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-β1, Forkhead box protein 3, IFN-γ, 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.