Prostaglandin E2-EP2/EP4 signaling induces the tumor-infiltrating Treg phenotype for tumor growth

Foxp3+ regulatory T cells (Tregs) heavily infiltrate malignant tumors and restrict anti-tumor immunity. These tumor-infiltrating Tregs (TI-Tregs) adopt a distinct phenotype by expressing a unique set of genes including those encoding various co-stimulatory and co-inhibitory molecules. This TI-Treg gene expression signature is conserved in TI-Tregs across species and the types and stages of tumors, suggesting the presence of a common inducing mechanism or mechanisms in tumor microenvironment (TME). However, identity of such a mechanism remains elusive. Here, we show that prostaglandin E2 (PGE2) produced in TME directly acts on its receptor EP2/EP4 on Tregs to induce the TI-Treg phenotype. Indeed, PGE2 added to TCR-activated Tregs induces a set of genes, many of which are included in the reported TI-Treg signature genes, in both induced Tregs (iTregs) and naturally occurring Tregs (nTregs), in a manner dependent on EP2 and EP4 via cAMP-PKA pathway. This is not limited to phenotypic changes, as EP4 agonist-treated Tregs exhibit potent suppressive activity to CD8+ T cells and strongly inhibit their proliferation and survival. Consistently, selective loss of EP2 and EP4 in mouse Tregs and pharmacological inhibition of EP2 and EP4 reduce expression of those genes in Tregs infiltrating LLC1 mouse syngeneic tumor, and significantly delay the tumor progression. In human FOXP3+iTregs, the expression of Treg signature genes, FOXP3, CD25 and CTLA4 as well as a typical TI-Treg signature gene, 4-1BB are significantly upregulated by PGE2-EP4 signaling. Furthermore, analysis of scRNA-seq of nasopharyngeal cancer patients demonstrates preferential expression of the TI-Treg signature genes in Tregs infiltrating the PTGShi tumor group compared to the PTGS2lo tumor group. These findings suggest that PGE2-EP2/EP4 signaling is one of the molecular mechanisms inducing the TI-Treg phenotype in TME.