INVITED REVIEW
Pemphigus: An Autoimmune Disease Model for Understanding the Role of Autoreactive T Cells
2025 Volume 74 Issue 4 Pages 177-188
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2025 Volume 74 Issue 4 Pages 177-188
Finding cures is the ultimate goal of research on autoimmune diseases. Pemphigus is an autoantibody-mediated autoimmune skin disease in which specific autoantibodies target desmogleins 1 and 3 as autoantigens. The condition leads to painful blisters and erosions in the skin and oral mucosa, impacting patients’ ability to eat and other daily activities, significantly affecting quality of life. The molecular mechanisms by which these pathogenic autoantibodies induce blisters have been extensively studied and understanding has advanced considerably. However, many critical questions remain, such as the exact cause of the disease, the mechanisms that normally prevent autoimmunity, and the pathogenic cells involved, other than autoantibodies. This article focuses on the role of autoreactive T cells in pemphigus and uses the pemphigus model to answer some of these questions. Research into pemphigus has enhanced our understanding of both the pathogenic and regulatory mechanisms involved, not only in pemphigus but also in other skin diseases caused by cellular autoimmunity. The growing body of scientific evidence on pemphigus has made it a model disease, paving the way for the development of novel therapeutic approaches, including antigen-specific immunotherapy for autoimmune diseases and chronic inflammatory disorders.
Pemphigus is an autoimmune bullous disease. The target autoantigens are desmogleins (Dsgs) 1 and 3, which are cadherin-type cell–cell adhesion molecules expressed in stratified squamous epithelium such as the skin and oral mucosa.1,2,3,4,5 The autoantibodies produced in pemphigus directly bind Dsg1 and/or Dsg3 in these tissues, disrupting their adhesive functions. This leads to the dissociation of keratinocytes and the formation of intraepidermal blisters, a process known as acantholysis (Fig. 1). As a result, patients typically develop painful blisters and erosions in the skin and oral mucosa, making it difficult to eat and significantly impacting their daily lives.
Immunological and clinical features of pemphigus.
(A) Histopathology of a skin lesion in a patient with pemphigus vulgaris reveals intraepidermal blisters with acantholytic cells. (B) Direct immunofluorescent test reveals that IgG antibodies (green) are deposited on the surfaces of keratinocytes. (C, D) Clinical images of pemphigus vulgaris. Flaccid blisters (arrows) and erosions (arrowheads) are observed in the oral mucosa (C) and on the back (D).
There are three clinical subtypes of pemphigus: pemphigus foliaceus (PF), pemphigus vulgaris (PV), and paraneoplastic pemphigus (PNP).1 PF and PV are the classical types, generally causing bullous lesions in the skin and oral mucosa, respectively. In PV, about half of patients also develop skin lesions, further dividing them into mucosal-dominant and mucocutaneous subtypes. Anti-Dsg1 immunoglobulin G (IgG) autoantibodies are detected in the serum of patients with PF, whereas anti-Dsg3 IgG are detected in the mucosal-dominant type of PV and both anti-Dsg1 and Dsg3 IgG are detected in the mucocutaneous type of PV.3 In other words, anti-Dsg1 and Dsg3 IgG are associated with skin and oral lesions, respectively, in pemphigus. The reason for this association between autoantibody profile and clinical manifestation may be clear when considering expression location of each target molecule within the epidermis or epithelium of oral mucosa and the compensatory role of each molecule. The detailed molecular explanation can be found in other articles.3,4 In addition to Dsg1 and 3 as target autoantigens, desmocollins, other desmosomal adhesive proteins, are also targeted by autoantibodies in some rare skin diseases such as herpetiform pemphigus and IgA pemphigus.6,7 In both diseases, acantholysis is not necessarily observed in the skin histopathology; instead, eosinophilic and neutrophilic infiltrates are likely involved as secondary inflammation after binding of the autoantibodies to keratinocytes, reflecting the different clinical manifestations of these rare diseases from classical pemphigus. Anti-desmocollin autoantibodies are also found in PNP and atypical pemphigus.6,8 Therefore, autoimmunity to desmosomal proteins other than Dsg3 may develop in autoimmune skin diseases.
Although the target autoantigens of PV and PF were identified more than 30 years ago,9,10 specific pemphigus therapies have not been introduced into clinical practice.1,5 This article reviews advancements in our understanding of pemphigus and the pemphigus model, focusing on Dsg3-specific immune responses and immune regulation, particularly the role of Dsg3-specific T cells.
