Fate and function of soluble CD14 at ocular and gastrointestinal surfaces and in transgenic tobacco seeds

Mucosal surfaces are constantly challenged with a variety of microorganisms. The pathogen invasion of mucosal surfaces is initially countered by the innate immune system via recognition of microbe-specific motifs. Of these microbial motifs, lipopolysaccharide (LPS) is known to be one of the most powerful bacterial virulence factors. This thesis is an attempt to understand the roles of the LPS receptor complex at mucosal surfaces and the ways by which it discriminates between commensal and pathogenic Gram-negative bacteria. The LPS receptor components studied were the LPS binding protein (LBP), cluster of differentiation 14 (CD14), toll-like receptor 4 (TLR4) and myeloid differentiation protein 2 (MD-2). In the first section of this thesis, biological analyses of the LPS receptor complex at the tear-corneal interface suggested that the LPS receptor components are strategically and spatially expressed to fine-tune and even restrain their LPS response to invasive pathogenic Gram-negative bacteria. To study its function in the developing gastrointestinal mucosal surface, soluble CD14 in breast milk was tracked in human neonates. The lack of detection of CD14 in stools of breast-fed neonates and the in vitro proteolysis assays suggested that CD14 is likely to survive its gastrointestinal passage in the low bacteria density lumen of the upper digestive system, but is likely to be absent from the LPS-rich environment of the distal gastrointestinal tract. This controlled and limited spatial distribution could be a strategy to prevent an overzealous immune response against the commensal flora of the distal bowel. The results obtained from these two studies concluded that the function and fate of CD14 at mucosal surfaces was dynamic enough to merit further investigations on a much larger scale. For this to happen, recombinant expression systems needed to be first explored. The successful production of human CD14 in transgenic tobacco proved to be a promising source of a stable and active CD14 to further elucidate the mechanisms of the mucosal LPS response system.