By Dean A. Haycock
Special to BioWorld Today
Of all the possible hosts on this planet, Neisseria meningitidis favors humans and only humans. The bacterium lives in the noses of up to 15 percent of the population in many countries, according to the American Public Health Association's "Control of Communicable Disease Manual." In most of these people, the bacterium produces no symptoms. But when it invades the bloodstream or the membrane that surrounds the brain and spinal cord, the symptoms are serious.
Symptoms produced by infection of the membranes in the central nervous system, called meningococcal meningitis, include fever, severe headaches, stiff neck, nausea and sometimes vomiting. This happens to between 0.5 and 10 people out of 100,000 each year. In an epidemic in which the pathogen is spread from person to person by direct contact, the incidence rate can reach 400 persons per 100,000. Between 5 percent and 15 percent of those infected die. A quarter of them may experience lasting neurological problems. Death for children and young adults from meningococcal meningitis and sepsis can come very quickly, in less than a day.
The mortality rate was once even higher, above 50 percent. One of the reasons for the drop was the development of vaccines for four of the five subtypes, or serotypes, of N. meningitidis. One serotype, N. meningitidis B or MenB, however, has resisted for 30 years the efforts of medical scientists to prepare an effective vaccine.
There are two reasons for this, according to Rino Rappuoli, vice president of vaccine research at Chiron Corp. in Siena, Italy. One is the fact that some sugar molecules on the surface of MenB, capsular polysaccharides, are very similar to sugar molecules found in humans. That means a vaccine would cross-react with human tissue and would render it unsafe. Second, there are different strains of MenB that each has what microbiologist call "sequence variations of surface-exposed proteins." That means that even though they are all classified as MenB bacteria, different strains of MenB have significant molecular variations in crucial proteins on their surfaces. Hence, a vaccine that works against a strain causing an outbreak in, say, Cuba, very likely would not work against the strain responsible for an outbreak in New York.
To get around this problem, researchers from three different groups agreed in 1997 to sequence the entire genome of N. meningitidis Serogroup B and then go looking for promising bacterial proteins that might finally lead to an effective vaccine. The results of their collaboration appear in two papers in the March 10 issue of Science, "Complete Genome Sequence of Neisseria meningitidis Serogroup B Strain MC58," and "Identification of Vaccine Candidates Against Serogroup B Meningococcus by Whole-Genome Sequencing."
Sequencing Only Way To Get Answer
Rappuoli is a co-author on both papers. "We thought of doing the sequencing because we knew it was the only way to solve a problem that could not be solved with all the other technology," he told BioWorld Today. In addition to Chiron, the collaborating scientists represent The Institute for Genomic Research (TIGR) in Rockville, Md., and the University of Oxford in Oxford, UK.
Chiron said it "has the exclusive right to commercialize vaccine products resulting from the collaboration. TIGR will receive milestone payments and royalties on any such commercialized pharmaceutical products."
Together, the research teams identified 350 candidate antigens. They expressed them in Escherichia coli, purified them and injected them into mice. Study of sera from the mice indicated which of the candidates were proteins found on the surfaces of MenB bacterial cells, which were conserved in many different strains of MenB and which induced the production of antibodies that killed bacteria, a good sign they might make effective vaccines.
Seven particularly promising candidates emerged from this series of tests. "All seven proteins, which include four lipoproteins and two outer membrane proteins, were accessible to antibodies and evoked efficient antibacterial antibody responses, suggesting that they hold promise as vaccine antigens," Xavier Nassif of INSERM (the French equivalent of the NIH), Faculti de Midecine Necker-Enfants Malades in Paris, wrote in a commentary. His piece, "A Furtive Pathogen Revealed," appears in the same issue of Science.
The relatively high number of promising protein vaccine candidates that are conserved in many different strains of MenB was a bit of a surprise to the researchers. Rappuoli suggests an explanation. "With [traditional] biochemical technology, you always purified proteins that were very abundant. It is very difficult to purify proteins that are not abundant. So in the past three or four decades, I don't think they were able to identify more than 20. The power of the genome is that it is not really important how abundant the protein is in the bacteria because you start from the gene. And all genes are identical in terms of abundance. So we really managed to identify all of the possibilities which are out there."
Advances Expected From Bacterial Genomics
The authors point out that while sequencing the genomes of pathogenic bacteria has increased the understanding of their basic biology, it has not yet led to any advances in therapies. Their work has already pointed to conserved cell-surface proteins that appear to be viable targets for the preparation of vaccines. The work stresses "the enormous potential of bacterial genomics for discovering new therapeutic strategies to fight infectious disease," according to Nassif.
Rappuoli added, "With the technology of genomics becoming easier and easier, and faster and faster, most of the pathogens will be sequenced in a very short time. But I'm not sure there will be another example like MenB where the genome was done to solve one problem."
Chiron hopes to start testing vaccine candidates in humans by next year, according to Rappuoli.