Reversing PD-1 Resistance in B16F10 Cells and Recovering Tumour Immunity Using a COX2 Inhibitor

SIMPLE SUMMARY: Some patients develop drug resistance to programmed cell death protein 1/programmed death-ligand 1(PD-1/PD-L1) therapy but the mechanism is unclear. Therefore, the study of drug resistance to PD-1 therapy is quite important. In this sense, we obtained B16F10-R tumours resistant to anti-PD-1 therapy through multiple rounds of drug resistance screening in vitro. We found that COX2 expression was significantly elevated and COX2 inhibitors in combination with anti-PD-1 monoclonal antibodies (mAbs) could reverse this resistance phenomenon. Knockout of the ptgs2 gene in B16F10-R tumours also restored tumour sensitivity to anti-PD-1 therapy. Therefore, we believe that the combination of COX2 inhibitors and anti-PD-1 mAbs may become a new choice for the drug resistance of anti-PD-1 therapy in the future. ABSTRACT: Immunotherapy is an effective method for tumour treatment. Anti-programmed cell death protein 1 (PD-1) and anti-programmed death-ligand 1 (PD-L1) monoclonal antibodies play a significant role in immunotherapy of most tumours; however, some patients develop drug resistance to PD-1/PD-L1 therapy. Cyclooxygenase-2 (COX2) is expressed in various solid tumours, and prostaglandin E2 (PGE2) drives the development of malignant tumours. We developed a drug-resistant B16F10 (B16F10-R) tumour mouse model through four rounds of selection in vivo. Subsequently, we investigated changes in PD-L1 expression and lymphocyte infiltration in B16F10-NR and B16F10-R tumours. Additionally, we explored the role of COX2 in acquired resistance to pembrolizumab, an anti-PD-1 treatment. Immune cell infiltration was significantly decreased in resistant tumours compared to B16F10-NR tumours; however, ptgs2 gene expression was significantly elevated in resistant tumours. Aspirin or celecoxib combined with pembrolizumab can effectively reverse tumour drug resistance. In addition, ptgs2 knockout or the use of the EP4 inhibitor E7046 abrogated drug resistance to anti-PD-1 treatment in B16F10-R tumour cells. Our study showed that inhibition of the COX2/PGE2/EP4 axis could increase the number of immune cells infiltrating the tumour microenvironment and recover drug-resistant tumour sensitivity to pembrolizumab. Thus, we highlight COX2 inhibition as a promising therapeutic target for drug-resistant tumours for future consideration.