Jesus Arenas Busto, Jesus Perez Ortega, Eline de Jonge
Neisseria meningitidis is a natural inhabitant of the nasopharynx. Normally, about 10 % of the population harbour N. meningitidis asymptomatically. In a small number of carriers, mucosal colonization results in invasive meningococcal disease with severe clinical syndromes, such as bacterial meningitis and septicaemia. The high incidence in early childhood, the rapid onset of the invasive disease and the high case fatality rate make vaccine development against this pathogen of utmost importance. Therefore, the meningococcal surface antigens are being studied intensively. OMPs are generally considered as potential candidates for the development of a vaccine against N. meningitidis serogroup B strains. However, the major OMPs suffer from high antigenic variability. Therefore, we are evaluating the vaccine potential of several minor OMPs with more conserved cell-surface-exposed parts. Recently, we succeeded to solve the structure of one promising vaccine candidate, i.e. the NspA protein (1). However, the function of NspA is not known, and it is also not known whether the protein is essential during infection. Therefore, there is a risk of selecting for escape mutants when NspA is used as the predominant vaccine component. Therefore, we are also studying the vaccine potential of several other OMPs, particularly OMPs that are essential for the viability of the bacteria, such as Omp85 (see project Biogenesis of the outer membrane), and proteins required during infection.
Because of the presence of iron-binding proteins, such as transferrin and lactoferrin, the human tissues form an iron-restricted environment, which limits the growth of microorganisms. Pathogenic bacteria cope with these conditions by producing virulence factors, dedicated to the uptake of iron. N. meningitidis produces receptors under iron limitation for the iron-binding proteins of the host. These receptors are important vaccine candidates, because they are essential for growth in the iron-restricted environment of the host and because they must expose conserved domains, i.e. the ligand-binding domains, to the cell surface. If the immune response can be directed to these ligand-binding domains, an effective, broadly cross-reactive vaccine can be developed. We are particularly interested in studying the vaccine potential of the lactoferrin receptor, as well as in the mechanism of iron acquisition from lactoferrin (2).
Like the bacterial receptors, adhesins are presumed to expose conserved domains at the cell surface, i.e. domains with which they bind to receptors on the human target cells. Autotransporters (see project Protein secretion mechanisms) often function as adhesins. Screening of the available genome sequences revealed the presence in N. meningitidis of eight different autotransporters (3), many of which indeed show a high degree of similarity to known autotransporter adhesins of other bacteria. The function and vaccine potential of these autotransporters are under investigation.
Besides the meningococcal vaccine, we work on a new vaccine against Bordetella pertussis, the causative agent of whooping cough. The current whole-cell vaccine is under debate because of its side effects. In close collaboration with the Netherlands Vaccine Institute, we are trying to improve the vaccine by modifying the LPS (endotoxin), which is most likely responsible for these side effects. Apart from these adverse effects, LPS has also beneficial effects in vaccine formulations, i.e. it is a strong adjuvant. Both the toxic and the adjuvant activity of LPS are determined by its lipid A moiety. In N. meningitidis, we have recently demonstrated that it is possible by genetic engineering to reduce the endotoxic activity, while retaining the adjuvant activity of LPS (4). Along similar ways, we are now trying to reduce the toxicity of the B. pertussis LPS in order to create a new whole-cell vaccine with reduced reactogenicity. Recently, we identified in many bacteria an outer membrane enzyme (lipid A 3-O-deacylase) that deacylates LPS, thereby reducing its toxicity drastically (5). Such enzymes will be extremely useful for the construction of vaccine strains with drastically reduced reactogenicity.
- van de Putte-Rutten L, Bos MP, Tommassen J, Gros P. 2003. Crystal structure of Neisserial surface protein A (NspA), a conserved outer membrane protein with vaccine potential. J. Biol. Chem. 278:24825-24830.
- Pettersson A, Prinz T, Umar A, van der Biezen J, Tommassen J. 1998. Molecular characterization of LbpB, the second lactoferrin-binding protein of Neisseria meningitidis. Mol. Microbiol. 27:599-610.
- van Ulsen P, van Alphen L, Hopman C Th P, van der Ende A, Tommassen J. 2001. In vivo expression of Neisseria meningitidis proteins homologous to theHaemophilus influenzae Hap and Hia autotransporters. FEMS Immunol. Med. Microbiol. 32:53-64.
- Steeghs L, Berns M, ten Hove J, de Jong A, Roholl P, van Alphen L, Tommassen J, van der Ley P. 2002. Expression of foreign LpxA acyltransferases in Neisseria meningitidis results in modified lipid A with reduced toxicity and retained adjuvant activity. Cellular Microbiol. 4:599-611.
- Geurtsen J, Steeghs L, Ten Hove J, van der Ley P, Tommassen J. 2005. Dissemination of lipid A deacylases (PagL) among Gram-negative bacteria: identification of active-site histidine and serine residues. J. Biol. Chem. 280:8248-59.