Aged human skin accumulates mast cells with altered functionality that localize to macrophages and vasoactive intestinal peptide-positive nerve fibres

Background Skin health declines with age and this is partially attributed to immunosenescence. Mast cells (MCs) are innate immune cells which coordinate tissue immune responses integral to skin homeostasis and disease. Objectives To understand how MCs 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 (≤30yrs) and aged (≥75yrs) individuals immunostaining and spatial morphometry were performed to identify changes in MC phenotype, number, distribution and interaction with the vasculature and nerve fibres. Quantitative PCR was used to measure changes in gene expression related to immune cell activity and neuropeptide signalling. Results Skin MCs, macrophages and CD8+ T‐cells increased in number in intrinsically aged, as compared to young skin, by 40%, 44% and 90% respectively (p<0.05), whilst CD4+ T‐cells and neutrophils were unchanged. In the aged, MCs were more numerous in the papillary dermis, showed a reduced incidence of degranulation (50% lower than in young; p<0.01), a conserved tryptase/chymase phenotype and co‐expression of granzyme B. In aged skin, MCs increased their association with macrophages (~48% cf ~27%; p<0.05) and nerve fibres (~29% cf 16%; p<0.001), whilst reducing their interactions with blood vessels (~34% cf 45%; p<0.0001). Additionally, we observed modulation of vasoactive intestinal peptide (VIP; increased) and substance P (decreased) gene expression with age; this was associated with an increased frequency of VIP+ nerve fibres (~3x higher in aged skin; p<0.05), which were strongly associated with MCs (~19% in aged cf 8% in young; p<0.05). Conclusions Hence, in photoprotected skin, we observe an accumulation of MCs with increasing age; these MCs have both altered functionality and distribution within the skin which supports a role for these cells in altered tissue homeostasis during ageing.