Transmissibility of characteristic lesions to experimental animals may help us understand the pathomechanism of human autoimmune disease. Here we show that human autoimmune disease can be reproduced using genetically engineered model mice. Bullous pemphigoid (BP) is the most common serious autoimmune blistering skin disease, with a considerable body of indirect evidence indicating that the underlying autoantigen is collagen XVII (COL17). Passive transfer of human BP autoantibodies into mice does not induce skin lesions, probably because of differences between humans and mice in the amino acid sequence of the COL17 pathogenic epitope. We injected human BP autoantibody into Col17-knockout mice rescued by the human ortholog. This resulted in BP-like skin lesions and a human disease phenotype. Humanization of autoantigens is a new approach to the study of human autoimmune diseases.

In most stem cell systems, the organization of the stem cell niche and the anchoring matrix required for stem cell maintenance are largely unknown. We report here that collagen XVII (COL17A1/BP180/BPAG2), a hemidesmosomal transmembrane collagen, is highly expressed in hair follicle stem cells (HFSCs) and is required for the maintenance not only of HFSCs but also of melanocyte stem cells (MSCs), which do not express Col17a1 but directly adhere to HFSCs. Mice lacking Col17a1 show premature hair graying and hair loss. Analysis of Col17a1-null mice revealed that COL17A1 is critical for the self-renewal of HFSCs through maintaining their quiescence and immaturity, potentially explaining the mechanism underlying hair loss in human COL17A1 deficiency. Moreover, forced expression of COL17A1 in basal keratinocytes, including HFSCs, in Col17a1-null mice rescues MSCs from premature differentiation and restores TGF-β signaling, demonstrating that HFSCs function as a critical regulatory component of the MSC niche.

Epidermolysis bullosa (EB) is classified into major types -EB simplex (EBS), junctional EB (JEB) and dystrophic EB (DEB) -on the basis of the level of tissue separation within the cutaneous basement membrane zone. Recent advances in research on EB have led to the identification of 10 genes responsible for EB. The Japanese Ministry of Health, Labor and Welfare has designated JEB and DEB, but not EBS, as specified diseases. However, EBS has a lethal variant and should also be registered as a specified disease. In the Third Consensus Meeting on the Diagnosis and Classification of EB held in Vienna in 2007, it was recommended that Kindler syndrome should be classified as a subtype of EB. Corrective gene therapy is the most ideal therapy for EB, but much more research is required before it can be developed and used in clinical practice. Cell-based therapies using fibroblasts and bone marrow cells have recently attracted considerable attention.

Until about 20 years ago, the term lamellar ichthyosis (LI) represented all nonbullous autosomal recessive ichthyoses except for harlequin ichthyosis and ichthyosis syndromes. Since the 1980s, nonbullous autosomal recessive ichthyoses have been divided into two major clinical entities, nonbullous congenital ichthyosiform erythroderma (NBCIE) and LI. The nature of scaling and intensity of erythroderma are important clinical features that distinguish between NBCIE and LI. However, a considerable number of cases show an intermediate phenotype between the two classic clinical features. Histologically, parakeratosis and inflammatory cell infiltration are seen more frequently in NBCIE than in LI and the stratum corneum is usually thicker in LI than in NBCIE. However, neither histopathological findings nor ultrastructural features seem to help clearly distinguish between NBCIE and LI. Mutations in any of the three known causative genes, TGM1, ALOXE3 or ALOX12B, can lead either to NBCIE or LI. Candidate genes specific to either NBCIE or LI alone have not been identified. Based on these facts, it might be better to consider NBCIE and LI as variations of a single keratinization disorder, although the classification of these autosomal recessive congenital ichthyosis patients into NBCIE or LI depending on their clinical features is still useful for practical patient management.

In bullous pemphigoid (BP), the most prevalent autoimmune blistering disease, type XVII collagen (COL17) is targeted by circulating autoantibodies. BP is thought to be an autoantibody-mediated complement-fixing blistering disease, and a juxtamembranous noncollagenous 16A (NC16A) domain spanning Glu490 to Arg566 was proved to be the main pathogenic region on COL17, although precise pathogenic epitopes within NC16A have not been elucidated. In this study, we showed that injection of rabbit IgG Abs targeting Asp522 to Gln545 induced skin fragility associated with in vivo deposition of IgG and complement in neonatal COL17-humanized mice. Notably, immunoadsorption of rabbit anti-NC16A IgG Ab with this epitope (Asp522 to Gln545) or the anti-NC16A IgG administered together with the peptides of this epitope as a decoy ameliorated skin fragility in the injected neonatal COL17-humanized mice compared with the anti-NC16A IgG alone even though all of the mice showed both IgG and complement deposition. These results led us to investigate an additional, complement-independent mechanism of skin fragility in the mice injected with anti-COL17 Abs. The rabbit anti-NC16A IgG depleted the expression of COL17 in cultured normal human keratinocytes, whereas immunoadsorption of the same IgG with this epitope significantly suppressed the depletion effect. Moreover, passive transfer of F(ab′)2 fragments of the human BP or rabbit IgG Abs against COL17 demonstrated skin fragility in neonatal COL17-humanized mice. In summary, this study reveals the importance of Abs directed against distinct epitopes on COL17, which induce skin fragility in complement-dependent as well as complement-independent ways.