Prostaglandin E₂ Modulates Monocytes and Dendritic Cells Specific Progenitor Cell (MDP) Fate by Regulating M-CSF Receptor and Flt3 Receptor Expression.

Abstract 2636

Prostaglandin E₂ (PGE2), the predominant metabolite of arachidonic acid metabolism by cyclooxygenase (COX) enzymes, is an important physiological regulator of hematopoiesis. We have previously shown that PGE₂ negatively regulates proliferation of macrophage colony formation (CFU-M) (Pelus et al. JEM 1979), however the mechanism through which PGE₂ inhibits monocyte/macrophage generation is not known. Recently we showed that blockade of endogenous PGE₂ synthesis in vivo in mice decreased overall dendritic cell (DC) number in lymphoid organs and bone marrow (Singh et al. ASH 2009). Since monocytes and dendritic cells originate in bone marrow from common hematopoietic progenitor cells termed monocyte and DC progenitor cells (MDP), having the phenotype (Lin⁻ cKit^hi CD115⁺CX3CR1⁺ Flt3⁺), we hypothesized that PGE₂ signaling may modulate MDP fate. Treatment of mice with Indomethacin, a dual COX 1 and COX2 inhibitor, for 6 days simultaneously increased bone marrow monocytes and decreased dendritic cells number compared to vehicle treated control mice. To determine whether the observed in vivo change in monocytes and dendritic cells after Indomethacin treatment was due to fate switch of MDP, we cultured FACS sorted MDP cells with Flt3L plus M-CSF, which simultaneously supports both DC and monocyte differentiation, in the absence or presence of Indomethacin for 9 days. Indomethacin reduced DC differentiation by 42±3.8% compared to vehicle control, and concomitantly increased monocyte generation in the same cultures by 25±2.8%. Moreover, as expected, addition of exogenous PGE₂ to these cultures reverted the Indomethacin mediated alteration in dendritic cell and monocyte generation from MDP. To confirm whether the reduction in dendritic cells and increase in monocytes generated from MDP upon the blockade of PGE₂ was due to preferential differentiation of MDP into monocyte-committed precursors rather than DC-committed precursors, we analyzed specific DC committed precursors (lin⁻c-kit^int M-CSFR⁺ Flt3⁺) and monocyte-committed precursors (CD11⁻CD11b⁺F4/80^intM-CSFR⁺) generated from MDP in vitro in the presence of Indomethacin. Fewer DC committed precursors were detected in Flt3L plus M-CSF cultures in the presence of Indomethacin, however monocyte precursor cell number increased compared to vehicle treated control. To further investigate whether a change in fate decision of MDP by blockade of PGE₂ is due to modulation of Flt3 and M-CSF receptors on MDP, we evaluated Flt3 receptor and M-CSF receptor on bone marrow MDP from Indomethacin treated mice. Flt3 receptor expression on MDP was decreased by 30±5.2% after Indomethacin treatment compared to control, however M-CSF receptor expression on MDP was increased by 51±4.5. In conclusion, our data suggest that PGE₂ regulates monocytes and dendritic cell generation by switching the differentiation fate of MDP by modulating M-CSF receptor and Flt3 receptor expression.