Molecular studies on streptococcal surface proteins

Abstract: Streptococcus agalactiae and Streptococcus pyogenes are two related human pathogens causing different diseases. This thesis focuses on a number of surface proteins expressed by S. agalactiae and S. pyogenes and, more specifically, the molecular and biological characterization of these proteins. The first paper describes the novel Blr protein of S. agalactiae, which together with the previously described Slr protein of S. pyogenes identifies a family of streptococcal proteins with leucine-rich repeats (LRRs). Characterization of Blr and Slr revealed that the two proteins are efficiently camouflaged by other surface components. In S. agalactiae, exposure of Blr was increased ~100-fold in a mutant lacking the polysaccharide capsule, whereas S. pyogenes mutants lacking M protein and/or protein F displayed ~20-fold increased surface exposure of Slr. It seems possible that the camouflaging structures are downregulated during some parts of the infection process, thus exposing the Blr and Slr proteins on the streptococcal surface. In the second paper, we analyzed a key problem in the vaccine field, the identification of antigens that elicit protective immunity. Our studies were focused on the immune response to the S. agalactiae proteins Rib and alpha. These proteins contain a unique N-terminal region and long repetitive C-terminal sequences. The immune response against pure Rib or alpha was almost exclusively directed against the repeat region, i.e. very few antibodies were directed against the N-terminal regions. Thus, the N-terminal region is nonimmunodominant in both Rib and alpha. Nevertheless, a fusion protein comprising the N-terminal regions of Rib and alpha elicited antibodies that were protective. Importantly, studies of the S. pyogenes M22 protein showed that the N-terminal region, which is targeted by opsonic antibodies, was also nonimmunodominant. Together, these results indicate that nonimmunodominant regions are of general interest for vaccine development. The third paper addresses the role of fibrinogen (Fg) in bacterial virulence. We characterized the Fg-binding B repeat region of the S. pyogenes M5 protein. In a mouse model, a mutant lacking the Fg-binding domain was severely attenuated, indicating that binding of Fg plays a key role in virulence. Similarly, a bacterial mutant lacking the most N-terminal part of the M5 protein was avirulent. Together, these data indicate that two separate regions of M5 are important for virulence. The function of bacteria-bound Fg is not known, but our data support the notion that bound Fg protects the bacteria against complement deposition and thereby against phagocytosis.