Detection of PD-L1-Expressing Myeloid Cell Clusters in the Hyaluronan-Enriched Stroma in Tumor Tissue and...

Expression of the transmembrane protein PD-L1 is frequently upregulated in cancer. Because PD-L1-expressing cells can induce apoptosis or anergy of T lymphocytes through binding to the PD1 receptor, the PD-L1-mediated inhibition of activated PD1⁺ T cells is considered a major pathway for tumor immune escape. However, the mechanisms that regulate the expression of PD-L1 in the tumor microenvironment are not fully understood. Analysis of organotypic tumor tissue slice cultures, obtained from mice with implanted syngeneic tumors (MBT2 bladder tumors in C3H mice, Renca kidney, and CT26 colon tumors in BALB/c mice), as well as from patients with cancer, revealed that tumor-associated hyaluronan (HA) supports the development of immunosuppressive PD-L1⁺ macrophages. Using genetically modified tumor cells, we identified epithelial tumor cells and cancer-associated mesenchymal fibroblast-like cells as a major source of HA in the tumor microenvironment. These HA-producing tumor cells, and particularly the vimentin-positive fibroblast-like cells of bone marrow origin, directly interact with tumor-recruited myeloid cells to form large stromal congregates/clusters that are highly enriched for both HA and PD-L1. Furthermore, similar cell clusters composed of HA-producing fibroblast-like cells and PD-L1⁺ macrophages were detected in tumor-draining, but not in distant, lymph nodes. Collectively, our findings indicate that the formation of multiple large HA-enriched stromal clusters that support the development of PD-L1-expressing APCs in the tumor microenvironment and draining lymph nodes could contribute to the immune escape and resistance to immunotherapy in cancer.