Immune response accelerated telomere shortening during early life stage of a passerine bird, the blue tit (Cyanistes caeruleus)

Dealing with infections is a daily challenge for wild animals. Empirical data show an increase in reactive oxygen species (ROS) production during immune response. This could have consequences on telomere length, the end parts of linear chromosomes, commonly used as proxy for good health and ageing. Telomere length dynamics may reflect the costs associated with physiological responses. In this study, immune system of blue tit (Cyanistes caeruleus) nestlings was experimentally challenged through a polyinosinic:polycytidylic acid (poly I:C) injection, a synthetic double-stranded RNA that mimics a virus, activating the pathway of immune response triggered via the toll-like receptors 3. This path is known to form ROS downstream. Immune response was quantified by white cell counts in blood, while brain lipoperoxidation has been evaluated as an indicator of oxidative damage. Finally, individuals' telomere length shortening between days 8 and 15 after hatching was measured in erythrocytes. Challenged nestlings showed increased leukocyte number when compared with control (treated with a saline solution), lower brain lipid peroxidation (likely as a result of a compensatory mechanism after oxidative stress burst) and accelerated telomere shortening. These findings support the 'ageing cost of infections pathway' hypothesis, which supposes a role for infections in quick biological ageing.