Skin triggering of TLR7 modulates respiratory dendritic cells and natural killer cells - identification of a novel mode for manipulation of respiratory leukocytes

Abstract

Objective: Dendritic cell (DC) maturation and activation in situ is a critical element of successful DC vaccination protocols whereas suppression of functional DC activation represents an important step in the development of immunoregulatory cellular therapy protocols. The TLR7 agonist imiquimod has been used successfully as novel topical immune adjuvant but the systemic use has been limited due to significant toxicity. We hypothesized that skin application of a synthetic TLR7 ligand may represent a novel means to modulate respiratory leukocytes. Methods: In a placebo-controlled experimental animal study we have used 7-colour flow cytometry to systematically analyze the modulation of murine respiratory leukocyte subsets after skin administration of the synthetic TLR7 ligand imiquimod. Results: Compared to placebo, skin administration of imiquimod markedly increased respiratory dendritic cells and innate lymphocytes within 24h, whereas total respiratory leukocytes, alveolar macrophages, T helper and killer lymphocyte numbers were not affected or reduced. DC subpopulation analyses revealed that elevation of respiratory DC was related to an increase of monocytic DC (CD11c+CD11b+MHC-classIIdimSiglecFneg,CD45+). Lymphocyte subpopulation analyses indicated a marked elevation of respiratory natural killer cells and a reduction of B cells. In contrast, bronchoalveolar cell counts and histopathology were not affected suggesting that skin TLR7 stimulation markedly modulated respiratory leukocyte composition without inducing a respiratory inflammatory response. Conclusion: These data suggest for the first time the possibility to modulate respiratory leukocyte subsets after skin administration of a clinically approved TLR7 ligand. Skin administration of synthetic TLR7 ligands may represent a novel and safe means to modulate respiratory immunity. DC vaccination trials targeting the respiratory system may be modulated by skin TLR7 triggering.