Abstract: 【Abstract】 Objective To investigate the therapeutic effect of human umbilical cord mesenchymal stem cells （MSCs） on psoriasis-like mouse models induced by imiquimod and the underlying mechanisms. Methods Eighteen C57BL/6 mice were randomly and equally divided into vaseline group, model group and treatment group according to a random number table. The mice in the model group and treatment group received topical treatment with 5% imiquimod cream at a dose of 62.5 mg once a day for 6 consecutive days on the shaved back, and those in the vaseline group received the treatment with the same amount of vaseline ointment; the mice in the treatment group were injected with 1.5×106 human umbilical cord MSCs via the caudal vein on days 1 and 4. The severity of skin lesions on the back of the mice was assessed everyday according to the psoriasis area and severity index （PASI）. Twenty-four hours after the last treatment, that is, on day 7, blood samples were taken, and the mice were sacrificed. The dorsal skin tissues were resected and subjected to hematoxylin and eosin （HE） staining. A single cell suspension of the resected spleen was prepared, and flow cytometry was performed to detect the Th1 and Th17 cell subsets in the spleen cells. Enzyme-linked immunosorbent assay was conducted to detect serum levels of cytokines interleukin （IL）-17A and tumor necrosis factor （TNF）-α. One-way analysis of variance was used for comparisons among groups, Tukey test for multiple comparisons, and repeated measures analysis of variance for the analysis of changes in the PASI score over time. Results On day 7, there was obvious scaly erythema on the back of the mice in the model group, and the skin thickness and number of infiltrating inflammatory cells were significantly higher in the model group （78.73 ± 23.11 μm, 36.16 ± 2.95 cells/mm2） than in the vaseline group （13.28 ± 4.57 μm, 13.33 ± 1.15 cells/mm2, q = 19.25, 7.21, respectively, both P < 0.001）. The treatment group showed significantly decreased PASI score, epidermal thickness and number of infiltrating inflammatory cells compared with the model group （all P < 0.001）. The percentage of Th17 cell subsets in the spleen cells and serum level of TNF-α were significantly lower in the treatment group than in the model group （both P < 0.05）. There were no significant differences in the spleen weight, spleen index, spleen cell count, Th1 cell percentage or serum IL-17A level between the treatment group and the model group （all P＞0.05）. Conclusion Human umbilical cord MSCs can effectively alleviate skin inflammation induced by imiquimod in the psoriasis-like mouse models, likely by inhibiting Th17 cell formation and TNF-α expression.

Key words: Psoriasis, Models, animal, Mesenchymal stem cells, Th17 cells, Psoriasis-like murine model, Imiquimod