Effect of chronic or cyclic exposure to estradiol on the humoral immune response and the thymus.

Estradiol (E2) is known to enhance humoral immunity. Previous studies examining serum immunoglobulin levels on days within the estrous cycle or in the ovariectomized, chronically E2-implanted rat have determined a positive correlation between serum E2 concentrations and antibody responsiveness. No previous study has investigated the effect of fluctuating E2 concentrations, as during the estrous cycle, throughout the course of a specific humoral immune response. Therefore, in the present study we addressed the influences of cyclic and tonic E2 exposure on the anti-fluorescein response in the rat. Ovariectomized rats were submitted to a 2 x 2 factorial design of chronic E2 implants and 4-day E2 injection (and vehicle control) regimens. Anti-fluorescein antibody production and thymic weights were measured. Animals given a 4-day cycle of E2 showed significantly (p less than 0.001) greater anti-fluorescein responses than did animals given tonic or no E2 replacement. Animals which were given implants, with or without cyclic E2 injections, experienced significant (p less than 0.001) thymic involution relative to those without implants. The 4-day E2 injections alone did not lead to thymic atrophy. These results indicate that while chronic E2 treatment induced thymic atrophy, cyclic exposure to E2 exerted a greater immunoenhancing influence than did tonic E2 exposure.