Introduction: Mammalian early-stage embryo is protected from viral infection by zona-pellucida, but a hatched blastocyst has no physical defense. It is not clear how blastocyst protects from the infection of viruses by itself. We tried to study innate immune responses in the in vitro model of blastocyst using murine embryonic stem (ES) and trophoblast stem (TS) cell lines as inner cell mass and trophectoderm of blastocyst, respectively.
Methods: GFP-expressing ES cell line, GST1, was established by introduction of EGFP-expression vector into BALB/c derived ES cell (ST1). TS cell line, TR1-2, was established from the trophectoderm of the blastocyst of DsRed2 transgenic BALB/c mouse. These cell lines were cultured separately in the cell-culture insert system. The cells were stimulated by transfection with 100 µg/ml poly[I:C], or addition of 10³ unit/ml IFN-β. After total RNA was isolated, RT-PCR or real-time PCR were carried out for gene expression analysis.
Results: The expression of PRRs was detected in both GST1 and TR1-2. The expression level of Ifn-β and protein kinase R (Pkr) were increased in TR1-2 after poly[I:C] stimulation, but not in GST1. Pkr expression was upregulated in IFN-β-treated TR1-2. In the co-culture of poly[I:C]-treated TR1-2 and non-treated GST1, interestingly, 2'-5'-oligoadenylate synthetase1 (Oas1) expression was induced only in GST1 but not in TR1-2.
Conclusion & Discussion: These results suggest that IFN-β in poly[I:C]-transfected TR1-2 may induce Pkr expression in both cells in the cell-culture insert system, and may induce Oas1 expression only in non-treated GST1. In the blastocyst, trophectoderm layer is the outermost layer and first barrier for infection. Virus-infected trophectoderm, i.e. the outer layer of the embryo may produce type I interferon and this may give anti-viral signals to both trophectoderm and inner cell mass. Pkr expresses in both cell types, but Oas1 expresses only in inner cell mass for anti-viral defense.