Tumour copper levels regulate PD-L1 driven immune evasion

Abstract

Cancer immune evasion is recognised as a central hallmark of tumour development. One mechanism that cancer cells use to protect themselves from anti-tumour immune responses is the over-expression of Programmed Death Ligand 1 (PD-L1). The immune checkpoint protein Programmed Death receptor 1 (PD-1) expressed by lymphocytes negatively regulates T-cells effector functions against target cells, including tumour cells. Several therapeutic monoclonal antibodies targeting PD-L1/PD-1 have been approved by the FDA for adult melanoma and lung cancer. However, their efficacy is limited by acquired resistance and immune-related adverse events in many patients. Additionally, the regulation of PD-L1 expression on tumour cells is still poorly understood.

Copper transporter 1 (CTR-1) and copper levels are elevated in tumours and the use of copper targeting agents is currently under intense investigations. It has been also reported that copper plays a major role in the immune system, but its activity is unclear. In this study, we demonstrated for the first time that copper plays a key role in the expression of PD-L1 in cancer cells. Tissue microarrays from neuroblastoma and glioblastoma patients showed a significant correlation between CTR-1 and PD-L1 expression (p<0.0001 and p=0.006 respectively). Deep analysis of the TCGA database showed that CTR-1 and PD-L1 association also occurs across many cancer types characterised by high copper levels, but not in the corresponding normal tissues. In vitro experiments showed that copper supplementation enhanced PD-L1 expression at mRNA and protein levels in cancer cells. Consistently, Dextran-Catechin (DC) and TEPA, copper-lowering drugs, were able to down-regulate PD-L1 expression in cancer cells, both post-transcriptionally by inhibiting EGFR-phosphorylation and promoting ubiquitin-mediated PD-L1 degradation and transcriptionally by reducing STAT signalling pathway. In vivo studies showed that copper-lowering drugs slowed tumour growth and improved mice survival, by down-regulating PD-L1 expression, which in turn caused a significantly increase in number of tumour-infiltrating CD8+ T cells and natural killer cells.

This study reveals an important role for copper in regulating PD-L1 expression and it raises the potential of repurposing copper-lowering drugs to enhance immune checkpoint blockade. This novel therapeutic approach could target a wide spectrum of copper-dependent cancers.