Introduction
The skin is the largest immune organ facing the outside of the body. The skin exerts an immune responses to maintain homeostasis when stimulated with viruses, pathogens, air pollutants, also chemical materials including cosmetics. This immune function is indispensable for skin homeostasis, however excessive exertion of immune function causes overproduction of cytokines and unbalanced cell differentiation / proliferation, which may cause skin diseases. Therefore, it is widely desired to elucidate the mechanism of the regulation mechanism(s) of the skin immunity, also to establish the method how to analyze skin inflammation quantitatively / qualitatively by using various indexes.
Primary cilia are cell organelles present in almost all cell types. Primary cilia protrude to extracellular space showing like ‘antenna’ to sense extracellular environment. Because various signal pathways are transduced in it, it is thought to play an essential role in maintaining cell homeostasis. However, it is not clear the physiological function of primary cilia in controlling skin immunity. Therefore, we firstly investigated the presence of primary cilia in human adult epidermis/dermis, and confirmed that only langerhans cells (LCs) and keratinocytes have primary cilia. Our results suggested that primary cilia in LCs promoted proliferation while suppressing maturation, suggesting the physiological function of primary cilia in maintaining homeostasis of skin immunity.

Materials and Methods
We employed human dendritic cells or monocyte isolated from human peripheral blood mononuclear cells (PBMC), and human skin samples derived from healthy donor or atopic dermatitis patients. We performed immunofluorescent imaging analysis to visualize primary cilia by using anti-acetylated tubulin antibody. To measure the cell proliferation activity, we performed CCK8 assay. To analyze the maturation marker expression, we used cell sorter. All experiments with human tissue were approved by the Ethics committee of Osaka University.
Results
We first examined whether healthy adult human epidermal/dermal skin cells have primary cilia. Our immunofluorescent imaging data suggested that LCs and keratinocyte, but not CD8 T cells or CD4 T cells, have primary cilia. LCs are known as a kind of dendritic cells (DCs) in epidermis. To examine the physiological function of primary cilia in DCs, we employed human DCs derived from human PBMC. The stimulation of DCs with maturation agent TNFa significantly decreased primary cilia rate and proliferation activity. Knockdown of ciliary component gene namely IFT88 decreased primary cilia rate with increasing maturation marker CCR7, suggesting that primary cilia function on cell maturation and proliferation. When examined the atopic dermatitis skin, we found that excess formation of primary cilia in epidermal keratinocyte and LCs. In atopic epidermis, proliferating, but immature cells were increased compared to healthy epidermis, suggesting the primary cilia function in skin homeostasis.

Conclusion
1. Langerhans cells and keratinocyte have primary cilia as cellular antenna.
2. Primary cilia formation in DCs are related to proliferation and maturation.
3. Atopic dermatitis skin increased primary cilia rate, with increasing immature/proliferating cells.

Reference
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