T regulatory cells and other lymphocyte subsets in patients with bullous pemphigoid

From Molecular Insights to Clinical Perspectives in Drug-Associated Bullous Pemphigoid

Bullous pemphigoid (BP), the most common autoimmune blistering disease, is characterized by the presence of autoantibodies targeting BP180 and BP230 in the basement membrane zone. This leads to the activation of complement-dependent and independent pathways, resulting in proteolytic cleavage at the dermoepidermal junction and an eosinophilic inflammatory response. While numerous drugs have been associated with BP in the literature, causality and pathogenic mechanisms remain elusive in most cases. Dipeptidyl peptidase 4 inhibitors (DPP4i), in particular, are the most frequently reported drugs related to BP and, therefore, have been extensively investigated. They can potentially trigger BP through the impaired proteolytic degradation of BP180, combined with immune dysregulation. DPP4i-associated BP can be categorized into true drug-induced BP and drug-triggered BP, with the latter resembling classic BP. Antineoplastic immunotherapy is increasingly associated with BP, with both B and T cells involved. Other drugs, including biologics, diuretics and cardiovascular and neuropsychiatric agents, present weaker evidence and poorly understood pathogenic mechanisms. Further research is needed due to the growing incidence of BP and the increasing identification of new potential triggers.

Detection of a natural antibody targeting the shed ectodomain of BP180 in mice

BACKGROUND

Pemphigoid diseases are characterized by subepidermal blister formation accompanied by autoantibodies targeting skin component molecules, such as BP180. It is suggested that an epitope-phenotype correlation exists among autoantibodies recognizing BP180. However, it is unclear which regions of BP180 are likely targets for autoantibodies.

OBJECTIVE

To elucidate the portions of BP180 where antibodies tend to react under the breakdown of immune tolerance.

METHODS

We immunized mice with full-length mouse BP180 (mBP180) to produce anti-mBP180 antibodies. Using the immunized mice, hybridoma cells were established to produce anti-mBP180 antibodies. We analyzed the characteristics of the anti-mBP180 antibodies that were produced in terms of epitopes, immunoglobulin subclasses, and somatic hypermutations.

RESULTS

Hybridoma cells derived from immunized mice with full-length mBP180 produced antibodies targeting the intracellular domain (IC) and the shed ectodomain (EC) of mBP180. Using the domain-deleted mBP180 recombinant protein, we revealed that monoclonal anti-mBP180 EC antibodies react to neoepitopes on the 13th collagenous region of cleaved mBP180, which corresponds to the epitopes of linear IgA bullous dermatosis antibodies in human BP180. Furthermore, the subclasses of these antibodies could be distinguished by epitope: The subclass of the anti-mBP180 IC monoclonal antibodies was IgG, whereas that of the anti-mBP180 EC antibodies was IgM. Of note, a clone of these IgM mBP180 EC antibodies was a germline antibody without somatic hypermutation, which is also known as a natural antibody.

CONCLUSION

These data suggest that mice potentially have natural antibodies targeting the neoepitopes of cleaved mBP180 EC.

Autoimmune bullous diseases in pregnancy: an overview of pathogenesis, clinical presentations, diagnostics and available therapies

Autoimmune bullous diseases (AIBDs) are rare organ-specific diseases characterized by the appearance of blisters and erosions on the skin and mucous membranes. These dermatoses are marked by the development of autoantibodies targeting the autoantigens located in intercellular junctions, i.e., between keratinocytes or in the basement membrane area. Therefore, the fundamental division of AIBDs into the pemphigus and pemphigoid groups exists. Although AIBDs are uncommon in the general population, their overall incidence is somewhat higher in women of all ages, for which a pregnant women can be likely affected too. While the pemphigoid gestationis is exclusive bullous dermatosis of pregnancy, the other AIBDs can also start or worsen during this period. The appearance of AIBDs in childbearing women is a particularly sensitive situation requiring exceptional clinicians' caution due to the possibility of pregnancy complications with adverse effects and risks to the mother and the child. Also, there are numerous management difficulties in the period of pregnancy and lactation related to the drugs' choice and safety. This paper aimed to outline the pathophysiologic mechanisms, clinical manifestations, diagnostic approach and therapy of the most commonly recognized AIBDs in pregnancy.

