EXTH-06. DOWN-REGULATION OF PD-L1 VIA FKBP5 LOWERED BY A CYCLOOXYGENASE-2 INHIBITOR IN GSCs AND GBM CELLS MAY BE ATTRIBUTABLE TO ENHANCE ANTITUMOR EFFECTS OF IMMUNOTHERAPY

BACKGROUND Antitumor therapies targeting programmed cell death-1 (PD-1)/its ligand-1 (PD-L1) are influential at present stage. However, in glioblastoma (GBM), the expression of PD-L1 is variable and the role of anti-PD-1 antibody therapy is still unclear. The high expression of PD-L1 affects cell proliferation and invasion in GBM cells. As COX-2 modulates PD-L1 expression in cancer cells, we tested our hypothesis that a COX-2 inhibitor, celecoxib may play a role on anti-PD-1 antibody treatment via down-regulation of PD-L1. METHODS Six weeks old male C57BL/6 mice subjected to intracranial injection of mice glioma stem cells (GSCs) were randomly divided into four treatment groups; vehicle control (VC), celecoxib, anti PD-1 antibody or the combination of celecoxib and anti-PD-1 antibody groups and examined antitumor effects. To verify the mechanisms underlying antitumor effects, mice GSCs and human GBM cells were used. RESULTS Compared to each single treatment in the glioma model, the combination therapy of anti PD-1 antibody and celecoxib significantly decreased the tumor volume and improved the survival period. Importantly, the high expression of PD-L1 in the glioma model, mice GSCs and human GBM cells was decreased by celecoxib. Interestingly, the reduction of PD-L1 was associated with post-transcriptional regulation of co-chaperone FK506-binding protein 5 (FKBP5) by celecoxib. The combination therapy of anti PD-1 antibody with celecoxib could be a promising therapeutic strategy targeting PD-L1 in GSCs and GBM. CONCLUSIONS Down-regulation of PD-L1 via FKBP5 by celecoxib may play a role on the antitumor effects under the overwhelmed expression of PD-L1.