MENINGOCOCCAL MENINGITIS

shooting at a moving target

Meningitis is a disease that attracts considerable public disquiet and steps to combat it are an important health priority. Development of a vaccine against meningococcal meningitis is hampered by the high degree of variation of the proteins at the surface of the bacterium. Research is now revealing how this variation occurs at the molecular level.

The main cause of bacterial meningitis in the UK is an organism called Neisseria meningitidis. N. meningitidis causes septicaemia, a severe blood infection that gives rise to the characteristic skin rash that occurs on patients with meningitis (Figure 1). Around 2,000 cases of N. meningitidis infection are confirmed every year in the UK, affecting mainly very young children and adolescents. It is also a severe problem in developing countries such as sub-Saharan Africa, where it causes endemic disease.

Figure 1. The characteristic skin rash of meningococcal septicaemia, caused by Neisseria meningitidis. (Courtesy of Wellcome Trust Photographic Library)

Meningococcal meningitis is characterised by an extremely rapid onset of symptoms. These include a high fever, a skin rash and severe headache. An apparently symptomless individual can progress to a serious state within 24 hours. The availability of an effective universal vaccine against all forms of N. meningitidis is therefore an important goal. The difficulty is that there are many different strains of N. meningitidis that cause meningitis and the development of a vaccine against all of them has not been possible. Different strains of Neisseria meningitidis have different PorA amino acid sequences, with the greatest degree of variation occurring within the loop region. As many as 10% of the population in the UK will be carrying the bacterium at any one time on the mucosal surfaces of the nose and throat. The majority of these carriers will not have any symptoms of the disease at all. It is this continual exposure to the human immune system that puts pressure on the bacterium to mutate its surface components in order to survive. A vaccine may give effective protection against one strain but will not prevent the bacterium from mutating to evade the body's protective antibodies.

Research scientists at UMIST, led by Dr Jeremy Derrick, are using protein crystallography to explore what effects these mutations have at the molecular level. They have studied the interactions between antibodies and the proteins that coat the surface of the bacterium. N. meningitidis is covered with an outer membrane that contains many proteins that are targets for the body's immune response. Antibodies from the blood bind to portions of these proteins and trigger a response that eventually leads to the death of the bacterium.

A protein called PorA was selected for further study because it is a major component of the outer membrane of N. meningitidis and a constituent of several vaccines that have been used in recent clinical trials. PorA is a pore-forming protein and its principal function is to make small channels in the outer membrane of the bacterium. It has several large 'loop' regions that protrude from the surface and it is these that are targets for antibody binding.

By making crystals of an antibody, termed a 'Fab' fragment, with a portion of the loop region, information can be obtained on the molecular basis for antibody binding to PorA. Data from protein crystals of a Fab fragment bound to a short stretch of loop protein from PorA were collected on SRS Station 9.5. The 3-dimensional structure of the complex shows that the loop adopts a 'U' shape, which fits into a crevice on the surface of the antibody. When differences in the loop region were examined, it was found that the bacterium disrupts antibody binding in some strains by inserting amino acid residues at the tip of the loop, which would alter or remove many of the interactions with the antibody (Figure 2).

Figure 2. Image of the antibody molecular surface, with the PorA antigen superimposed. The dark coloured groove on the surface of the antibody matches precisely the shape of the PorA antigen; hence any changes in the sequence of PorA in this region can disrupt antibody binding.

Introducing changes into portions of the PorA protein that are exposed at the surface illustrates how well how the bacterium can evade the attentions of the immune system. Furthermore, these alterations are apparently introduced without compromising the biological function of PorA, as a pore-forming protein. Designing vaccines that are able to take account of these changes is a huge challenge but the availability of more information on their effects at the molecular level is certainly a positive step towards a more rational approach to vaccine design.