Ectopic expression of epidermal antigens renders the lung a target organ in paraneoplastic pemphigus.

Paraneoplastic pemphigus (PNP) is an autoimmune disease of the skin and mucous membranes that can involve fatal lung complications. IgG autoantibodies target the cell adhesion molecules desmoglein (Dsg)3 and plakins, but the nature and targets of infiltrating T cells are poorly characterized. Moreover, the lung involvement in this skin Ag-specific autoimmune condition represents a paradox. To mimic autoimmunity in PNP, we grafted wild-type skin onto Dsg3(-/-) mice, which resulted in graft rejection and generation of anti-Dsg3 IgG and Dsg3-specific T cells. Transfer of splenocytes from these mice into Rag2(-/-) mice induced a combination of suprabasilar acantholysis and interface dermatitis, a histology unique to PNP. Furthermore, the recipient mice showed prominent bronchial inflammation of CD4(+) and CD8(+) T cells with high mortality. Intriguingly, ectopic Dsg3 expression was observed in the lungs of PNP mice, mirroring the observation that squamous metaplasia is often found in the lungs of PNP patients. Dsg3 and other epidermal Ags were ectopically expressed in the lungs after pulmonary injuries by naphthalene, which was sufficient for recruitment of Dsg3-specific CD4(+) T cells. These findings demonstrate that squamous metaplasia after pulmonary epithelial injury may play a crucial role in redirecting the skin-specific autoimmune reaction to the lungs in PNP.