Introduction
The immune system is of primary importance to control diseases
throughout an individual’s life, and therefore crucial to its fitness.
In vertebrates, the immune system involves different immune functions
which are classically divided into innate and adaptive components (Hoebe
et al., 2004); in close interaction with each other (Iwasaki &
Medzhitov, 2010, 2015). The innate immune functions are the first
defence against pathogens, involving phagocytic cells (e.g. neutrophils,
macrophages and dendritic cells) and molecules such as cytokines, also
able to activate other components of the immune system (Akira et al.,
2006; Mantovani et al., 2011; Nathan, 2006; Vivier et al., 2011). The
adaptive immune functions comprise a cell-mediated immune response, with
the stimulation of T lymphocytes, and a humoral immune response,
controlled by activated B lymphocytes that can produce immunoglobulins
against specific antigens (Iwasaki & Medzhitov, 2010; Mantovani et al.,
2011; Vivier et al., 2011).
Mounting an immune response
carries costs (Graham et al., 2005; Lochmiller & Deerenberg, 2000;
Maizels & Nussey, 2013) and trade-offs with other life-history traits
are likely to emerge (Eraud et al., 2009; Graham et al., 2010; Hanssen
et al., 2004; Lemaitre et al., 2015; Viney et al., 2005). Therefore,
immunity is likely to change during an individual’s life. Changes of
immunity with age has been mainly studied in humans and laboratory
animals (Bektas et al., 2017; Frasca et al., 2005; Gayoso et al., 2011;
Larbi et al., 2008; Noreen et al., 2011; Solana et al., 2012), with the
general pattern being a decline in adaptive immunity with age, while
innate immunity remains unchanged and inflammatory markers increase
(Bauer & De la Fuente, 2016; Franceschi, Bonafe, et al., 2000a;
Franceschi, Bonafe, et al., 2000b; Franceschi et al., 2007; Frasca et
al., 2011; Panda et al., 2009; Shaw et al., 2013; Simon et al., 2015).
In non-model organisms, a recent review found similar trends (Peters et
al., 2019).
Some studies indicate
that the decrease in the immune functions with age could impaired
survival (e.g. Froy et al., 2019; Hanssen et al., 2004; Schneeberger et
al., 2014). However, others suggest that variations in immune functions,
characterised by changes in the proportion of the different cells
involved in the immune response, could be adaptive (i.e. immune
remodelling ) and could fit with the different immune challenges faced
throughout life (Fulop et al., 2018, p. 2018; Mueller et al., 2013;
Nikolich-Zugich, 2018). It could even be a combination of both (Fulop et
al., 2020).
Because the immune system is
complex, involving many cell types and pathways, its characterization in
non-model organisms is challenging, thus limiting the study of
age-related variation of immunity in free-ranging animals (Boughton et
al., 2011; Demas et al., 2011). Nevertheless, cross-sectional studies
investigated the variations in the immune function with age (mammals:
Abolins et al., 2018; Cheynel et al., 2017; Nussey et al., 2012; birds:
Hill et al., 2016; Lecomte et al., 2010; Palacios et al., 2007; Saino et
al., 2003; Vermeulen et al., 2017; reptiles: Massot et al., 2011; Ujvari
& Madsen, 2011; Zimmerman et al., 2013; see Peters et al. 2019 for a
review), and seem to confirm the pattern observed in humans and
laboratory animals (see above). However, these studies cannot
disentangle whether the observed variations arise from within-individual
changes or from processes like selective disappearance, which supposedly
eliminate individuals with poor (or unappropriate) immune defences from
the population (van de Pol & Verhulst, 2006; van de Pol & Wright,
2009). Longitudinal studies investigating variations in immune functions
with age exist, but are still very limited (to the best of our
knowledge, seven studies: Beirne et al., 2016; Bichet et al., 2022; Froy
et al., 2019; Graham et al., 2010; Roast et al., 2022; Schneeberger et
al., 2014; Vermeulen et al., 2017). Therefore, we are far to understand
the evolutionary consequences of such variations, and, more broadly how
proximate mechanisms, like immunity, could explain (even partly)
processes such as ageing (Bouwhuis & Vedder, 2017; Lemaitre et al.,
2013; Peters et al., 2019).
In the present study,
we recorded the age-specific leukocyte concentration and profile in 52
dominant individuals (i.e. fully grown and reproductive individuals)
repeatedly sampled between 2011 and 2015 (for a total of 169
measurements) from a wild and long-term studied (1992-2018) population
of Alpine marmots. We first tested whether leukocyte concentration and
profile changed within individuals’ ages (i.e. within-individual level).
We then investigated if changes in these immune parameters could also be
explained by selective (dis)appearance of individuals (i.e.
among-individual level) with particular immune parameters, influencing
their risk of death (survival analysis). Based on the previous studies,
we expected the relative number of lymphocytes (mainly involved in
acquired immunity) to decrease with age, while the others relative
numbers of leukocytes (neutrophils, monocytes, eosinophils; mainly
involved in innate immunity) to increase with age, at the
within-individual level. We further expect both leukocyte concentration
and leukocyte profile to compromise individual age-specific survival.
Material and methods