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.

Giardia VSPAS7 protein attenuates Giardia intestinalis-induced host macrophage pyroptosis

“…This protein family is typically characterized by a varied N-terminal region, a conserved transmembrane region and a CRGKA intracellular tail [ 8 ]. Every 6–13 generations, each VSP on the trophozoite's surface undergoes a spontaneous conversion, which may be a self-defense mechanism used by Giardia to prevent being identified and removed by the immunological system of the host [ 9 ]. In addition, VSP1267, VSP9B10 and VSPH7 have been found to activate the host innate immune responses [ 10 ].…”