Changes in immune phenotype with age impaired survival: a longitudinal approach in Alpine marmots

Recent findings suggest that immunosenescence should not be considered as a unidirectional deterioration, and that the remodeling of the immune system with age could also be adaptive. Longitudinal studies on immunosenescence in wild populations are scarce, and therefore, processes like selective disappearance cannot be easily torn apart from senescence. Using a long-term and longitudinal monitoring of a wild population of Alpine marmots, we aimed to understand within and between individual variation in the immune phenotype with age, in order to improve our knowledge about the occurrence and the consequence of immunosenescence in the wild. We tested, once controlled for a potential selective disappearance, whether individuals’ immune function only decreases as they age, as expected from the disposable soma theory, or whether remodelling of the immune system does occur. Therefore, we recorded the age-specific leukocyte concentration and counts in repeatedly sampled dominant individuals and we tested the potential changes with age as well as their association with survival probabilities. The overall leukocyte concentration was stable with age, but the lymphocyte count decreased, while the neutrophil count increased, over the course of an individual’s life. The leukocyte counts also predicted survival: at a given age, individuals with fewer lymphocytes but more neutrophils were more likely to die. Longitudinal studies, like the present one, are required to properly understand the patterns and consequences of immunosenescence in the wild.