Innate Immune Responses to Engineered Nanomaterials During Allergic Airway Inflammation

The field of nanotechnology is continually advancing, and increasing amounts of consumer goods are being produced using engineered nanomaterials (ENMs). The health risks of occupational and/or consumer exposure to ENMs are not completely understood, although significant research indicates that pulmonary exposure to nanomaterials induces toxic effects in the lungs of exposed animals. Multi-walled carbon nanotubes (MWCNTs) are a specific category of ENMs and consist of sheets of graphene rolled into cylinders that are multiple layers thick in order to strengthen their rigidity. MWCNTs have a fiber-like shape, similar to that of asbestos, which allows for a high aspect ratio and makes them difficult to clear from the lung. Studies with rodent models have demonstrated that pulmonary exposure to ENMs, in particular MWCNTs, results in acute lung inflammation and the subsequent development of chronic fibrosis, suggesting a potential human health risk to individuals involved in the manufacturing of products utilizing these nanomaterials. Induction of IL-1beta secretion via activation of the inflammasome is a prime mechanism of MWCNT-induced inflammation. The inflammasome is a multi-protein scaffold found in a variety of cell types that forms in response to a variety of immune signals, including particulates.;Sensitization with allergens, such as house dust mite (HDM), increases levels of the T helper 2 (Th2) cytokines IL-4 and IL-13 in mice and in humans, and there is particular cause for concern in cases of MWCNT exposure in individuals with pre-existing allergic airway disease, such as asthma. MWCNT exposure exacerbates airway inflammation and fibrosis in animal models of pre-existing allergic asthma, suggesting that individuals suffering from asthma are more susceptible to the toxic pulmonary effects of MWCNT exposure. Asthma is an exceptionally prominent human disease, and therefore the goal of this research was to better understand how pre-existing allergic airway disease would modulate the innate immune response to MWCNTs. We hypothesized that Th2 cytokines and the allergic asthmatic microenvironment would alter MWCNT-induced inflammasome activation and IL- 1beta secretion both in vitro and in vivo.;In vitro, THP-1 cells, a human monocytic cell line, were differentiated into macrophages and exposed to MWCNTs and or recombinant Th2 cytokines, specifically IL-4 and/or IL-13. Exposure of THP-1 cells to MWCNTs alone caused dose-dependent secretion of IL-1beta, while co-exposure to IL-4 and/or IL-13 suppressed MWCNT-induced IL-1beta. Further analysis determined that IL-4 and IL-13 were phosphorylating the protein signal transducer and activator of transcription 6 (STAT6) and subsequently inhibiting inflammasome activation and function through suppression of caspase-1, a cysteine protease responsible for cleavage of pro-IL-1beta into an active, secretable form.;In vivo, wild-type C57BL6 mice were sensitized intranasally with HDM allergen and exposed to MWCNTs via oropharyngeal aspiration. Treatment with MWCNTs alone induced secretion of IL-1beta in the bronchoalveolar lavage fluid (BALF) one day post-exposure, while sensitization with HDM prior to MWCNT exposure suppressed MWCNT-induced IL-1beta. Immunohistochemical (IHC) analysis of lung sections from exposed animals showed that HDM sensitization inhibited MWCNT-induced pro-casapse-1 protein expression, responsible for inflammasome activation, in the airway epithelium and macrophages. MWCNT exposure combined with HDM sensitization increased inflammatory cell infiltration and subsequent acute lung inflammation and chronic fibrosis. Analysis of the systemic effects of MWCNT exposure during allergic airway sensitization showed that MWCNTs and/or HDM allergen upregulated STAT3 mRNA expression in the lungs, liver, and spleen of exposed animals, and at the same induced mixed T helper (Th) responses in the different tissues.;Collectively, these data suggest that the allergic microenvironment induced during asthma can modulate the innate inflammatory response to MWCNTs through inhibition of caspase-1 and inflammasome activation in the lung and through alteration of the transcription factors involved in the T helper immune responses systemically.