A35 Dendritic Cell in Situ Vaccination Potentiates Anti-PD-1 Efficacy and Induces Immunoediting in a Murine Model of NSCLC

Abstract

Studies reveal that responses to checkpoint blockade in non-small cell lung cancer (NSCLC) are associated with high tumor mutational burden (TMB), preexisting CD8+ T-cell infiltration, and high baseline PD-L1 expression within the tumor microenvironment (TME). In contrast, co-occurring KRAS/LKB1 mutation is associated with primary resistance to PD-1 blockade and decreased overall survival. In preclinical studies as well as a phase I clinical trial, we have discovered that intratumoral (IT) vaccination with gene-modified dendritic cells expressing CCL21 (CCL21-DC) promotes tumor effector T-lymphocyte infiltration, PD-L1 upregulation, and systemic tumor-specific immune responses. We hypothesized that in situ vaccination with CCL21-DC could restore tumor antigen presentation and promote T-cell priming and activation, thereby sensitizing nonresponsive NSCLC tumors to checkpoint blockade. Although genetically engineered murine models (GEMMs) of NSCLC bear driver mutations of the disease, recent studies reveal that these GEMMs possess low mutational burden. We established novel GEMMs of NSCLC [KrasG12D (K), KrasG12DP53-/- (KP), KrasG12DP53+/-Lkb1-/- (KPL)] bearing common driver mutations and varying mutational loads by in vitro exposure of tumor cell lines to the carcinogen N-methyl-N-nitrosourea (MNU). Our preclinical KPL model with high TMB recapitulates the immunologic phenotype of human disease, and contains a predominance of myeloid-derived suppressor cells (MDSC), low tumor-infiltrating lymphocytes (TILs), and low PD-L1 expression within the TME. As anticipated, the KPL tumors are resistant to anti-PD-1 therapy, even with increased mutational load. We evaluated IT CCL21-DC combined with anti-PD-1 therapy in immunocompetent mice bearing KPL tumors with high TMB, and observed that IT CCL21-DC vaccination induces infiltration of autologous T lymphocytes and conventional type I DCs (cDC1s) into the TME and sensitizes the tumors to anti-PD-1 therapy. Combination therapy also reprogrammed the myeloid compartment, resulting in a significant reduction of MDSCs and a concurrent increase in CD11b+Ly6GhiLy6Clo monocyte/myeloid population. Whole-exome sequencing (WES) of tumors revealed immunoediting and selective depletion of tumor subclones post IT CCL21-DC and anti-PD1 combination therapy. Future studies will evaluate the evolution of the T-cell receptor (TCR) repertoire in response to the combination treatment and define functional responses to neoepitopes. These studies will enhance our understanding of the molecular mechanisms of tumor vaccination and facilitate the development of rational combination strategies.