Immune and Stromal Features of Durable Complete Response to Radiation and Dual Immune Checkpoint Blockade in Pancreatic Cancer

Microsatellite stable (MSS) pancreatic ductal adenocarcinoma (PDAC) is refractory to immune checkpoint blockade. We conducted a single-arm phase II trial ( NCT04361162 ) combining nivolumab, ipilimumab and radiation therapy to treat patients with pre-treated metastatic MSS PDAC (n=30). We integrated longitudinal profiling of 32 pre- and on-treatment tumor biopsies from 22 patients, yielding 245,529 single-nucleus and 128,295 single-cell transcriptomes including 27,215 T-cells with paired TCR clonotypes, as well as Visium spatial transcriptomics from 13 biopsies, and peripheral blood TCR-sequencing from 25 patients. While clinical activity was limited overall, one patient achieved a durable complete response with no evidence of disease 4 years after trial enrollment. This response was marked by a therapy-associated shift in the state composition of pre-existing CD8 T cell clonotypes from GZMK+ to exhausted and predicted tumor-reactive states, durable maintenance of associated clonotypes in the blood after 1 year, interferon-polarized macrophage and fibroblast programs, and high levels of ACKR1+ venous endothelium. Across independent PDAC cohorts, high ACKR1 expression was associated with improved survival, greater intratumoral TCR richness and clonality, and increased tumor-blood TCR sharing. These findings suggest that productive immunotherapy responses in PDAC require not only tumor-reactive T cells, but also a stromal-vascular niche capable of supporting their recruitment, recirculation and persistence. This may have implications for the design of future immunotherapy and vaccine strategies for PDAC.