Morphologic and immunologic characterization of a canine isthmus mural folliculitis resembling pseudopelade of humans

The clinical, histopathologic and immunopathologic features of a novel form of isthmus mural folliculitis in dogs, which resembles pseudopelade in hum ans, were characterized. Clinically, dogs exhibited variably distributed fo ci of alopecia that persisted without treatment or did not respond to immun osuppressive therapy. Histopathologically, mixed mononuclear cell infiltrat es, largely lymphocytes, infiltrated the follicular isthmus. Occasionally, inflammation extended above and below the follicular isthmus but did not in volve the hair bulb or the epidermis. Severe follicular atrophy and variabl e atrophy of sebaceous glands occurred in all dogs. Folliculotropic lymphoc ytes exhibited most commonly CD3 and CD8 (cytotoxic T cells). Autoantibodie s specific for the lower hair follicle were detected in the serum of affect ed patients. Western immunoblotting demonstrated binding of these antibodie s to multiple follicular keratinocyte proteins, including hair keratins and trichohyalin. Lack of hair regrowth (in contrast to canine alopecia areata ), as well as location of inflammation and extreme atrophy of adnexal units are similar to findings seen in human pseudopelade.