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Abstract

Cancer immunotherapy has made strong impact on the treatment of many cancers, particularly malignant melanoma. However, primary and secondary resistance remains a major challenge. Tumour-associated macrophages (TAMs) represent the most abundant immune cell type within the tumour, and despite their inherent potential for immune stimulation, TAMs predominantly exert tumour-promoting functions through immune suppression. This occurs due to signals from the tumour microenvironment (TME) that drive TAMs towards a pro-tumour phenotype, thus promoting a tumour-permissive milieu. Recently, tumour-derived glucocorticoids (GCs) were identified as a mechanism for evading the immune response in tumours, impeding both anti-tumour T cell responses and the effectiveness of cancer immunotherapy. The impact of tumour-derived GCs on the function of TAMs remains largely unknown. The present thesis aimed to investigate the role of GC-responsive TAMs in melanoma that is resistant to aPD-1 checkpoint immunotherapy. Genetic ablation of the glucocorticoid receptor (GR) in monocyte-macrophage lineage cells decreased tumour growth and restored effectiveness of checkpoint immunotherapy in aPD-1–resistant melanoma. This was accompanied by reprogramming of immunosuppressive TAMs towards a more anti-tumour phenotype, however only moderate changes were observed in the T cell activation state. Bone marrow-derived macrophages (BMDM) were polarized with TCM/DEX/IL4 towards a TAM-like phenotype, characterized by strong upregulation of M2 phenotypic markers and high functional capacity to inhibit T cell proliferation. This was associated with increased HSD11B1 activity, amplifying local GC levels in a positive feed-forward mechanism. Notably, strong upregulation of HSD11B1 expression was also observed in checkpoint immunotherapy-resistant melanoma upon PD-1 mAb treatment. The small molecule GR inhibitor mifepristone (MF) readily inhibited TAM-like polarization and interrupted the self-reinforcing mechanism of GCs and HSD11B1 in BMDM. To test the capability of MF to repolarize GC-response immunosuppressive TAMs in vivo, lipid nanoparticles (LNPs) incorporating MF were produced by microfluidic mixing and conjugated to CD169 antibodies via click chemistry for active targeting of TAMs in the TME. MF-aCD169 LNPs efficiently reprogrammed immunosuppressive TAMs and improved the anti-tumour T cell response, restoring efficiency of checkpoint immunotherapy in aPD-1–resistant melanoma. Collectively, this thesis demonstrates that GC-responsive TAMs play a prominent role in suppressing anti-tumour immunity in aPD-1–resistant melanoma. Additionally, this thesis represents the first approach to reprogram immunosuppressive TAMs by GR blockade, sensitizing melanoma to checkpoint immunotherapy.