Animal models are essential for furthering our understanding of disease mechanisms. In the case of pemphigus, an early passive transfer model was established by injecting IgG from patients into neonatal mice.11,12 This model demonstrated the direct pathogenicity of human circulating autoantibodies in inducing pemphigus pathology in mice. However, this model only reflects the final stage of the disease mechanism after autoantibody production. Progression of the disease involves several immunological steps, including the breakdown of immunological tolerance to harmful immune cells and the interactions of autoreactive B and T cells. Although forced immunization of recombinant autoantigens in wild-type mice can sometimes break the tolerance mechanism and induce autoimmune responses, this approach was unsuccessful for pemphigus. A unique methodological approach using autoantigen knock-out mice has overcome these issues.
To create an active disease model of pemphigus, Dsg3–/– mice are immunized with recombinant Dsg3 protein.13 Because these mice do not establish immunological tolerance against Dsg3, once immunized, they produce anti-Dsg3 IgG antibodies but do not develop pemphigus autoimmunity because they do not possess the target antigen. Consequently, splenocytes containing B and T cells are isolated from immunized mice and are adoptively transferred into Rag2–/– mice, which lack B and T cells but express endogenous Dsg3. The transferred lymphocytes then produce anti-Dsg3 antibodies in response to the endogenously expressed Dsg3 in the recipient Rag2–/– mice for more than 2 months. These antibodies bind Dsg3 and induce the pemphigus phenotype, which includes acantholysis as determined via histology and skin erosions.
Unlike autoimmune models that use artificial autoantigens or forced immunization with recombinant proteins, the active disease model is expected to mimic the pathophysiological process of antigen exposure to the immune system as it likely occurs in the patient’s body. This model’s advantage lies in its physiological processes, including autoantigen expression in tissues, antigen uptake and processing by specific antigen-presenting cells, and immune responses at the appropriate sites.
Autoreactive B cells that produce anti-Dsg3 antibodies play a crucial role in pemphigus, and extensive studies have been conducted on anti-Dsg3 IgG and pathogenic B cells. Here, we focus on autoreactive T cells that control autoantibody production.1 Several lines of evidence support the involvement of CD4+ T cells in the pathogenesis of pemphigus. PV is associated with specific human leukocyte antigen (HLA) class II alleles such as HLA-DRB1*0402 in Ashkenazi Jewish patients and HLA-DRB1*14 and HLA-DQB1*0503 in non-Jewish European and Asian patients, implying that Dsg3 peptides are presented to CD4+ T cells in these HLA class II molecules.14,15,16 Somatic hypermutations observed in the complementary determining region 3 (CDR3) of anti-Dsg3 antibodies indicate T cell-dependent affinity maturation of immunoglobulin.17 The predominant subclass of anti-Dsg3 antibodies is IgG4, suggesting T cell-dependent class switching.18,19 In an animal model, CD4+ T cells derived from Dsg3–/– mice are necessary to produce anti-Dsg3 IgG antibodies.20 To further understand the disease mechanisms of pemphigus, autoreactive T cells have been investigated in both humans and mice.
Although Dsg3-reactive CD4+ T cells are found in both healthy individuals and PV patients, their cytokine expression profiles differ.21 In healthy individuals, T cell reactivity to Dsg3 is detected only in those carrying the PV-related HLA class II alleles HLA-DRB1*0402 or DQB1*0503, or both, and is limited to interferon (IFN)-γ production, not interleukin (IL)-4. In PV patients, IL-4-producing T cell clones are detected in all disease phases, whereas IFN-γ-producing clones significantly increase in the chronic active phase but not in the acute or remittent phases. Another study also supported IL-4 production from Dsg3-reactive T cell clones isolated from PV patients.22 These results suggest that autoimmune responses against Dsg3 differ between patients and healthy individuals, and a specific regulatory process that limits Dsg3-specific Th2 responses associated with the disease might prevent the development of pemphigus in healthy individuals.
In contrast with human studies, animal studies can evaluate the in vivo pathogenicity of autoreactive T cells. Several Dsg3-specific T cell clones and lines have been established from Dsg3–/– mice immunized with recombinant Dsg3 protein.23 The in vivo pathogenicity of these T cells in terms of their ability to induce the pemphigus phenotype is assessed by adoptively transferring them into Rag2–/– mice along with B cells from Dsg3–/– mice immunized with recombinant Dsg3 protein. In all, seven clones and lines have been found to induce anti-Dsg3 antibody production and the pemphigus phenotype, identifying them as pathogenic T cells; all other lines are not pathogenic. This indicates the heterogeneity of the pathogenicity of Dsg3-specific T cells.
