Tumor PD-L1 induces β2m ubiquitylation and degradation for cancer cell immune evasion.

Resistance to anti-PD-1/PD-L1 immune checkpoint blockade continues to be a critical challenge undermining its therapeutic efficacy in clinical applications. Most of the resistance mechanisms characterized to date have predominantly involved external factors beyond PD-L1. Here, we unexpectedly discovered that PD-L1 itself possesses E3 ubiquitin ligase activity to induce β2m ubiquitylation and subsequent degradation, which notably reduces MHC-I levels on the surface of tumor cells and antigen-presenting cells, thereby contributing to tumor cell evasion of recognition by CD8(+) T cells and ultimately resulting in resistance to anti-PD-1/PD-L1 immunotherapy, particularly in tumors with low basal β2m expression. Disrupting the E3 ubiquitin ligase activity of PD-L1 or interfering with the PD-L1-β2m interaction dramatically enhanced the sensitivity of tumor cells to PD-L1 blockade therapy. Our study reveals a previously unknown function of PD-L1 in the immune evasion of tumor cells, expanding our understanding of intrinsic resistance mechanisms to immune checkpoint blockade therapy.