Tumor microenvironment dictates function of tumor-expressed PD-L1

Abstract

Abstract To evade immune clearance by cytotoxic T lymphocytes (CTLs), tumors express immunosuppressive checkpoints such as PD-L1. Tumor-expressed PD-L1 is thought to suppress CTL effector functions by binding CTL-expressed PD-1. To investigate the molecular consequences of CTL engagement with tumor PD-L1, we generated PD-L1 knockouts in murine sarcoma, breast and colorectal cancer cell lines by Crispr/Cas9. Surprisingly, PD-L1 expression conferred no protection from CTL-mediated cytotoxicity in an in vitro co-culture model. Similarly, loss of PD-L1 had no effect on in vivo growth following subcutaneous or orthotopic challenge in immunocompetent mice. However, tumor PD-L1 facilitated metastasis to the lung in both spontaneous and experimental metastasis models. Bulk RNA-seq of tumor-bearing lungs revealed global enrichment in interferon-stimulated gene expression following loss of tumor PD-L1. Flow cytometric analysis and single-cell RNA-sequencing localized this signature to an inflammatory macrophage population that expressed PD-1 and selectively expanded following injection of PD-L1 deficient cell lines. In vitro and in vivo experimentation showed that engagement of macrophage PD-1 by tumor PD-L1 reduced the expression of interferon-stimulated genes and antigen processing machinery. We propose a model by which tumor-expressed PD-L1 regulates CTL function indirectly in the lung microenvironment by suppressing the recruitment and activation of CTLs through myeloid cells.