The cytokine milieu of bullous pemphigoid: Current and novel therapeutic targets

Bullous pemphigoid (BP) is the most common autoimmune bullous disease, characterized by severe pruritus and skin blistering. The loss of tolerance against Collagen XVII, also referred to as BP180, is the main pathogenic event of BP, leading to production of IgG autoantibodies which mainly target the juxtamembranous extracellular non-collagenous 16th A (NC16A) domain of BP180. A complex inflammatory network is activated upon autoantibody binding to the basement membrane zone; this inflammatory loop involves the complement cascade and the release of several inflammatory cytokines, chemokines and proteases from keratinocytes, lymphocytes, mast cells and granulocytes. Collectively, these events disrupt the integrity of the dermal-epidermal junction, leading to subepidermal blistering. Recent advances have led to identify novel therapeutic targets for BP, whose management is mainly based on the long-term use of topical and systemic corticosteroids. As an example, targeting type-2 T-helper cell-associated cytokines, such as Interleukin-4 and interleukin-13 has shown meaningful clinical efficacy in case series and studies; targeting IL-17 and IL-23 has also been tried, owing to an important role of these cytokines in the chronic maintenance phase of BP. In this review article, we discuss the complex cytokine milieu that characterized BP inflammation, highlighting molecules, which are currently investigated as present and future therapeutic targets for this life-threatening disease.

What's new in the pathogeneses and triggering factors of bullous pemphigoid

Bullous pemphigoid (BP) is a subepidermal blistering disease induced by autoantibodies to type XVII collagen (COL17, also called BP180) and BP230. Previous studies using patients' samples and animal disease models elucidated the complement-dependent and complement-independent pathways of blister formation, the pathogenic roles of immune cells (T and B cells, macrophages, mast cells, neutrophils, eosinophils), and the pathogenicity of IgE autoantibodies in BP. This review introduces the recent progress on the mechanism behind the epitope-spreading phenomenon in BP, which is considered to be important to understand the chronic and intractable disease course of BP, and the pathogenicity of anti-BP230 autoantibodies, mainly focusing on studies that used active disease models. To clarify the pathogenesis of BP, the mechanism behind the breakdown of immune tolerance to BP antigens should be investigated. Recent studies using various experimental models have revealed important roles for regulatory T cells in the maintenance of self-tolerance 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 peptidase-IV inhibitors and immune checkpoint inhibitors have been reported as triggering factors for BP. These factors and certain underlying conditions such as genetic background, regulatory T-cell dysfunction or aging might synergistically affect some individuals and eventually induce BP. Further studies on the breakdown of self-tolerance and on the identification of key molecules that are relevant to blister formation and inflammation may expand our understanding of BP's etiology and may lead to the development of novel therapeutic approaches.

Understanding CD4+ T cells in autoimmune bullous diseases

Autoimmune bullous diseases (AIBDs) are a group of life-threatening blistering diseases caused by autoantibodies that target proteins in the skin and mucosa. Autoantibodies are the most important mediator in the pathogenesis of AIBDs, and various immune mechanisms contribute to the production of these pathogenic autoantibodies. Recently, significant progress has been made in understanding how CD4⁺ T cells drive autoantibody production in these diseases. Here, we review the critical role of CD4⁺ T cells in the production of pathogenic autoantibodies for the initiation and perpetuation of humoral response in AIBDs. To gain an in-depth understanding of CD4⁺ T-cell pathogenicity, antigen specificity, and mechanisms of immune tolerance, this review covers comprehensive mouse and human studies of pemphigus and bullous pemphigoid. Further exploration of pathogenic CD4⁺ T cells will potentially provide immune targets for improved treatment of AIBDs.

Regulatory T cell subsets in bullous pemphigoid and dipeptidyl peptidase-4 inhibitor-associated bullous pemphigoid

BACKGROUND

Regulatory T (Treg) cells play an essential role in peripheral immune tolerance. Bullous pemphigoid (BP) is the most common blistering disease and is caused by autoantibodies to two BP antigens: type XVII collagen and BP230. Recently, we reported that Treg cell dysfunction may cause the production of autoantibodies to BP antigens. Several studies have suggested an association between Treg cells and BP pathogenesis. However, Treg cells are heterogeneous in humans, leading to inconsistent results in previous studies.

OBJECTIVE

To assess functional Treg subsets in BP.

METHODS

We examined three distinct Treg subsets in conventional BP (cBP) patients before versus after systemic corticosteroid treatment, dipeptidyl peptidase-4 inhibitor-associated BP (DPP-4i-BP) patients, younger controls and older controls.

