Antigenic variation in Giardia lamblia and the host's immune response.

Giardia lamblia, a protozoan parasite of the small intestine of humans and other animals, undergoes surface antigenic variation. The antigens involved belong to a family of variant-specific surface proteins (VSPs), which are unique, cysteine-rich zinc finger proteins. The patterns of infection in humans and animals fail to show the expected cyclical waves of increasing and decreasing numbers of parasites expressing unique VSPs. Nevertheless, changes in VSP expression occur within the population in vivo owing to selection of VSPs by both immune and non-immune mechanisms. After inoculation of a single G. lamblia clone (able to persist in the absence of immune pressure) expressing one VSP (> or = 90%) into mice or humans, the original VSP continues to be expressed until 2 weeks post inoculation (p.i.), when many other VSPs gradually replace it. Selection by immune-mediated processes is suggested because switching occurs at the same time that humoral responses are first detected. In most mouse strains, switching also occurs at about two weeks. Almost all trophozoites are eliminated at three weeks (p.i.), but a barely detectable infection persists over months. In neonatal mice, apparent self-cure is delayed until the sixth or seventh week. Antigenic switching does not occur in adult or neonatal severe combined immunodeficiency disease (SCID) mice, but does occur in neonatal nude mice, thus implicating B-cell-mediated mechanisms in immune switching. Not all VSPs are expressed to the same degree in vivo. Some VSPs appear to be preferentially selected whereas others are eliminated on a non-immune basis. In infections in which immunity does not play a role, such as in SCID mice, and during the first week of infection in immunocompetent mice or gerbils, persisting VSPs are preferentially expressed and maintained whereas non-persisting VSPs are replaced within the first week of infection. The purpose of antigenic variation may be presentation of a wide assortment of VSPs to hosts, increasing the chance of a successful initial infection or reinfection. Immune selection of variants comes into play following biological selection.