Internalization via plasmalemmal vesicles: a route for antidesmoplakin autoantibodies into cultured human keratinocytes.

Recently, autoantibodies to desmoplakin I and II have been identified in a subset of patients with a severe form of erythema multiforme. These autoantibodies recognize a specific peptide sequence at the carboxy terminal domain of desmoplakin I and II responsible for interaction with keratin filaments. Desmoplakins are major constitutive proteins of the inner dense desmosomal plaque of keratinocytes and are entirely localized within the cells. With the assumption of pathogenecity for circulating autoantibodies, the question arose how antidesmoplakin autoantibodies enter keratinocytes. Utilizing immunhistochemical procedures for cell motility and time kinetic studies at the light- and electron-microscopic level, we found that autoantibodies are bound at the cell surface of cultured human keratinocytes, internalized via plasmalemmal vesicles, and are found consecutively within tubulovesicular structures inside the cells. At the same time, a fraction of antibodies can be detected at the inner dense desmosomal plaques. Immunogold labeling reveals internalization of autoantibodies in small non-coated plasmalemmal vesicles positive for caveolin. These observations indicate that vesicular transport may represent a relevant biological mechanism for antidesmoplakin autoantibodies to enter keratinocytes and allow access to their corresponding antigenic target in vivo.