During normal pregnancy, the maternal immune system remains systemically immunocompetent [33, 34]. However, it is faced with a dilemma that it must continue to protect the mother against infection while inhibiting immune activation against the semiallogeneic fetus. There is clear evidence that in the later stages of pregnancy women are uniquely susceptible to microbial infection. The prevailing view, though not shared by all, is that the predominant immune response is shifted from a gamma interferon based Th1 response to a less inflammatory Th2 response. The period of enhanced sensitivity to infection corresponds to the extremely high concentrations of E2 and PG that are reached in the later stages of pregnancy [16, 17]. Both E2 and PG have been shown to exert a powerful effect on various aspects of the innate and adaptive immune response [13, 14]. In a previous publication we investigated the effect of these hormones on the activation of DCs cultured from monocytes as well as those isolated directly from the blood. PG did not exert any effect on DCs in either the steady state or after activation by viruses. In contrast, E2 was a potent inhibitor of DC activation triggered by a number of human viruses but not by toll-like receptors ligands (TLRs) .
Alpha-defensins 1-3 are small peptides that can act both as natural antibiotics by directly killing a wide range of microorganisms and by participating in adaptive immunity by attracting monocytes, T cells, or immature DCs [35, 36]. Moreover, α-defensins 1-3 have been detected throughout the female reproductive tract (FRT) in non-pregnant women [5, 6] as well as in the amnion, chorion, placenta, amniotic fluids and cervical mucus plug from pregnant women [7–9]. Previous reports by us and others demonstrate the expression of α-defensins 1-3 by immune cells populations including MDDCs , NK cell, B cells, γδTcells, mast cells, neutrophils . Most human DC studies are performed using MDDCs produced by culturing these cells in the presence of the cytokines GM-CSF and IL4. It is not known whether cells produced this way simulate in vivo DC populations. However, mDCs as well as pDCs can be isolated directly from human blood by the expression of the CD1c marker (mDC) or BCDA4 (pDC) on their surface . In this report we analyzed directly isolated mDCs and pDCs and show that they both secrete significantly more α-defensins 1-3 than MDDCs. These results suggest that culturing of DCs may reduce their α-defensin 1-3 secretion and that previous work may have underestimated the contribution of DCs to in vivo α-defensin 1-3 production .
17-β-estradiol (E2), one of the most important forms of estrogen, acts through binding to two known hormone receptors, estrogen receptors ERα and ERβ , which are expressed in specific tissues as well as in immune cells, including macrophages, CD8+ T cells, CD4+ T cells, B cells, monocytes, and DCs . Cells pre-treated with ICI 182, 780, a specific ER antagonist, became insensitive to the inhibitory effect of E2, demonstrating a direct and specific effect of E2 on the release of α-defensins 1-3 by DCs. However, intrinsic or subsequently acquired resistance to the therapy remains a major obstacle in treatment with this compound as has been describe by Zhao et al., using a cell line of breast cancer . These open new questions that should be addressed in further investigations with dendritic cells. In our system, ICI 182,780 reduces the expression of the ER, as a result of the induction of a proteasome-dependent degradation induced by this compound . However, ICI 182,780 treatment does not affect the differentiation of monocytes into MDDCs as measured by percentage of CD11c+CD14- cells.
E2 but not PG suppressed the α-defensins 1-3 production from the mDCs at high concentration but had no effect on pDC α-defensins 1-3 secretion. This result is consistent with our earlier observation that E2 did not inhibit activation or cytokine secretion from pDCs. A possible explanation for these observations comes from the work of Hartman et al who demonstrated that murine pDCs derive exclusively from estrogen-resistant myeloid progenitors  which may explain the failure of E2 to alter α-defensin 1-3 production by these cells.
Based upon our in vitro studies we expected that mDCs collected from the blood in the later part of pregnancy would show a reduction in α-defensins 1-3 production. Moreover, evidence suggests that elevated levels of α-defensins 1-3 in serum correlated with adverse pregnancy outcomes [43, 44]. While we observed a trend towards less α-defensins 1-3 production from DCs collected in the 3rd trimester relative to those collected from the same patient in the 1st trimester, the difference was not statistically significant. Almost 40% of the patients did show a very substantial drop in α-defensins 1-3 release from their mDCs at third trimester, while the remaining patients secreted α-defensins 1-3 at nearly identical levels to what was observed in the 1st trimester. It is worth noting, however, that the levels of estrogen detected in serum were lower than the dose that most effectively inhibited α-defensin 1-3 secretion in vitro.
Among the 50% failed to show a reduction, the consistency of the mDC α-defensin 1-3 secretion within patients and variation between patients (ranging from 10 ng/ml to 60 ng/ml) was significant. This high inter-donor variability has been well described and likely corresponds to variations in the copy number of the genes that encode for α-defensins 1-3 between individuals .
The sum of the α-defensin 1-3 production by mDCs suggested a net loss in the later stages of pregnancy. If other cells that produce α-defensins 1-3 were likewise affected, serum α-defensins1-3 during pregnancy would be expected to drop.
In this report we document that myeloid and plasmacytoid DCs directly isolated from the blood produce substantial amounts of α-defensins 1-3 constitutively. pDCs are not affected, but both mDCs and MDDCs produce less α-defensins 1-3 in the presence of high levels of E2. Alpha-defensins 1-3 release from mDCs collected in the third trimester of pregnancy either fell or stayed the same in the majority of the patients.