Antigenic Variation in Vector-Borne Pathogens
Members of the genus Babesia cause one of the most common parasitic infections worldwide in wild and domestic animals. Some of the species, such as B. microti, have been transmitted to humans. Like Plasmodium, Babesia are intraerythrocytic parasites, but they are transmitted by ticks, not mosquitoes. While several multigene families have been described for various species of Babesia, clonal antigenic variation of B. bovis, a parasite of cattle, is best documented. The variant erythrocyte surface antigen (VESA1) of B. bovis is a heterodimeric protein expressed on the surface of infected red blood cells. The rapid variation of these polymorphic proteins likely contributes to chronic infection in cattle by prolonging the parasite's survival through immune evasion and sequestration of the infected red blood cells in peripheral organs. The VESA1 proteins, which have an approximate molecular weight of 128 kDa, are expressed on the external tips of the membrane knobs of infected erythrocytes. Their cytoadhesive phenotype depends on the antigenic and structural changes of the VESA1 proteins. The gene encoding the VESA1a subunit has been recently shown to belong to the ves multigene family. The predicted protein does not seem to have cleavable signal sequence, but it does have a predicted transmembrane segment and a cysteine/lysine-rich domain. The molecular events that determine the switching mechanism in B. babesia are unknown.