High Expression of Collagen XVII Compensates for its Depletion Induced by Pemphigoid IgG in the Oral Mucosa

Bullous pemphigoid induced by IgG targeting type XVII collagen non-NC16A/NC15A extracellular domains is driven by Fc gamma receptor- and complement-mediated effector mechanisms and is ameliorated by neonatal Fc receptor blockade

Bullous pemphigoid (BP) is an autoimmune blistering disease characterized by autoantibodies targeting type XVII collagen (Col17) with the noncollagenous 16A (NC16A) ectodomain representing the immunodominant site. The role of additional extracellular targets of Col17 outside NC16A has not been unequivocally demonstrated. In this study, we showed that Col17 ectodomain-reactive patient sera depleted in NC16A IgG induced dermal-epidermal separation in a cryosection model indicating the pathogenic potential of anti-Col17 non-NC16A extracellular IgG. Moreover, injection of IgG targeting the murine Col17 NC14-1 domains (downstream of NC15A, the murine homologue of human NC16A) into C57BL/6J mice resulted in erythematous skin lesions and erosions. Clinical findings were accompanied by IgG/C3 deposits along the basement membrane and subepidermal blistering with inflammatory infiltrates. Disease development was significantly reduced in either Fc-gamma receptor (FcγR)- or complement-5a receptor-1 (C5aR1)-deficient mice. Inhibition of the neonatal FcR (FcRn), an atypical FcγR regulating IgG homeostasis, with the murine Fc fragment IgG2c-ABDEG, a derivative of efgartigimod, reduced anti-NC14-1 IgG levels, resulting in ameliorated skin inflammation compared with isotype-treated controls. These data demonstrate that the pathogenic effects of IgG targeting the Col17 domain outside human NC16A/murine NC15A are partly attributable to antibody-mediated FcγR- and C5aR1 effector mechanisms while pharmacological inhibition of the FcRn represents a promising treatment for BP. The mouse model of BP will be instrumental in further investigating the role of Col17 non-NC16A/NC15A extracellular epitopes and validating new therapies for this disease. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Mouse gingival single-cell transcriptomic atlas identified a novel fibroblast subpopulation activated to guide oral barrier immunity in periodontitis

Periodontitis, one of the most common non-communicable diseases, is characterized by chronic oral inflammation and uncontrolled tooth supporting alveolar bone resorption. Its underlying mechanism to initiate aberrant oral barrier immunity has yet to be delineated. Here, we report a unique fibroblast subpopulation activated to guide oral inflammation (AG fibroblasts) identified in a single-cell RNA sequencing gingival cell atlas constructed from the mouse periodontitis models. AG fibroblasts localized beneath the gingival epithelium and in the cervical periodontal ligament responded to the ligature placement and to the discrete topical application of Toll-like receptor stimulants to mouse maxillary tissue. The upregulated chemokines and ligands of AG fibroblasts linked to the putative receptors of neutrophils in the early stages of periodontitis. In the established chronic inflammation, neutrophils, together with AG fibroblasts, appeared to induce type 3 innate lymphoid cells (ILC3s) that were the primary source of interleukin-17 cytokines. The comparative analysis of Rag2-/- and Rag2-/-Il2rg-/- mice suggested that ILC3 contributed to the cervical alveolar bone resorption interfacing the gingival inflammation. We propose the AG fibroblast-neutrophil-ILC3 axis as a previously unrecognized mechanism which could be involved in the complex interplay between oral barrier immune cells contributing to pathological inflammation in periodontitis.

Zonula occludens-1 distribution and barrier functions are affected by epithelial proliferation and turnover rates

Zonula occludens-1 (ZO-1) is a scaffolding protein of tight junctions, which seal adjacent epithelial cells, that is also expressed in adherens junctions. The distribution pattern of ZO-1 differs among stratified squamous epithelia, including that between skin and oral buccal mucosa. However, the causes for this difference, and the mechanisms underlying ZO-1 spatial regulation, have yet to be elucidated. In this study, we showed that epithelial turnover and proliferation are associated with ZO-1 distribution in squamous epithelia. We tried to verify the regulation of ZO-1 by comparing normal skin and psoriasis, known as inflammatory skin disease with rapid turnover. We as well compared buccal mucosa and oral lichen planus, known as an inflammatory oral disease with a longer turnover interval. The imiquimod (IMQ) mouse model, often used as a psoriasis model, can promote cell proliferation. On the contrary, we peritoneally injected mice mitomycin C, which reduces cell proliferation. We examined whether IMQ and mitomycin C cause changes in the distribution and appearance of ZO-1. Human samples and mouse pharmacological models revealed that slower epithelial turnover/proliferation led to the confinement of ZO-1 to the uppermost part of squamous epithelia. In contrast, ZO-1 was widely distributed under conditions of faster cell turnover/proliferation. Cell culture experiments and mathematical modelling corroborated these ZO-1 distribution patterns. These findings demonstrate that ZO-1 distribution is affected by epithelial cell dynamics.

