Antibodies to variable surface antigens induce antigenic variation in the intestinal parasite Giardia lamblia.

The genomes of most protozoa encode families of variant surface antigens. In some parasitic microorganisms, it has been demonstrated that mutually exclusive changes in the expression of these antigens allow parasites to evade the host's immune response. It is widely assumed that antigenic variation in protozoan parasites is accomplished by the spontaneous appearance within the population of cells expressing antigenic variants that escape antibody-mediated cytotoxicity.

Antigenic variation in the intestinal parasite Giardia lamblia.

Giardia lamblia trophozoites undergo antigenic variation, where one member of the Variant-specific Surface Protein (VSP) family is expressed on the surface of proliferating trophozoites and periodically replaced by another one. Two main questions have challenged researchers since antigenic switching was discovered in Giardia: What are the mechanisms involved? How are they influenced by other cellular processes or by the environment? Two molecular mechanisms have been proposed, both involving small non-coding RNAs. Here we postulate that (a) chromatin remodeling, triggered by environmental factors, also plays an important role in selecting the VSP that will be expressed and (b) the particular VSP structure may not only protect the parasite in the small intestine but also signal the need to exchange the existing VSP for another.