ROLE OF HORIZONTAL GENETIC EXCHANGE IN THE ANTIGENIC VARIATION OF THE CLASS-1 OUTER-MEMBRANE PROTEIN OF NEISSERIA-MENINGITIDIS

The nucleotide sequences of the genes encoding the class 1 outer membrane protein of Neisseria meningitidis (PorA) from 15 meningococcal isolates have been examined. These strains, isolated over a number of years, represented a variety of serological types, clonal groups, and geographical locations. Analysis of the aligned nucleotide sequences showed that the known serological relationships between these proteins were not necessarily reflected throughout the nucleotide sequences of their genes. The uneven distribution of base substitutions, revealed by a comparison of the informative bases, suggested that these genes possessed a mosaic structure. This structure probably resulted from the horizontal transfer of DNA between strains and would have contributed to both the generation and the spread of novel antigenic variants of the protein. In addition, the nucleotide differences between porA genes from different strains were not consistent with the nucleotide sequence divergence of the whole chromosome, as indicated by pulsed-field gel electrophoresis (PFGE) fingerprinting techniques: some strains with divergent PFGE fingerprints shared porA genes with extensive regions of nucleotide sequence identity and, conversely, some strains with similar chromosome structures possessed porA genes with different nucleotide sequences and serological properties. This suggested that entire genes had been exchanged between strains. Given that the meningococcal class 1 OMP is a major component in novel vaccines, some of which are currently undergoing field trials, the potential of horizontal genetic exchange to generate antigenic diversity has implications for the design of such vaccines.