Role of TLR ligands in epicutaneously induced contrasuppression

Our previous work showed that epicutaneous (EC) immunization in mice with protein antigen (Ag) induced an Ag-independent unresponsiveness mediated by suppressor CD4(+)8(+) T cells (Ts), which inhibited contact hypersensitivity (CS). Simultaneous EC immunization with Ag and various Toll-like receptor (TLR) ligands reversed skin-induced suppression. Our present study shows that this process activates Ag-specific T contrasuppressor (Tcs) cells and leads to the protection of CS effector T cells from suppression. Epicutaneous immunization with Ag and the TLR4 ligand lipopolysaccharide (LPS) led to a significant increase in IFN-gamma production by lymph node and spleen cells. Ag and TLR ligands, like LPS, CpG or lipoteichoic acid did not need to be applied concomitantly to the skin. An identical contrasuppressive effect was observed when the Ag and TLR ligands were deposited on distant skin areas, suggesting that both the generation of Ts and Tcs are independent. To corroborate this finding, we used a model system that uses macrophages (Mf) as Ag-presenting cells. Mf labeled in vitro with Ag (Mf-Ag) induced, upon intravenous (iv) administration, an unresponsiveness reaction that was mediated by Ts cells. When treated simultaneously with LPS-treated Mf (Mf-Ag-LPS), a TLR-ligand could induce CS. Both the Ag and the LPS signal could be uncoupled i.e., Mf-Ag and Mf-LPS given at separate time points (with an 1 h interval between injections) induced immunity.We also found that LPS-treated Mf also produced significant amounts of IL-12, a cytokine that has well-known anti-tolerogenic properties. Our experiments suggest that reversal of EC-induced suppression by TLR-ligands may be a potential tool to increase the immunogenicity of weakly immunogenic antigens.