Intratumoral mregDC and Tfh-like niches enable local reinvigoration of CD8 T cells following PD-1 blockade

While T cell accumulation in tumors is associated with response to immune checkpoint blockade (ICB), many T cell-rich tumors fail to respond to ICB. Here we leveraged a large neoadjuvant PD-1 blockade trial in hepatocellular carcinoma (HCC) to search for correlates of ICB response within T cell-rich tumors. Paired scRNAseq/TCRseq of nearly one million immune cells revealed that tumor responses to ICB correlated with significant clonal expansion of intratumoral CH25H+CXCL13+IL-21+CD4 T cells and GranzymeK+PD-1+CD8 effector T cells, whereas terminally exhausted CD39hiToxhiPD-1hi CD8 clones dominated in non-responders. Notably, proliferating and progenitor CD8 T cells clones were found in tumors of responders and non-responders. However, tumors from responders were highly enriched in mregDCs, a DC state triggered by capture of tumor debris, which formed physically interacting cellular triads with Tfh-like CD4 and progenitor-like CD8 T cells. Receptor-ligand analysis revealed unique interactions within these triads that may promote progenitor CD8 T cell differentiation into effector cells upon PD-1 blockade. These results suggest that discrete cellular niches that include mregDCs and Tfh-like CD4 T cells might control the differentiation of tumor-specific progenitor CD8 T cell clones into effective anti-tumor T cells in human tumors.