The disclosure relates to transglutaminase 4 as a prostate specific autoantigen, and methods for diagnosis of autoimmune polyendocrune syndrome type 1, autoimmune prostatitis in autoimmune polyendocrine syndrome type 1, and isolated autoimmune prostatitis. The disclosure further relates to use of transglutaminase 4 and transglutaminase 4 antibodies and antibody conjugates for the treatment of prostate cancers.