Researchers here propose that the skin is a significant source of the systemic chronic inflammation that is observed in older individuals. Setting aside the range of other mechanisms that contribute to inflammation to only consider the accumulation of senescent cells with age, and the fact that these errant cells are a potent source of inflammatory signaling, this proposition doesn't seem unreasonable. The skin is a sizable organ, after all, and even if it produces senescent cells at much the same pace as the rest of the body, it will still represent a large and quite distributed pool of such cells, positioned to delivery their inflammatory signals throughout the body.
Increasing evidence points to a provocative role of sustained, sub-clinical inflammation, often termed "inflammaging," in the development of these chronic disorders. In support of this notion, chronologically aged humans (≥50 years) display elevated circulating levels of pro-inflammatory cytokines, particularly IL-6, IL-1β, and TNFα. Moreover, subjects with chronic cutaneous inflammatory diseases, such as psoriasis and eczematous dermatitis, also display an increased prevalence of aging-associated disorders, including atherosclerotic cardiovascular disease, obesity, and type 2 diabetes. Though anti-inflammatory regimens, such as inhibitors of IL-1βα and TNFα, as well as methotrexate, have been deployed in the management of these aging-associated disorders, the outcomes of treatments with these agents have been inconclusive.
While many chronologically aged humans merely display marked evidence of inflammation, they nonetheless display elevated circulating levels of cytokines, suggesting that one or more, as yet identified organs, could account for the aging-associated increase in circulating cytokines. It seems reasonable to postulate that the responsible organs must be large enough to sustain such an increase in circulating cytokines, even without noticeable inflammation. Although the musculoskeletal system is the largest organ in humans, most chronologically aged humans display no evidence of musculoskeletal inflammation.
Other relatively large organs to be considered include the skin, intestines, lungs, and liver. The skin weighs about 20 lbs (with an additional, variable contribution from subcutaneous adipose tissues), while the weights of the intestines, lungs, and liver represent ≈7.5, 5.0 and 3.3 lbs, respectively. Because of their relatively lesser size, inflammation of the lungs, intestines, and liver likely would not only need to be apparent, but also sustained if any of these organs could account for the increase in circulating levels of cytokines. Yet again, the majority of otherwise normal aged humans display few clinical signs or symptoms of inflammation in these organs. Hence, it seems unlikely that they could contribute substantially to "inflammaging" unless multiple organs simultaneously exhibit mild inflammation. Notably, the aged skin commonly exhibits signs and symptoms of inflammation, such as pruritus and senile xerosis.
Because of its relatively large size, we hypothesized that the skin could be an important contributor to the elevated levels of circulating cytokines in chronologically aged humans, despite the fact that it typically displays little evidence of inflammation. Not only its size, but also its unique anatomic site, serving as the interface between the body and external environment, supports our hypothesis. In this site, it is continuously exposed to external physical and chemical stressors, which themselves can provoke inflammation, even as other less-exposed organs remain quiescent. In addition, chronologically aged humans display alterations in several key epidermal functions, each of which can provoke low-grade, chronic inflammation in the skin.