Discussion
The immune phenotype of marmots varies with their age. While the
leukocyte concentration remains stable over the course of an individual
life, the lymphocyte count decreases, and the neutrophil count
increases. In Mammals, lymphocytes and neutrophils make up the majority
(80%) of the leukocytes
(Jain,
1993). Lymphocytes play a central role in acquired immunity, being
involved in immunoglobulin and memory cell production and in the
modulation of immune defence
(Jain,
1993; Roitt et al., 2001; Vandervalk & Herman, 1987). Involved in the
innate immune response, neutrophils are the primary phagocytic
leukocytes, and circulating phagocytes proliferate in response to
infections, inflammation and stress
(Jain,
1993).
A decrease in lymphocytes, together with an increase in neutrophils
(Cheynel
et al., 2017; Kirk et al., 2010), and more broadly, a decrease in the
acquired immune system combined with an increase (or upkeep) in the
innate immune system with age, was observed in various vertebrate
species
(reviewed
in
Peters
et al., 2019). A decrease in the acquired immune system with age is
often interpreted as a consequence of the thymus involution
(Hakim
& Gress, 2007). The observed increase in neutrophil count 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 increasing their number could be an adaptive
compensatory mechanism. However, this increase in neutrophil count could
also be a compensation for a decrease in the efficacy of the acquired
immunity. A remodeling of the immune system could indeed occur due to
changes in the selective pressures when getting old. Given the lower
probability to encounter new pathogens at old ages, downregulating the
acquired immune system could be adaptive
(Fulop
et al., 2018). Immunosenescence should not be considered as a
unidirectional deterioration, and would probably be better described by
taking into account remodeling and reshaping of the immune functions
with age
(Fulop
et al., 2018).
We observed less lymphocytes for marmot males than for females. Various
hypotheses such as sex-differences in 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 patterns of
immunosenescence remain equivocal: some suggested sex differences (e.g.
(Gubbels Bupp et al., 2018; Tidière et al., 2020; van Lieshout et al.,
2020; Bichet et al., submitted ), 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; A. Cohas et al., 2006).
In our study, individuals with fewer lymphocytes but more neutrophils
were more likely to die. This result was further confirmed by a
significant selective disappearance of individuals with this phenotype.
Innate cellular response (involving neutrophils) is considered as costly
in terms of energy and autoimmune damage
(Lee, 2006).
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 have an excessive cost to this response
and die.
Our study also illustrates the importance of longitudinal analyses and
the use of appropriate statistical tools to avoid misleading conclusions
regarding immunosenescence
(Peters
et al., 2019; van de Pol & Wright, 2009). At the population level, our
analyses revealed quadratic age effects on immune parameters, probably
due to a combination of variations in the strength of selective
disappearance with age and of within-individual variations (Figure 1).
Our current knowledge derived from cross-sectional studies thus has to
be taken with caution
(Peters
et al., 2019). So far, only three studies investigated longitudinal
variations with age in the immune functions (Beirne et al., 2016; Andrea
L. Graham et al., 2010; Schneeberger et al., 2014) which is clearly not
enough to understand senescence in a complex system like immunity. For
instance, in the Greater Sac-Winged Bat (Saccopteryx bilineata ),
it was found that the number of leukocytes decreased with age, both
within and between 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). More longitudinal studies, like the present one, are highly
necessary to properly understand the patterns and consequences of
immunosenescence for wild individuals and populations.