Toll-like receptor 4 ligation enforces tolerogenic properties of oral mucosal Langerhans cells

Background: Despite high bacterial colonization, acute infections are rare in the oral mucosa, implicating tolerogenic predominance. Bacterial antigens like LPSs are recognized by innate immunity receptors such as Toll-like receptor 4 (TLR4), associated with LPS receptor (CD14). Objectives: Toll-like receptor 4 agonist monosphoryl lipid A has been successfully used as adjuvant in subcutaneous immunotherapy, suggesting reinforcement of allergen-specific tolerance. Recently sublingual immunotherapy (SLIT) has been shown to be an effective alternative to subcutaneous immunotherapy. We observed CD14 expression on human oral Langerhans cells (oLCs), representing a major target of SLIT. However, not much is known about TLR4 expression and its effect on oLCs. Methods: Cell suspensions were obtained by trypsinization of human oral mucosa and analyzed by flow cytometry, RT-PCR, cytometric bead arrays, ELISA, and mixed lymphocyte reactions. Results: We could show that oLCs express TLR4, and its ligation by monosphoryl lipid A upregulated expression of coinhibitory molecules B7-H1 and B7-H3 while surface expression of costimulatory molecule CD86 was concomitantly decreased. Furthermore, TLR4 ligation on oLCs increased their release of the anti-inflammatory cytokine IL-10 and decreased their stimulatory capacity toward T cells. Moreover, TLR4-ligation on oLCs induced IL-10, TGF-beta 1, Forkhead box protein 3, IFN-gamma, and IL-2 production in T cells. Conclusion: In view of these data, TLR4-ligation on oLCs might not only play a role in pathogen recognition for efficient immunity but also contribute to the tolerogenic state predominating in the oral cavity.