9: Progesterone promotes the expansion of proangiogenic immature myeloid cells and prevents their differentiation into inflammatory cells

Abstract

Immature myeloid cells (IMCs) are bone marrow-derived cells that normally differentiate into granulocytes, macrophages, and dendritic cells (DCs) but expand in pathological conditions such as malignancy. DCs are antigen-presenting cells that regulate the immune response. We have shown that IMCs accumulate in the placenta and actively promote angiogenesis. IMCs peak in concentration during mid-pregnancy and their presence correlates with neonatal birthweight. Furthermore, labor and delivery are preceded by a decrease in IMCs and an increase in DCs populating the placenta. We hypothesized that progesterone may promote IMC population expansion while preventing their differentiation into inflammatory cells. We derived bone marrow cells from C57Bl6 pregnant mice. We isolated the CD11b+ myeloid subset by magnetic based immuno-separation, and cultured them in the presence of GMCSF and progesterone (10-7, 10-8, and 0 mM). After 5 days we analyzed cultures for the presence of Ly6CintLy6Ghi granulocytic IMCs, Ly6ChiLy6Gint monocytic IMCs and CD11c+MHCII+ DCs by flow cytometry. We also compared their presence in Lewis Lung Carcinoma tumors that were implanted subcutaneously in mice to that in placentas derived from pregnant mice. Progesterone treatment caused a dose dependent increase in granulocytic IMCs (n=3, p<0.05), accompanied by a concomitant decline in the monocytic IMCs (n=3, p<0.001). Importantly, these changes were paralleled by a decrease in DCs (n=3, p<0.05). When analyzing CD45+ hematopoietic cells in tumors and placentas, we detected a significant enrichment (P<0.01) of monocytic IMCs subpopulation in tumors compared to that in placentas. This was paralleled by a concomitant, more than 2-fold decrease (P<0.01) in granulocytic IMCs. Progesterone enhances proliferation and/or survival of placenta specific- granulocytic IMCs but not that of tumor specific- monocytic IMCs, in a dose dependent manner. Importantly, the differentiation of IMCs into DCs was abrogated by progesterone. We thus speculate that progesterone may play role in the maintenance of proangiogenic IMCs in the placenta. Inhibition of their differentiation into inflammatory cells such as DCs and neutrophils might explain, at least in part, the protective effect of progesterone in preventing preterm labor.