Inhibition of PCSK9 potentiates immune checkpoint therapy for cancer

Despite its success in achieving the long-term survival of 10–30% of treated individuals, immune therapy is still ineffective for most patients with cancer 1 , 2 . Many efforts are therefore underway to identify new approaches that enhance such immune ‘checkpoint’ therapy 3 – 5 (so called because its aim is to block proteins that inhibit checkpoint signalling pathways in T cells, thereby freeing those immune cells to target cancer cells). Here we show that inhibiting PCSK9—a key protein in the regulation of cholesterol metabolism 6 – 8 —can boost the response of tumours to immune checkpoint therapy, through a mechanism that is independent of PCSK9’s cholesterol-regulating functions. Deleting the PCSK9 gene in mouse cancer cells substantially attenuates or prevents their growth in mice in a manner that depends on cytotoxic T cells. It also enhances the efficacy of immune therapy that is targeted at the checkpoint protein PD1. Furthermore, clinically approved PCSK9-neutralizing antibodies synergize with anti-PD1 therapy in suppressing tumour growth in mouse models of cancer. Inhibiting PCSK9—either through genetic deletion or using PCSK9 antibodies—increases the expression of major histocompatibility protein class I (MHC I) proteins on the tumour cell surface, promoting robust intratumoral infiltration of cytotoxic T cells. Mechanistically, we find that PCSK9 can disrupt the recycling of MHC I to the cell surface by associating with it physically and promoting its relocation and degradation in the lysosome. Together, these results suggest that inhibiting PCSK9 is a promising way to enhance immune checkpoint therapy for cancer.