Objective:To investigate different regulatory effects of S100A8/A9 expressed by keratinocytes in 3 common inflammatory skin injury modes: UVB-induced skin injury, allergic contact dermatitis, and psoriasis.Methods:Wild-type C57BL/6JGpt mice aged 6 to 8 weeks were selected for the following experiments: (1) mouse models of UVB-induced skin injury were established by single exposure to ultraviolet B (UVB) radiation on the shaved dorsal skin of mice (UVB group, n = 4), with the mice receiving no UVB radiation serving as a control group ( n = 4) ; (2) mouse models of allergic contact dermatitis were established by application of 2,4-dinitrochlorobenzene (DNCB) to the right ears of mice (DNCB group, n = 4), with the left ears of mice treated with a vehicle control serving as a control group ( n = 4) ; (3) mouse models of psoriasis-like skin inflammation were established by topical application of imiquimod cream to the depilated dorsal skin of mice (imiquimod group, n = 4), with the mice treated with vaseline serving as a control group ( n = 4). Hematoxylin-eosin (HE) staining was performed to assess histopathological changes in mouse skin tissues obtained from each group, and immunohistochemical study and Western blot analysis were performed to determine the expression of S100A8 and S100A9 in the mouse dorsal epidermis or ear skin lesions. In vitro cultured HaCaT cells were subjected to the following experiments: (1) cells in the UVB group were treated with a single UVB irradiation at a dose of 50 mJ/cm 2, and cells in the control group received no irradiation; (2) some cells were treated with tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) (collectively referred to as TI), and named as the TI group, which simulated the inflammatory environment in allergic contact dermatitis, while cells treated with corresponding solvents served as the control group; (3) cells were treated with 5 cytokines (interleukin 17A [IL-17A], IL-22, IL-1α, oncostatin M, and TNF-α, collectively referred to as M5), and named as the M5 group, which simulated the inflammatory environment in psoriasis, while cells treated with corresponding solvents served as the control group. Real-time fluorescence-based quantitative PCR, Western blot analysis, and enzyme-linked immunosorbent assay were performed to determine the mRNA and protein expression of S100A8 and S100A9, and to detect the extracellular secretion level of S100A8/A9, respectively. Results:Immunohistochemical study and Western blot analysis revealed that S100A8 and S100A9 expression levels were significantly higher in the skin lesions of mouse models of UVB-induced skin injury, allergic contact dermatitis, and psoriasis-like skin inflammation than in their corresponding control groups; immunohistochemical study further demonstrated that the increase in the expression of the two proteins was more pronounced in the mouse models of psoriasis-like skin inflammation. In the in vitro cell experiments, the mRNA expression of S100A8 and S100A9 in HaCaT cells at 12 and 24 hours were markedly higher in the UVB group (e.g., at 24 hours, 6.14 ± 0.60 vs. 1.00 ± 0.08, 2.58 ± 0.06 vs. 1.02 ± 0.22, respectively, both P < 0.01), TI group (e.g., at 24 hours, 3.90 ± 0.75 vs. 1.00 ± 0.02, 2.42 ± 0.30 vs. 1.01 ± 0.13, respectively, both P < 0.05), and M5 group (e.g., at 24 hours, 157.59 ± 9.30 vs. 1.00 ± 0.11, 251.37 ± 6.63 vs. 1.00 ± 0.03, both P < 0.001) than in the corresponding control groups, so were the extracellular secretion levels of S100A8/A9 at 24 and 48 hours (all P < 0.001), with some differences observed in their response patterns; notably, the response was more pronounced in the mouse model of psoriasis-like skin inflammation. Additionally, the protein expression levels of S100A8 and S100A9 in HaCaT cells were significantly higher in the M5 group than in the control group ( t = 4.66, 4.63, respectively, both P < 0.01), but were significantly lower in the UVB group ( t = -3.75, -3.34, P = 0.020, 0.029, respectively) and TI group ( t = -3.30, -4.50, P = 0.030, 0.011, respectively) than in the control groups. Conclusion:Keratinocytes exhibited active responses following 3 common inflammatory skin injuries, with their effector molecules S100A8 and S100A9, as damage-associated molecular patterns, playing crucial roles in UVB-induced skin injury, allergic contact dermatitis, and psoriasis, and the response seemed to be more pronounced in psoriasis-like dermatitis.