Respiratory effects of endotoxin exposure : Individual susceptibility and gene-environment interactions

Abstract

Endotoxins are cell wall components of Gram-negative bacteria that occur commonly on plants and plant products and as gut commensals. A large variability in airborne endotoxin exposure levels has been measured in a range of agricultural and other occupational environments. Inhalation of endotoxins may cause respiratory and inflammatory effects characterized by increased levels of infiltrating neutrophils and production of pro-inflammatory cytokines. Conversely, several studies have shown inverse associations between domestic endotoxin exposure and atopy and atopic asthma in children from farming families, but also in children living in urban areas. A number of studies has shown that childhood farm exposures continue to protect against atopic sensitization into adulthood, and recent studies suggested that endotoxin exposure during adulthood still protects against atopy and atopic asthma. Evidently, not all endotoxin-exposed workers develop respiratory or inflammatory responses, even at very high exposure levels. Genetic and non-genetic susceptibility factors may interact with endotoxin exposure, resulting in inter-individual heterogeneity in type and magnitude of responses within groups of individuals with apparently identical exposures. The main aim of this thesis was to investigate associations between endotoxin exposure and the occurrence of respiratory and allergic effects in adults with sometimes extremely high work-related exposure. Cytokine responsiveness in an ex vivo whole blood assay and genetic variation in innate immunity genes were evaluated as susceptibility factors in associations between exposure and respiratory and allergic outcomes. A total of 249 full-shift personal inhalable dust samples were collected for endotoxin exposure measurements among Dutch farmers and agricultural industry workers (flower bulb, animal feed, vegetable seed, and onion processing workers). A wide range of endotoxin levels were found (~10 to 10,000 EU/m3) ranging from slightly elevated levels close to background to extremely high levels usually not encountered in the home and general environment. A job-exposure matrix was constructed to assign endotoxin exposure levels to all 878 participants in the study. Significant and positive exposure-response relationships were found between endotoxin exposure and asthma-related outcomes, which were characterized by a predominantly non-atopic phenotype. In contrast, inverse associations between occupational endotoxin exposure and hay fever and atopic sensitization were found. An important role for individual susceptibility to the adverse effects of endotoxin was implied by the finding that associations between occupational endotoxin exposure and respiratory effects were only observed in the subgroup of subjects with above median cytokine response by ex vivo LPS-stimulated blood monocytes. A nested case-control study in young farmers, and a case-control and family-based study in adults suggested gene-environment interactions between genes involved in innate immunity responses such as CD14 and Toll-like receptor genes (TLR2, TLR4, TLR9) and occupational and environmental exposure to endotoxin or other microbial agents, in their associations with asthma or atopy. Although endotoxin might protect against allergies, this thesis emphasizes the need to lower endotoxin exposure levels in the agricultural environment to protect workers against chronic respiratory health effects. Regulations should not only focus on the average worker but should, where possible, also provide sufficient protection for the most susceptible individuals in any given context.