Analysis of these Dsg3-specific T cell clones and lines has revealed the importance of cytokine profiles in inducing the pemphigus phenotype. For example, they express high levels of IL-4 and IL-10, which are not significantly expressed in nonpathogenic lines.23 Blocking IL-4, but not IL-10, significantly suppresses anti-Dsg3 IgG production induced by the pathogenic clones. These findings suggest that IL-4 expression in Dsg3-specific T cells is critical for inducing the pemphigus phenotype in vivo and might be a target for limiting the autoimmune response (Fig. 2A).
Pathogenic roles of Dsg3-specific CD4+ T cells and immunological tolerance in mice.
(A) Dsg3-specific CD4+ T cells promote anti-Dsg3 antibody production from B cells and induce acantholytic blisters in an IL-4-dependent manner. Follicular helper T (Tfh) cells also cause this phenotype. (B) Dsg3-specific CD4+ T cells are deleted in the presence of Dsg3 in the thymus (negative selection in the central tolerance), and they develop as CD4+ T cells in the absence of Dsg3. (C) Dsg3-specific Th1 cells induce interface dermatitis in an IFN-γ-dependent manner. (D) Dsg3-specific Th17 cells cause psoriasiform dermatitis. (E) Regulatory T cells (Treg) constrain autoreactivity of Dsg3-specific CD4+ T cells by facilitating deletion of these autoreactive T cells in the extrathymic peripheral tissue, a process that is contingent upon expression of OX40 in Tregs.
In another study, pathogenic T cell activity inducing the pemphigus phenotype was detected in ICOS+CXCR5+PD-1+ T follicular helper (Tfh) cells within the active disease model of pemphigus.24 This is logical because Tfh cells are specialized helper T cells that contribute to germinal center formation through CXCR5 expression, facilitating efficient antibody production through CD40–CD40L engagement.25 This interaction is necessary for the IgM-to-IgG isotype switch and somatic hypermutation, which are critical for immunoglobulin affinity maturation. These steps are essential to the autoantibody-mediated autoimmune pathogenesis of pemphigus. Consistently, I-Ab tetramer with Dsg3 peptide identified Dsg3-specific Tfh cells with high ICOS expression, and blocking ICOS reduced anti-Dsg3 antibody production in the model. These findings highlight the pivotal roles of Dsg3-specific ICOS+ Tfh cells in the pemphigus mouse model (Fig. 2A). Another model, mouse major histocompatibility complex (MHC) class II-deficient HLA-DRB1*0402 transgenic mouse, is useful for assessing the crucial role of the interaction between human Dsg3 or Dsc3 peptides and pemphigus-related HLA molecules in autoantibody production and is able to provide epitope information without obtaining Dsg3-specific T cell clones.26,27,28 Therefore, the study of autoreactive T cells in animal models provides important insights into disease mechanisms.
Regulatory mechanisms that prevent Dsg3-specific immune cells from attacking the body’s own tissues are thought to be compromised in patients with pemphigus. However, the physiological mechanisms that function in healthy individuals are not yet fully understood. Central tolerance in the thymus is a major mechanism preventing the development of autoreactive T cells.29 Progenitor T cells develop in the bone marrow and migrate to the thymic cortex, where they start to express T cell receptors (TCRs).30 During central tolerance, T cells expressing TCRs that are moderately reactive to self-antigens can survive through positive selection.31 These T cells then move to the thymic medulla, where their fate is determined through interactions with medullary thymic epithelial cells (mTECs). mTECs promiscuously express tissue-restricted antigens via the transcription factors autoimmune regulator (AIRE) or FEZ family zinc finger 2 (FEGF2) and present these autoantigens to developing T cells. T cells with TCRs of high avidity for these self-antigens are eliminated as self-damaging immune cells through negative selection, whereas those with relatively low avidity differentiate into regulatory T cells (Tregs) and others become normal naive T cells for future immune responses.32,33 In the context of pemphigus, Dsg3 is recognized as an autoantigen expressed in mTECs via AIRE.34 This suggests that thymic central tolerance likely plays a role in preventing pemphigus by maintaining immunological tolerance.
