Differential activation of dendritic cells by toll-like receptors causes diverse differentiation of naïve CD4+ T cells from allergic patients

Background: To avert the differentiation of allergen-specific Th2 cells in atopic individuals is a major goal in the prevention and therapy of IgE-mediated allergy. We aimed to compare different toll-like receptor (TLR) agonists regarding their effects on antigen-presenting cells and the differentiation of naïve T cells from allergic patients.

Methods: Monocytes and monocyte-derived dendritic cells (mdDC) from allergic patients were stimulated with Pam3CSK4 (TLR1/2 ligand), FSL-1 (TLR2/6 ligand), monophosphoryl lipid (MPL)-A, lipopolysaccharide (LPS, both TLR4 ligands), and flagellin (TLR5 ligand). Allergen uptake and upregulation of CD40, CD80, CD83, CD86, CD58, CCR7 and PD-L1 were analyzed by flow cytometry. Functional maturation of mdDC was tested in mixed leukocyte reactions, and the synthesis of proinflammatory cytokines, IL-10 and members of the IL-12 family was assessed. TLR-ligand-activated mdDC were used to stimulate naïve CD4(+) T cells, and cytokine responses were assessed in supernatants and intracellularly.

Results: All TLR ligands except flagellin enhanced allergen uptake. All TLR ligands induced functional maturation of mdDC with differential expression of surface molecules and cytokines and promoted the differentiation of IFN-γ-producing T cells. LPS-matured mdDC exclusively induced Th1-like responses, whereas mdDC stimulated with the other TLR ligands induced both Th1- and Th0-like cells. Pam3CSK4 and flagellin additionally induced Th2-like cells. Th1-like responses were associated with higher expression levels of co-stimulatory molecules, PD-L1, IL-6, TNF-α, and IL-12p70. None of the TLR-ligand-stimulated mdDC induced IL-10- or IL-17-producing T cells.

Conclusion: Different TLR ligands differently influence T-cell responses due to varying activation of the three signals relevant for T-cell activation, that is, antigen presentation, co-stimulation and cytokine milieu.