Background: Nitrogen dioxide (NO2) is an air pollutant associated with poor respiratory health, asthma exacerbation, and an increased likelihood of inhalational allergies. NO2 is also produced endogenously in the lung during acute inflammatory responses. NO2 can function as an adjuvant, allowing for allergic sensitization to an innocuous inhaled antigen and the generation of an antigen-specific Th2 immune response manifesting in an allergic asthma phenotype. As CD11c+ antigen presenting cells are considered critical for naïve T cell activation, we investigated the role of CD11c+ cells in NO2-promoted allergic sensitization.
Methods: We systemically depleted CD11c+ cells from transgenic mice expressing a simian diphtheria toxin (DT) receptor under of control of the CD11c promoter by administration of DT. Mice were then exposed to 15 ppm NO2 followed by aerosolized ovalbumin to promote allergic sensitization to ovalbumin and were studied after subsequent inhaled ovalbumin challenges for manifestation of allergic airway disease. In addition, pulmonary CD11c+ cells from wildtype mice were studied after exposure to NO2 and ovalbumin for cellular phenotype by flow cytometry and in vitro cytokine production.
Results: Transient depletion of CD11c+ cells during sensitization attenuated airway eosinophilia during allergen challenge and reduced Th2 and Th17 cytokine production. Lung CD11c+ cells from wildtype mice exhibited a significant increase in MHCII, CD40, and OX40L expression 2 hours following NO2 exposure. By 48 hours, CD11c+MHCII+ DCs within the mediastinal lymph node (MLN) expressed maturation markers, including CD80, CD86, and OX40L. CD11c+CD11b- and CD11c+CD11b+ pulmonary cells exposed to NO2 in vivo increased uptake of antigen 2 hours post exposure, with increased ova-Alexa 647+ CD11c+MHCII+ DCs present in MLN from NO2-exposed mice by 48 hours. Co-cultures of ova-specific CD4+ T cells from naïve mice and CD11c+ pulmonary cells from NO2-exposed mice produced IL-1, IL-12p70, and IL-6 in vitro and augmented antigen-induced IL-5 production.
Conclusions: CD11c+ cells are critical for NO2-promoted allergic sensitization. NO2 exposure causes pulmonary CD11c+ cells to acquire a phenotype capable of increased antigen uptake, migration to the draining lymph node, expression of MHCII and co-stimulatory molecules required to activate naïve T cells, and secretion of polarizing cytokines to shape a Th2/Th17 response.