Effective CD4 T cell priming requires repertoire scanning by CD301b migratory cDC2 cells upon lymph node entry.

During the initiation of adaptive immune responses, millions of lymphocytes must be scanned to find the few cognate clones. The activation mechanisms of CD4 T cells have been extensively studied, but the cellular mechanisms that drive selection of cognate clones are not completely understood. Here, we show that recently homed naïve polyclonal CD4 T cells are temporarily retained before leaving the lymph node. This stop-and-go traffic of CD4 T cells provides an adequate time window for efficient scanning and timely priming of antigen-specific cognate clones. CD301b DCs, a major subset of migratory cDC2 cells, localize to the areas around high endothelial venules, where they retain incoming polyclonal CD4 T cells through MHCII-dependent but antigen-independent mechanisms, while concurrently providing cognate stimuli for priming. These results indicate that CD301b DCs function as an immunological “display window” for CD4 T cells to efficiently scan their antigen specificity.