing cells early in an infection and of the relevant naive T cells are both low. This suggests that an active mechanism facilitates the necessary cell–cell associations. Here we demonstrate that after immunization but before antigen recognition, naive CD8+ T cells in immunogen-draining lymph nodes upregulate the chemokine receptor CCR5, permitting these cells to be attracted to sites of antigen-specific dendritic cell–CD4+ T cell interaction where the cognate chemokines CCL3 and CCL4 (also known as MIP-1α and MIP-1β) are produced. Interference with this actively guided recruitment markedly reduces the ability of CD4+ T cells to promote memory CD8+ T-cell generation, indicating that an orchestrated series of differentiation events drives nonrandom cell–cell interactions within lymph nodes, optimizing CD8+ T-cell immune responses involving the few antigen-specific precursors present in the naive repertoire.