Abstract 586: Ionizing radiation-induced PD-L1 upregulation in glioma: a crucial role for the molecular chaperone FKBP5

Glioblastoma can avoid immune surveillance and induce tumor tolerance, through inhibitory molecules, e.g. PD-L1. Ionizing radiation (IR), used to treat this tumor, is known to increase tumor expression of PD-L1, thus inducing resistance mechanisms. Finding molecular determinants involved in IR-induced PD-L1 may provide a target for preventing such an effect and improve radiotherapy outcomes. We demonstrated that the short isoform of the cochaperone FKBP51 (FKBP51s) regulated PD-L1 expression in melanoma. In glioma, FKBP51s was expressed at high levels, together with PD-L1 and its silencing reduced PD-L1 levels. Conversely, overexpression of FKBP51s increased PD-L1. Different PD-L1 isoforms were observed by immunoblot. A lower band (~37 kDa) corresponding to the naïve protein and two upper bands (~50, ~68 kDa) ascribable to post-translationally modified isoforms. FKBP51s was found mainly bound to the heaviest bands of PD-L1, reasonably mature protein, while the canonical isoform FKBP51 appeared to bind only the naïve protein. Mature PD-L1 protein consists in carbohydrates addition, the principal chemical modification to most plasma membrane proteins, and, particularly, N-glycosylation. Treatment of immunoprecipitated PD-L1 protein with PNGaseF produced a decrease of the highest band and the appearance of a lower band, corresponding to the naïve PD-L1, in accordance with the concept that the heaviest band of PD-L1 is a glycosylated form. Moreover, following subcellular fractionation to obtain extracts from ER and Golgi compartments, we found that naïve 37 kDa PD-L1 was detectable in the ER, but not in the Golgi. The PD-L1 glycosylated band was expressed in ER in a small proportion and mostly in the Golgi. FKBP51s, but not the canonical FKBP51, was found in ER. Co-IP of FKBP51s and PD-L1 from ER extract confirmed the two proteins interacted each other in ER. Our results show that naïve PD-L1 colocalized in the ER of glioma cell complexed with FKBP51s, while the PD-L1 glycosylated form was measured in the Golgi apparatus. Treatment of glioma cell with increasing doses of IR upregulated PD-L1 expression, in a dose-response manner. Particularly, we found a significant increase in PD-L1 expression at 4 and 8 Gy, in comparison with unirradiated glioma cell. Moreover, IR induction of mature PD-L1 was efficiently counteracted by FKBP51s silencing. Subcellular fractionation of glioma cell subjected to IR in kinetics showed an early and transitory decrease in FKBP51s ER levels at 3hrs, in line with a reduction of the glycosylated band in the whole lysate. After 8 hrs from IR, FKBP51s rose up again in the ER inducing a full maturation of PD-L1. These findings suggested that FKBP51s has a role in catalyzing PD-L1 folding, an essential step to glycosylation, through which it controls the affinity for PD1. This study identifies FKBP51s as an essential element that regulates PD-L1 expression on glioma cell, which is exploited by the tumor to resist to IR.