Destroyed Melanocytes Manifested with Poliosis and Halo Nevi in Children: Proposed Immunopathophysiological Mechanisms with Emphasis on Hair Follicle Immune Privilege

Both poliosis (circumscribed depigmentation of the hair in any area of the body) and halo nevi (a melanocytic nevus which acquires a surrounding depigmented rim) are characterized by depigmentation driven by a T-cell-mediated loss of functional melanocytes within the epidermis and hair bulbs. Halo nevi and poliosis are a relatively frequent phenomenon occurring in 1% of the children and adolescents with an average age of onset of 15 years and has no gender predisposition. Both can be associated with vitiligo and are occasionally seen together as the same cytotoxic immune response (specifically CD8+ T cells) that destroys nevus melanocytes which can attack the same melanocyte-specific targets surrounding epidermal and follicular melanocytes. While poliosis is frequently seen on the scalp, halo nevi on the scalp have been scarcely reported in the literature. The timing of occurrence of the destruction of both epidermal and follicular melanocytes may be supported by a case of a 13-year-old Caucasian male with three halo nevi on the trunk which preceded the development of four scalp patches of poliosis. This may indicate that the halo nevi on the trunk induced a temporary systemic autoimmune reaction during the halo phenomenon, collaterally destroying melanocytes at distance on the scalp causing poliosis.