Based on epidemiological data, the hygiene hypothesis associates poor hygienic living conditions during childhood with a lower risk for the development of allergic diseases such as
bronchial asthma. The role of
viral infections, and especially of viral TLR
ligands, within this context remains to be clarified. Viral TLR
ligands involve dsRNA and ssRNA which are recognized by TLR-3 or TLR-7, respectively. In this study, we evaluated the impact of TLR-3 or TLR-7 activation on experimental
asthma in mice. Systemic application of the synthetic TLR-3 or TLR-7
ligands polycytidylic-polyinosinic
acid (p(I:C)) or
R-848, respectively, during the sensitization phase prevented the production of OVA-specific
IgE and
IgG1 Abs and subsequently abolished all features of experimental
asthma including
airway hyperresponsiveness and allergic airway
inflammation. Furthermore, administration of p(I:C) or
R-848 to animals with already established primary allergic responses revealed a markedly reduced secondary response following
allergen aerosol rechallenges. In contrast to wild-type animals, application of p(I:C) or
R-848 to IL-12p35(-/-) mice had no effect on airway
inflammation, goblet cell
hyperplasia, and
airway hyperresponsiveness. However, in the absence of
IL-12, the numbers of eosinophils and lymphocytes in bronchoalveolar lavage fluids were still significantly reduced. These partial effects could also be abolished by neutralizing anti-IL-10 Abs in IL-12p35(-/-) mice. These data indicate that TLR-3 or TLR-7 activation by viral TLR
ligands has both preventive as well as suppressive effects on experimental
asthma which is mediated by the additive effects of
IL-12 and
IL-10.