Antioxidant protection, carotenoids and the costs of immune challenge in greenfinches

Costs accompanying immune challenges are believed to play an important role in life-history trade-offs and warranting the honesty of signal traits. We performed an experiment in captive greenfinches (Carduelis chloris L.) in order to test whether and how humoral immune challenge with non-pathogenic antigen [sheep red blood cells (SRBC)] affects parameters of individual condition including intensity of coccidian infection, estimates of total antioxidant protection, plasma carotenoids and ability to mount a cell-mediated immune response. We also asked whether the potential costs of immune challenge can be alleviated by dietary carotenoid supplementation. None of the treatments affected intensity of coccidiosis. Humoral immune challenge suppressed the cell-mediated response to phytohemagglutinin (PHA), suggesting a trade-off between the uses of different arms of the immune system. Immune challenge reduced body-mass gain, but only among the carotenoid-depleted birds, indicating that certain somatic costs associated with immune system activation can be alleviated by carotenoids. No evidence for oxidative stress-induced immunopathological damages could be found because immune activation did not affect total antioxidant protection or carotenoid levels. Carotenoid supplementation inclined birds to fattening, indicating that lutein interfered with lipid metabolism. Altogether, our results support the hypotheses of biological importance of carotenoids and exemplify the overwhelming complexity of their integrated ecophysiological functions.