To investigate how Dsg3-specific T cells are regulated under physiological conditions, a Dsg3-specific T cell receptor transgenic mouse (Dsg3H1 mouse) was generated.35 The genes encoding the Dsg3-specific T cell receptor were isolated from pathogenic Dsg3-specific T cell clones capable of inducing the pemphigus phenotype. Because TCR transgenic mice express endogenous TCR α-chains in addition to the transgenic one,36,37,38 the Dsg3H1 mouse was crossed with an Rag2−/− mouse to prevent expression of endogenous TCR α-chains, ensuring that the transgenic T cells are specific to Dsg3. In those studies, to analyze thymic tolerance of Dsg3-specific T cells, bone marrow cells from Dsg3H1-Rag2−/− mice were transplanted into wild-type (WT) and Dsg3−/− mice. In the thymus of Dsg3−/− mice, Dsg3-specific transgenic CD4+ T cells (Dsg3H1-Rag2−/− T cells) were detected 2 months after bone marrow transplantation; these T cells were not detected in the thymus of WT mice, which express endogenous Dsg3. This finding clearly demonstrated that the mechanism for eliminating harmful autoreactive T cells, which functions as negative selection, also applies to Dsg3-specific T cells in mice (Fig. 2B).
When Dsg3H1 CD4+ T cells are isolated from Dsg3H1 mice and adoptively transferred into Rag2−/− mice along with B cells derived from Dsg3−/− mice to evaluate their ability to induce pemphigus, the recipient mice start to develop skin erosions several weeks after the transfer. However, histopathology and immunofluorescence staining of the palate have shown that Dsg3H1 CD4+ T cells directly infiltrate the dermal–epidermal junction, causing liquefaction degeneration and keratinocyte cell death, but no acantholytic blister formation is observed.35 This histopathology induced by Dsg3H1 CD4+ T cells is considered interface dermatitis, a histopathological change often seen in various skin conditions such as lichen planus, adverse reactions to drugs, and collagen diseases including lupus erythematosus. In addition, the induction of interface dermatitis by Dsg3H1 CD4+ T cells is entirely dependent on IFN-γ production by pathogenic T cells, not IL-17A. The exact cause of interface dermatitis is not fully understood, but this finding clearly demonstrates that Th1 autoimmunity to epidermal autoantigen can induce interface dermatitis as a disease mechanism (Fig. 2C).
The pathogenicity of the original Dsg3-specific CD4+ T cell clone that provided the Dsg3H1 transgene is based on humoral immunity, inducing anti-Dsg3 IgG production and the pemphigus phenotype. By contrast, the pathogenicity of Dsg3H1 transgenic T cells is based on cellular immunity, inducing interface dermatitis, a type of T cell-mediated inflammation. One reason for the difference in pathogenicity could be different developmental conditions; the original Dsg3-specific CD4+ T cell clone was isolated from Dsg3−/− mice, whereas the Dsg3H1 transgenic T cells were isolated from Dsg3H1 mice that expressed endogenous Dsg3.
To understand how the absence of Dsg3 during T cell development affects the effector phenotype of Dsg3H1 T cells, bone marrow cells from Dsg3H1 mice were transferred to Dsg3−/− and WT mice, allowing the Dsg3H1 transgenic T cells to develop in the absence and presence of Dsg3, respectively. Then, the developed Dsg3H1 T cells can be isolated from both Dsg3−/− and WT recipient mice and adoptively transferred into Rag2−/− mice along with B cells derived from Dsg3−/− mice. Dsg3H1 T cells from WT recipients induce interface dermatitis as expected, whereas Dsg3H1 T cells from Dsg3−/− recipients induce both interface dermatitis by directly infiltrating the epidermis and acantholytic blisters by promoting anti-Dsg3 IgG production.35 These findings suggest that T cells with a particular range of TCR avidity are likely deleted during negative selection in WT mice but can develop as naive T cells in Dsg3−/− mice without undergoing Dsg3-specific immunological selection. These T cells subsequently acquire helper activity when activated upon encountering and recognizing Dsg3. This implies that differences in TCR avidity resulting from immunological selection during T cell development dictate differences in the effector phenotype acquired in the future because naive T cells should not be functionally different other than having different TCR avidity prior to antigen stimulation.
