A highly expressed
prostaglandin E2 (
PGE2) in
tumor tissues suppresses antitumor immunity in the tumor microenvironment (TME) and causes tumor immune evasion leading to
disease progression. In animal studies, selective inhibition of the
prostaglandin E receptor 4 (EP4), one of four
PGE2 receptors, suppresses
tumor growth, restoring the
tumor immune response toward an antitumorigenic condition. This review summarizes
PGE2/EP4 signal inhibition in relation to the
cancer-immunity cycle (C-IC), which describes fundamental
tumor-immune interactions in
cancer immunotherapy.
PGE2 is suggested to slow down C-IC by inhibiting natural killer cell functions, suppressing the supply of conventional dendritic cell precursors to the TME. This is critical for the
tumor-associated
antigen priming of CD8+ T cells and their translocation to the
tumor tissue from the
tumor-draining lymph node. Furthermore,
PGE2 activates several key immune-suppressive cells present in
tumors and counteracts tumoricidal properties of the effector CD8+ T cells. These effects of
PGE2 drive the
tumors to non-T-cell-inflamed
tumors and cause refractory conditions to
cancer immunotherapies, e.g.,
immune checkpoint inhibitor (ICI) treatment. EP4 antagonist
therapy is suggested to inhibit the immune-suppressive and tumorigenic roles of
PGE2 in
tumors, and it may sensitize the
therapeutic effects of ICIs in patients with non-inflamed and C-IC-deficient
tumors. This review provides insight into the mechanism of action of EP4 antagonists in
cancer immunotherapy and suggests a C-IC modulating opportunity for EP4 antagonist
therapy in combination with ICIs and/or other
cancer therapies.