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 t estosterone is immunosuppressive. Although many studies in birds and mammal s have supported this assumption, conflicting results have been reported in a variety of species. We investigated the effects of testosterone manipula tion on both cell-mediated and humoral immunity in adult songbirds, Europea n 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 day s later via an enzyme-linked immunosorbant assay. Cell-mediated immunity wa s assessed 7 weeks after implantation via intradermal injection of the T-ce ll mitogen phytohemagglutinin into the wing web and measuring the degree of swelling 24 h later. Antibody responses to antigenic challenge were signif icantly suppressed in testosterone-treated females 10 days postinjection an d in both sexes 15 days post-injection. Furthermore, there was a significan t inverse relationship between individual variability in antibody responsiv eness and plasma testosterone concentrations. Cell-mediated responses to ph ytohemagglutinin stimulation were also significantly suppressed in testoste rone-treated males compared to same-sex controls. Testosterone treatment si gnificantly increased plasma corticosterone concentrations compared to cont rols, and the possibility of this effect mediating the immunosuppressive ef fects of testosterone is discussed. The present study is among the first to demonstrate testosterone-induced suppression of both cell-mediated and hum oral immunity in a species of songbird.