Skin TLR7 Triggering Promotes Accumulation of Respiratory Dendritic Cells and Natural Killer Cells

Holger Hackstein,Nelli Baal,Sabine Kranz,Gregor Bein,Werner Von Wulffen,Achim D Gruber,Nicole Hagel,Jürgen Lohmeyer,Angela Knoche,Olivia Kershaw

doi:10.1371/journal.pone.0043320

Abstract

The TLR7 agonist imiquimod has been used successfully as adjuvant for skin treatment of virus-associated warts and basal cell carcinoma. The effects of skin TLR7 triggering on respiratory leukocyte populations are unknown. In a placebo-controlled experimental animal study we have used multicolour flow cytometry to systematically analyze the modulation of respiratory leukocyte subsets after skin administration of imiquimod. Compared to placebo, skin administration of imiquimod significantly increased respiratory dendritic cells (DC) and natural killer cells, whereas total respiratory leukocyte, alveolar macrophages, classical CD4+ T helper and CD8+ T killer cell numbers were not or only moderately affected. DC subpopulation analyses revealed that elevation of respiratory DC was caused by an increase of respiratory monocytic DC and CD11bhi DC subsets. Lymphocyte subpopulation analyses indicated a marked elevation of respiratory natural killer cells and a significant reduction of B lymphocytes. Analysis of cytokine responses of respiratory leukocytes after stimulation with Klebsiella pneumonia indicated reduced IFN-γ and TNF-α expression and increased IL-10 and IL-12p70 production after 7 day low dose skin TLR7 triggering. Additionally, respiratory NK cytotoxic activity was increased after 7d skin TLR7 triggering. In contrast, lung histology and bronchoalveolar cell counts were not affected suggesting that skin TLR7 stimulation modulated respiratory leukocyte composition without inducing overt pulmonary inflammation. These data suggest the possibility to modulate respiratory leukocyte composition and respiratory cytokine responses against pathogens like Klebsiella pneumonia through skin administration of a clinically approved TLR7 ligand. Skin administration of synthetic TLR7 ligands may represent a novel, noninvasive means to modulate respiratory immunity.

Highlights

Imiquimod is a synthetic Toll-like receptor TLR7 ligand stimulating innate and adaptive immunity against viruses and tumors [1,2]
To assess the effects of skin TLR7 triggering on respiratory leukocytes, mice were treated with imiquimod creme (AldaraH) applied to the back skin for up to seven days
Sustained TLR7 skin triggering over 7 days did not affect total respiratory leukocyte numbers, whereas single exposure resulted in reduced leukocyte numbers (Fig. 1A)

Summary

Introduction

Imiquimod is a synthetic Toll-like receptor TLR7 ligand stimulating innate and adaptive immunity against viruses and tumors [1,2]. TLR7 activation of these professional antigen presenting cells bridges innate and adaptive immunity through induction of inflammatory cytokines type I interferon, TNF-a, IL12, chemokines and DC migration [7,8,9]. Different groups demonstrated effectiveness of TLR7 triggering to suppress allergic airway inflammation in experimental asthma models suggesting immunomodulatory potential of synthetic TLR7 ligands [11,12,13]. Experimental skin vaccination studies with imiquimod revealed significant co-induction of T regulatory cells and IL-10 signalling suppressing the activity of cytotoxic CD8+ T cells [15]

Methods

Results

Conclusion