Immunosuppressive function of estradiol via PGE2induction in vitro.
To investigate the mechanism of PGE2 induction in blood before calving, estradiol concentrations in the sera of BLV-infected pregnant cattle were measured using ELISA. Interestingly, in the same manner as PGE2 kinetics in the pregnant cattle, estradiol concentrations tended to be increased on day 0 (Figure 2d, Supplemental Figures 1d). Previous studies also have shown that the production of estrogen in blood is induced soon before parturition [22–24]. On the basis of these findings, we hypothesized that estradiol plays important roles in the induction of PGE2production, which causes the suppression of Th1 responses, in pregnant cattle. In order to examine this hypothesis, PBMCs from BLV-uninfected cattle were cultivated with estradiol in the presence of T-cell stimulation. IFN-γ concentrations in culture supernatants were reduced by the treatment of estradiol (Figure 3a). Additionally, estradiol impaired IFN-γ production from PBMCs of BLV-infected cattle in the presence of BLV-antigen (Figure 3b). To assess whether estradiol induces PGE2 production, PBMCs from BLV-uninfected cattle were cultivated with estradiol. After incubation, both COX2 expression and PGE2 production were significantly increased by treatment with estradiol (Figures 3c and 3d). Furthermore, PBMCs from BLV-uninfected cattle were pretreated with each EP antagonist, and then cultured with estradiol in the presence of T-cell stimulation. The suppressive effect of estradiol was not observed when estradiol incubation was performed in combination with EP4 antagonist (Figure 3e), but not with antagonists of other EP receptors, thus suggesting that estradiol inhibits Th1 responses via PGE2/EP4 signaling.