Anti-VSG antibodies induce an increase inTrypanosoma evansiintracellular Ca2+concentration

Abstract

SUMMARYTrypanosoma evansiandTrypanosoma vivaxhave shown a very high immunological cross-reactivity. Anti-T. vivaxantibodies were used to monitor changes in theT. evansiintracellular Ca2+concentration ([Ca2+]i) by fluorometric ratio imaging from single parasites. A short-time exposure ofT. evansiparasites to sera fromT. vivax-infected bovines induced an increase in [Ca2+]i, which generated their complete lysis. The parasite [Ca2+]iboost was reduced but not eliminated in the absence of extracellular Ca2+or following serum decomplementation. Decomplemented anti-T. evansiVSG antibodies also produced an increase in the parasite [Ca2+]i, in the presence of extracellular Ca2+. Furthermore, this Ca2+signal was reduced following blockage with Ni2+or in the absence of extracellular Ca2+, suggesting that this response was a combination of an influx of Ca2+throughout membrane channels and a release of this ion from intracellular stores. The observed Ca2+signal was specific since (i) it was completely eliminated following pre-incubation of the anti-VSG antibodies with the purified soluble VSG, and (ii) affinity-purified anti-VSG antibodies also generated an increase in [Ca2+]iby measurements on single cells or parasite populations. We also showed that an increase of theT. evansi[Ca2+]iby the calcium A-23187 ionophore led to VSG release from the parasite surface. In addition,in vivoimmunofluorescence labelling revealed that anti-VSG antibodies induced the formation of raft patches of VSG on the parasite surface. This is the first study to identify a ligand that is coupled to calcium flux in salivarian trypanosomes.