-induced positive cyclooxygenase 2-prostaglandin E
-E-prostanoid receptor 2/4 feedback loop allows for ex vivo generation of high numbers of myeloid-derived suppressor cells and their functional stability. (A) Prostaglandin E2 (PGE2) (via E-prostanoid receptor (EP)2- and EP4-dependent signals) drives the early induction of cyclooxygenase (COX)2 in local myeloid cells (monocytes, macrophages, immature dendritic cells (iDCs)), promoting their production of suppressive factors (indoleamine 2,3-dioxygenase (IDO)1, IL-10, arginase 1, nitric oxide synthase (NOS)2, and PGE2 itself (current data and ), and acquisition of suppressive functions . These processes are further amplified by the de novo production of endogenous PGE2, now produced at high levels by myeloid-derived suppressor cells (MDSCs) themselves, thereby creating a positive feedback loop leading to persistence of MDSCs. The key role of the EP2- and EP4-mediated COX2-PGE2 feedback to control multiple aspects of MDSCs function provides convenient targets to generate MDSC-associated immune regulation in tolerogenic therapies. (B) PGE2 induces high numbers of MDSCs (48.6%), with yields similar to iDCs (40.2%) and TNF-α matured DCs (36.9%). Percentages indicate the yields of the cells generated in day 6 monocyte cultures performed in the presence of granulocyte macrophage colony stimulating factor (GM-CSF) and IL-4 in the absence of PGE2 (iDC, CD1a+ DCSIGN+CD14-CD80-CD83-) or presence of PGE2 (MDSCs, CD1a-DCSIGN-CD14+CD33+CD34+CD80-CD83-) and after additional 48 h maturation of iDC with TNF-αmDC, (CD1a+ DCSIGN+CD14-CD80+CD83+). Bars present data (mean ± s.d.) from 12 different experiments with different donors. (C) Dose-dependent induction of immunosuppressive factors IL10, IDO1, IL4Rα and COX2 in PGE2-induced MDSCs, generated in the presence or absence of IL-4 (relative mRNA levels normalized for hypoxanthine phosphoribosyltransferase 1 and expressed as fold increase (2-ΔCT), where ΔCT = CT (Target gene) - CT (HPRT1)). Bars present data (mean ± s.d) of a single representative experiment with different donors. *P <0.05, **P <0.01, ***P <0.001, statistically significant differences relative to medium alone.