Effects of testosterone on cell-mediated and humoral immunity in non-breeding adult European starlings

One of the primary assumptions of the immunocompetence hypothesis is that testosterone is immunosuppressive. Although many studies in birds and mammals have supported this assumption, conflicting results have been reported in a variety of species. We investigated the effects of testosterone manipulation on both cell-mediated and humoral immunity in adult songbirds, European starlings (Sturnus vulgaris). Male and female starlings were wild-caught, housed in the laboratory,and implanted with either empty silastic capsules or capsules containing testosterone. Six weeks after implantation, humoral immunity was assessed by injecting the birds with a novel antigen, keyhole limpet hemocyanin, and measuring specific antibody responses 10 and 15 days later via an enzyme-linked immunosorbant assay. Cell-mediated immunity was assessed 7 weeks after implantation via intradermal injection of the T-cell mitogen phytohemagglutinin into the wing web and measuring the degree of swelling 24 h later. Antibody responses to antigenic challenge were significantly suppressed in testosterone-treated females 10 days postinjection and in both sexes 15 days post-injection. Furthermore, there was a significant inverse relationship between individual variability in antibody responsiveness and plasma testosterone concentrations. Cell-mediated responses to phytohemagglutinin stimulation were also significantly suppressed in testosterone-treated males compared to same-sex controls. Testosterone treatment significantly increased plasma corticosterone concentrations compared to controls, and the possibility of this effect mediating the immunosuppressive effects of testosterone is discussed. The present study is among the first to demonstrate testosterone-induced suppression of both cell-mediated and humoral immunity in a species of songbird.