Batf3-dendritic cells and 4-1BB/4-1BB ligand axis are required at the effector phase within the tumor microenvironment for PD-1/PD-L1 blockade efficacy

The cellular source of positive signals that reinvigorate T cells within the tumor microenvironment (TME) for the therapeutic efficacy of PD-1/PD-L1 blockade has not been clearly defined. We now show that Batf3-lineage dendritic cells (DCs) are essential in this process. Flow cytometric analysis, gene-targeted mice, and blocking antibody studies revealed that 4-1BBL was a major positive costimulatory signal provided by these DCs within the TME, that translated to CD8+ T cell functional reinvigoration and tumor regression. Immunofluorescence and spatial transcriptomics on human tumor samples revealed clustering of Batf3+ DCs and CD8+ T cells, which correlated with anti-PD-1 efficacy. In addition, proximity to Batf3+ DCs within the TME was associated with CD8+ T cell transcriptional states linked to anti-PD-1 response. Our results demonstrate that Batf3+ DCs within the TME are critical for PD-1/PD-L1 blockade efficacy, and indicate a major role for the 4-1BB/4-1BBL axis during this process.