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 (E-3), 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. E-3 was found to have very different effects on antigen-induced production of specific antibodies in animals immunized with these two antigens. While E-3 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 E-3 on the production of antigen-specific antibodies depends on the dose of E-3 used. For BSA-induced antibody production, E-3 had a dose-dependent inhibitory effect, whereas for PPS-14-specific antibody (Ab) production, E-3 exerted the strongest stimulation at a lower dose, and produced less stimulation at higher doses. E-3 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 E-3 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.