Mouse gingival single-cell transcriptomic atlas: An activated fibroblast subpopulation guides oral barrier immunity in periodontitis

Periodontitis, one of the most common non-communicable diseases, is characterized by chronic oral inflammation and uncontrolled tooth supporting alveolar bone resorption. Its underlying mechanism to initiate aberrant oral barrier immunity has yet to be delineated. Here, we report a unique fibroblast subpopulation activated to guide oral inflammation (AG fibroblasts) identified in a single-cell RNA sequencing gingival cell atlas constructed from the mouse periodontitis models. AG fibroblasts localized beneath the gingival epithelium and in the cervical periodontal ligament responded to the ligature placement and to the discrete application of Toll-like receptor stimulants to mouse maxillary tissue. The upregulated chemokines and ligands of AG fibroblasts linked to the putative receptors of neutrophils in the early stages of periodontitis. In the established chronic inflammation, neutrophils together with AG fibroblasts appeared to induce type 3 innate lymphoid cells (ILC3s) that were the primary source of interleukin-17 cytokines. The comparative analysis of Rag2-/- and Rag2γc-/- mice suggested that ILC3 contributed to the cervical alveolar bone resorption interfacing the gingival inflammation. We propose that AG fibroblasts function as a previously unrecognized surveillant to initiate gingival inflammation leading to periodontitis through the AG fibroblast-neutrophil-ILC3 axis.

[Immunopathogenesis of mucous membrane pemphigoid]

A detailed understanding of the immunopathogenesis of mucous membrane pemphigoid (MMP) is of particular importance in view of the mostly difficult diagnostics and treatment of this blistering autoimmune dermatosis. A still unknown disturbance of the body's own immune tolerance leads to the formation of autoreactive cells. As the disease progresses these produce autoantibodies which are directed against structural proteins in the basement membrane zone (BMZ). After they bind to the target antigen, complement factors are deposited along the BMZ and inflammatory cells invade the underlying tissue and produce the characteristic subepithelial blistering. This inflammatory response is associated with fibrosis and scarring in many affected tissues. Most phases of MMP pathogenesis are poorly understood; however, the last few years have shed more light on this processes. These advances are mostly the result of animal and cell culture models. Typical clinical and immunopathological characteristics of MMP, such as oral, conjunctival and skin lesions, are reflected, for example, in an antibody transfer-induced mouse model for anti-laminin 332 MMP in adult mice. Dapsone, as first-line treatment for MMP patients, significantly reduced the severity of these symptoms, and fibrosis in the skin and mucous membranes was also found histologically, which makes the model well-suited for testing new therapeutic approaches for MMP patients and might be of help for further elucidation of the immunopathogenesis of MMP.

Mucous membrane pemphigoid

Mucous membrane pemphigoid (MMP) is a clinically and immunopathologically heterogenous disease with an incidence of about 2/million inhabitants/year in central Europe. Pemphigoid diseases are characterized by autoantibodies against structural proteins of the epidermis and/or surface-close epithelia. MMP has been defined as pemphigoid disease with predominant mucosal lesions. Most frequently, the oral cavity and the conjunctivae are affected. Lesions outside the mouth tend to heal with scarring leading to visual impairment and finally blindness, as well as, more rarely, impairment of breathing and food intake. Autoantibodies target BP180 (collagen type XVII), laminin 332, BP230 (nearly always in conjunction with other antigens), and type VII collagen in about 75%, 10-20%, 10-30%, and <5% of MMP patients, respectively. While the main autoantibody isotype is IgG, additional, and less frequently, exclusive IgA autoantibodies can be detected in the majority of patients. Assaying for anti-laminin 332 reactivity is pivotal, since in about a quarter of patients with anti-laminin 332 MMP, a malignancy, mainly solid cancers, is associated. The pathophysiology of MMP is yet incompletely understood. A recent mouse model of anti-laminin 332 MMP replicating characteristic clinical and immunopathological findings of the human disease may be helpful to close this knowledge gap. Diagnosis is established by the clinical picture with predominant mucosal lesions and visualization of tissue-bound anti-basement membrane zone antibodies by direct immunofluorescence microscopy. In recent S3 guidelines initiated by the European Academy of Dermatology and Venereology, the clinical spectrum and diagnostic strategies are detailed. In addition, treatment regimens for different clinical situations including patients with exclusive oral or ocular involvement are outlined. Future studies are needed to better understand the clinal complexity and associations as well as to establish widely available diagnostic assays and evidence-based therapeutic strategies.

