Date of Thesis



Exposure to testosterone during embryonic development can suppress immune function in many avian species, including chickens. However, it is unclear whether this is caused by direct stimulation of androgen receptors. At least two lines of evidence suggest that estradiol may be involved: estrogen receptors are expressed on B- and T-lymphocytes and their progenitor cells, and estradiol can inhibit lymphocyte production both in vitro and in vivo. Estradiol is synthesized from testosterone by the enzyme aromatase and this conversion is a necessary step in many testosterone-dependent signaling pathways. I hypothesized that immunosuppressive effects of in ovo testosterone exposure are mediated by conversion to estradiol by aromatase. To test this, I inhibited aromatization of endogenous testosterone during a crucial period of pre-hatch immune system development and measured post-hatch immune activity (total IgY antibodies, response to PHA challenge, and size of thymus and bursa of Fabricius). On day 13 of incubation, chicken eggs were injected with 0.1mg of the aromatase inhibitor fadrozole in saline or saline only. On day 14 post-hatch, chicks were injected in the wing web with 0.1mg of phytohemagglutinin (PHA) in 0.1mL of PBS buffer. 24 hours later, swelling, an indicator of inflammation due to T-cell recruitment, was measured. Blood samples were taken on post-hatch day 3, day 13, and day 18 and analyzed for total IgY antibody count. Thymus and bursa were weighed on day 18. I predicted that if immunomodulation by testosterone were dependent on aromatization to estradiol, then fadrozole treatment would promote immune activity by inhibiting the pathway. Conversely, if testosterone were acting on immune tissues directly by binding to androgen receptors, then fadrozole 2 treatment would instead suppress immune activity by increasing testosterone levels. Fadrozole decreased day 3 IgY antibody titers but increased day 18 thymic mass. Furthermore, fadrozole treatment tended to increase day 18 IgY levels for a given bursal mass in females. There was no effect on PHA response or bursal mass. My results suggest that developmental aromatase inhibition does not affect the immune system in a uniform way, and that testosterone may be immunomodulatory in this context, rather than simply immunosuppressive. Finally, I have demonstrated that aromatase inhibition can alter retention of maternal IgY antibody, and further study is needed to determine the mechanism by which this occurs.

Access Type

Honors Thesis (Bucknell Access Only)



First Advisor

Z. Morgan Benowitz-Fredericks