Proximity-dependent labeling identifies dendritic cells that prime the antitumor CD4+ T cell response

Dendritic cells (DCs) are uniquely capable of transporting tumoral antigens to tumor-draining lymph nodes (tdLNs), and also interact with effector T cells within the tumors themselves, mediating both natural antitumor immunity and the response to checkpoint blockade immunotherapy. Using LIPSTIC (Labeling Immune Partnerships by SorTagging Intercellular Contacts)-based single-cell transcriptomics, we identify individual DCs capable of presenting antigen to CD4+ T cells in the tdLN as well as inside tumors. These represent only a fraction of all DCs present and display a distinctive activated phenotype that in the LN includes production of cytokine IL-27, required for efficient T cell priming and tumor rejection. Tumor progression results in loss of effective priming of naïve CD4+ T cells, downstream of transcriptional changes in DCs that are manifested already when they arrive at the tdLN, which can be rescued by CTLA4 checkpoint blockade.