Risk Factors for Mucosal Involvement in Bullous Pemphigoid and the Possible Mechanism: A Review

Bullous pemphigoid (BP) is the most common type of autoimmune bullous disease and is characterized by the presence of circulating anti-BP180 and/or anti-BP230 autoantibodies. Patients with BP often present with tense blisters and erythema, mainly on the trunk and limbs, but a few patients also have mucosal involvement. In this article, we discuss the fact that BP patients with mucosal involvement tend to have more serious conditions and their disease is more difficult to control. Potential risk factors for mucous involvement include earlier age at onset, drugs such as dipeptidyl peptidase-4 inhibitors, cancer, and blood/serum biomarkers, including lower eosinophil count, higher erythrocyte sedimentation rate, IgG autoantibodies against both the NH₂- and COOH-termini of BP180, and the absence of anti-BP230 antibodies. IgA and C3 deposition at the dermo-epidermal junction may also be present. Understanding these risk factors may benefit earlier diagnosis of these patients and promote the development of novel treatments. What's more, it's helpful in deeper understanding of BP development and the relationship between BP and mucous membrane pemphigoid (MMP).

Zonula occludens-1 demonstrates a unique appearance in buccal mucosa over several layers

Tight junctions (TJs) firmly seal epithelial cells and are key players in the epithelial barrier. TJs consist of several proteins, including those of the transmembrane claudin family and the scaffold zonula occludens (ZO) family. Epithelial tissues are exposed to different conditions: to air in the stratified epithelium of the skin and to liquids in the monolayer of the intestine. The TJs in stratified oral mucosal epithelium have remained insufficiently elucidated in terms of distributions, appearances and barrier functions of TJ proteins in normal buccal mucosa. We investigated these and ZO-1 and claudin-1 were found to be expressed in the top third and in the bottom three quarters of the mucosal epithelium. ZO-1 in the buccal mucosa was found to have an irregular linear appearance. ZO-1 in the buccal mucosa continuously existed in several layers. Electron microscopy revealed the buccal mucosa to have kissing points. In a biotin permeation assay that sought to investigate inside-outside barrier function, the biotin tracer penetrated several ZO-1 layers but did not pass through all the ZO-1 layers. We found that the oral mucosal cell knockdown of TJP1 or CLDN1 resulted in decreases of TER but no significant change in FITC-dextran leakage. Our results suggest that the distribution and appearance of ZO-1 in the buccal mucosa differ from those in the skin. We were unable to prove barrier function in this study but we did show barrier function against small molecules in vivo and against ions in vitro.

Normal human skin is superior to monkey oesophagus substrate for detection of circulating BP180-NC16A-specific IgG antibodies in bullous pemphigoid

BACKGROUND

Bullous pemphigoid (BP) is the most common autoimmune subepidermal blistering skin disease. Two antigens have been identified as targets of circulating autoantibodies (autoAbs) - BP180 and BP230 - with BP180 being a critical transmembrane adhesion protein of basal keratinocytes of the epidermis. The noncollagenous domain 16A (NC16A) of BP180 is the immunodominant epitope in patients with BP, and anti-BP180-NC16A IgG antibodies (Abs) correlate to disease activity. Routine serological testing and follow-up of BP relies on indirect immunofluorescence (IIF) of serum Abs, commonly performed on monkey oesophagus (ME), and/or enzyme-linked immunosorbent assay (ELISA) testing on recombinantly produced fragments of BP180 and BP230 (BP180-NC16A, BP230-C/N).

OBJECTIVES

To determine if NC16A epitopes are well represented on ME substrate.

METHODS

Sera from different BP cohorts were tested by IIF on ME and normal human skin (NHS). To confirm findings, affinity-purified anti-BP180-NC16A/BP230 polyclonal Abs and recombinant anti-BP180-NC16A/BP230 monoclonal antibodies (mAbs) were used.

RESULTS

For sensitive detection of BP180-NC16A-specific IgG Abs, sections of NHS are superior to the widely used ME. Confirmation comes from polyclonal affinity-purified anti-BP180-NC16A/BP230 Abs, and by mAbs cloned from a patient with active BP.

CONCLUSIONS

Use of NHS is preferable over ME in routine IIF testing for BP. These results are of clinical relevance because anti-BP180-NC16A IgG titres are correlated to disease activity and detecting them reliably is important for screening, diagnosis and follow-up of patients with BP.

