TLR
ligands and other
allergen-nonspecific immunostimulatory molecules are ubiquitous in ambient air and have profound modulatory activities in animal models of allergic
asthma. However, several of these molecules have been shown to promote exaggerated Th2-biased airway
hypersensitivity responses (AHRs), whereas others attenuate the asthmatic phenotype. Therefore, it has proven difficult to extrapolate experimental results with purified molecules toward a more general understanding of the
allergen-nonspecific immunomodulatory influence of living environments on the natural history of allergic
asthma. These investigations determined how regular and intermittent airway exposures to an unpurified, but sterile
house dust extract standard (HDEst) affected the OVA-specific AHR and immune status of previously Th2-sensitized mice. Low-dose daily and high-dose intermittent HDEst exposures modulated ongoing AHRs considerably, reducing eosinophil recruitment and
methacholine responsiveness, while increasing neutrophilic
inflammation. However, only daily airway delivery of low-dose HDEst attenuated OVA-specific Th2
cytokine production and Th2-biased AHRs to
allergen challenge 1 mo later. Finally, whereas LPS mimicked many of the immunomodulatory characteristics of HDEst in this murine
asthma model, daily airway HDEst delivery was highly effective in attenuating the AHR of
OVA/alum-sensitized TLR4-deficient mice. Taken together, these investigations provide direct evidence that living environments contain
allergen-nonspecific immunostimulatory molecules that influence the airway
hypersensitivity status of
allergen-sensitized mice by TLR4-dependent and independent mechanisms.