Many skin diseases manifest different symptoms because of their unique disease mechanisms. In the studies on Dsg3-specific autoimmunity mentioned above, autoimmune responses to epidermal autoantigen were investigated in detail using T cell clones and TCR transgenic mice. Specifically, Dsg3-specific CD4+ T cells were found to differentiate into IL-4-producing Th2 cells during development of the pemphigus phenotype, and IL-4 was shown to be important for this phenotype in mice.23 Conversely, interface dermatitis was found to be induced by cellular autoimmunity to Dsg3 in mice, with IFN-γ playing a pivotal role. This demonstrates the critical involvement of Th1 cells in the disease mechanism.35
Other Th subsets, such as Th17 and Treg, have not been studied in the context of Dsg3-specific autoimmunity. In one study, when naive Dsg3H1 CD4+ T cells were forced to differentiate into Th17 cells in vitro and were then adoptively transferred into Rag2−/− mice,39 the recipient mice developed clinical skin lesions characterized by pronounced scaling, more so than in recipients showing interface dermatitis in histology. Reflecting this difference in clinical symptoms, histopathology of the recipient skin lesions revealed acanthosis (thickening of the epidermis) with parakeratosis and neutrophil accumulation in the cornified layer, which are characteristic features of psoriasis pathology. Liquefaction degeneration and keratinocyte cell death, characteristic features of interface dermatitis, were not observed (Fig. 2D). At the molecular level, Stat3 activation and upregulation of psoriasis-related cytokines and genes (IL-1β, TNF-α, IL-23, IL-36, S100a8, and S100a9) were observed only in recipients transferred with Dsg3H1 Th17 cells, not in recipients with interface dermatitis. This is consistent with results from other studies on psoriasis patients and models.40,41
T cells with the same antigen specificity against an epidermal autoantigen but different effector phenotypes can experimentally induce completely different pathological features in the skin. Therefore, the effector phenotype of T cells is a major determinant of the type of tissue inflammation, and understanding the effector phenotype and autoreactivity of lesion-infiltrating T cells is crucial for dissecting disease mechanisms in patients with chronic inflammatory disorders and autoimmune diseases.
Dsg3H1 mice were originally generated to understand immunoregulation to Dsg3-specific T cells under physiological conditions. In addition to the central tolerance mechanism against Dsg3 described above, peripheral tolerance mechanisms against Dsg3-specific T cells are also of great interest to understand how harmful immune cells are restricted in healthy individuals. A major issue in the study of immune regulation in extrathymic tissues is that Dsg3-specific thymic tolerance always precedes peripheral tolerance. As a result, it is difficult to analyze peripheral immune regulation without the influence of central tolerance. However, if thymic tolerance could be eliminated in a Dsg3-specific manner, only peripheral tolerance would be effective against Dsg3-specific T cells. To prepare such an immunological condition, thymus transplantation was performed from Dsg3–/– mice to nude mice that congenitally lack the thymus (NudeDsg3–KO Thy mouse, Fig. 3A).42 At the same time, WT thymus-transplanted nude mice (NudeWT Thy) and Dsg3–/– mice were prepared as Dsg3-bearing and Dsg3-lacking controls, respectively. Then, bone marrow cells from Dsg3H1-Rag2–/– mice were transplanted into these models to allow the transgenic Dsg3-specific (Dsg3H1-Rag2–/–) CD4+ T cells to develop under the recipient conditions (Fig. 3B). Thymic transplantation did not interfere with the process of immunological selection for Dsg3-specific T cells, because the development of Dsg3H1-Rag2−/− CD4+ T cells was hardly observed in the transplanted WT thymus in NudeWT Thy mice, as expected, after bone marrow transplantation.
Schematic overview and results of experiments analyzing Dsg3-specific peripheral tolerance.
(A) The thymus from Dsg3−/− mice was transplanted into athymic nude mice (NudeDsg3−KOThy). In these mice, Dsg3-specific thymic tolerance was absent but Dsg3-specific peripheral tolerance functioned normally (top). When a WT thymus was transplanted into athymic nude mice (NudeWT Thy), both Dsg3-specific thymic and peripheral tolerance were present (bottom). In Dsg3−/− mice, neither Dsg3-specific thymic nor peripheral tolerance was present (middle). (B) Bone marrow cells were transplanted from Dsg3H1-Rag2−/− mice into thymic chimera mice to observe the development of Dsg3-specific CD4+ T cells. In NudeDsg3−KOThy mice, Dsg3-specific CD4+ T cells developed in the thymus but not in peripheral tissues (top). In Dsg3−/− mice, these cells developed in both the thymus and peripheral tissues (middle). In NudeWT Thy mice, Dsg3-specific CD4+ T cells did not develop in the thymus (bottom).
