OWEN-ASHLEY, N. T.; HASSELQUIST, D.; WINGFIELD, J. C.; Univ. of Washington, Seattle; Lund Univ., Lund, Sweden; Univ. of Washington, Seattle: Indirect mechanisms of testosterone-induced immunosuppression in songbirds: a new paradigm

We investigated the effect of different androgens on immune function in song sparrows (Melospiza melodia). The immunocompetence handicap hypothesis predicts a tradeoff between suppressive effects of testosterone (T) on immune function and enhancing effects of T on some traits that are attractive to females. In seasonally breeding birds, T and 5&alpha-dihydrotestosterone (DHT) levels are high in the spring during territorial establishment, but are low or nondetectable in winter. A precursor of T, dehydroepiandrosterone (DHEA), is the only androgen elevated in the blood during the winter and may regulate nonbreeding aggression in song sparrows. Captive male song sparrows on short days (8L:16D) were implanted with either T, DHT, DHEA, or control (empty) silastic implants, and the acute phase response and cell-mediated and humoral immunity were assessed in vivo. T-implanted subjects suppressed both humoral and cell-mediated immune responses compared to control-implanted birds. Birds implanted with DHT or DHEA showed robust humoral and cell-mediated immune responses that were similar to controls. DHT binds to the same androgen receptor as T, suggesting that immunosuppression occurs through an indirect pathway. Basal corticosterone levels were elevated in T-implanted birds compared to DHT-, DHEA-, or control-implanted subjects. Birds did not differ in an acute phase challenge, once baseline corticosterone levels were taken into account . Hormone implantation also affected condition. T-induced immunosuppression likely occurs through a combination of three indirect mechanisms: (1) a rise in glucocorticosteroids, (2) aromatization to estradiol, and/or (3) a tradeoff between energy expenditure and immune function.