Antigenic Variation in Giardia

Abstract

Giardia lamblia trophozoites undergo surface antigenic variation where one member of a family of related proteins, variant specific surface proteins (VSPs), is expressed but periodically replaced by another. It is the only intestinal dwelling organism to do this. Switching occurs spontaneously in culture in vitro in the absence of defined environmental triggers. The two major Giardia groups infecting humans differ in rates of VSP switching, which vary from once every 6.5 to 12–13 generations, nature of the vsp repertoires and occurrence of switching during encystation/excystation. VSPs are cysteine-rich with many CXXC motifs whose positions are indicative of the VSP subgroup. The cysteines are non-reactive most likely because of disulfide bond formation, which creates conformational epitopes. The most characteristic feature of VSPs is an absolutely conserved carboxyl terminal CRGKA tail, which is post-translationally modified by palmitoylation of the cysteine and modification of the arginine to citrulline. The function of these modifications is unclear but both affect antibody-mediated cytotoxicity. A well-conserved membrane-spanning region is immediately proximal to the tail. VSPs vary dramatically in size and a subset has tandem repeating amino acid units near the amino terminus that can differ in size, number, sequence, and antigenicity. Some are known to be immunodominant. Unique surface Zn finger and GGCY motifs of unclear function are present in a majority. VSPs are secreted into the medium by an undefined proteolytic process that cleaves the VSP close to the membrane-spanning region; the fate of the residual palmitate-containing fragment has not been studied. With the exception of closely related VSPs, they are antigenically distinct and possess individual features that impart differentiating physical chemical characteristics. Similar to the role of surface proteins of other organisms that undergo antigenic variation, VSPs are essential for the development of immunity, immune selection, and immune evasion as well as biological selection. The latter operates in humans and experimental animals in the absence of an adaptive immune system or before adaptive immune responses develop. Only certain VSPs are compatible in the intestine of specific host and organisms that express these survive and grow. Additionally, populated VSPs are also unrecognized by the host’s immune system. These constraints likely limit the number of VSPs that can be expressed in an infected individual. VSPs are most diverse at the amino terminus, the portion of the VSPs that interfaces most directly with the intestine and the immune responses of the host. Antibodies directed to the amino terminus are cytotoxic in vitro and lead to immune selection and replacement with VSPs unrecognized by the host in vivo. Experimental animals immunized to all VSPs are resistant to challenge infection. The WB isolate has a repertoire of about 270 vsp genes, only one of which is present on the surface of an organism. How switching occurs is unclear. Unlike the process in African trypanosomes, the process is epigenetic since there is no gene movement and vsps are not overly represented at the telomeres. There is good evidence that RNAi-based post-transcriptional mechanisms are involved in the control of VSP expression but how switching occurs and only one VSP is expressed remain unclear.