| Summary: | The honey bee Apis mellifera is an extremely beneficial insect due to its role in pollination and honey production. Honey bees are vulnerable to many diseases, one of which is caused by the bacterium Paenibacillus larvae. Ingestion of its spores by honey bees early in their larval stage results in death before adulthood. This disease is known as American foulbrood. Interestingly, this deadly consequence does not occur when older larvae or adults consume the spores, a phenomenon which we hypothesize to be the result of an underdeveloped immune system in young larvae. To address this issue, we mainly employed mass spectrometry-based proteomics techniques to learn about the protein effectors that are present in the host. We focused on the hemolymph (insect blood) where a significant repertoire of immunity-related proteins exists. In our comparison of adults and larvae, we saw age-correlated differences in the levels of the antimicrobial peptide hymenoptaecin, the phenoloxidase enzyme which lead to the production of cytotoxic free radicals, and several bacterial recognition proteins. This prompted a detailed study of the larval development in which many new expression trends were revealed; for example, the age-related decrease of antioxidant proteins, proteins associated with translational machinery, and enzymes related to protein turnover. Unexpectedly, most immunity-related proteins showed no significant age correlation, except the antimicrobial peptide apisimin and phenoloxidase. The latter protein was particularly intriguing, given that its increased expression and activity with respect to age agreed with the buildup of resistance against P. larvae. Furthermore, this protein was upregulated in P. larvae infected larvae compared to healthy controls. This enzyme may be an important factor of the host immune response. The decrease of nutrient storage proteins in the diseased state which we observed also implied that the defense response was mounted with an associated energetic cost. Data from the work covered in this thesis has helped explain, from a molecular point-of-view, the pathogenesis of American foulbrood. Although our focus was on honey bee immunity, the proteomics-based approach taken here has provided protein evidence and expression data that will serve as a resource for the wider scientific community.
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