Abstract B096: Immune and stromal features of durable complete response to radiation and dual immune checkpoint blockade in pancreatic cancer

Abstract

Microsatellite stable (MSS) pancreatic ductal adenocarcinoma (PDAC) is refractory to immune checkpoint blockade, reflecting an immunosuppressive stroma, impaired lymphocyte trafficking, and low neoantigen burden. 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). While clinical activity was limited overall (3.3% overall response rate, 10% disease control rate, 2.17 months median progression-free survival), one patient achieved a durable complete response (CR) with no evidence of disease after 5 years. To understand the cellular drivers of treatment response, we integrated longitudinal profiling of 33 pre- and on-treatment tumor biopsies from 23 patients, yielding 245,529 single-nucleus and 128,295 single-cell transcriptomes with 27,215 paired single-cell TCR clonotypes, as well as Visium spatial transcriptomics from 13 biopsies, and peripheral blood TCR-sequencing from 25 patients. Across the cohort, treatment was associated with expansion of T, NK, and myeloid cell populations. Lineage tracing of expanded T cell clonotypes revealed a therapy-associated remodeling of pre-existing GZMK+ CD8 clonotypes into CD8 exhausted T cells in the CR patient alone, in contrast to phenotypically stable clones in non-responders. Using a benchmarked tumor-reactivity gene signature (AUROC 0.71–0.95 across validation cohorts), we found predicted tumor-reactive clonotypes were highly concentrated in the CR patient (71% of tumor-reactive cells, 78% of tumor-reactive clonotypes), with peripheral blood TCR-sequencing showing selective expansion of these clonotypes in circulation one year after treatment. The CR patient also uniquely displayed enrichment of interferon-polarized C1Q+ macrophages, an interferon-responsive fibroblast population, and ACKR1+ venous endothelial cells. The CR patient was further characterized by distinct coordination across cell types, reflected in a multicellular interferon hub linking CXCL13+ T cells, CXCL10+ macrophages, and POSTN+FAP+ fibroblasts with increased activity on treatment. Spatial transcriptomics confirmed increased macrophage–T cell co-localization and chemokine signaling (CCL3-CCR1, CXCL9-CXCR3) activity on-treatment, and it additionally nominated ACKR1+ endothelial cells (via CXCL9-ACKR1) as a site of immune cell recruitment. 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.