Imaging of the cross-presenting dendritic cell subsets in the skin-draining lymph node

Dendritic cells (DCs) are antigen-presenting cells specialized for activating T cells to elicit effector T-cell functions. Cross-presenting DCs are a DC subset capable of presenting antigens to CD8⁺ T cells and play critical roles in cytotoxic T-cell-mediated immune responses to microorganisms and cancer. Although their importance is known, the spatiotemporal dynamics of cross-presenting DCs in vivo are incompletely understood. Here, we study the T-cell zone in skin-draining lymph nodes (SDLNs) and find it is compartmentalized into regions for CD8⁺ T-cell activation by cross-presenting DCs that express the chemokine (C motif) receptor 1 gene, Xcr1 and for CD4⁺ T-cell activation by CD11b⁺ DCs. Xcr1-expressing DCs in the SDLNs are composed of two different populations: migratory (CD103^hi) DCs, which immigrate from the skin, and resident (CD8α^hi) DCs, which develop in the nodes. To characterize the dynamic interactions of these distinct DC populations with CD8⁺ T cells during their activation in vivo, we developed a photoconvertible reporter mouse strain, which permits us to distinctively visualize the migratory and resident subsets of Xcr1-expressing DCs. After leaving the skin, migratory DCs infiltrated to the deep T-cell zone of the SDLNs over 3 d, which corresponded to their half-life in the SDLNs. Intravital two-photon imaging showed that after soluble antigen immunization, the newly arriving migratory DCs more efficiently form sustained conjugates with antigen-specific CD8⁺ T cells than other Xcr1-expressing DCs in the SDLNs. These results offer in vivo evidence for differential contributions of migratory and resident cross-presenting DCs to CD8⁺ T-cell activation.