Antigenic variation of the class I outer membrane protein in hyperendemic Neisseria meningitidis strains in the netherlands.

Since 1980, the number of cases of meningococcal disease caused by serogroup B isolates with the P1.4 serosubtype has greatly increased in The Netherlands. Screening for this serosubtype in the strain collection of The Netherlands Reference Laboratory for Bacterial Meningitis revealed that a low number of P1.4 strains had been present in the Dutch meningococcal population since 1965. Genotyping of P1.4 strains showed that one cluster of strains, the hyperendemic lineage III (D. A. Caugant et al., J. Infect. Dis. 162:867-874, 1990), is responsible for the increase since 1980. The diversity of the porA genes, which encode the P1 protein on which serosubtyping is based, was studied for genotypically different P1.4 strains and for lineage III strains expressing antigenically different P1 proteins. Sequence analysis showed that porA genes of genotypically distinct strains that express antigenically indistinguishable P1 proteins are identical only in the epitope-encoding region, suggesting that this region has spread through the meningococcal population via horizontal gene transfer. Analysis of porA genes of lineage III strains showed that both horizontal gene transfer and partial deletion of the epitope-encoding region may contribute to the different antigenic properties for P1 of these strains. Phase variation of expression of the porA gene seems to account for most nonreacting strains. These results show that serosubtyping may underestimate the rise of a hyperendemic clone.