Genetic mechanisms and biological implications of phase variation in pathogenic neisseriae

A sophisticated strategy used by a number of procaryotic and eucaryotic microorganisms to escape host defense mechanisms is that of varying the expression of their major surface antigens.This immune escape strategy has been nicely documented for African trypanosomes (6) and Borrel- ia hermsii (2).Antigen variation in pathogenic neisseriae leads to another intriguing view of the same phenomenon: the variation of functional properties of the respective sur- face proteins.There are two classes of proteins in patho- genic neisseriae that undergo antigenic variation, (i) pilin, the major subunit of the pili, and (ii) the opacity-associated proteins (OPAs), previously P.II or class 5 proteins, a family of heat-modifiable outer membrane proteins.Both classes of proteins are substantially immunogenic and have an essen- tial role in bacterial adherence.Conceivably, variation of these proteins not only leads to immunological changes but also gives rise to alterations of their functional (i.e., adhe- sive) properties.Although little is known about the biologi- cal role of antigen variation in Neisseria species, extensive molecular genetic investigations performed during the past several years might lead us to gain better insight into this field.