Background: Ectopic lymphoid tissues (eLTs) and associated follicular helper T (T FH ) cells contribute to local immunoglobulin hyperproduction in nasal polyps (NPs). Follicular regulatory T (T FR ) cells in secondary lymphoid organs counteract T FH cells and suppress immunoglobulin production; however, the presence and function of T FR cells in eLTs in peripheral diseased tissues remain poorly understood. Objective: We sought to investigate the presence, phenotype, and function of T FR cells in NPs. Methods: The presence, abundance, and phenotype of T FR cells in NPs were examined using single -cell RNA sequencing, immunofluorescence staining, and flow cytometry. Sorted polyp and circulating T -cell subsets were cocultured with autologous circulating na & imath; <euro> ve B cells, and cytokine and immunoglobulin production were measured by ELISA. Results: T FR cells were primarily localized within eLTs in NPs. T FR cell frequency and T FR cell/T FH cell ratio were decreased in NPs with eLTs compared with NPs without eLTs and control inferior turbinate tissues. T FR cells displayed an overlapping phenotype with T FH cells and FOXP3 1 regulatory T cells in NPs. Polyp T FR cells had reduced CTLA-4 expression and decreased capacity to inhibit T FH cell-induced immunoglobulin production compared with their counterpart in blood and tonsils. Blocking CTLA-4 abolished the suppressive effect of T FR cells. Lower vitamin D receptor expression was observed on polyp T FR cells compared with T FR cells in blood and tonsils. Vitamin D treatment upregulated CTLA-4 expression on polyp T FR cells and restored their suppressive function in vitro . Conclusions: Polyp T FR cells in eLTs have decreased CLTA-4 and vitamin D receptor expression and impaired capacity to suppress T FH cell-induced immunoglobulin production, which can be reversed by vitamin D treatment in vitro . (J Allergy Clin Immunol 2024;153:1025-39.)