Understanding the mechanisms of bacterial-induced exacerbation of allergic airways disease in a mouse model

Activation of innate immune responses in individuals with asthma by respiratory viral and bacterial infections or colonization with pathogenic bacteria can cause disease exacerbation. Exacerbations have a negative impact on quality of life and are characterised by persistent airway inflammation, worsening of disease symptoms and poor responsiveness to standard corticosteroid therapy. How activation of innate host defence pathways by bacterial infection triggers steroid-resistant inflammatory pathways and disease exacerbation is poorly understood. Better disease models are urgently required to identify mechanisms underlying disease exacerbation. We hypothesised that bacterial infection (mimicked by lipopolysaccharide (LPS) exposure) would exacerbate pre-existing allergic airways disease (AAD) in a mouse model, causing steroid-resistant airways inflammation and airway hyperresponsiveness (AHR). Mice were initially sensitized and subsequently challenged with nebulised ovalbumin (OVA) to induce AAD. LPS was then administered into the lung, in the presence or absence of dexamethasone (DEX) to assess steroid sensitivity. Disease outcomes were assessed by quantifying lung function (airways hyperresponsiveness; AHR), inflammatory cell infiltration, tissue cytokine levels and microarray profiling. LPS administration induced steroid-resistant AHR and increased inflammatory cytokine expression (including interleukin (IL)-27, interferon (IFN)γ, macrophage inflammatory protein (MIP)-1α and tumour necrosis factor α (TNFα)), while CD4⁺ T-helper 2 (Th2) cytokines (IL-5 and IL-13) were not altered compared to OVA-treated mice. Neutrophil and macrophage numbers were also increased in the bronchoalveolar fluid (BALF) following LPS administration. Targeted depletion of alveolar macrophages with 2-chloroadenosine (2-CA) significantly suppressed AHR. Further, IL-13 was required for exacerbation, as LPS failed to exacerbate AHR in IL-13 deficient mice or following administration of IL-13 blocking antibodies. Microarray profiling of lung samples revealed that microRNA (miR)-135b-5p expression was markedly increased following LPS administration in mice with pre-existing AAD and expression was only partially suppressed by corticosteroid treatment. Inhibition of miR-135b-5p function by antagomir treatment suppressed LPS-induced exacerbations AHR and markedly reduced inflammatory cell infiltration. In summary, we developed a novel mouse model of LPS-induced steroid-resistant exacerbation of AAD, which mimic critical features of infection-induced exacerbation of asthma. Our findings highlight key roles for pulmonary macrophages, IL-13 and miR-135b-5p in the development of disease symptoms. Targeting these pathways may be a useful treatment for acute bacterial-induced exacerbation of asthma.