In the transplanted Dsg3−/− thymus of NudeDsg3−KO Thy mice, the development of Dsg3H1-Rag2−/− CD4+ T cells was clearly observed with an efficacy similar to that in the thymus of Dsg3−/− recipients, indicating proper development of Dsg3H1-Rag2−/− CD4+ T cells in the transplanted Dsg3−/− thymus. In addition, as expected, Dsg3H1-Rag2−/− CD4+ T cells were abundant in the skin-draining lymph nodes and spleen of Dsg3−/− mice. By contrast, Dsg3H1-Rag2−/− CD4+ T cells were detected only as a small fraction in the peripheral lymphoid tissues of NudeDsg3−KO Thy mice, which was significantly reduced compared to those of Dsg3−/− mice (Fig. 3B). Furthermore, NudeDsg3−KO Thy mice did not show a dermatitis phenotype. These observations in thymic chimera mice clearly indicate there is an extrathymic regulatory mechanism that eliminates Dsg3-specific CD4+ T cells to avoid harmful autoimmunity in peripheral tissues.
Given that the immunoregulatory mechanism for the deletion of Dsg3H1-Rag2−/− CD4+ T cells exists in the peripheral tissues of the immune system, elucidating the detailed mechanism is the next focus for further investigation. To more easily mimic the tolerogenic conditions in peripheral tissues during thymic chimera experiments, peripheral naive Dsg3H1-Rag2−/− CD4+ T cells were isolated from the spleen and lymph nodes of Dsg3−/− mice that had undergone bone marrow transfer from Dsg3H1-Rag2−/− mice and adoptively transferred into WT mice. Then, lymphocytes in the periphery of the WT recipients were observed.42 The transferred Dsg3H1-Rag2−/− CD4+ T cells proliferated transiently and disappeared within 2 weeks but did not proliferate in the Dsg3−/− mice. During proliferation, Dsg3H1-Rag2−/− CD4+ T cells do not acquire a proper effector phenotype, such as IFN-γ production and CD44 upregulation, but do after transfer into Rag2−/− mice, where they can become pathogenic and induce interface dermatitis. Therefore, the deletion mechanism for Dsg3H1-Rag2−/− CD4+ T cells operates in the periphery of WT mice and clearly limits proper lymphocyte activation before deletion in an antigen-specific manner.
To further understand the deletion mechanism at the molecular and cellular levels, a previous study investigated the involvement of immunoregulatory molecules such as AIRE and PD-1 using an adoptive transfer approach. Deletion of Dsg3H1-Rag2−/− T cells was observed in AIRE−/− mice and WT mice treated with anti-PD-1 blocking antibody after adoptive transfer, suggesting that these two molecules are not involved in the deletion mechanism. By contrast, Dsg3H1-Rag2−/− T cells proliferated and persisted for 2 weeks without disappearing, subsequently inducing interface dermatitis after adoptive transfer into Treg-depleted recipients.42 This indicated Treg-dependent deletion of Dsg3H1-Rag2−/− T cells in the WT periphery.
To identify the genes responsible for the deletion mechanism involving Tregs, three Foxp3 mutant mice were used as recipients for adoptive transfer of Dsg3H1-Rag2−/− T cells, in which Foxp3-dependent Treg function was impaired to different degrees.43 Deletion of Dsg3H1-Rag2−/− T cells was observed in Foxp3I363V/Y and Foxp3A384T/Y recipient mice, which usually start to show mild to moderate tissue inflammation, including skin lesions, by 3 months of age. By contrast, Foxp3R397W/Y mice show the most severe phenotype and usually die from aberrant systemic inflammation at around 6 weeks of age, similar to Foxp3-deficient scurfy mice.44 To prolong their survival, Foxp3R397W/Y mice were crossed with Tbx21−/− mice and used as recipients in adoptive transfer experiments. Even in Foxp3R397W/Y-Tbx21−/− recipient mice, transferred Dsg3H1-Rag2−/− T cells unexpectedly disappeared in 14 days, suggesting that the genes regulated by Foxp3 in Tregs are not involved in the deletion mechanism, although the mechanism is indeed Foxp3+ Treg-dependent.42 Instead, Foxp3-independent molecular events operating specifically in Foxp3+ Tregs may be associated with the deletion mechanism.
