Background: Sublingual immunotherapy (SLIT) is used for the treatment of type 1 allergies, such as allergic rhinitis. SLIT leads to tolerance against allergens possibly via the redirection of allergen-specific T helper 2 cells to T helper 1 cells and the generation of peripheral regulatory T (Treg) cells. However, the detailed mechanisms remain unclear. Systemic tolerance to orally administered antigens (oral tolerance) has been extensively investigated. Recent studies have recognized the central role of Treg cells and classical dendritic cells (cDCs) in oral tolerance development. Highlight: This review focuses on recent advances in the understanding of the underlying mechanisms of SLIT compared with those of oral tolerance. The sublingual administration of soluble protein antigens has been reported to induce antigen-specific Treg cells in oral mucosa-draining submandibular lymph nodes in mice. The generation of Treg cells is critical for SLIT efficacy because the transfer of SLIT-induced Treg cells confers tolerance against the antigens. A large number of oral cDCs with the CD103−CD11b+ phenotype exert retinoic acid-producing activity and convert naïve CD4+ T cells into Foxp3+ Treg cells in vitro in a transforming growth factor-β-dependent and retinoic acid-dependent manner. Oral CD103−CD11b+ cDCs transport sublingual antigens to submandibular lymph nodes and induce antigen-specific Treg cells. Sublingual antigens enter the mucosa most likely by crossing the sublingual ductal epithelium and are captured by oral antigen-presenting cells, especially macrophages. Conclusion: Oral CD103−CD11b+ cDCs are specialized for the induction of Treg cells in mice; thus, targeting their human counterpart may enhance the therapeutic effects of SLIT.
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