Abstract
Recent findings suggest that immune functions do not unidirectionally
deteriorate with age but that a potentially adaptive remodelling, where
functions of the immune system get down regulated while others get
upregulated with age could also occur. Scarce in wild populations,
longitudinal studies are yet necessary to properly understand the
patterns and consequences of age variations of the immune system in the
wild. In the meanwhile, it is challenging to understand if the observed
variations in immune parameters with age are due to changes at the
within-individual level or to selective (dis)appearance of individuals
with peculiar immune phenotypes.
Thanks to a long-term and longitudinal monitoring of a wild Alpine
marmot population, 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 evolutionary consequences of such
age-variations in the wild. To do so, we recorded the age-specific
leukocyte concentration and profile in repeatedly sampled dominant
individuals. We then tested whether the potential changes with age were
more likely due to within-individual variations and/or selective
(dis)appearance. Finally, we investigated if the leukocyte concentration
and profiles were correlated to the probability of death at a given age.
The leukocyte concentration was stable with age, but the relative number
of lymphocytes decreased, while the relative number of neutrophils
increased, over the course of an individual’s life. Moreover, between
individuals of the same age, individuals with fewer lymphocytes but more
neutrophils were more likely to die. Therefore, selective disappearance
seems to play a role in the age variations of the immune parameters in
this population. Further investigations linking age variations in immune
phenotype to individual fitness are needed to understand whether
remodelling of the immune system with age could or could not be
adaptive.