Discussion
The immune parameters
(leukocyte concentration and leukocyte profile) measured on the marmots
varied with their age. At the within-individual level, although the
leukocyte concentration remains stable over the course of an individual
life, the relative number of lymphocytes decreased, while the relative
numbers of neutrophils and eosinophils increased with age. Lymphocytes
play a central role in acquired immunity, being involved in
immunoglobulin and memory cell production (Jain, 1993; Roitt et al.,
2001). Neutrophils and eosinophils are both involved in the innate
immune response, and more specifically in the inflammatory process
(Jain, 1993). While neutrophils are the primary phagocytic leukocytes
and eosinophils are often associated with defences against internal
parasites (Jain, 1993).
The decrease in the number of lymphocytes with age is often interpreted
as a consequence of the gradual decline over age in the generation, in
the thymus, of new naïve T lymphocytes, responsible for generating new
input in the immune memory. Such gradual decline is often suggested to
lead to a decrease in the efficiency of the acquired immune system
(Dowling & Hodgkin, 2009; Hakim & Gress, 2007; Shanley et al., 2009).
Similarly, the observed increase in the relative number of neutrophils
does not necessarily mean a higher performance of the innate immune
system with age. Indeed, the phagocytic ability of neutrophils could
decrease with age (Gomez et al., 2008) and an adaptive compensatory
mechanism for such decline in neutrophil performance could lead to an
increase in their relative number.
However, a decrease in
lymphocytes, together with an increase in neutrophils (Cheynel et al.,
2017; Kirk et al., 2010; and in Roast et al., 2022, even if not
significant), and more broadly, a decrease in the acquired immune system
combined with an increase (or upkeep) in the innate immune system, with
age, has been observed in various vertebrate species (Franceschi et al.,
2000a; Franceschi et al., 2000b; reviewed in Peters et al., 2019). Such
modification of the acquired/innate immune balance with age (McDade et
al., 2016), called immune remodelling , could be interpreted as an
increased resource allocation strategy towards the production of cheaper
immune components, at the expense of the more costly immune components
(Klasing, 2004). Indeed, the acquired immune components are generally
thought to be more expensive to maintain (Lee, 2006), and therefore,
more prone to decline with age than the innate immune components (Peters
et al., 2019). This could reflect that immune parameters are traded-off
with other life history traits, and/or that terminal investment is
unlikely to be turned to these parameters since their costs of
production would outweigh their future benefit when age-related
mortality becomes imminent. In such a scenario, given the lower
probability to encounter new pathogens at old ages, downregulating the
acquired immune system would not necessarily be the sign of any
malfunction, but could be adaptive (Fulop et al., 2018). Consequently,
immune systems should not be considered to undergo unidirectional
deterioration with age (i.e.senescence) but would probably be better
described by taking into account remodelling and reshaping of the immune
functions with age (Fulop et al., 2018).
Apart from the
hypotheses exposed above, age-related changes in the level of
environmental stress endorsed by an individual could also affect the
relative numbers of lymphocytes and neutrophils. Indeed, stress hormones
such as glucorticoïds stimulate an influx of neutrophils from tissues
into the blood; concomitantly, it causes a migration of lymphocytes from
the blood circulation to other compartments (Dhabhar, 2002). Thus, a
rise of plasma glucocorticoïds caused by stress increases the
neutrophils to lymphocytes ratio over a time span of hours (Davis et
al., 2008; Lopez-Olvera et al., 2007). Nevertheless, to date, no link
has been clearly established between environmental stress, levels of
glucocorticoids and patterns of age variations of lymphocytes and
neutrophils (see for instance Roast et al., 2022; Watson et al., 2016).
We observed fewer lymphocytes for marmot males than for females. Various
hypotheses such as sex-differences in resource allocation strategy,
intra-sexual competition (Metcalf & Graham, 2018; Sheldon & Verhulst,
1996) or inhibition of the immune system by some steroid hormones were
often suggested to induce differences between males and females (Gubbels
Bupp et al., 2018; Klein & Flanagan, 2016; Taneja, 2018). However, we
did not observe sex-specific differences in the variation of the immune
phenotype with age. So far, studies of sex-specific variation on immune
parameters with age remain equivocal: some suggested sex differences
(e.g. Bichet et al., 2022; Gubbels Bupp et al., 2018; Tidière et al.,
2020; van Lieshout et al., 2020), while others did not (e.g. Brooks &
Garratt, 2017; Cheynel et al., 2017; Kelly et al., 2018; Peters et al.,
2019). For instance, van Lieshout et al. (2020) found a decrease in the
proportion of lymphocytes with age in male badgers (Meles meles ),
but not in females. The authors argued that this result could be
explained by the high testosterone levels observed in male badgers, due
to their polygynandrous mating system (Buesching et al., 2009), contrary
to monogamous species (Sugianto et al., 2019) such as the Alpine marmot
(Allainé, 2000; Cohas et al., 2006).
In our study, individuals
with fewer lymphocytes but more neutrophils were more likely to die
(Table 3), as also indicated by a significant selective disappearance of
individuals with this phenotype (Table 2). Innate cellular response
(involving neutrophils) can be costly in terms of energy, as well as
autoimmune (Lee, 2006) and inflammatory damages (Franceschi et al.,
2018; Goto, 2008). Individuals with neutrophil-oriented response may be
unable to mount an appropriate immune response against challenges
encountered at old ages (Froy et al., 2019), and/or may pay an excessive
cost to this response and die (Pawelec, 2018). Studies investigating the
potential links between age variation in immune phenotype and individual
fitness are still scarce and show contrasting results (see also Froy et
al., 2019). For instance, in the greater sac-winged Bat
(Saccopteryx bilineata ), the number of leukocytes decreased with
age, both within- and among-individuals, while the immunoglobulin G
concentration was higher in older individuals, but did not vary within
individuals, and the bacterial killing capacity of the plasma did not
vary with age, at both levels (Schneeberger et al., 2014). These
variations with age also impacted the short-term survival probability
(Schneeberger et al., 2014). However, in a study on purple-crowned
fairy-wrens (Malarus coronatus ), Roast et al. (2020) found no
evidence that high levels of innate immune functions impaired short-term
survival, nor any other fitness traits (annual reproduction and
dominance acquisition). In our marmot population, more investigations on
the link between fitness and immune variations with age are needed to
better understand the evolutionary consequences of the within-individual
age variations and the selective disappearance we observed.
More generally, to
understand the complexity of age-related changes in immune functions, as
well as their evolutionary causes and consequences, we must not only
supplement the existing longitudinal studies focused on age-related
pattern of immune parameters (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), but also relate the observed patterns to
individual fitness. Such studies are crucial to disentangle whether
remodelling of the immune system with age could or could not be
adaptive.