ENFERMEDADES INFECCIOSAS EMERGENTES

What Is Antigenic Variation? Immunodominant antigens are commonly used to distinguish strains of a species of pathogens. These antigens can vary from strain to strain to the extent that the strain-specific immune responses of vertebrate reservoirs determine the population structure of the pathogen. One such strain-defining antigen is the OspC outer membrane protein of the Lyme disease spirochete Borrelia burgdorferi in the northeastern United States (1). Different strains express different OspC surface proteins in the rodent reservoirs of B. burgdorferi. The single type of ospC gene in a cell does not vary during infections of immunocompetent mammals (2). OspC sequences are diverse, and the immune responses to them appear to provide for balancing selection. This diversity between strains in an immunodominant antigen is often called antigenic variation. True antigenic variation, however, arises in a single clone or genotype in a single host and “involves the loss, gain, or change in a particular antigenic group, usually by loss, gain, or change in one of the polypeptide or polysaccharide antigens...” (3). In most cases, this change is reversible, i.e., the information for producing the original antigen is archived in the cell and can be used in the future. The adaptive immune system of an infected vertebrate selects against the original infecting serotype, but that specific response is ineffective against new variants. One example of antigenic variation occurs in B. hermsii, a cause of tickborne relapsing fever (4), which has a protein homologous to the OspC protein of B. burgdorferi. However, instead of a single version of this gene, each cell of B. hermsii has several copies of silent genes (alleles) that may be expressed during infection. The sequences of these alleles within a single strain of B. hermsii vary as widely as the ospC alleles of different strains of B. burgdorferi. We review infectious pathogens that undergo clonal antigenic variation and, like B. hermsii, depend on arthropod vectors for transmission. These pathogens are not free-living and do not form spores or have equivalent means for survival outside an animal. Vertical transmission in the arthropod or the vertebrate either does not occur or is too rare to maintain the pathogen in nature. Without access to another vertebrate host through an arthropod, the pathogen will die with the host. We restrict this review to situations in which an immune response against an antigen is synonymous with selection for another allele in the population. Many pathogens have repetitivegene families. A multimember family may Antigenic Variation in Vector-Borne Pathogens