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Dysfunctional T cells in the tumour microenvironment have abnormally high expression of PD-1 and antibody inhibitors against PD-1 or its ligand (PD-L1) have become commonly used drugs to treat various types of cancer.sup.1-4. The clinical success of these inhibitors highlights the need to study the mechanisms by which PD-1 is regulated. Here we report a mechanism of PD-1 degradation and the importance of this mechanism in anti-tumour immunity in preclinical models. We show that surface PD-1 undergoes internalization, subsequent ubiquitination and proteasome degradation in activated T cells. FBXO38 is an E3 ligase of PD-1 that mediates Lys48-linked poly-ubiquitination and subsequent proteasome degradation. Conditional knockout of Fbxo38 in T cells did not affect T cell receptor and CD28 signalling, but led to faster tumour progression in mice owing to higher levels of PD-1 in tumour-infiltrating T cells. Anti-PD-1 therapy normalized the effect of FBXO38 deficiency on tumour growth in mice, which suggests that PD-1 is the primary target of FBXO38 in T cells. In human tumour tissues and a mouse cancer model, transcriptional levels of FBXO38 and Fbxo38, respectively, were downregulated in tumour-infiltrating T cells. However, IL-2 therapy rescued Fbxo38 transcription and therefore downregulated PD-1 levels in PD-1.sup.+ T cells in mice. These data indicate that FBXO38 regulates PD-1 expression and highlight an alternative method to block the PD-1 pathway.PD-1 undergoes internalization, FBXO38-mediated ubiquitination and proteasome degradation in activated T cells, and inhibition of this pathway dampens anti-tumour immunity of T cells., Author(s): Xiangbo Meng [sup.1] , Xiwei Liu [sup.1] , Xingdong Guo [sup.1] , Shutan Jiang [sup.1] , Tingting Chen [sup.2] , Zhiqiang Hu [sup.3] [sup.4] , Haifeng Liu [sup.5] , [...]