Estrogen and progesterone modulate monocyte cell cycle progression and apoptosis

Pregnancy is characterized by dramatic immunologic changes most commonly characterized as suppression of cell-mediated immunity. Mechanisms of this immunosuppression are obscure but may be caused by increases in pregnancy-associated sex steroids such as 17-beta-estradiol or progesterone.

Using five myelomonocytic cell lines in various stages of differentiation, the effects of 17-beta-estradiol and progesterone on cell cycling, apoptosis, and bcl-2 expression in randomly cycling cells before and after lipopolysaccharide (LPS) activation were examined.

Lipopolysaccharide alone inhibited cell cycle progression in THP-1 monocyte-like cells and U-937 histiocyte-like cells. Estrogen alone produced cell cycle arrest in all myelomonocytic cells except HL-60 pro-myelocyte-like cells. Progesterone had effects predominantly on pro-myelocytic-like HL-60 cells, inducing apoptosis. Estrogen and progesterone both decreased levels of bcl-2 in KG-1alpha, HL-60, and THP-1 cells. LPS partially antagonized both estrogen-induced THP-1 apoptosis and its suppression of bcl-2 protein.

Sex steroid-induced effects on cell cycle transition and apoptosis are potential mechanisms by which pregnancy-induced cell-mediated immune suppression may occur. Further investigation should provide a better understanding of pregnancy-induced immune changes and, perhaps, sex-based differences in monocyte function and immunologic responses.