Fc-binding proteins enhance autoantibody-induced BP180 depletion in pemphigoid

Immunoglobulins (Igs) consist of two antigen-binding regions (Fab) and one constant region (Fc). Protein A and protein G are bacterial proteins used for the purification of IgG by virtue of their high affinities for the Fc fragment. Rheumatoid factors are autoantibodies against IgG Fc fragments, which are present in the body under physiological conditions. Little is known about the influence of Fc-binding proteins on the pathogenicity of antibody-induced autoimmune diseases. Pemphigoid diseases are a group of autoimmune subepidermal blistering disorders that includes bullous pemphigoid and mucous membrane pemphigoid. IgGs targeting the non-collagenous NC16A domain of the 180-kDa bullous pemphigoid antigen (BP180) are known to induce skin fragility in mice and the depletion of BP180 in keratinocytes. In this study, mAb against NC16A in combination with Fc-binding proteins was found to enhance BP180 depletion. Although mAb against the C-terminus of BP180 does not show pathogenicity in vivo or in vitro, mAb treatment with Fc-binding proteins clearly induced skin fragility in mice and BP180 depletion in keratinocytes. Anti-BP180 mAbs and Fc-binding proteins were colocalized in the cytoplasm and at the basement membrane zone. Cell adhesion strengths were decreased in parallel with BP180 amounts. Clinically, bullous pemphigoid patients had higher rheumatoid factor titers than controls. Anti-BP180 mAb in combination with high-titer rheumatoid factor serum was found to enhance BP180 depletion. Furthermore, saliva from mucous membrane pemphigoid patients contained larger quantities of bacteria and Fc-binding proteins than controls. Our results suggest that Fc-binding proteins (rheumatoid factor or protein G) may enhance the pathogenicity of autoantibodies in pemphigoid diseases. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The Diagnosis and Blistering Mechanisms of Mucous Membrane Pemphigoid

Mucous membrane pemphigoid (MMP) is a mucous membrane-dominated autoimmune subepithelial blistering disease that is caused by autoantibodies against various autoantigens in basement membrane zone (BMZ) proteins, including collagen XVII (COL17). Clinicians face diagnostic problems in detecting circulating antibodies and targeted antigens in MMP. The diagnostic difficulties are mainly attributed to the low titers of MMP autoantibodies in sera and to heterogeneous autoantigens. Additionally, no unanimous diagnostic criteria have been drawn for MMP, which can result in delayed diagnoses or misdiagnoses. This review aims to integrate and present currently available data to clarify diagnostic strategies and to present diagnostic criteria for MMP. The ultimate blistering mechanism in MMP has not been elucidated, and such mechanism is especially obscure in COL17-type MMP. In bullous pemphigoid (BP), which is the most common autoimmune subepidermal blistering disease, some patients show oral lesion as well as predominant skin lesions. However, there is no fundamental explanation for the onset of oral lesions in BP. This article summarizes innovative research perspectives on the pathogenesis of oral lesions in pemphigoid. Finally, we propose a potential pathogenesis for COL17-type MMP.

The direct binding of collagen XVII and collagen IV is disrupted by pemphigoid autoantibodies

The basement membrane zone (BMZ) is framed by hemidesmosomes and extracellular matrix (ECM) including collagen IV (COL4). Hemidesmosomes are multiprotein complexes that include collagen XVII (COL17). BMZ proteins can be targeted in autoimmune subepidermal blistering diseases, e.g., pemphigoid targeting COL17. The blistering mechanisms in pemphigoid have not been fully elucidated, especially in mucous membrane pemphigoid (MMP), which mainly affects the mucosa. In this study, we showed that oral lesions in pemphigoid may be attributed to the inhibition of protein-protein interactions by autoantibodies. Using immunoprecipitation, we revealed that COL17 directly binds to COL4 in normal human keratinocytes and normal human oral keratinocytes. In particular, the C-terminus of COL17 is binding site to COL4 in oral keratinocytes. The precise COL4-binding region on COL17 was determined by protein-protein binding assay to be from amino acid Gly1175 to Asp1340 on the C-terminus. MMP-IgG or mAb recognizing the C-terminus hindered the interaction of COL17 with COL4 in oral keratinocytes. Furthermore, keratinocyte adhesion strength to COL4-coated plates was significantly reduced by the treatment of mAb against the C-terminus. In addition, the inflammatory infiltrates around perilesions were significantly less in MMP compared to BP. These results indicate that pemphigoid IgG targeting the C-terminus plays a pathogenic role in blister formation in the oral mucosa to inhibit protein interactions with less inflammation.