Immune function and structural analysis of recombinant bovine conglutinin and human lung surfactant protein-D
Recognition of sugar moieties on the surface of microorganisms is one of the ways the body distinguishes potential pathogens from self-cells. The sugarbinding proteins, lectins, mediate this recognition role of the first line of defence against infections, preceding the antibody-mediated (adaptive) immune response. Collectins are calcium-dependent carbohydrate-binding proteins that have been implicated in innate immunity. Bovine conglutinin (BC) and lung surfactant protein-D (SP-D), belong to the family of 'collectins' which are characterised by four domains: an N-terminal cysteine-rich region, a collagenlike region linked with the carbohydrate recognition domain (CRD) via an ahelical neck region. BC and SP-D show remarkable similarity in their amino acid sequence (79% identity), function and biological characteristics. They have been shown to mediate microbial clearance either by directly binding to bacteria leading to phagocytosis or interacting with complement system components. The present study aims to elucidate the biological function of these proteins more precisely. Recombinant fragments (r) of BC and SP-D consisting of their CRDs and neck regions have been cloned in pET-21a and pMal-c2 vectors respectively, for expression in Escherichia coli. Recombinant conglutinin was expressed in BL21(DE3)pLysS and isolated by a denaturation-renaturing procedure. Binding of rBC(N/CRD) to mannan and complement component, iC3b, was assessed in real-time by BIAcore. The dissociation constants were calculated by Scatchard analysis. The carbohydrate structures present on the surface of the microorganisms play an important role in mediating the interactions with the immune cells. The recombinant molecules showed calcium-dependent binding to lipopolysaccharides (LPS) from gram-negative bacteria Pseudomonas aeruginosa, Klebsiella pnuemonia and Salmonella typhosa, which was inhibited in presence of sugars. rBC(N/CRD) also bound to whole bacteria as assessed by ELISA and retained its capacity to recognise various complement system components and the carbohydrate moieties on the surface of various pathogenic microorganisms. The recombinant protein retained its ability to bind various sugar residues, although with lower affinity than that of the native molecule. rBC(N/CRD) is able to bind and aggregate bacteria and cause agglutination of bacterial cell suspensions. A novel model has been used to describe the interactions of the collectins at the molecular level based on specificity of carbohydrate-recognition by the collectins. The pyocin mutant strain 1291 series of Neisseria gonorrhoeae has sequential deletions of the terminal sugars in their lipooligosaccharides (LOS). Conglutinin showed a preferential high affinity binding to 1291a mutant that expresses GlcNAc as the terminal hexose, in comparison to other mutants. This provides a unique system to understand the specific cell-surface interactions in relevance to a particular lectin. Further elucidation of the function of CRD and neck region at a structural level is in progress, using X-ray crystallography. Since the submission of the thesis, the structure of the monomeric CRD has been solved, which revealed a remarkable similarity to the SP-D and MBL structure. Trials are underway to get the structure of the trimeric CRDs. These studies aim to provide a better understanding of the collectinpathogen interaction at the biological and structural levels. The ultimate aim is to determine if the recombinant forms of these proteins can be used therapeutically to enhance the uptake and killing of pathogens.