Tumor PD-L1 engages myeloid PD-1 to suppress type I interferon to impair cytotoxic T lymphocyte recruitment (bulk)

A hallmark of PD-1/L1 blockade is long-term, sustained remission of metastatic disease. How the immune system coordinates the destruction of macro- and micro-metastases following checkpoint blockade, however, remains unclear. Here, we show that tumor-expressed PD-L1 (tPD-L1) enhanced metastasis in a mechanism distinct from and independent of its role in primary tumor growth. This difference in metastatic growth was mediated by cytotoxic T lymphocytes (CTLs), however, tPD-L1 did not promote effector CTL exhaustion or suppress lytic activity in vivo. Instead, single cell RNA sequencing revealed that tPD-L1 engaged macrophage-expressed PD-1 to antagonize type I interferon production and signaling, creating an immunologically ‘cold’ microenvironment. Loss of tPD-L1 eliminated metastases by driving interferon-mediated sensitization of tumor cells to CTL lysis and CTL recruitment. Bulk RNA-sequencing: Balb/c mice were injected i.v. through the lateral tail vein with 2.5*105 4T1 WT or PDL1-KO cells. After seven days, mice were sacrificed, lungs were perfused with 20mL of PBS. Lungs were mechanically homogenized and RNA was extracted using Direct-zol RNA microprep kit according to manufacturer’s instructions (Zymo Research, Irvine CA). RNA was sequenced by Novogene. Reference genome (mm10) and gene model annotation files were downloaded from Ensembl directly. Indices were built and paired-end clean reads were aligned to the reference genome using STARv2.5. Reads were quantified using HTSeqv0.6.1.