This study establishes that uNK cells play a central role in inflammation-induced preterm labor and delivery. IL-10 likely prevents preterm parturition by regulation uNK cell function in the presence of an inflammatory insult. Unlike wild type mice, very low doses of LPS can induce preterm birth in IL-10−/− mice where uNK cells become cytotoxic, increase in number, and migrate to the placenta. Our findings suggest a maternal immune-mediated mechanism of LPS-induced premature birth with cytotoxic and migratory uNK cells invading fetal tissue resulting in pathology.
Evidence for an indispensable role for uNK cells in inflammation-induced preterm labor and delivery derives from NK cell depletion experiments in IL-10−/−
mice. This result is surprising given the population kinetics of uNK cells. In both mice and human beings, specialized uNK cells invade the uterus during decidualization, peak at midgestation, and decline until they are nearly absent at term.31,33
In LPS-treated IL-10−/−
mice, the uNK cell number was significantly higher than in wild type control groups during late gestation (). Thus, it appears that LPS-induced inflammatory signals promote either proliferation of resident uNK cells or recruitment of uNK cell precursors to the uterus in IL-10−/−
mice. In vitro studies demonstrate LPS-mediated effects on human NK cells, including proliferation, cytotoxicity, and IFN-γ production.39–42
Our data provide in vivo evidence for LPS-mediated uNK cell activation and show invasion of uNK cells into the placenta as a necessary step to induce preterm delivery. An alternative explanation for LPS-induced preterm delivery could be functional loss of progesterone (P4) as P4 is required for implantation and maintenance of pregnancy.43
Because NK cell-depleted IL-10−/−
mice do not experience preterm birth in response to LPS and spectrum P4 levels are similar between LPS-treated IL-10−/−
and wild type mice (), it appears that preterm labor and delivery in our model is independent of P4 activity. uNK cells appear to be a key cell population linking inflammatory insult to preterm labor and delivery.
The sensitivity of IL-10−/−
mice to LPS-induced uNK cell-associated preterm birth suggests that IL-10 plays a key regulatory role in uNK cell function and proliferation. Single dose administration of recombinant IL-10 restored pregnancy to term in these mice (). Invasion of uNK cells into the placenta of LPS-treated IL-10−/−
mice suggest aberrant production or regulation of chemotactic factors by the placenta. LPS can trigger the expression of various chemokines in macrophages and trophoblasts,44,45
whereas IL-10 acts to down-regulate these chemokines.46,47
The absence of this regulation in IL-10−/−
mice may contribute to abnormal cellular trafficking.
In vivo TNFα neutralization in LPS-treated IL-10−/−
mice prevents uNK cell invasion into the placenta, placental cell apoptosis, and preterm labor and delivery (; and ). TNFα induces the production of prostaglandins and metalloproteinases, molecules associated with cellular infiltration.48
During normal pregnancy, uNK cells remain noncytotoxic despite the presence of cytotoxic granules. We hypothesized that in LPS-treated IL-10−/−
mice, uNK cells may kill through perforin/granzyme granule-mediated mechanisms. Human uNK cells express molecules required for targeted cell lysis and are able to form conjugates and immune synapses with target cells, yet remain noncytotoxic.49
Because TNFα was found to be a regulator of uNK cell cytotoxicity and migration ( and ), we conclude that the uNK cell-TNFα axis is an important mechanism for preterm labor and delivery in IL-10−/−
Our data suggest a regulatory role for IL-10 on uNK cells during an inflammatory insult leading to preterm parturition. It is plausible that pregnant women with premature reductions in the levels of IL-10 will experience preterm labor and delivery in response to even subclinical infection/inflammation. Thus, our results using IL-10−/− mice provide a cogent framework for explanation of inflammation-associated preterm labor and delivery, leading to further studies in human beings and revealing potential targets for clinical interventions.