An investigation into the role of autoimmunity in vitiligo
Vitiligo is an acquired depigmenting disorder characterised by the loss of functional melanocytes from the cutaneous epidermis. A role for autoimmunity is supported by the presence of circulating antibodies and T lymphocytes which react against melanocyte antigens in patients with vitiligo. The identification and characterisation of these autoantigens will improve understanding of the immune response in vitiligo, and may allow development of better therapies and diagnostic tools. Candidate immunoregulatory genes may predispose to vitiligo. However, this study failed to find an association between vitiligo and a polymorphism of the cytotoxic T lymphocyte antigen-4 (CTLA-4), although the polymorphism increases the likelihood of autoimmune endocrinopathy patients developing vitiligo. The epitopes on melanocyte-specific antigens tyrosinase and Pmel17 which are recognised by antibodies in vitiligo patient sera were identified by molecular mapping. Multiple regions of tyrosinase and at least two domains on PmeI17 were identified as B cell epitopes. Sequence analysis revealed that the tyrosinase epitopes are likely to be cross-reactive with tyrosinase-related proteins but that the antibody response to Pmel17 is distinct. Antibody reactivity to a melanocyte protein, MelanA, targeted by a cellular immune response in vitiligo and melanoma was investigated by immunoblotting and radioimmunoassay. No MelanA-specific antibodies were isolated suggesting that either it is not a target of the humoral immune response in vitiligo, or that antibody reactivity was not detectable by the methods used. To identify novel vitiligo autoantigens, a melanoma cDNA expression library was constructed in a phage-display cloning system and immunoscreened with vitiligo patient IgG. Several possible autoantigens were enriched by this technique, including proteins previously characterised as autoantigens in other disorders. Additionally, humoral reactivity was identified to a protein with a possible role in pigmentation, the melanin-concentrating hormone receptor 1 (MCHR1). MCHR1-specific antibodies were detected in 16.4% (9/55) of vitiligo patients but not in other diseases or healthy control subjects. The study demonstrates the usefulness of phage-display for further autoantigen identification in vitiligo.