| Summary: | <i>Equisetum arvense</i> L. (Equisetaceae), widely known as ‘horsetail’, is a perennial plant found extensively across Asia. Extracts of <i>E. arvense</i> have been used in traditional medicine, particularly for the treatment of inflammatory disorders. This study aimed to determine the phytochemical compounds in <i>E. arvense</i> ethanolic extract and their anti-inflammatory properties. Subsequently, we isolated and identified nine secondary metabolites, including kaempferol 3,7-di-<i>O</i>-<i>β</i>-D-glucopyranoside (<b>1</b>), icariside B<sub>2</sub> (<b>2</b>), (<i>Z</i>)-3-hexenyl <i>β</i>-D-glucopyranoside (<b>3</b>), luteolin 5-<i>O</i>-<i>β</i>-D-glucopyranoside (<b>4</b>), 4-<i>O</i>-<i>β</i>-D-glucopyranosyl caffeic acid (<b>5</b>), clemastanin B (<b>6</b>), 4-<i>O</i>-caffeoylshikimic acid (<b>7</b>), (7<i>S</i>,8<i>S</i>)-<i>threo</i>-7,9,9′-trihydroxy-3,3′-dimethoxy-8-<i>O</i>-4′-neolignan-4-<i>O</i>-<i>β</i>-D-glucopyranoside (<b>8</b>), and 3-<i>O</i>-caffeoylshikimic acid (<b>9</b>). The chemical structures of the isolated compounds (<b>1–9</b>) were elucidated using HR-ESI-MS data, NMR spectra, and ECD data. Next, the anti-inflammatory effects of the isolates were evaluated in tumor necrosis factor (TNF)α/interferon (IFN)γ-induced HaCaT, a human keratinocyte cell line. Among the isolates, compound <b>3</b> showed the highest inhibitory effect on the expression of pro-inflammatory chemokines, followed by compounds <b>6</b> and <b>8</b>. Correspondingly, the preceding isolates inhibited TNFα/IFNγ-induced activation of pro-inflammatory transcription factors, signal transducer and activator of transcription 1, and nuclear factor-κB. Collectively, <i>E. arvense</i> could be employed for the development of prophylactic or therapeutic agents for improving dermatitis.
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