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

Abstract

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. Here we show, both in vitro and in animal infections, that antibodies to Variant-specific Surface Proteins (VSPs) of the intestinal parasite Giardia lamblia are not cytotoxic, inducing instead VSP clustering into liquid-ordered phase membrane microdomains that trigger a massive release of microvesicles carrying the original VSP and switch in expression to different VSPs by a calcium-dependent mechanism. This novel mechanism of surface antigen clearance throughout its release into microvesicles coupled to the stochastic induction of new phenotypic variants not only changes current paradigms of antigenic switching but also provides a new framework for understanding the course of protozoan infections as a host/parasite adaptive process.

Antigenic variation in protozoan parasites is considered a spontaneous process and antibodies to these variant antigens were cytotoxic. The clearance of the former antigen during antigenic switching is thought to occur by dilution of the original antigen. In this work, the authors provide in vitro and in vivo evidence, that low concentrations of antibodies against Variant-specific Surface Proteins (VSPs) of the intestinal parasite Giardia lamblia are not cytotoxic but induce a robust stimulation of the switching process and the release of the former antigen into extracellular microvesicles. This process is mediated by antibody-induced clustering of VSPs into L_o-phase membrane microdomains through the raftophilic capability of the highly conserved transmembrane domain of the VSPs.