However, autoantibodies in scurfy sera have no reactivity to the NC14A domain of murine COL17, the domain that is responsible for subepidermal blister formation. roles for regulatory T cells in the maintenance of selftolerance to COL17 and BP230 as well as in the suppression of inflammation triggered by the binding of antibodies to COL17. Notably, physical stresses such as trauma, thermal burns, bone fractures, irradiation and ultraviolet exposure, some pathologic conditions such as neurological diseases and hematological malignancies, and the use of dipeptidyl peptidaseIV inhibitors and immune checkpoint inhibitors have been reported as triggering factors for BP. These factors and certain underlying conditions such as genetic background, regulatory Tcell dysfunction or aging might synergistically affect some individuals and eventually induce BP. Further studies around the breakdown of selftolerance and on the identification of key molecules that are relevant to blister formation and inflammation APS-2-79 HCl may expand our understanding of BP’s etiology and may Rcan1 lead to the development of novel therapeutic approaches. Keywords:BP180, BP230, epitope spreading, immune checkpoint inhibitor, regulatory T cell == 1. INTRODUCTION == Bullous pemphigoid (BP) is the most common autoimmune blistering disease. It characteristically affects the elderly, and recent studies have reported a trend of increased incidence of BP.1Clinically, it is characterized by tense blisters, urticarial plaques, and erythema on APS-2-79 HCl the whole body; histologically, subepidermal blisters with prominent eosinophilic infiltration are usually observed.2Autoantibodies in BP react with two structural components of the dermalepidermal junction (DEJ): type XVII collagen (COL17, also called BP180 or BPAG2) and BP230 (also called dystonin or BPAG1).3COL17 is a hemidesmosomal transmembrane protein that spans the lamina lucida and projects into the lamina densa of the DEJ.4,5,6The extracellular portion of COL17 contains 15 collagenous domains that are separated from one another by noncollagenous domains.5The noncollagenous 16A (NC16A) domain is located at the membraneproximal extracellular region of COL17 and is preferentially recognized by autoantibodies in BP.7,8In fact, 80%90% of BP sera react with the NC16A domain of COL17.7,9,10Previous studies have demonstrated that this serum levels of autoantibodies to the NC16A domain of COL17 correlate with BP disease activity.7,8,11,12The passive transfer of IgG antibodies to the NC16A domain of human COL17 or its murine counterpart into neonatal mice has been shown to induce subepidermal separation.13,14Thus, the NC16A domain name of COL17 contains the major pathogenic epitope for BP, and an ELISA using recombinant NC16A protein is widely used for detecting and quantifying BP autoantibodies. Notably, the antiCOL17 antibody can also recognize other epitopes on COL17, other than the NC16A domain name. To detect all antibodies to COL17, we established an ELISA that uses mammalian cellderived recombinant human fulllength COL17 protein, which improved the BP autoantibody detection rate from 82.6% (NC16A ELISA alone) to 94.2% (combined use of NC16A ELISA and fulllength COL17 ELISA).15Intriguingly, nonNC16A BP in which the antibodies were detected by the fulllength COL17 ELISA alone significantly showed a noninflammatory phenotype that is characterized by few erythema and few eosinophilic infiltration in periblistering dermis and likely to receive dipeptidyl peptidaseIV (DPP4) inhibitors (DPP4i) before BP onset.15 BP230, a cytoplasmic component of hemidesmosomes that belongs to the plakin family, is another autoantigen of BP, and it is targeted by autoantibodies in about 50%80% of BP cases.16,17,18The autoantibodies preferentially target the Cterminal domain of BP230.16,17Although several studies have noted the pathogenicity of autoantibodies to BP230,19,20it remains uncertain whether antiBP230 autoantibodies directly contribute to blister formation or whether they are just byproducts of epitope spreading associated with disease extension. Recent clinical research and ex vivo and in vivo experiments have gradually elucidated the pathomechanisms and triggering factors APS-2-79 HCl of BP. The present review focuses on the epitopespreading phenomenon in BP, the pathogenicity of autoantibodies to BP230, the impact of regulatory Tcell (Treg) dysfunction on BP, and the triggering factors for BP. == 2. EPITOPE SPREADING IN BP == Epitope spreading is a phenomenon in which the targets of cellular and/or humoral immune responses can extend from the initial dominant epitope to other epitopes APS-2-79 HCl on the same protein (intramolecular epitope spreading) or to other proteins in the same tissue (intermolecular epitope spreading) over time.21,22Some autoimmune disorders such as multiple sclerosis23and myasthenia gravis24are known to show intramolecular epitope spreading. It is well known that epitope spreading frequently occurs in BP. APS-2-79 HCl In vivo experiments using a human COL17expressing skingrafted BP mouse model showed that IgG antibodies to human COL17 initially react to the extracellular domains and subsequently target additional extracellular domains and intracellular domains.25A prospective multicenter study demonstrated that 17 of 35 (49%) BP patients showed.