Mesenchymal PGD₂ activates an ILC2-Treg axis to promote proliferation of normal and malignant HSPCs

Cyclooxygenase (COX)-dependent production of prostaglandins (PGs) is known to play important roles in tumorigenesis. PGD₂ has recently emerged as a key regulator of tumor- and inflammation-associated functions. Here we show that mesenchymal stromal cells (MSCs) from patients with acute myeloid leukemia (AML) or normal MSCs overexpressing COX2 promote proliferation of co-cultured hematopoietic stem and progenitor cells (HSPCs), which can be prevented by treatment with COX2 knockdown or TM30089, a specific antagonist of the PGD₂ receptor CRTH2. Mechanistically, we demonstrate that PGD₂-CRTH2 signaling acts directly on type 2 innate lymphoid cells (ILC2s), potentiating their expansion and driving them to produce Interleukin-5 (IL-5) and IL-13. Furthermore, IL-5 but not IL-13 expands CD4⁺CD25⁺IL5Rα⁺ T regulatory cells (Tregs) and promotes HSPC proliferation. Disruption of the PGD₂-activated ILC2-Treg axis by specifically blocking the PGD₂ receptor CRTH2 or IL-5 impedes proliferation of normal and malignant HSPCs. Conversely, co-transfer of CD4⁺CD25⁺IL5Rα⁺ Tregs promotes malignant HSPC proliferation and accelerates leukemia development in xenotransplanted mice. Collectively, these results indicate that the mesenchymal source of PGD₂ promotes proliferation of normal and malignant HSPCs through activation of the ILC2-Treg axis. These findings also suggest that this novel PGD₂-activated ILC2-Treg axis may be a valuable therapeutic target for cancer and inflammation-associated diseases.