In vitro induction of T helper 17 cells by synergistic activation of human monocyte-derived langerhans cell-like cells with bacterial agonists

In the case of epidermal barrier disruption, pathogens encounter skin-resident Langerhans cells (LCs) and are recognized by pathogen recognition receptors such as Toll-like receptors (TLRs). As the majority of microorganisms exhibit more than one TLR ligand, the mechanisms of subsequent T cell differentiation are complex and far from clear. In this study, we investigated combinatory effects on Th cell polarization by bacterial cell wall compounds peptidoglycan (PGN) and lipopolysaccharide (LPS) and by bacterial nucleic acid (DNA). Expression of maturation markers CD40, CD80, HLA-DR and CCR7 and the release of IL-1β, IL-6 and IL-23 was strongly enhanced by simultaneous exposure to PGN, LPS and DNA in LCs. As all these factors were potential Th17 driving cytokines, we investigated the potency of combinatory TLR stimuli to induce Th17 cells via LC activation. High amounts of IL-17A and IL-22, key cytokines of Th17 cells, were detected. By intracellular costaining of IL-17⁺ T cells, IL-22− (Th17) and IL-22⁺ (immature Th17) cells were identified. Interestingly, one population of LPS stimulated cells skewed into IL-9⁺ Th cells, and LPS synergized with PGN while inducing high IL-22. In conclusion, our data indicates that when mediated by a fine-tuned signal integration via LCs, bacterial TLR agonists synergize and induce Th17 differentiation.