Antigenic Variation of the VSP Genes of Giardia Lamblia

Abstract

Giardia lamblia, a protozoan parasite inhabiting the small intestine, is a common infection worldwide that frequently results in chronic diarrhea, malabsorption and upper gastrointestinal symptoms. Giardia undergoes surface antigenic variation in humans and animal model infections, a phenomenon that may account for both chronicity of infections and the relatively broad mammalian host specificity with genotypically identical organisms found in humans, cats, beavers, and other mammals. The variant-specific surface proteins (VSPs) are an unusual family of related cysteine-rich proteins, from 50 kD to over 200 kD in size, that coat the surface of the trophozoite. Only one VSP of the estimated 150 or more vsp genes is expressed on an individual at any specific time. However, the repertoires of vsp genes may differ depending on the genetic group. VSPs switch spontaneously every 6–12 generations although some Giardia also switch during encystation/excystation. All VSPs have a high cysteine content of about 12 % cysteines that are mostly present as CXXC motifs as well as a highly conserved C-terminus, a surface Zn finger motif, a GGCY motif and other common features. The biological function(s) of VSPs are uncertain but they undergo both immune and biological selection. The molecular mechanism(s) involved in antigenic variation are unknown but because there is the absence of gene movement or gene mutations and only one of 4 practically identical alleles (Giardia are tetraploid) is expressed; epigenetic processes are likely involved. Further studies of the mechanism of antigenic variation and the biological role of the VSPs promise to contribute to our understanding of G. lamblia and its pathogenesis.