037 The intra-niche activation of clonal CXCL13+CD4+ T cells in tertiary lymphoid structures associated with non-healing blisters of pemphigus

Pemphigus is a rare and life-threatening autoimmune bullous disease mediated by autoreactive B cells targeting desmogleins (DSGs) in keratinocytes. Anti-DSG autoantibodies cause loss of cell-cell adhesion, which results in suprabasal acantholytic blisters. Here, we found that tertiary lymphoid structures (TLSs) were detected in non-healing skin blisters of pemphigus patients. TLSs resemble B-cell follicles of secondary lymphoid organs and are detected in various inflammatory diseases. We found that DSG-specific B and plasma cells were observed in TLSs of non-healing skin blisters. Skin blisters were decreased after intralesional corticosteroid injection with a decrease of skin TLSs, suggesting that skin TLSs are associated with non-healing state of the blisters. CXCL13 is an important chemokine for recruiting CXCR5-expressing naïve B and T follicular helper cells, but how CXCL13-secreting cells are expanded and functionally regulated within TLSs remains unclear. Through immunofluorescence studies, we found that CXCL13+ cells were mainly produced by CD4+ T cells. In single-cell RNA sequencing, CXCL13+CD4+ T cells are clonally expanded and contain DSG3-specific memory T cells. These cells are characterized by activated Th1-like features but possess a paradoxical attenuation of T-cell receptor signaling when they highly express CXCL13. By using spatial proteomics, we found that regulatory T cells (Tregs) directly contact CXCL13+CD4+ T cells. In vitro studies, CXCL13+CD4+ T cells decreased after depletion of Tregs and increased after co-culture with induced Tregs. In conclusion, clonal CXCL13+CD4+ T cells are activated by Tregs in skin TLSs associated with non-healing blisters in pemphigus.