Biological selection of variant-specific surface proteins in Giardia lamblia

Immune evasion is frequently cited as the main reason for antigenic variati on in pathogenic microorganisms. To better understand the role of switching of variant-specific surface proteins (VSPs) in Giardia lamblia-host intera ctions, antigenic variation during infections of mice and gerbils was exami ned, using clones that predominantly expressed unique VSPs. As expected, VS Ps were selected against during infections of immunocompetent hosts. In con trast, in immunodeficient hosts, some VSPs were selected for and others wer e selected against. These diverse patterns of selection demonstrate that th ere are host-VSP interactions that exert both positive and negative selecti ve pressures on parasites, independent of the adaptive immune response. Fur thermore, selection was dependent on both the particular VSP and the host. Thus, the large number of VSP genes in G. lamblia may allow the parasite to infect multiple different hosts, and antigenic variation could be a mechan ism to expand the parasite's host range.