Differential localization of PD-L1 and Akt-1 involvement in radioresistant and radiosensitive cell lines of head and neck squamous cell carcinoma

Abstract

Immunotherapy by blockade of the PD-1/PD-L1 checkpoint demonstrated amazing tumor response in advanced cancer patients including head and neck squamous cell carcinoma (HNSCC). However, the majority of HNSCC patients still show little improvement or even hyperprogression. Irradiation is currently investigated as synergistic treatment modality to immunotherapy as it increases the number of T-cells thereby enhancing efficacy of immunotherapy. Apart from this immunogenic context a growing amount of data indicates that PD-L1 also plays an intrinsic role in cancer cells by regulating different cellular functions like cell proliferation or migration. Here, we demonstrate opposing membrane localization of PD-L1 in vital and apoptotic cell populations of radioresistant (RR) and radiosensitive (RS) HNSCC cell lines up to 72 h after irradiation using flow cytometry. Moreover, strong PD-L1 expression was found in nuclear and cytoplasmic cell fractions of RR. After irradiation PD-L1 decreased in nuclear fractions and increased in cytoplasmic fractions of RR cells. In contrast, RS cell lines did not express PD-L1, neither in the nucleus nor in cytoplasmic fractions. Additionally, overexpression of PD-L1 in RS cells led to a proportional increase of vital PD-L1 positive cells after irradiation. Moreover, co-immunoprecipitation experiments revealed an interaction between Akt-1 and PD-L1, mostly in irradiated RR cells compared to RS cells suggesting a differential influence of PD-L1 on cell signaling. In summary, our data imply the need for different therapeutic strategies dependent on the molecular context in which PD-L1 is embedded.

Immune checkpoint expression in HNSCC patients before and after definitive chemoradiotherapy

“…This is in line with our observation of decreased disease‐free survival. Moreover, they saw a time‐dependent increase of PD‐L1 in rs cells and decrease in rr cells after irradiation 33 . It has to be assumed that cancerous tissue consists of both, rr and rs cells and this might be the explanation for the fact that we could measure no significant changes of PD‐L1 by CRT.…”

“…Our review revealed no obvious consensus about the prognostic role of PD‐L1 expression (Table 2). 26‐33 Among these studies, Schulz et al were the only group to look at changes of PD‐L1 expression by radiation. They compared known radio‐resistant (rr) with radio‐sensitive (rs) cell lines and could measure high PD‐L1 expression in rr cells before radiation.…”

Posttranslational Modifications in PD-L1 Turnover and Function: From Cradle to Grave

“…PD-L1 is mainly located on the cell membrane and in the cytoplasm, yet it is also found in the nucleus to a lesser extent. Nuclear PD-L1 is detected in many cancer tissues, including renal cell carcinomas, colorectal cancer, prostate cancer, lung cancer, and hepatocellular carcinomas [ 57 , 58 , 59 ]. Its expression is significantly correlated with tumor invasion, radioresistance, and overall survival, suggesting that nuclear PD-L1 could be a potential prognostic biomarker in cancer patients [ 57 , 60 ].…”

Differential Expression of PD-L1 during Cell Cycle Progression of Head and Neck Squamous Cell Carcinoma

“…In our studies, we explored the underlying reason behind these findings. In recent years, a growing number of studies have concluded that PD-L1 may not only have immunogenic but at the same time cell-intrinsic functions [ 23 , 24 , 38 ]. This study was conducted because we observed a confluence- and time-dependent fluctuation of PD-L1 expression in all our HNSCC cell lines.…”

“…In previous studies, we demonstrated the strong influence of PD-L1 on proliferation, migration, invasion, and survival after irradiation [ 23 ]. Furthermore, we revealed differential expression and localization of PD-L1 depending on the sensitivity to irradiation [ 24 ].…”