Tumor-Infiltrating Myeloid Cells Confer De Novo Resistance to PD-L1 Blockade through EMT–Stromal and Tgfβ-Dependent Mechanisms

Abstract Most bladder cancers are poorly responsive to immune checkpoint blockade (ICB). With the need to define mechanisms of de novo resistance, including contributions from the tumor microenvironment (TME), we used single-cell transcriptional profiling to map tumor-infiltrating lymphocytic and myeloid cells in 10 human bladder tumors obtained from patients with a history of smoking either with or without previous ICB. Human datasets were qualitatively compared with single cell datasets from the BBN carcinogen-induced mouse model of bladder cancer, which was poorly responsive to PD-L1 blockade. We applied an established signature of acquired ICB resistance to these human and murine datasets to reveal conservation in EMT and TGFβ ICB resistance signatures between human–mouse stromal and myeloid cells. Using TCGA transcriptional datasets and deconvolution analysis, we showed that patients with a history of smoking and bladder tumors high in M2 macrophage tumor content had a significantly worse survival outcome compared with nonsmokers who were M2 high. Similarly, BBN-induced tumors were high in M2 macrophage content and contained exhausted T–NK cells, thereby modeling the identified TCGA patient subpopulation. The combined targeting of TGFβ + PD-L1 reverted immune cell exclusion and resulted in increased survival and delayed BBN-induced tumor progression. Together, these data support a coordinated role for stromal and myeloid cell populations in promoting de novo resistance to PD-L1 blockade, particularly in patients with a history of smoking. Significance: Most patients with bladder cancer do not respond to ICB targeting of the PD-L1 signaling axis. Our modeling applied a de novo resistance signature to show that tumor-infiltrating myeloid cells promote poor treatment response in a TGFβ-dependent mechanism.