Role of Siglec‐E in <scp>MC903</scp>‐Induced Atopic Dermatitis

Atopic dermatitis (AD) is a common skin disease. Although AD pathogenesis has been widely researched, inhibitory mechanisms in AD are still unclear. Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of receptors recognising sialic acids; most Siglecs work as inhibitory receptors. Among Siglecs, Siglec-E is expressed on dendritic cells (DCs) and eosinophils, important immune cells in AD. Although Siglec-E inhibits Type 1 inflammatory diseases, how it influences AD is unknown. Thus, we investigated the role of Siglec-E in AD mouse model by using Siglec-E knockout (KO) mice. We demonstrated that Siglec-E attenuated AD-like inflammation of mice caused by topical application of MC903 on ear skin (MC903-induced AD). To reveal the role of Siglec-E in MC903-induced AD, we focused on Siglec-E on DCs and eosinophils. We first showed that Sigle-E was expressed on cutaneous DCs and migratory DCs of draining lymph nodes. Moreover, OX40L expression on cutaneous DCs was reduced in the presence of Siglec-E. In vitro experiments using cultured spleen DCs (SpDCs), highly expressing Siglec-E, revealed that IL-33 was involved in the induction of Siglec-E and confirmed that Siglec-E inhibited OX40L expression on SpDCs induced by IL-33. Moreover, CD4⁺ T cell-SpDC coculture revealed that Siglec-E inhibited Th2 polarisation under IL-33 stimulation. We finally revealed that Siglec-E was expressed on eosinophils and reduced the eosinophils infiltration to the MC903-treated ear skin with the suppression of CD49d, a necessary integrin for eosinophil migration to skin tissue [1], expression on eosinophils. These findings elucidated the inhibitory role of Siglec-E in MC903-induced AD.