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

Abstract Background Pemphigus is a life-threatening autoimmune disease mediated by anti-desmoglein(Dsg) autoantibodies. Ectopic lymphoid-like structures (ELSs) 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 ELSs 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 to identify the myeloid subpopulation 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 sequencing and trans-well chemotaxis experiments were conducted to assess the effects of IL-34 and tumor 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 ELSs in pemphigus skin lesions, including PV and PF. Single-cell RNA sequencing revealed that 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 IL1B, IL6, and chemokines such as CCL20, CCL3, CCL5, and CXCL5, promoting leukocyte infiltration. Ex-vivo experiments showed that IL1B_Macro differentiation is enhanced by the synergistic action of IL-34 and TNF-α, and this can be attenuated by a CSF-1R inhibitor. IL-34 alone also promotes IL1B and CCL20 expression, and keratinocytes were found to be the major source of elevated IL-34 in pemphigus lesions. Bulk RNA sequencing data indicated that high IL-34 expression in pemphigus keratinocytes correlates with increased levels of CCL5, IL6, and IL23A. Conclusions IL-1β-producing myeloid cells play a crucial role in forming ELSs 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.