Anti-cytokine autoantibodies have been widely reported to be present in human 
plasma, both in healthy subjects and in patients with underlying autoimmune 
conditions, such as autoimmune polyendocrinopathy candidiasis ectodermal 
dystrophy (APECED) or thymic epithelial neoplasms. While often asymptomatic, 
they can cause or facilitate a wide range of diseases including opportunistic 
infections. The potential therapeutic value of specific neutralizing 
anti-cytokine autoantibodies has not been thoroughly investigated. Here we used 
mammalian cell display to isolate IL17A-specific antibodies from a thymoma 
patient with proven high-titer autoantibodies against the same. We identified 3 
distinct clonotypes that efficiently neutralized IL17A in a cell-based in vitro 
assay. Their potencies were comparable to those of known neutralizing 
antibodies, including 2, AIN457 (secukinumab) and ixekizumab that are currently 
in clinical development for the treatment of various inflammatory disorders. 
These data clearly demonstrate that the human autoantibody repertoire can be 
mined for antibodies with high therapeutic potential for clinical development.