The success of immune checkpoint blockade adds a new therapeutic category to the cancer therapy repertoire. Despite efforts made on cancer cell and immune cell interaction, how cancer cells initiate immune escape is less understood. Here we reveal PD-L1’s immunosuppression activity is stringently modulated by the spatial restraints between ubiquitination and N-linked glycosylation. We identified GSK3beta as a novel PD-L1 interacting protein capable of inducing phosphorylation dependent proteasome degradation. The essence of PD-L1 N192, N200 and N219 glycosylation suggests it antagonizes GSK3beta binding. In this regard, only non-glycosylated PD-L1 forms a complex with GSK3beta and beta-TrCP. More importantly, activation of GSK3beta-mediated PD-L1 degradation by TKI can reduce membrane PD-L1 expression and thereby enhance anti-tumor immunity. To test T cell-mediated cancer cell killing in vitro, we employed a time lapse microscopy, IncuCyte, to monitor RFP-tagged BT549 cells and activated PBMC interaction over a five day period. Massive cancer cell death was observed in glycosylation deficient cancer cells, suggesting glycosylation of PD-L1 is required for its immunosuppression. Together, our results link ubiquitination and glycosylation pathways with stringent regulation of PD-L1, proposing a new therapeutic strategy to enhance cancer immune therapy efficacy.

Learning Objectives:

• Molecular mechanism of PD-L1 regulation in cancer cells
• Immunosuppressive nature of triple negative breast cancer
• Using an in vitro co-culture system to analyze T cell-mediated cancer cell killing effect