Pregnant women are at high risk for severe influenza disease outcomes, yet insights into the underlying mechanisms are limited. Here, we present models of H1N1 infection in syngenic and allogenic pregnant mice; infection in the latter mirrors the severe course of 2009 pandemic influenza in pregnant women. We found that the anti-viral immune response in the pregnant host was significantly restricted as compared to the non-pregnant host. This included a reduced type I interferon response as well as impaired migration of CD8 T cells into the lung. The multi-faceted failure to mount an anti-viral response in allogenic pregnant mice resulted in a less stringent selective environment that promoted the emergence of 2009 H1N1 virus variants that specifically counteract type I interferon response and mediate increased viral pathogenicity. These insights underscore the importance of influenza vaccination compliance in pregnant women and may open novel therapeutic avenues.

Up to 10% of pregnancies in Western societies are affected by intrauterine growth restriction (IUGR). IUGR reduces short-term neonatal survival and impairs long-term health of the children. To date, the molecular mechanisms involved in the pathogenesis of IUGR are largely unknown, but the failure to mount an adequate endocrine and immune response during pregnancy has been proposed to facilitate the occurrence of IUGR. A cross talk between the pregnancy hormone progesterone and innate immune cell subsets such as dendritic cells (DCs) is vital to ensure adequate placentation and fetal growth. However, experimental strategies to pinpoint distinct immune cell subsets interacting with progesterone in vivo have long been limited. In the present study, we have overcome this limitation by generating a mouse line with a specific deletion of the progesterone receptor (PR) on CD11c + DCs. We took advantage of the cre/loxP system and assessed reproductive outcome in Balb/c-mated C57Bl/6 PR flox/flox CD11c cre/wt females. Balb/c-mated C57Bl/6 PR wt/wt CD11c cre/wt females served as controls. In all dams, fetal growth and development, placental function and maternal immune and endocrine adaptation were evaluated at different gestational time points. We observed a significantly reduced fetal weight on gestational day 13.5 and 18.5 in PR flox/flox CD11c cre/wt females. While frequencies of uterine CD11c + cells were similar in both groups, an increased frequency of co-stimulatory molecules was observed on DCs in PR flox/flox CD11c cre/wt mice, along with reduced frequencies of CD4 + FoxP3 + and CD8 + CD122 + regulatory T (Treg) cells. Placental histomorphology revealed a skew toward increased junctional zone at the expense of the labyrinth in implantations of PR flox/flox CD11c cre/wt females, accompanied by increased plasma progesterone concentrations. Our results support that DCs are highly responsive to progesterone, subsequently adapting to a tolerogenic phenotype. If such cross talk between progesterone and DCs is impaired, the generation of pregnancy-protective immune cells subsets such as CD4 + and CD8 + Treg cells is reduced, which is associated with poor placentation and IUGR in mice.

Problem Steroid hormones such as progesterone and glucocorticoids rise during pregnancy and are accountable for the adaptation of the maternal immune system to pregnancy. How steroid hormones induce fetal tolerance is not fully understood. We hypothesized that steroid hormones selectively regulate the T‐cell response by promoting T‐cell death. Method of study We incubated murine spleen cells isolated from non‐pregnant and pregnant mice with physiological concentrations of steroid hormones in vitro and analyzed T‐cell subsets after 48 h of incubation. ResultsWe found that progesterone and the synthetic glucocorticoid dexamethasone induce T‐cell death. CD4+ regulatory T (Treg) cells were refractory toward progesterone‐induced cell death, in contrast to conventional CD4+ T cells, which resulted in a preferential enrichment of CD4+ Tregcells in culture. T cells isolated from pregnant mice at early and late gestation showed comparable sensitivity to steroid‐induced cell death. The target receptor for progesterone in immune cells is controversially discussed. We provide here support of progesterone binding to the glucocorticoid receptor as only T cells lacking the glucocorticoid but not the progesterone receptor showed resistance against progesterone‐induced death. ConclusionsOur results indicate that high levels of progesterone during pregnancy can induce selective T‐cell death by binding the glucocorticoid receptor. Although physiological hormone concentrations were used, due to different bioavailability of steroid hormones in vivo these results have to be validated in an in vivo model. This mechanism might ensure immunological tolerance at the feto‐maternal interface at gestation.