Abstract 2710: Depletion of CXCR2-dependent myeloid-derived suppressor cells (MDSCs) overcomes anti-PD-1 resistance in a murine model of LKB1-deficient non-small cell lung cancer (NSCLC) with high mutational load
Authors: Li Rui , Salehi-Rad Ramin , Ong Stephanie , Momcilovic Milica , Liu Bin , Lim Raymond , Tran Linh , Huang Ziling , Jing Zhe , Paul Manash , Krysan Kostyantyn , Park Stacy , Minna John , Teitell Michael , Shackelford David , Dubinett Steven 
Published: August 2019

Abstract
Checkpoint inhibitors such as PD-L1/PD-1 blockade have rapidly integrated into the paradigm of NSCLC treatment. However, a majority of patients do not benefit from monotherapy with checkpoint inhibitors. High tumor mutational burden (TMB), along with pre-existing intratumoral T cell infiltration and baseline high PD-L1 expression, predicts response to checkpoint blockade. Furthermore, a recent retrospective study identified LKB1 alterations as the most prevalent genomic driver of resistance to PD-1 axis inhibitors in KRAS-mutant lung adenocarcinoma. In this study, we investigate the mechanisms underlying LKB1-mediated immunosuppression in NSCLC. We show that loss of LKB1 in human bronchial epithelial cells (HBECs) and NSCLC cells leads to increased secretion of CXCR2 ligands, including CXCL1, CXCL2, CXCL3, CXCL5 and CXCL8. These CXCR2 ligands are also elevated in LKB1-deficient tumors from patient-derived xenografts and genetically-engineered murine models. We find abundant tumor infiltrating MDSCs in murine Lkb1-deficient NSCLC, consistent with the capacity for CXCR2 ligands to recruit MDSCs. MDSCs mediate potent immune suppressive activities at multiple levels including release of immunosuppressive cytokines, recruitment of regulatory T cells (Tregs), inhibition of CD8 T cell tumor infiltration and upregulation of PD-L1 expression. Although MDSC depletion activates interferon gamma signaling and decreases systemic Tregs in murine KrasK12D;Tp53-/-;Lkb1-/- (KPL) tumors, it does not sensitize KPL tumors to anti-PD-1 therapy. One of the major challenges in the preclinical assessment of lung cancer immunotherapy is that the commonly utilized murine models lack the mutational burden of human NSCLC. To assess this combination therapy in the context of a mutational burden that more accurately reflects the clinical disease, we generated tumors with high TMB by exposing KPL cells in vitroto the tobacco carcinogen N-methyl-N-nitrosourea. In the context of high TMB, MDSC depletion demonstrates remarkable anti-tumor effects in combination with anti-PD-1 therapy. Finally, we delineate the regulation of CXCR2 ligands by LKB1 which is dependent on the MARK-mediated NF-κB pathway. In conclusion, we find that LKB1 deficiency leads to increased CXCR2 ligand production and tumor infiltrating MDSCs. MDSC depletion enhances the efficacy of anti-PD-1 blockade in LKB1-deficient tumors bearing high TMB.
Citation Format: Rui Li, Ramin Salehi-Rad, Stephanie Ong, Milica Momcilovic, Bin Liu, Raymond Lim, Linh Tran, Ziling Huang, Zhe Jing, Manash Paul, Kostyantyn Krysan, Stacy Park, John Minna, Michael Teitell, David Shackelford, Steven Dubinett. Depletion of CXCR2-dependent myeloid-derived suppressor cells (MDSCs) overcomes anti-PD-1 resistance in a murine model of LKB1-deficient non-small cell lung cancer (NSCLC) with high mutational load [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2710.