RESULTS

We found that total Treg cells and all Treg cell subsets were increased in cBP patients before treatment and decreased by systemic corticosteroid treatment. In contrast, neither total Treg cells nor all Treg subsets were increased in DPP-4i-BP. Notably, CD45RA- Foxp3hi effector Treg cells positively correlated with disease severity in cBP, whereas CD45RA+Foxp3lo naïve Treg cells positively correlated with the disease severity in DPP-4i-BP.

CONCLUSION

These findings suggest that Treg cells are differently involved in the pathogeneses of cBP and DPP-4i-BP.

Coagulation and Skin Autoimmunity

Several lines of evidence indicate that the immune system, inflammation, and coagulation are simultaneously activated in autoimmune and immune-mediated skin diseases. Pro-inflammatory cytokines such as interleukin-6 and tumor necrosis factor-alpha induce the expression of the main initiator of coagulation, i.e., tissue factor. The proteases of coagulation in turn act on protease-activated receptors inducing the expression of various pro-inflammatory cytokines triggering inflammation. The cross-talk among immune system, inflammation, and coagulation amplifies and maintains the activation of all three pathways. This review focuses on three skin disorders as chronic spontaneous urticaria (CSU), angioedema, and bullous pemphigoid (BP), in which the relationships among the three systems have been investigated or their clinical consequences are relevant. Markers of thrombin generation, fibrinolysis, and inflammation have been reported to be increased in the plasma during flares of CSU and angioedema, as well as in the active phase of BP, with the marker levels reverting to normal during remission. The coagulation activation seems to be important only at local level in CSU and angioedema while both at local and systemic levels in BP which is the only condition associated with an increased thrombotic risk. The prothrombotic state in autoimmune skin diseases raises the question of the indication of anticoagulant treatment, particularly in the presence of other cardiovascular risk factors.

Regulatory T cells in autoimmune skin diseases

CD4⁺ Foxp3⁺ regulatory T cells (Tregs) are suppressors of immune activation and play a crucial role in the maintenance of peripheral tolerance. Mutations of Foxp3 result in fatal autoimmunity in multiple organs, including the skin, in both humans and mice. Many studies have demonstrated the altered frequency and functions of Tregs, changes in cytokine and chemokine levels related to Tregs and the differences in genetic background regarding Tregs in autoimmune skin disorders. Recent studies have extended our knowledge of certain properties of Tregs, especially skin-resident Tregs. In addition, some novel therapies have been performed by modulating the number and the function of Tregs. This review focuses on the role of Tregs in some autoimmune skin disorders, including alopecia areata, vitiligo, pemphigoid and pemphigus, and systemic sclerosis, and discusses questions that remain to be addressed.

Anti-Type VII Collagen Antibodies Are Identified in a Subpopulation of Bullous Pemphigoid Patients With Relapse

Bullous pemphigoid (BP) is an autoimmune bullous skin disease characterized by anti-BP180 and anti-BP230 autoantibodies (AAbs). Mucous membrane involvement is an uncommon clinical feature of BP which may evoke epidermolysis bullosa acquisita, another skin autoimmune disease characterized by anti-type VII collagen AAbs. We therefore evaluated the presence of anti-type VII collagen AAbs in the serum of BP patients with and without mucosal lesions at time of diagnosis and under therapy. Anti-BP180, anti-BP230, and anti-type VII collagen AAbs were measured by ELISA in the serum of unselected patients fulfilling clinical and histo/immunopathological BP criteria at baseline (n = 71) and at time of relapse (n = 24). At baseline, anti-type VII collagen AAbs were detected in 2 out of 24 patients with BP presenting with mucosal involvement, but not in patients without mucosal lesions (n = 47). At the time of relapse, 10 out of 24 BP patients either displayed a significant induction or increase of concentrations of anti-type VII collagen AAbs (P < 0.01), independently of mucosal involvement. Those 10 relapsing BP patients were also characterized by a sustained high concentration of anti-BP180 AAb, whereas the serum anti-BP230 AAb concentrations did not vary in BP patients with relapse according to the presence of anti-type VII collagen AAbs. Thus, our study showed that anti-type VII collagen along with anti-BP180 AAbs detection stratified BP patients at time of relapse, illustrating a still dysregulated immune response that could reflect a potential epitope spreading mechanism in those BP patients.