Inhibition of microsomal prostaglandin E2 synthase reduces collagen deposition in melanoma tumors and may improve immunotherapy efficacy by reducing T cell exhaustion

The arachidonic acid pathway participates in immunosuppression in various types of cancer. Our previous observation detailed that microsomal prostaglandin E2 synthase 1 (mPGES-1), an enzyme downstream of cyclooxygenase 2 (COX-2), limited antitumor immunity in melanoma; in addition, genetic depletion of mPGES-1 specifically enhanced immune checkpoint blockade therapy. The present study set out to distinguish the roles of mPGES-1 from those of COX-2 in tumor immunity and determine the potential of mPGES-1 inhibitors for reinforcing immunotherapy in melanoma. Genetic deletion of mPGES-1 showed different profiles of prostaglandin metabolites from that of COX-2 deletion. In our syngeneic mouse model, mPGES-1–deficient cells exhibited similar tumorigenicity to that of COX-2–deficient cells, despite a lower ability to suppress PGE2 synthesis by mPGES-1 depletion, indicating the presence of factors other than PGE2 that are likely to regulate tumor immunity. RNA sequencing analysis revealed that mPGES-1 depletion reduced the expressions of collagen-related genes, which have been found to be associated with immunosuppressive signatures. In our mouse model, collagen was reduced in mPGES-1–deficient tumors, and phenotypic analysis of tumor-infiltrating lymphocytes indicated that mPGES-1-deficient tumors had fewer TIM3+ exhausted CD8+ T cells compared with COX-2–deficient tumors. CAY10678, an mPGES-1 inhibitor, was equivalent to celecoxib, a selective COX-2 inhibitor, in reinforcing anti–PD-1 treatment. Our study indicates that mPGES-1 inhibitors represent a promising adjuvant for immunotherapies in melanoma by reducing collagen deposition and T cell exhaustion. In order to evaluate the specific roles of mPGES-1 in melanoma, we established ptgs1, ptgs2, and ptges genes (corresponding to COX-1, COX-2, and mPGES-1 proteins, respectively) knockout murine BrafV600E melanoma cells derived from C57BL/6 Braf+/LSL-V600E;Tyr::CreERT2+/o;p16INK4a-/- mouse using CRISPER/CAS9 system.