PD-1/PD-L1 expression and its interaction with interferon-γ in Toxoplasma gondii-infected mice at middle and late pregnancy

Abstract

To explore the dynamic expression of programmed cell death-1 (PD-1) and its ligand PD-L1 at the maternal-fetal interface of mice post-infection with Toxoplasma gondii at early pregnancy and examine its interaction with interferon-γ (IFN-γ). A total of 20 mice at day 0 of pregnancy were randomly assigned into 4 groups, including the 12-day pregnancy control group (12 dpn group), 12-day pregnancy and infection group (12 dpi group), 18-day pregnancy control group (18 dpn group) and 18-day pregnancy and infection group (18 dpi group), respectively. On the 6th day of the pregnancy, mice in the 12 dpi and 18 dpi groups were injected intraperitoneally with 150 tachyzoites of the T. gondii PRU strain, while mice in the 12 dpn and 18 dpn groups were injected with the same volume of PBS. All mice in the four groups were sacrificed on 12th and 18th day of the pregnancy, and the number of placenta and fetus was counted and the weight of placenta and fetus was measured. Then, the placental and uterine tissues of the pregnant mice in each group were sampled for pathological examinations. The mRNA expression of PD-1, PD-L1, T. gondii surface antigen SAG-1 and IFN-γ genes was quantified using a quantitative real-time PCR (qPCR) assay, and the correlation between PD-1 and IFN-γ expression was examined. In addition, the 12 dpn group, 12 dpi group, 18 dpn group, 18 dpi group, PBS negative control of the 12 pdi group and PBS negative control of the 18 dpi group were assigned, and the PD-1 expression was determined in the uterine and placenta tissues of the pregnant mice. Adverse pregnant outcomes were seen in mice in the 12 dpi and 18 dpi groups, including placental dysplasia and fetal maldevelopment, and the placental weights and fetal body weights were significantly lower in mice in the 12 dpi and 18 dpi groups than those in the 12 dpn and 18 dpn groups (t = 5.52, 11.44, 12.63 and 11.67, all P < 0.01). The histopathological examinations showed that the decidua and junctional regions of the placental tissues were loosely connected in the 12 dpi and 18 dpi groups, and a large number of inflammatory cells infiltration and congestion were seen in the placental and uterine tissues. qPCR assay detected significant differences in PD-1, PD-L1, IFN-γ and SAG-1 expression in the placental and uterine tissues among the 12 dpn, 12 dpi, 18 dpn and 18 dpi groups (F = 22.48, 51.23, 9.61, 47.49, 16.08, 21.52, 28.66 and 238.90, all P < 0.05), and the PD-1, PD - L1, IFN - γ and SAG - 1 expression was all significantly higher in the placental and uterine tissues of mice in the 12 dpi group than in the 12 dpn group (all P values < 0.05). The PD-1 and PD-L1 expression was significantly lower in the placental tissues of mice in the 18 dpi group than in the 18 dpn group (all P values < 0.05), and the IFN-γ and SAG-1 expression was significantly higher in the placental and uterine tissues of mice in the 18 dpi group than in the 18 dpn group (all P values < 0.05), while the PD-1 and PD-L1 expression was significantly lower in the placental and uterine tissues of mice in the 18 dpi group than in the 12 dpi group (all P values < 0.05). Immunohistochemical staining showed PD-1 expression in the inflammatory cells of the placental tissues of mice in the 12 dpi group, and no apparent PD-1 expression in the 18 dpi group, while strongly positive PD-1 expression was found in the uterine epithelium of mice in the 12 dpi group, and mildly strong expression was in the 18 dpi group. In addition, the IFN-γ mRNA expression was positively correlated with the PD-1 mRNA expression in placental (rs = 0.99, P < 0.01) and uterine tissues of mice in the 12 dpi group (rs = 0.97, P < 0.01) and in placental (rs = 0.82, P < 0.01) and uterine tissues of mice in the 18 dpi group (rs = 0.81, P < 0.01). Following T. gondii infection at early pregnancy, the PD-1 and PD-L1 expression shows a remarkable rise at middle pregnancy and a reduction at late pregnancy in placental and uterine tissues of mice, which appears the same tendency with IFN-γ expression during the same time period, and PD-1 expression positively correlates with IFN-γ expression. The dynamic expression of PD-1 and PD-L1 on the maternal-fetal interface of mice may be mutually mediated by IFN-γ induced by T. gondii infection.