Susceptibility of MHC class I expressing extravillous trophoblast cell lines to killing by natural killer cells.

Purified human first trimester extravillous trophoblast (EVT) cell lines HTR-8 and HT-116 were examined for susceptibility to natural killer (NK) cell-mediated lysis. Based upon nucleic acid sequencing of an amplified fragment of cDNA, Western blot analysis and immunostaining of fixed and live cells, it was shown that both EVT cell lines expressed HLA-G mRNA and protein within the cytoplasm when cultured on laminin-coated plates. Very weak HLA-G expression was detectable on the cell surface under these conditions. However, strong cell surface expression of a classical MHC class I molecule (most likely HLA-C) was exhibited by these EVT cell lines when grown on laminin, as indicated by W6/32 FACS analysis (Ab recognizing pan MHC class I), and Western immunoblotting with HC10 (Ab recognizing HLA-B/C). When these EVT cells, cultured on laminin, were used as targets for peripheral blood natural killer (NK) cells in a standard chromium release assay, both HTR-8 and HT-116 cells were lysed by NK cells in a dose-dependent manner. The respective percentage specific lysis at an effector to target (E/T) ratio of 100 was 28+/-7, and 48+/-14. The choriocarcinoma cell lines JAR and JEG-3 which were respectively MHC class I negative and HLA-G positive were resistant to NK cell lysis. Thus, there was no clear relationship between the MHC class I expression and NK cell resistance or susceptibility among the EVT cell lines and choriocarcinoma cells. These findings raise the possibility that NK cells may take part in the surveillance of the invasive EVT cells during normal placentation.