Unique effect of the pregnancy hormone estriol on antigen-induced production of specific antibodies in female BALB/c mice

We investigated the modulatory effect of estriol (E3), an estrogen predominantly produced during human pregnancy, on the antigen-induced production of specific antibodies in female BALB/c mice, and its effect was compared with 17β-estradiol (E2). Estriol (E3) had a very different effect than E2 on the antigen-induced production of specific antibodies in animals immunized with two different antigens, i.e., the bovine serum albumin (BSA) and pneumococcal polysaccharide serotype-14 (PPS-14). While E2 strongly stimulated the production of BSA-specific antibodies (mostly IgG1), E3 had little or no effect on their production. In comparison, when the bacterial PPS-14 was the immunogen, E3 and E2 both strongly increased the production of PPS-14-specific antibodies (mostly IgM). E3 and E2 also had a similar effect on the thymus weight reduction and on the spontaneous antibody production in these animals. Our results provided an example demonstrating that the pregnancy hormone E3 has a distinctly different profile of modulatory actions in the immune system compared to E2, while the former strongly enhanced the body's ability to produce bacteria-specific IgM antibodies, it had no effect on the production of specific antibodies against a soluble protein. This differential effect of E3 may be beneficial for reducing the risk of developing antibody-mediated immune attack against the maternal and fetal elements during pregnancy.