Unique dose-dependent effects of the human pregnancy hormone estriol on the ratio of blood IgM to IgG in female mice

The present study aimed to investigate the dose-dependent modulating effect of estriol (E3), an estrogen predominantly produced during human pregnancy, on antigen‑induced production of specific antibodies in female BALB/c mice. The animals were immunized either with bovine serum albumin (BSA) or the pneumococcal polysaccharide serotype-14 (PPS-14), and the levels of specific serum antibodies were determined using ELISA kits. E3 was found to have very different effects on antigen-induced production of specific antibodies in animals immunized with these two antigens. While E3 stimulated the production of PPS-14-specific antibodies, it suppressed the production of BSA-specific antibodies. The results also demonstrated that the modulating effect of E3 on the production of antigen‑specific antibodies depends on the dose of E3 used. For BSA‑induced antibody production, E3 had a dose‑dependent inhibitory effect, whereas for PPS‑14‑specific antibody (Ab) production, E3 exerted the strongest stimulation at a lower dose, and produced less stimulation at higher doses. E3 caused thymus atrophy in animals immunized with either PPS‑14 or BSA, but only induced spleen atrophy in BSA‑injected mice. These observations suggest that E3 increases the ability of a pregnant female to avoid bacterial infections while decreasing the incidence of autoimmune responses against circulating components from either the fetus or pregnant female.