Progesterone via its receptor antagonizes the pro-inflammatory activity of estrogen in the mouse uterus.

Populations of macrophages and neutrophils in the uterus are under the control of the female sex steroids estrogen and progesterone (P4). Their influx is induced by estrogen, while P4 can both stimulate and inhibit leukocyte influx depending on the timing of P4 with respect to estrogen. Regulation of leukocytes has been implicated in changes in uterine immune responses during the estrous cycle, pregnancy, and implantation. This work demonstrates that P4 given concurrently with estrogen to ovariectomized mice for 4 days antagonizes the ability of estrogen to recruit macrophages and neutrophils into the mouse uterus. Using progesterone receptor knockout (PRKO) mice, we show that this effect is dependent on progesterone receptors (PR). In the absence of PR, neutrophils recruited by estrogen were found to be degranulated, partially explaining the edema that is observed with long-term treatment of PRKO mice with estrogen and P4. Populations of B lymphocyte cells were shown to be unchanged by estrogen and P4 treatment in both wild-type and PRKO mice. The neutrophil chemotactic chemokine MIP-2 was examined for down-regulation by P4 but was found to be unaffected by hormonal treatment. Together, these observations demonstrate that PR has a strong anti-inflammatory role in the mouse uterus when estrogen and P4 are present together.