Summary
Background
Skin health declines with age and this is partially attributed to immunosenescence. Mast cells (
MC
s) are innate immune cells that coordinate tissue immune responses integral to skin homeostasis and disease.
Objectives
To understand how
MC
s contribute to human skin ageing, we investigated how intrinsic ageing impacts
MC
phenotype and
MC
relationships with other immune cells and skin structures.
Methods
In photoprotected skin biopsies from young (≤ 30 years) and aged (≥ 75 years) individuals, immunostaining and spatial morphometry were performed to identify changes in
MC
phenotype, number, distribution and interaction with the vasculature and nerve fibres. Quantitative polymerase chain reaction was used to measure changes in gene expression related to immune cell activity and neuropeptide signalling.
Results
Skin
MC
s, macrophages and
CD
8
+
T cells increased in number in intrinsically aged vs. young skin by 40%, 44% and 90%, respectively (
P
< 0·05), while
CD
4
+
T cells and neutrophils were unchanged. In aged skin,
MC
s were more numerous in the papillary dermis and showed a reduced incidence of degranulation (50% lower than in young,
P
< 0·01), a conserved tryptase–chymase phenotype and coexpression of granzyme B. In aged skin,
MC
s increased their association with macrophages (~ 48% vs. ~27%,
P
< 0·05) and nerve fibres (~29% vs. 16%,
P
< 0·001), while reducing their interactions with blood vessels (~34% vs. 45%,
P
< 0·001). Additionally, we observed modulation of gene expression of vasoactive intestinal peptide (
VIP
; increased) and substance P (decreased) with age; this was associated with an increased frequency of
VIP
+
nerve fibres (around three times higher in aged skin,
P
< 0·05), which were strongly associated with
MC
s (~19% in aged vs. 8% in young,
P
< 0·05).
Conclusions
In photoprotected skin we observed an accumulation of
MC
s with increasing age. These
MC
s have both altered functionality and distribution within the skin, which supports a role for these cells in altered tissue homeostasis during ageing.