Pemphigus is an organ-specific autoimmune bullous disease characterized by the presence of IgG autoantibodies (autoAbs) targeting desmoglein (Dsg), a cell adhesion molecule in the epidermis [1,2]. This disease leads to the disruption of cell-cell adhesion between keratinocytes, resulting in intraepidermal blister formation in the skin and mucous membranes. Pemphigus is classified into two main types: pemphigus foliaceous (PF), which primarily affects the skin and is caused by IgG autoAbs against Dsg1, and pemphigus vulgaris (PV), which affects both the skin and mucosa and is induced by IgG autoAbs against Dsg1 and Dsg3 [1].

Experimental evidence from passive transfer models has established the critical role of circulating anti-Dsg autoAbs in the pathogenesis of pemphigus [3]. Clinical observations have shown a correlation between the initial disease activity of pemphigus and the levels of serum anti-Dsg autoAbs. A number of human pathogenic anti-Dsg autoAbs have been identified in circulating memory B cells (Bmems) from pemphigus patients [4,5]. Dsg-specific Bmems are present in the peripheral blood of patients with pemphigus [[6], [7], [8], [9]], and their number is significantly correlated with the serum anti-Dsg IgG titer in patients [10]. Long-term remissions following the treatment of glucocorticoid or rituximab, a monoclonal antibody against CD20+ B cells, are associated with the disappearance of circulating anti-Dsg autoAbs and Dsg-specific Bmems [[11], [12], [13]]. It is concluded that anti-Dsg autoAbs are produced by antibody-secreting cells (ASCs) derived from Dsg-autoreactive B cells. Although the major sites of Dsg-specific ASC differentiation and anti-Dsg autoAb production are thought to be lymph nodes, spleen, and bone marrow, how and where Dsg-specific ASCs are generated has not been fully elucidated.

We have reported that Dsg-specific B cells reside in pemphigus cutaneous lesions and can produce Dsg-specific autoAbs with the assistance of T cells in vitro in recent studies [14,15]. We also observed diffuse ectopic lymphoid-like structures (ELSs) and ongoing B cell responses in the cutaneous lesions of patients with active PV and PF [16]. ELSs are commonly induced by self-antigens in non-lymphoid tissues under autoimmune conditions, such as rheumatoid arthritis, and provide a suitable environment for the pathogenic B cell response and production of autoAbs [17]. These findings suggest that circulating Dsg-specific Bmems could migrate to pemphigus cutaneous lesions and locally differentiate into ASCs upon Dsg re-encounter. Dsg-specific autoAbs produced by lesion-resident ASCs may not only deposit in the nearby epidermis and exacerbate acantholysis but also replenish the systemic circulating pool [18]. However, less is known about the cellular origin, characteristics, and pathogenic roles of these cells.

The involvement of CD4+ T cells in pemphigus pathogenesis has long been recognized. Dsg-reactive CD4+ T cells have been proven essential for autoAb production and disease development [19,20]. Early studies demonstrated that Dsg-reactive, IFN-γ-producing Th1 and IL-4-producing Th2 cells were present in the peripheral blood of pemphigus patients [21]. However, a recent study suggested that circulating Dsg-reactive T cells were mainly composed of IL-17/IL-21-producing CD4+ T cells [22]. Large-scale meta-analysis confirmed significant increased levels of IL-17 and IL-10 in serum of pemphigus patients, but revealed conflicting altered levels of Th1/Th2-related cytokines [23]. Patients with active pemphigus display higher levels of circulating IL-17-producing T follicular helper (Tfh) cells that can promote anti-Dsg autoAb production by B cells in co-culture experiments [24]. IL-17 may play crucial roles in the pathogenesis of pemphigus as a key mediator of inflammatory reaction contributing to local inflammation and autoAbs production [25]. In addition to CD4+ T cells, several CD8+ T cell subpopulations expressing cytotoxic and/or pro-inflammatory programs have been identified in various inflammatory disorders, including autoreactive CD8+ cytotoxic T cells targeting melanocytes in vitiligo-affected skin and CD8+ T cell subsets producing IFN-γ and TNF-α mediating local inflammatory responses [26,27]. Growing evidences suggest that pemphigus lesions exhibit increased expression of Th17-related cytokines and IL-17-skewed inflammatory signature [15,24,28,29]. However, comprehensive characteristics of CD4+ and CD8+ T cell subsets within pemphigus cutaneous lesions remain elusive despites their important roles in disease pathogenesis.

Recently, single-cell sequencing techniques have been successfully employed to investigate the highly complex immune microenvironment and heterogeneity of several human inflammatory skin diseases [[30], [31], [32]]. To gain a deeper understanding of the immune mechanisms underlying pemphigus pathogenesis, we conducted a single-cell study, including single-cell RNA sequencing (scRNA-seq) and B/T cell receptor sequencing (scBCR/TCR-seq), to explore the adaptive immune landscape within pemphigus lesional skin. Through this analysis, we identify and characterize several potentially pathogenic T and B-lineage subsets in pemphigus lesions. Our findings provide novel insights into the pathogenesis of pemphigus and offer valuable information for the development of potential therapeutic strategies for this disease.