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.