Redirection of allergen-specific TH2 responses by a modified adenine through Toll-like receptor 7 interaction and IL-12/IFN release

Background: Natural or synthetic ligands of Toll-like receptors (TLRs), such as CpG-containing oligodeoxynucleotides and imidazoquinolines, affect the functional phenotype of antigen-specific human T lymphocytes by inducing cytokine release by cells of the innate immunity. Objective: In vitro investigation of the ability of substitute adenines (SAs) to affect antigen-presenting cells and shift the functional phenotype of specific human T(H)2 cells was performed. Methods: The functional profile of hapten- and allergen-specific T-cell lines obtained in the absence or presence of modified adenines was assessed by means of quantitative real-time PCR, flow cytometry, and ELISAs. Activation of TLRs was evaluated by means of nucleofection of HEK293 cells. Results: The synthetic heterocycle, chemically related to adenine with substitution in positions 2-, 8-, and 9- (SA-2), but not its related derivative lacking 2- and 8- substitutions, stimulated the production of high amounts of IL-12, IL-10, TNF-alpha, and IL-6 by CD14(+) cells and IFN-alpha and CXCL10 by blood dendritic cell antigen (BDCA)-4(+) plasmacytoid dendritic cells. A nuclear factor kappa B-dependent signaling pathway mediated by SA-2 ligation of TLR7 was responsible for these effects. SA-2 also redirected the in vitro differentiation of either Dermatophagoides pteronyssinus group 1 or amoxicillin-specific TH2 cells toward the T(H)1/T(H)0 phenotype, with parallel downregulation of GATA-3 and upregulation of T-box expressed in T cells transcription factors. Conclusion: Critical substitutions of the adenine backbone confer the ability to activate TLR7, inducing the production of modulatory cytokines able to shift human allergen-specific T(H)2 cells to a T(H)1/T(H)0 phenotype. Clinical implications: Appropriately modified adenines might be used as effective adjuvants for the development of novel immunotherapeutic strategies of allergic disorders.