Pancreatic ductal adenocarcinoma (PDA) is rarely cured, and single-agent immune checkpoint inhibition has not demonstrated clinical benefit despite the presence of large numbers of CD8⁺ T cells. We hypothesized that tumor-infiltrating CD8⁺ T cells harbor latent antitumor activity that can be reactivated using combination immunotherapy.

Preserved human PDA specimens were analyzed using multiplex IHC (mIHC) and T-cell receptor (TCR) sequencing. Fresh tumor was treated in organotypic slice culture to test the effects of combination PD-1 and CXCR4 blockade. Slices were analyzed using IHC, flow cytometry, and live fluorescent microscopy to assess tumor kill, in addition to T-cell expansion and mobilization.

mIHC demonstrated fewer CD8⁺ T cells in juxtatumoral stroma containing carcinoma cells than in stroma devoid of them. Using TCR sequencing, we found clonal expansion in each tumor; high-frequency clones had multiple DNA rearrangements coding for the same amino acid binding sequence, which suggests response to common tumor antigens. Treatment of fresh human PDA slices with combination PD-1 and CXCR4 blockade led to increased tumor cell death concomitant with lymphocyte expansion. Live microscopy after combination therapy demonstrated CD8⁺ T-cell migration into the juxtatumoral compartment and rapid increase in tumor cell apoptosis.

Endogenous tumor-reactive T cells are present within the human PDA tumor microenvironment and can be reactivated by combined blockade of PD-1 and CXCR4. This provides a new basis for the rational selection of combination immunotherapy for PDA.

See related commentary by Medina and Miller, p. 3747