The strategy of T cell antigen-presenting cell encounter in antigen-draining lymph nodes revealed by imaging of initial T cell activation

The development of an immune response critically relies on the encounter of rare antigen (Ag)-specific T cells with deudritic cells (DCs) presenting the relevant Ag. How two rare cells find each other in the midst of irrelevant other cells in lymph nodes (LNs) is unknown. Here we show that initial T cell activation clusters are generated near high endothelial venules (HEVs) in the outer paracortex of draining LNs by retention of Ag-specific T cells as they exit from HEVs. We further show that tissue-derived DO preferentially home in the vicinity of HEVs, thus defining the site of cluster generation. At this location DCs efficlently scan all incoming T cells and selectively retain those specific for the major histocompatibility complex-peptide complexes the DCs present. Such strategic positioning of DCs oil the entry route of T cells into the paracortex may foster T cell--DC encounter and thus optimize initial T cell activation in vivo.