Uterine natural killer cells in a three-dimensional tissue culture model to study trophoblast invasion

The high numbers of CD56(+) cells with natural killer (NK) functions present in the uterine mucosa during the late secretory phase of the menstrual cycle and during early pregnancy have been considered to be implicated in implantation and in the regulation of trophoblast invasion. A three-dimensional organ culture model was used to study the interactions of these uterine NK cells with Jeg-3 and BeWo choriocarcinoma cells as a model of the invasive trophoblast. For this purpose, fragments of endometrial and decidual tissue were put in close contact with multicellular spheroids of choriocarcinoma cells in small silicon funnels. After the formation of stable contacts, the confrontation cultures were transferred to spinner flasks, cultivated for up to 6 days, and prepared for immunohistochemistry. During 2 days of cocultivation, the first cells started to move forward into the stromal component of the confrontation culture as demonstrated by staining of the choriocarcinoma cells using anti-human cytokeratin. Invasion advanced until, after a total of 6 days, some choriocarcinoma cells had already penetrated deeply into the host tissue. After a cultivation period of 1 week, both the endometrial and decidual tissue fragments still contained several CD56(+) uterine NK cells, and some of them expressed the proliferation-associated marker Ki-67 without any exogenous activation. A few CD56(+) cells were found directly at the invasion front, as well as between the choriocarcinoma cells. These cells also contained the cytolytic granule protein perforin indicating a migration of NK cells with cytolytic potential toward the potentially invasive cells. In conclusion, this human system closely resembles the in vivo conditions during trophoblast invasion and provides an appropriate in vitro model for studying dynamic processes involving various cell types during trophoblast invasion at the experimental level. Moreover, it enables us to study the effects of cytokines and growth factors that possibly regulate trophoblast invasion.