Interleukin (IL)-34 promotes the inflammatory role of IL-1β-producing myeloid cells in pemphigus lesions

BACKGROUND: Pemphigus is a life-threatening autoimmune disease mediated by anti-desmoglein (Dsg) autoantibodies. Ectopic lymphoid-like structures (ELS) are frequently found in chronic skin lesions and are thought to contribute to local autoantibody production. However, the mechanisms driving ELS formation at lesion sites remain unclear. OBJECTIVES: To investigate the role of myeloid cells in the formation of ELS in pemphigus lesions, and to identify potential therapeutic targets by better understanding the underlying mechanisms that contribute to this process. METHODS: We used single-cell RNA sequencing (scRNA-seq) to identify the myeloid subpopulations in pemphigus lesions and study their functions. Immunohistochemistry (IHC), immunofluorescence and flow cytometry were used to validate the presence of interleukin (IL)-1β-producing myeloid cells. Culture, bulk RNA-seq and transwell chemotaxis experiments were conducted to assess the effects of IL-34 and tumour necrosis factor (TNF)-α on monocytes. Additionally, the high expression of IL-34 in pemphigus keratinocytes was validated by IHC. RESULTS: We first confirmed the abundant presence of myeloid cells within ELS in pemphigus skin lesions, including pemphigus vulgaris and pemphigus foliaceus. Single-cell RNA-seq revealed that IL-1β-producing macrophages ('IL1B_Macro') is the dominant myeloid subpopulation in pemphigus lesions, originating from classical monocytes. These cells have a strong inflammatory and chemotactic transcriptomic profile, expressing high levels of IL-1β, IL-6 and chemokines such as CCL20, CCL3, CCL5 and CXCL5, promoting leucocyte infiltration. Ex vivo experiments showed that IL1B_Macro differentiation is enhanced by the synergistic action of IL-34 and TNF-α, which can be attenuated by a colony-stimulating factor 1 receptor (CSF-1R) inhibitor. IL-34 alone also promotes IL-1β and CCL20 expression, and keratinocytes were found to be the major source of elevated IL-34 in pemphigus lesions. Bulk RNA-seq data indicated that high IL-34 expression in pemphigus keratinocytes correlates with increased levels of CCL5, IL-6 and IL-23α. CONCLUSIONS: IL-1β-producing myeloid cells play a crucial role in the formation of ELS in pemphigus lesions through inflammatory and chemotactic pathways. Keratinocytes contribute to this process by producing IL-34, which fuels local inflammation. These findings offer new insights into pemphigus immunopathogenesis and suggest the IL-34/CSF-1R pathway as a potential therapeutic target.