Immunolocalization of calcineurin and FKBP12, the FK506-binding protein, in Hassall's corpuscles of human thymus and epidermis

Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase, plays an important role in various physiological functions including T cell activation. This enzyme is a target molecule for the immunosuppressants, cyclosporin A and FK506. In the present study, we investigated immunohistochemical localization of calcineurin and FKBP12, an FK506-binding protein, in human thymus and epidermis. The catalytic subunit (calcineurin A) of calcineurin was abundantly expressed in Hassall's corpuscles which were localized in the thymic medulla and represented the terminal stages of thymic medullary epithelium. The regulatory subunit (calcineurin B) of calcineurin was also expressed in high amounts in Hassall's corpuscles. In the epidermis, which shows similarities to Hassall's corpuscles, both subunits were also abundantly expressed, and their expression increased with the differentiation of keratinocytes. FKBP12 was observed to be expressed abundantly, both in Hassall's corpuscles and the entire epidermis. These findings suggest that the differentiated forms of the two cell types, which are the thymic medullary epithelial cell and the keratinocyte, are the target for pharmacological actions of FK506.