Mats Wahlgren

Cross-reactive antigenic determinants present on different Plasmodium falciparum blood-stage antigens.

Summary A gene encoding a previously undescribed antigen of Plasmodium falciparum has been isolated from a genomic expression library by use of a pool of human immune sera. Northern blot analysis indicated that the gene is expressed at the late stages of the intra–erythrocytic cycle. This antigen, 332, contains a series of degenerated amino acid repeats. Human antibodies affinity–purified on the 332 recombinant antigen reacted with a family of parasite proteins that are products of different genes. We identified antigens 11.1 and Pf 155–RESA as members of this family and confirmed, using a human monoclonal antibody, the presence of cross–reacting determinants. The sequences of these antigens also share some structural homologies. The significance of this family of blood–stage antigens is discussed.

Malaria vaccines: immunogen selection and epitope mapping

In recent years major efforts have been made to characterize parasite antigens thought to be suitable candidates for malaria vaccines. Many of the relevant plasmodial antigens have been found to contain extensive areas of short amino acid sequences organized in tandem repeats. These are usually strongly antigenic, forming linear epitopes seen by antibodies of the infected host. Several such epitopes have been identified and subunit vaccines are being designed in which synthetic peptides or gene constructs serve as immunogens. However, as an efficient malaria vaccine should give rise to anamnestic T-dependent antibody responses following reinfection after vaccination as well as to antibody independent cell-mediated immunity, efforts are now also being made to identify T-cell epitopes on the vaccine candidate antigens. In this paper the current Plasmodium falciparum sporozoite vaccines and the merozoite antigen Pf155/RESA, a possible candidate for a P. falciparum blood stage vaccine, serve as examples to illustrate recent advances made in this area as well as some of the problems remaining to be resolved.

Cytoadherence of knobby and knobless Plasmodium falciparum -infected erythrocytes

Cytoadherence of Plasmodium falciparum-infected erythrocytes to melanoma cells was analysed using strains or isolates of parasites expressing or not expressing knobs (K+ or K- phenotype) on the erythrocyte surface. Both K+ and K- parasites had the capacity to cytoadhere to melanoma cells. Using a panel of melanoma cell lines with different surface expression of the cytoadherence receptors CD36, thrombospondin and ICAM-1 indicated that CD36 was the major receptor for parasites of both K+ and K- phenotypes. Binding competition experiments between K+ and K- -infected erythrocytes suggested that K+ cytoadherence is of higher affinity than that of K- parasites. However, some K- cytoadherence was also found in isolates containing mixed populations of K+ and K- parasites. The interaction of the two types of infected erythrocytes with melanoma cells also differed ultrastructurally, erythrocytes of K+ phenotype showing intimate interdigitations with microvilli on the melanoma cells, while erythrocytes of K- phenotype displayed more separated interactions with fewer sites of contact and involving only a few melanoma cell microvilli. One and the same infected erythrocyte may co-express the ligand for CD36-mediated cytoadherence and the structures mediating binding of uninfected erythrocytes to form rosettes.

Monoclonal antibodies to a synthetic peptide corresponding to a repeated sequence in the Plasmodium falciparum antigen Pf155

Mouse monoclonal antibodies were prepared against a synthetic peptide (EENVEHDA) corresponding to a tandemly repeated sequence in the C-terminus of the Plasmodium falciparum antigen Pf155. One antibody (IgG1) producing hybridoma was studied in detail. The specificity of the antibody was determined by enzyme-linked immunosorbent assays using bovine serum albumin-conjugated or free peptides as solid phase antigens and various synthetic peptides for inhibition. The antibody reacted with Pf155 as detected by immunofluorescence and immunoblotting. It was also an efficient inhibitor of merozoite invasion in P. falciparum in vitro cultures indicating that it defines a biologically important epitope present on the native Pf155 molecule.

Plasmodium falciparum: An invasion inhibitory human monoclonal antibody is directed against a malarial glycolipid antigen

A Plasmodium falciparum malaria blood stage antigen was detected using a human monoclonal antibody (MAb A52A6) obtained from a clinically immune donor. Immunofluorescence analysis showed that the MAb reacted with the intracellular parasite throughout the asexual blood stage cycle as well as with gametocytes. The MAb also reacted with the surface of erythrocytes containing late stage P. falciparum parasites. The antigen seen by the MAb was species- but not strain- or isolate-specific. At rupture of the infected erythrocytes, antigenic material was deposited on the membrane of uninfected cells surrounding the parasite. At merozoite invasion MAb reactive material was present on the invaginating erythrocyte membrane, indicating an involvement of the antigen in the invasion process. This was also indicated by the high capacity of the MAb to inhibit merozoite invasion in vitro. The antigen appears to be a phosphoglycolipid, sensitive to phospholipase and present in lipid extracts of P. falciparum-infected erythrocytes.