Immune response, oxidative stress and dietary antioxidants in great tit nestlings

The activation of immune defences counteracts pathogens, but mounting an immune response is costly and can negatively impact life-history traits. Immune activation releases highly reactive species that kill pathogens but can also cause oxidative damage to host tissues, and these negative effects may therefore constrain further investment in immune responses. To offset these toxic effects, animals rely on a complex system of antioxidants. Here, we tested if vitamin E, a dietary antioxidant, can reduce oxidative damage induced by an immune challenge and thus enhance the immune response. In a 2 × 2 experimental design, we supplemented great tit nestlings with either vitamin E or a placebo, and then injected them with either a bacterial lipopolysaccharide (LPS), or a buffer solution (PBS) as a control. LPS-treated nestlings mounted an inflammatory response and increased antioxidant capacity, without any change in ROM (reactive oxygen metabolites), an index of early oxidative damage. These results suggest that the likely transient increase in reactive species of the LPS injection was counteracted by a rise in endogenous antioxidant defences that was independent of supplementary dietary antioxidants. Indeed, vitamin E supplementation neither affected oxidative status nor enhanced the immune response, suggesting that in our experimental condition great tit nestlings were not limited in vitamin E and in antioxidants in general. Overall, our results show that birds can mount an effective antioxidant response to face an immune challenge, and can therefore avoid stress caused by a transient increase in reactive species generated by immune activation.