BACKGROUND:The levels of expression and membrane localization of programmed cell death ligand 1 (PD-L1), an immune checkpoint type I transmembrane glycoprotein, are related to the clinical response of anti-PD-L1/PD-1 therapy. Although the biologically relevant localization of PD-L1 is on the plasma membrane of cancer cells, it has also been reported to be in the cytoplasm and sometimes in the nucleus. Furthermore, it has been claimed that chemotherapeutics can modify PD-L1 expression and/or its nuclear localization. RESULTS:Data from our group suggest that the nuclear localization of PD-L1, and other plasma membrane proteins as well, could be an artifact resulting from inadequate experimental conditions during immunocytochemical studies. Mild detergent and rigorous fixation conditions should be used in order to preserve the membrane localization and to prevent an erroneous translocation of PD-L1 and other non-interconnected membrane proteins, such as CD24, into other cellular compartments including the nucleus, of untreated and chemotherapeutically treated breast cancer cells. CONCLUSION:We propose that well-specified and rigorously followed protocols should be applied to immunocytochemical diagnostic techniques, especially to those related to individualized diagnosis and treatment.

As the main immune checkpoint, PD-L1-PD-1 interaction plays a critical role in the dysregulation of effector T cells, which contributes to the failure of Chimeric Antigen Receptor T-cell (CAR-T) and other immunotherapies. Presently, most research focuses on the extracellular function of PD-L1. Membrane PD-L1 can interact with its receptor PD-1 and decrease T cell-induced cancer immunity. However, the function of PD-L1 in cancer cells is still unclear. Recent studies have shown the separated clinical significance of PD-L1 expression in various cancer types, showing the complexity of PD-L1 in cancer cell regulation. As a novel regulatory pathway, the nuclear translocation of PD-L1 in cancer cells receives more attention. Results of these preclinical studies demonstrated that nuclear PD-L1 has an essential role in cancer development and other immune checkpoint molecules transcription. Herein, we summarized the mechanisms involved in PD-L1 nuclear transportation and identify the key regulatory factors in this process. Furthermore, we also summarize the function of nuclear PD-L1 in cancer immunity. These findings suggested the novel PD-L1 regulation in cancer development, which showed that nuclear PD-L1 is a potential therapeutic target in future cancer therapy.

Accumulating evidence demonstrates that the expression levels of programmed cell death protein 1 (PD-1) and programmed death ligand 1 (PD-L1) are regulated at the various levels, including transcription, post-transcriptional modification and post-translational modifications (PTMs). The PTMs of PD-1/PD-L1 contain phosphorylation, ubiquitination, methylation, glycosylation and palmitoylation. Recently, PD-L1 was reported to be acetylated at Lys263 site by p300 and was deacetylated by histone deacetylase 2 (HDAC2). Acetylation of PD-L1 prevented its translocation to the nucleus and led to a reduction of the nuclear portion of PD-L1, resulting in evading immune surveillance of tumor cells. In this review article, we briefly describe the PTMs of PD-1/PD-L1 and mainly summarize the novel findings of PD-L1 acetylation in tumor cells. Moreover, we discuss the associations of PD-L1 acetylation and ubiquitination, phosphorylation and methylation. Furthermore, we highlight that targeting acetylation of PD-L1 by HDAC inhibitors might be useful for enhancing tumor immunotherapy.

Targeting programmed cell death protein ligand 1 (PD-L1) remains one of the most essential immunotherapies in cancer. PD-L1 has been detected in the nucleus in multiple malignancies, playing an oncogenic role independent of immune checkpoint regulation. Howbeit, the regulatory function of nuclear PD-L1 (nPD-L1) remains to be fully understood. Here, we report that nPD-L1 is an endogenous accelerator for cancer angiogenesis. First, we found that an abundant proportion of PD-L1 was distributed within the nucleus of uveal melanoma samples, which is associated with an unfavorable outcome. Moreover, the capacity of promoting angiogenesis was largely attenuated in the nPD-L1-deficient cells both in vivo and in vitro. Mechanistically, nPD-L1 facilitates p-STAT3 binding to the promoter of early growth response-1 (EGR1), resulting in the activation of EGR1-mediated angiogenesis. Therapeutically, the inhibition of histone deacetylase 2 restores the normal acetylation level of PD-L1, blocking its nuclear translocation and thereby attenuating tumor angiogenesis. Conclusively, we reveal that nPD-L1 promotes angiogenesis in malignancies, and provide a novel anti-vascularization strategy through blocking aberrant PD-L1 nuclear translocation for tumor therapy.

Immunotherapies that target programmed cell death protein 1 (PD-1) and its ligand PD-L1 as well as cytotoxic T-lymphocyte-associated protein 4 (CTLA4) have shown impressive clinical outcomes for multiple tumours. However, only a subset of patients achieves durable responses, suggesting that the mechanisms of the immune checkpoint pathways are not completely understood. Here, we report that PD-L1 translocates from the plasma membrane into the nucleus through interactions with components of the endocytosis and nucleocytoplasmic transport pathways, regulated by p300-mediated acetylation and HDAC2-dependent deacetylation of PD-L1. Moreover, PD-L1 deficiency leads to compromised expression of multiple immune-response-related genes. Genetically or pharmacologically modulating PD-L1 acetylation blocks its nuclear translocation, reprograms the expression of immune-response-related genes and, as a consequence, enhances the anti-tumour response to PD-1 blockade. Thus, our results reveal an acetylation-dependent regulation of PD-L1 nuclear localization that governs immune-response gene expression, and thereby advocate targeting PD-L1 translocation to enhance the efficacy of PD-1/PD-L1 blockade.

Although targeting programmed death 1/programmed death ligand 1 (PD-1/PD-L1) has achieved durable responses and disease remission in patients with certain cancers, relatively low response rates and emerging resistance limit its clinical application. Hence, a more thorough understanding of regulatory mechanisms of the PD-1/PD-L1 axis is vital for developing combined therapeutic strategies to overcome hurdles of PD-1/PD-L1 blockade. Increasing evidence has demonstrated that PD-L1 can be secreted into the extracellular space or translocated into the nucleus, which also plays a critical role in regulating cancer immune evasion, tumorigenesis, and immunotherapy. In this review, we summarize these emerging roles of extracellular and nuclear PD-L1 and discuss future research directions and potential opportunities in translational medicine.