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.