To identify Treg-specific but Foxp3-independent molecules, a previous study examined genes consistently expressed in both WT and Foxp3R397W/Y mice among genes specifically expressed in Tregs. They identified 24 candidate genes essential for the deletion mechanism. Among these, OX40 was found to be expressed on the cell surface of Tregs in a Foxp3-independent manner and was shown to be a key molecule for the deletion mechanism. Specifically, OX40 on Tregs deprived antigen-presenting cells of OX40L, leading to restricted OX40 signaling in Dsg3H1-Rag2−/− T cells and their subsequent elimination.42 The study demonstrated the essential role of Tregs in eliminating Dsg3-specific CD4+ T cells and their OX40 expression in maintaining peripheral immune homeostasis (Fig. 2E).
The clinical relevance of experimentally elucidating the ability of Dsg3-specific CD4+ T cells to induce interface dermatitis is most evident in PNP, a very rare subtype of pemphigus associated with neoplasms.1,45,46 Overall, 84% of underlying neoplasms are hematological disorders such as non-Hodgkin lymphomas, Castleman disease, and chronic lymphocytic leukemia.47,48,49 Unlike PV, patients with PNP present with a variety of cutaneous eruptions, ranging from blisters and erosions to erythema multiforme-like targetoid lesions and lichen planus-like erythematous papules and plaques.45,50 The oral mucosa is usually involved as refractory erosion, which is often more extensive and severe than that of PV.1,51,52,53,54
Reflecting this clinical manifestation, the histological findings are also highly variable. Because PNP is a subtype of pemphigus, acantholysis induced by anti-Dsg3 humoral autoimmunity of PNP is observed. However, unlike PV, PNP includes prominent dermal infiltration of inflammatory cells, Civatte body formation (degenerative changes in keratinocytes), liquefaction degeneration, and exocytosis (lymphocyte infiltration into epidermis), all of which are typical histological components of interface dermatitis. These histological changes are considered the main reason that PNP symptoms are highly variable when compared with those of PV. PNP is therefore a unique skin disorder involving acantholysis and interface dermatitis.
Although interface dermatitis is a key component of PNP pathogenesis, it is unknown how it occurs in patients with PNP, and, more specifically, the target of the infiltrating cells is unclear. Despite this, some clues to understanding these questions can be found in the pathological and immunological features of PNP. Tumors are associated with PNP but not with the classical form of pemphigus and are often derived from immune-related cells such as B lymphocytes and thymic epithelial cells. Therefore, interface dermatitis, a unique feature of PNP, may be initiated based on a tolerance mechanism that is dysregulated by tumor formation in the immune system. In addition, interface dermatitis may form via cellular immune responses to epidermal autoantigens in PNP, because Dsg3-specific T cells can simultaneously induce acantholysis and interface dermatitis, and humoral anti-Dsg3 autoimmunity is active in patients with PNP.
Several studies on the role of HLA in autoimmune responses and disease mechanisms support this idea. For example, HLA-DRB1*03 has been found to be associated with PNP in Caucasian patients, whereas HLA-DR4 and DR14 (PV- and PF-associated HLA alleles) are not.55 In one study, HLA-Cw*14 was reported to be significantly associated with PNP in Chinese patients.56 These results suggest that the immunological pathogenesis of PNP may be different from that of PV and PF, as predicted by the clinical and histological features described above. In addition, the association between HLA class I molecules and PNP may support the contribution of CD8+ T cells to the pathogenesis of PNP, particularly via cellular autoimmunity.
One of the most important clinical features of PNP is bronchiolitis obliterans (BO) as a pulmonary complication, which affects approximately 30% of PNP cases and can be fatal, accounting for the low survival rate of PNP.57 Inflammation and fibrosis of the bronchus lead to obstructive changes with clinical symptoms such as dry cough, dyspnea, and hypoxemia. BO may be observed in a number of clinical settings, including infection, inhalation injury, and bone marrow transplantation.58 Although PNP is a primary cause, the reason for BO development in the lungs remains unclear given that PNP primarily involves autoimmunity against skin-specific antigens.
