Haemophilus influenzae adhesins and the innate immune response

University dissertation from Medical Microbiology, Lund University

Abstract: Haemophilus influenzae is a human specific Gram-negative respiratory tract pathogen. Encapsulated H. influenzae strains exist in six different serotypes a-f, of which type b (Hib) is the most virulent. Hib is protected by the polysaccharide capsule when invading the blood circulaton and may cause diseases such as meningitis and epiglottitis. Non-encapsulated H. influenzae referred to as nontypeable H. influenzae (NTHi), are frequently associated with acute otitis media in children and chronic obstructive pulmonary disease (COPD) among the elderly. H. influenzae expresses several outer membrane proteins of which the adhesins play an important role in the initial interactions with the airway epithelium. In this thesis, we have studied two Haemophilus influenzae adhesins and their interactions with innate defense mechanisms. Vitronectin is a glycoprotein that regulates the terminal pathway of the complement system by inhibiting the membrane attack complex. We demonstrate that Haemophilus influenzae surface fibrils (Hsf) may increase the bacterial survival by interacting with vitronectin in serum. We also describe the isolation and characterization of a novel H. influenzae adhesin designated protein E (PE). PE was isolated by using an IgD myeloma serum which specifically recognized the protein on the bacterial surface. Furthermore, PE was found to be a 16 kDa protein and classified as a lipoprotein. PE displays adhesive activity to three respiratory epithelial cell lines and erythrocytes, both when expressed on the surface of H. influenzae and as a recombinant protein in E. coli. The active adhesive binding domain has been determined to be located within the central part of the molecule, PE amino acids 84-108. PE shows stimulatory effects when incubated with respiratory epithelial cells by inducing high levels of IL-8 and ICAM-1. These inflammatory mediators play an important role for the innate immune response and the recruitment of neutrophils. Finally, PE promotes bacterial invasion into epithelial cells. In summary, the work presented in this thesis shows that Hsf may prolong the bacterial survival by interacting with the complement regulator vitronectin. The novel protein PE has been isolated, characterized and shown to be important for interactions of H. influenzae with the innate immune system.