The epidermis, although compact in structure, contains a discrete extracellular matrix located within the 15- to 20-nanometer intercellular spaces surrounding keratinocyte junctions. This matrix is rich in hyaluronic acid, a linear, highly hydrated glycosaminoglycan that creates an environment conducive to the diffusion of nutrients, growth factors, and cytokines, as well as to the trafficking of immune cells. Half of the body's hyaluronic acid resides in the skin, mainly in the dermis but also in the living layers of the epidermis, where its functions remain only partially understood. Notably, increased hyaluronic acid production has been described in inflammatory conditions such as atopic dermatitis. This molecule interacts with various partners, including TSG-6, a soluble protein secreted into the extracellular matrix by several cell types in response to inflammatory stimuli and encoded by the TNFAIP6 gene.
To clarify the role of TSG-6 in the epidermis, the authors worked with in vitro reconstructed human epidermises subjected to two types of insult. The first reproduced the atopic dermatitis phenotype through exposure to T helper 2–type cytokines (IL-4, IL-13, with or without IL-25); the second consisted of a fungal infection with the dermatophyte Trichophyton rubrum, triggering the secretion of cytokines and antimicrobial peptides. In both cases, TSG-6 expression was strongly induced, and its increased release accompanied a rise in hyaluronic acid production. To investigate the protein's function, the team inactivated the TNFAIP6 gene in immortalized N/TERT keratinocytes using CRISPR/Cas9 technology, generating TSG-6–deficient cell lines.
Keratinocytes lacking TSG-6 tended to migrate more slowly in in vitro wound-healing assays but gave rise to reconstructed epidermises displaying normal morphology and differentiation, with transcriptomic analysis revealing only a few significant alterations. By contrast, these TSG-6-deficient reconstructed epidermises exhibited a reduced hyaluronic acid content, together with an increased release of this glycosaminoglycan into the culture medium. Such a phenotype became even more pronounced under the insult conditions. When TSG-6–producing cells were reintroduced into the reconstructed epidermises, this leakage of hyaluronic acid was reduced.
Taken together, these findings point to a role for TSG-6 in sequestering hyaluronic acid between epidermal cells in response to inflammation. By retaining this glycosaminoglycan within the epidermis rather than allowing it to diffuse into the surrounding environment, the protein is thought to help preserve the integrity of the intercellular extracellular matrix in inflammatory contexts.