Ectopic expression of epidermal autoantigens in the lungs may solve the puzzle of pulmonary complications in PNP. Normally, epidermal autoantigens are expressed in stratified squamous epithelia such as the epidermis but not in the pulmonary epithelium (Fig. 4A,B). However, a lung autopsy sample from a patient with PNP revealed squamous metaplasia.59 Furthermore, Dsg3 expression has been detected in the lung tissues of patients with squamous metaplasia. A mouse model of squamous metaplasia induced by intraperitoneal injection of naphthalene expressed epidermal autoantigens such as Dsg3 and keratin 5 and 14 in the lungs. These results suggest that the lung may become a target organ of skin-specific autoimmunity via squamous metaplasia, leading to ectopic expression of epidermal autoantigens (Fig. 4C).
Hypothesis overview: bronchiolitis obliterans in paraneoplastic pemphigus.
(A) Epidermal autoantigen-specific T cells recognize and attack the autoantigen in the epidermis, causing interface dermatitis in paraneoplastic pemphigus. (B) These T cells typically do not attack the lungs because epidermal autoantigen is not normally expressed there. (C) Squamous metaplasia allows ectopic expression of the epidermal autoantigen in the lungs. In this condition, the epidermal autoantigen-specific T cells can attack the lungs, leading to pulmonary complications such as bronchiolitis obliterans.
To test this hypothesis, a PNP mouse model was created.59 In the model, Dsg3–/– mice were immunized with wild-type skin grafts to elicit immune responses of CD4+ and CD8+ T cells against Dsg3. Then, the splenocytes from the immunized Dsg3–/– mice were adoptively transferred into Rag2–/– mice. Acantholytic blister formation and interface dermatitis with CD4+ and CD8+ T cell infiltration were simultaneously observed in the recipient mice, consistent with the pathological changes observed in skin lesions in PNP patients. In addition to skin lesions, ectopic expression of Dsg3 in the lungs and pulmonary infiltration were observed in the recipient mice. Furthermore, Dsg3-specific Dsg3H1 CD4+ T cells were able to infiltrate lungs with naphthalene-induced squamous metaplasia more efficiently than T cells unrelated to Dsg3. Although the antigen specificity of infiltrating immune cells has not been investigated in tissues, a dense infiltrate of CD8+ T cells was observed in the lung epithelium of BO in a patient with PNP.60 In summary, autoreactive T cells may target specific epidermal autoantigens, leading to the formation of interface dermatitis in PNP, and ectopic expression of epidermal autoantigens via squamous metaplasia may render the lung susceptible to autoimmune attack by epidermal antigen-specific T cells that do not normally attack the lung (Fig. 4C).
This article describes the progress in understanding the role of autoreactive T cells in pemphigus, particularly PV and PNP. One of the most important advances has been the isolation of Dsg3-specific T cell clones in mice, which provides many opportunities to address questions that remain unanswered. The ultimate goal of this research is undoubtedly to cure pemphigus. This will be a long process, but the immediate next step is to elucidate the disease mechanisms and develop antigen-specific therapies with fewer side effects.
One challenge that emerges is elucidating the mechanism(s) underlying pulmonary complications in PNP using patient samples and identifying the target autoantigens and responsible T-cell subpopulation. This will help determine how tissue-specific damage occurs in multiple organs and aid prediction and diagnosis of these complications.
In regard to antigen-specific therapy in pemphigus, the utility of Tregs can be considered because of their role in suppressing Dsg3-specific autoimmunity. The expansion of OX40+ Dsg3-specific Tregs, which are associated with the deletion of pathogenic T cells in skin-draining lymph nodes, has been observed using tetramers.42 This shows that Dsg3-specific Tregs from the natural polyclonal pool of WT Tregs can respond to antigen-specific immunoregulation in mice. Mobilizing Dsg3-specific Tregs in patients could be key to controlling pemphigus disease activity in an antigen-specific manner. Tackling each problem individually is necessary to cure pemphigus and other autoimmune diseases.
The author thanks Professor Masayuki Amagai (Keio University) for his fruitful advice on this article and his precise guidance on previous studies conducted at the Department of Dermatology, Keio University School of Medicine. The author also thanks the members of the department who have devotedly contributed to our projects. This research was performed with support through grants from the following agencies: the Ministry of Health, Labor, and Welfare of Japan (Health and Labor Sciences Research Grant for Research on Intractable Diseases 23FC1039); the Japanese Society for the Promotion of Science [Grants-in-Aid for Scientific Research (A) 19H01051 and 24H00634]; and the Japanese Agency for Medical Research and Development (Practical Research Project for Rare/Intractable Diseases JP22ek0109447 and JP24ek0109728).
The author declares that no conflict of interest exists.
