Optimal CD4T Cell Priming in Lymph Nodes Requires Repertoire Scanning by CD301b+ Migratory cDC2 Cells

Activation of CD4T cells by conventional dendritic cells (cDC) is pivotal in adaptive immunity. However, while the activation mechanism of antigen-specific CD4T cells has been extensively studied, the cellular mechanism that leads to the selection of cognate CD4T cell clones out of the polyclonal pool is incompletely understood. Here, we show that, in the reactive lymph nodes, newly homed naive polyclonal CD4T cells are temporarily retained before leaving the lymph node. This stop-and-go traffic of CD4T cells provides an adequate time window for efficient scanning and timely priming of antigen-specific clones. Mechanistically, upon immunization, CD301b+ DCs, a major subset of migratory cDC2 cells, quickly migrate to the draining lymph node and settle in the areas near the high endothelial venules, where they retain incoming polyclonal CD4T cells through MHCII-dependent but antigen-independent mechanisms while concurrently providing cognate stimuli to prime antigen-specific CD4T cells. These results indicate that CD301b+ DCs function as an immunological "display window" for CD4T cells to efficiently scan their antigen specificity. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=150 SRC="FIGDIR/small/276410v1_ufig1.gif" ALT="Figure 1"> View larger version (49K): org.highwire.dtl.DTLVardef@118d9b7org.highwire.dtl.DTLVardef@7daf2eorg.highwire.dtl.DTLVardef@17808c0org.highwire.dtl.DTLVardef@186780e_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LINewly homed polyclonal CD4T cells are temporarily retained in the reactive lymph nodes. C_LIO_LIDepletion of CD301b+ DCs results in shorter dwell time of CD4T cells in the draining lymph node and delayed priming of antigen-specific clones. C_LIO_LIThe transient retention of polyclonal CD4T cells in the draining lymph node requires MHCII expression on CD301b+ DCs but not cognate antigen. C_LIO_LICD301b+ DCs are required for robust expansion of rare antigen-specific CD4T cell clones and their skewing toward Th2 cells. C_LI