EPIDEMIOLOGICAL AND FUNCTIONAL APPROACHES TO EVALUATE THE EFFECTS OF PARTICULATE MATTER EXPOSURE IN INFANTS WITH VIRAL BRONCHIOLITIS

According to the World Health Organization (WHO), environmental air pollution is currently among the leading causes of death and disease globally. Particulate matter (PM) exposure has been linked to the exacerbation of acute respiratory diseases, especially in the most susceptible population’s subgroups, such as infants and chil\dren. Acute bronchiolitis, a common clinical condition characterized by inflammation and obstruction of bronchioles, is generally caused by a viral infection of the lower airways and is estimated to be the leading cause of hospitalization of infants worldwide. At present, the severity of the disease is only partially explained by current known predisposing risk factors but recent evidence suggests that short-term exposure to PM causing airway inflammation, might result in increased susceptibility to viral infections and bronchiolitis severity. Due to its anatomical location, the upper respiratory tract (URT) is persistently exposed to PM, which affects the airway microbiome: recent evidence has linked the effects of PM10 and PM2.5 to microbiota structure and composition. In this context, innate immunity plays a fundamental role in the first response to both exogenous agents and commensal/pathobionts components of microbiota. In particular, bacterial-derived Extracellular vesicles (bEVs) are emerging as essential interregnum messengers that could play a crucial role in communication with mammalian cells, by modulating the host immune system. The main aim of the work is to investigate how PM exposure might modulate the relation that occurs between the severity of acute bronchiolitis, the respiratory microbiota, and the host immune response mediated by bEVs. First, we showed that the exposure to PM2.5 from the Milan metropolitan area impaired the innate immunity response following an acute infectious stimulus in a zebrafish (Danio rerio) embryo model. In particular, PM induced a pro-inflammatory effect, which leads to a dampened leukocytic response to a subsequent inflammatory stimulus. In the second part of the thesis, we investigated the relationship between short-term exposure to PM (PM2.5 and PM10) and the severity of viral bronchiolitis through a population study involving infants with bronchiolitis (with and without Respiratory Syncytial Virus, RSV, infection) aged less than one year living in the area of Milan. A positive association was observed between the severity of the disease and PM2.5 and PM10 exposure levels in the third week before the bronchiolitis peak. Moreover, we performed a metataxonomic analysis, comparing the composition of the bacterial portion of the nasal microbiome (bNM) of infants with bronchiolitis and matched it to healthy controls, to unveil a plausible interplay of the URT microbiota in this framework. We observed a respiratory dysbiosis in bNM of infants with bronchiolitis. Furthermore, Haemophilus influenzae (Hi) was predicted as the most represented species and also positively modulated by PM exposure during bronchiolitis. Last, the immunomodulatory potential of Hi-deriving bEVs was investigated using the zebrafish embryo. With this model, we showed that Hi-bEVs promote a pro-inflammatory response by altering leukocyte trafficking in particular in the neutrophil population. Overall, our data support the role of PM exposure in influencing the interplay between respiratory viruses, such as RSV, and specific microbiota components of the URT, such as Hi, in modulating the host immune response, potentially determining the pathogenesis and severity of acute bronchiolitis.