Immunosuppressive crosstalk between tumour cells, myeloid cells and T cells in an anti-PD-1 plus anti-CTLA-4 therapy resistant mouse ccRCC model [RNA-seq]

Many patients respond poorly or develop resistance to immune checkpoint therapies for metastatic clear cell renal cell carcinoma (ccRCC). Here we demonstrate that ccRCCs in the Vhl/Trp53/Rb1 mutant autochthonous mouse model are resistant to anti-PD-1 plus anti-CTLA-4 therapy, providing a model of checkpoint therapy resistance. In vivo checkpoint therapy allowed isolated T cells to recognise cultured ccRCC cells, implicating the tumour microenvironment in suppression of T cell activation. Single cell RNA-seq identified putative immunosuppressive PMN-MDSC, M-MDSC and TAM populations that show similar features to myeloid cells in the microenvironment of human ccRCC. Conditioned medium of ccRCC cell lines attracted bone marrow-derived macrophages in a MIF-dependent manner and induced the mis-differentiation of bone marrow cells to M-MDSC and TAM2-like phenotypes. These cells suppressed the viability and proliferation of activated CD8+ T cells. These analyses identify new candidate cellular and molecular barriers to effective immune checkpoint therapy. We validate this concept by showing that GALECTIN-3 is the predominant checkpoint ligand expressed by ccRCC cells and myeloid cells and that anti-LAG-3 treatment increases T cell infiltration in this model.