T-cell regulation of immune responses to the Plasmodium falciparum antigen Pf155/RESA

Abstract: Plasmodium falciparum is the cause of the most severe malaria inhumans. Due to the rapid resurgence of malaria in many parts of theworld extensive efforts have been made during the last few years todefine several P.falciparum antigens for inclusion in a vaccine. Onesuch antigen, generally considered to be a candidate for inclusion in asubunit vaccine against the asexual blood stages of the parasite isPfl55/RESA. Several antibody-binding structures (B-cell epitopes) ofthe molecule have recently been identified. However, immunogens for asubunit (malaria) vaccine should also contain T-cell activating sitesderived from the parasite in order to induce good secondary responsesafter natural boosting.For this reason, we have investigated Pfl55/RESA for its T-cellactivating capacity. Here we have shown, in Pfl55/RESA seropositivedonors, that the antigen induced production of anti-Pfl55/RESAantibodies in vitro is T-cell dependent, implying that Pfl55/RESAcontains T-helper cell stimulating epitopes. We have also shown thatsynthetic peptides representing sequences from the amino-acid repeatregions of Pfl55/RESA stimulate T-cells from P.falciparum-primed donorsto proliferate, to release IFN-7 and/or to synthesize IL-4. The resultssuggest the presence of several T-cell epitopes in the conserved repeatregions of Pfl55/RESA. In individual donors, there was no correlationbetween these different activities. Rather, they were negativelyassociated, suggesting that they have exhibited by functionallydistinct T-cells perhaps similar to the TH1 and TH2 cells found inmice. The results also demonstrate the importance of examining multipleparameters of T-cell activation when estimating the proportion ofindividuals responding to any particular epitope. However, induction ofIL-4 was seen in donors who had elevated concentrations of serumantibodies to the peptide used for T-cell activation. These resultssuggest that IL-4 producing cells have a role in the induction of theformation of Pfl55/RESA specific antibodies.To measure the actual number of cells secreting IFN-7 in vitro inresponse to Pfl55/RESA peptides, a single cell assay (ELISPOT) wasdeveloped. The results show that small numbers of antigen specificIFN-7 producing cells could be detected by ELISPOT even in the absenceof detectable levels of IFN-7 secreted into the culture supernatant.Thus, the Elispot makes it possible to estimate the frequency of Tcellsproducing different lymphokines even when cell numbers arelimited. As it requires only small volumes of blood and is relativelysimple, it is a useful assay for large scale epidemiological studies ofT-cell responsiveness in malaria or other diseases.

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