Cell-intrinsic PD-L1 signaling drives immunosuppression by myeloid-derived suppressor cells through IL-6/Jak/Stat3 in PD-L1-high lung cancer.

Background: Some patients with non-small-cell lung cancer (NSCLC) benefit from immune checkpoint inhibitors (ICIs) despite programmed death-ligand 1 (PD-L1) expression. To address the mechanism of ICI resistance in PD-L1-positive NSCLC, we investigated the role of tumor-cell-intrinsic function of PD-L1 in interleukin (IL)-6-mediated immunosuppression.

Methods: Cohorts of NSCLC patients treated with ICI and public datasets were analyzed. PD-L1-overexpressing and PD-L1-knockdown NSCLC cells were submitted to RNA-seq, in vitro analyses, chromatin immunoprecipitation-qPCR, CUT&Tag, and biochemical assays. Human myeloid-derived suppressor cells (MDSCs) sorted from peripheral blood mononuclear cells were co-cultured with NSCLC cells and then assessed for their immunosuppressive activity on T-cells. Mouse Lewis lung carcinoma (LLC) cells with PD-L1 overexpression or knockdown were subcutaneously injected into wild-type or PD-1-knockout C57BL/6 mice in the presence of IL-6 and/or PD-1 blockade.

Results: In the ICI cohort with RNA-seq data, the IL-6/Jak/Stat3 pathway was enriched, and IL-6 expression was higher in patients with PD-L1-high NSCLCs who did not respond to ICIs. In another cohort, a higher baseline serum IL-6 level was associated with poor clinical outcomes after ICI therapy. IL-6 expression and the IL-6/Jak/Stat3 pathway were enhanced in PD-L1-high NSCLCs in the ICI cohorts and The Cancer Genome Atlas analysis. IL-6 expression correlated positively with tumor-infiltrating MDSCs in NSCLCs. In NSCLC cells, PD-L1 activated Jak2/Stat3 signaling by binding to and inhibiting protein tyrosine phosphatase 1B. PD-L1 also bound to p-Stat3 in the nucleus, thus promoting the activity of p-Stat3 in the transcription of several cytokines (IL-6, TGF-β, TNF-α, IL-1β) and chemokines. PD-L1-overexpressing NSCLC cells enhanced the migration and immunosuppressive activity of human MDSCs in vitro, mediated by IL-6 and CXCL1. In both wild-type and PD-1-knockout mice, PD-L1-overexpressing LLC tumors were infiltrated by increased MDSCs with high immunosuppressive function, increased Tregs, and decreased granzyme B or IFNγ CD8 T-cells. These responses were mediated by IL-6 secreted from PD-L1-overexpressing tumor cells. Combined blockade of PD-1 and IL-6 was effective in tumor control and decreased MDSCs while increasing granzyme B or IFNγ CD8 T-cells.

Conclusions: The tumor-cell-intrinsic function of PD-L1 drives immunosuppression and tumor progression through the PD-L1/Jak/Stat3/IL-6/MDSC axis. This pathway represents a potential therapeutic target to improve ICI efficacy in PD-L1-high NSCLC.