Crossreacting determinants in the C-terminal region of trypanosome variant surface antigens

Abstract

ANTIGENIC variation seems to be the primary mechanism enabling the pathogenic African trypanosomes to evade the immune systems of their mammalian hosts (reviewed in refs 1–3). Antigenic variation is mediated through sequential expression of an extensive repertoire of variant surface glycoproteins (VSGs). Several VSGs have been purified from cloned variants of Trypanosoma brucei^4,5. The diversity in amino acid composition, conformational features⁶ and N-terminal amino acid sequences⁷ implies extensive structural diversity in VSGs so far studied. Earlier studies failed to identify immunological crossreactivity between VSGs when purified or whilst on the surface of living trypanosomes. Some time ago, Dr Frans van Knapen and I, while performing enzyme-linked immunoabsorbant assays, were puzzled to observe extensive crossreactions between two purified VSGs and a wide range of antisera to different trypanosome species (unpublished experiments). A recent study by Barbet and McGuire⁸ provided additional evidence for the presence of strongly crossreacting determinants on a wide range of VSGs. In several cases, the VSGs examined by Barbet and McGuire originated from clones which I previously prepared and examined. I have therefore re-examined several VSGs and their corresponding antisera by radioimmunoassay. The results reported here are consistent with those of Barbet and McGuire. Additional observations suggest that the crossreacting determinants are located towards the C-terminal end of the VSG.