Antigenic Variation in African Trypanosomes

Abstract

African trypanosomes, which are extracellular parasitic hemoflagellates, are responsible for several diseases in humans and domestic animals. The most widely known of these diseases is human sleeping sickness which is found in both East and West Africa. Like most mammalian parasites, the African trypanosomes have evolved a mechanism for escaping the host immune system, thus allowing them to persist in the immune competent mammal. For the African trypanosomes this is done by a mechanism known as antigenic variation, which consists of periodically changing the predominant and perhaps unique antigen presented on the surface of the parasite. These variable antigens are composed of immunologically distinct glycoproteins and are called variable surface glycoproteins (VSG’s). Although the total repertoire of different VSG’s which a given trypanosome species is capable of expressing is not known, over 100 have been characterized in a single clone of T. equiperdum (Capbern et al. 1977). Individual trypanosomes express only one VSG at a time, and the frequency of switching has been estimated to be 10⁻⁴–10⁻⁶ per cell per generation (Doyle 1977).