PD-L1 MODIFICATIONS IN CANCER DIAGNOSIS AND TREATMENT

PD-L1 Modifications in Cancer Diagnosis and Treatment PROJECT SUMMARY Programmed death ligand 1 (PD-L1, also known as CD274 or B7-H1) is the principal ligand of programmed death 1 (PD-1), a coinhibitory receptor on activated T cells. In the tumor microenvironment, PD-L1 overexpression is an immune evasion mechanism exploited by tumor cells and is generally associated with a poor prognosis in cancer patients before the era of immunotherapy. PD-L1 is a type I transmembrane protein with immunoglobulin V-like and C-like structures in its extracellular domain. Interaction between PD-L1 and PD-1 is a key oncogenic process between tumors and the host system. Inhibiting antibodies to block PD-1 or PD-L1 has revolutionized cancer care, whereas PD-L1 detection is often used as a companion or complementary diagnostic biomarker. However, contradictory evidence exists as to its role across histotypes. PD-L1 detection in tumors is complex because of variable antibodies and platforms, the subjective nature of scoring, and non-interchangeable definitions of PD-L1 positivity. Our preliminary studies demonstrate that a E3 ligase mediates non-proteolytic ubiquitination of PD-L1, facilitating its translocation from the cytosol to the plasma membrane, where it binds PD-1 extrinsically to prevent tumor cell killing by T cells. In this application, we propose two specific aims to ascertain the pathobiological role and translational significance of the non- proteolytic ubiquitination of PD-L1 in evading tumoricidal T cells and cancer care using lung cancer as a model. Lung cancer is chosen because it causes over 100,000 deaths annually in the United States and most FDA- approved anti-PD-L1 or anti-PD-1 treatment antibodies can be used for lung cancer. More lung cancer patients benefit from immune checkpoint blockade (ICB) than patients with any other cancer type. ICB therapies are usually recommended for late-stage lung cancer. However, most lung cancer patients do not respond to ICB therapy. Aim 1 will evaluate the clinical significance of PD-L1 and the E3 ligase expression in lung cancer diagnosis and prognosis. Aim 2 will dissect the molecular mechanisms of PD-L1 protein modifications in immune suppression using lung cancer cells and mice. This project will reveal the interplay among ubiquitinated PD-L1, the E3 ligase, anti-PD-L1 antibodies, and PD-1, which represents critical tumor and immune cell interactions in the tumor microenvironment to promote or attenuate tumorigenesis and progression. We expect to provide new insights into the molecular regulation of PD-L1 endomembrane trafficking and the clinical values of membranous PD-L1 and the modifying enzyme for lung cancer diagnosis, prognosis, and treatment.