Persistence of antibodies to meningococcal IgA1 protease versus decay of antibodies to group A polysaccharide and Opc protein.

Sera were taken over a 5 year period from Gambian children vaccinated in 1983, when aged 1-4 years, with A + C meningococcal capsular polysaccharide, ELISA tests were devised to determine the concentrations of immunoglobulin A, G and M reacting with A polysaccharide and of IgG reacting with Opc protein, IgA1 protease and an internal 104 mer peptide derived from IgA1 protease. Vaccination resulted in a brief rise of antibodies to A polysaccharide followed by decline to pre-immunization levels. IgM levels were very high even before vaccination. Antibodies to Opc protein stimulated by natural exposure also declined over the 5 year period. In contrast, antibodies stimulated by natural exposure to IgA1 protease or to the internal peptide remained constant or increased (final geometric mean level of 47 micrograms IgG ml-1). We speculate that healthy carriage of Neisseria meningitidis or Haemophilus influenzae is responsible for this increase in IgG concentration.