IL-15 Complex Enhances Agonistic Anti-CD40 + Anti-PDL1 by Correcting the T-bet to Tox Ratio in CD8⁺ T cells Infiltrating Pancreatic Ductal Adenocarcinoma

Agonistic anti-CD40 with anti–PD-1 can elicit objective responses in a small number of patients with pancreatic ductal adenocarcinoma (PDA). Better understanding of their individual effects on the pancreatic tumor microenvironment will help inform new strategies to further improve outcomes. In this study, we map tumor-specific CD8⁺ T-cell differentiation following agonistic anti-CD40 and/or anti-PDL1 in PDA. Rare Tcf1⁺Slamf6⁺ tumor-specific CD8⁺ T cells are identified and likely seed memory precursors that transition into exhausted or effector T cells. In tumors, anti-PDL1 drove the clonal expansion of Gzmk⁺ progenitor exhausted CD8⁺ T cells, whereas antiCD40 promoted CD4⁺ T-cell clonal expansion and accumulation of Tcf1⁺Slamf6⁺ CD8⁺ T cells. Cloning the most frequent intratumoral T-cell receptors revealed identical neoepitope specificity, yet the top T-cell receptors from anti-PDL1 ± anti-CD40 cohorts lacked tetramer binding, suggesting lower affinity. Anti-CD40 + anti-PDL1 markedly drove the clonal hyperexpansion of a unique exhausted T-cell subset in the spleen. Exhausted T cells were enriched for IL-2Rβ, and provision of IL-15 complex (IL-15C) mitigated systemic and intratumoral T-cell exhaustion when combined with anti-CD40 + anti-PDL1, resulting in enhanced antitumor effects, prolongation of animal survival, and resistance to orthotopic tumor rechallenge. Mechanistically, anti-CD40 + anti-PDL1 decreased Tox, whereas IL-15C + anti-CD40 + anti-PDL1 increased T-bet, thereby conferring a higher T-bet:Tox ratio in tumor-specific CD8⁺ T cells. Collectively, agonistic anti-CD40 + anti-PDL1 drove systemic and intratumoral CD8⁺ T-cell clonal expansion and acquisition of exhaustion features in a tissue-specific manner. Provision of IL-15C altered the trajectory of T-cell differentiation induced by immunotherapy, resulting in PDA eradication and long-lived antitumor memory T cells.