Cloning and characterization of a novel Plasmodium falciparum sporozoite surface antigen, STARP

Cell invasion by the vertebrate stages of Plasmodium

Protozoans of the genus Plasmodium are the causative agents of malaria; they have a complex life cycle involving vertebrate and arthropod hosts and have three distinct invasive stages. Although the invasive stages probably invade cells using similar mechanisms, each stage has a different host cell specificity and utilizes different receptors to enter cells.

Class I HLA-restricted cytotoxic T lymphocyte responses against malaria--elucidation on the basis of HLA peptide binding motifs

In animal models, CD8+ T cells are a critical effector mechanism in the protective immunity against malaria. Conventional approaches to the development of many vaccines, including those against malaria, have however proved inadequate. In particular, an alternative approach is needed for the development of vaccines designed to induce a cellular immune response mediated by CD8+ T cells. Advances in the field of molecular immunology during the past decade have provided an insight into the presentation of peptides by MHC class I molecules and their recognition by CD8+ T cells. These studies have provided a conceptual basis for the development of efficacious parasitic and viral vaccines. By a combination of immunochemical and cellular immunologic analyses based on specific peptide binding motifs, a subunit malaria vaccine that includes CD8+ T cell epitopes restricted by the most common class I HLA alleles, including HLA-A2, can now be constructed.

Irradiated sporozoite vaccine induces HLA-B8-restricted cytotoxic T lymphocyte responses against two overlapping epitopes of the Plasmodium falciparum sporozoite surface protein 2

Vaccines designed to protect against malaria by inducing CD8+ cytotoxic T lymphocytes (CTL) in individuals of diverse HLA backgrounds must contain multiple conserved epitopes from various preerythrocytic-stage antigens. Plasmodium falciparum sporozoite surface protein 2 (PfSSP2) is considered an important antigen for inclusion in such vaccines, because CD8+ CTL against the P. yoelii SSP2 protect mice against malaria by eliminating infected hepatocytes. To develop PfSSP2 as a component of malaria vaccines, we investigated the presence of anti-PfSSP2 CTL in two HLA-B8+ volunteers immunized with irradiated P. falciparum sporozoites and characterized their CTL responses using PfSSP2-derived 15-amino acid peptides bearing the HLA-B8-binding motif. Peripheral blood mononuclear cells from both volunteers stimulated with recombinant vaccinia expressing PfSSP2 displayed antigen-specific, genetically restricted, CD8+ T cell-dependent CTL activity against autologous target cells expressing PfSSP2. Of the five HLA-B8 motif-bearing 15-mers identified in the PfSSP2 sequence, two peptides sharing a 10-amino acid overlap sensitized HLA-B8-matched target cells from both volunteers for lysis by peptide-stimulated effectors. The CTL activity was HLA-B8 restricted and dependent on CD8+ T cells. Analysis of the three shorter peptides representing HLA-B8 motif-bearing sequences within the two positive peptides for their ability to bind to HLA-B8 in vitro, and to sensitize target cells for lysis by effectors stimulated with the 15-mers, identified two overlapping HLA-B8-restricted CTL epitopes. Available data indicate that the sequence of one CTL epitope is conserved and the other is variant among P. falciparum isolates. Circulating activated CTL against the conserved epitope could be directly identified in one of the two volunteers. The identification of two HLA-B8-restricted CTL epitopes on PfSSP2 provides data critical to developing an epitope-based anti-liver stage malaria vaccine.

Identification of conserved antigenic components for a cytotoxic T lymphocyte-inducing vaccine against malaria

Several cellular and humoral mechanisms probably play a role in natural immunity to Plasmodium falciparum malaria, but the development of an effective vaccine has been impeded by uncertainty as to which antigens are targeted by protective immune responses. Experimental models of malaria have shown that cytotoxic T lymphocytes (CTL) which kill parasite-infected hepatocytes can provide complete protective immunity against certain species of Plasmodium in mice, and studies in The Gambia have provided indirect evidence that CTL play a protective role against P falciparum in humans. By using an HLA-based approach, termed reverse immunogenetics, we have previously identified peptide epitopes for CTL in liver-stage antigen-1 and the circumsporozoite protein of P falciparum. We have extended this work to identify CTL epitopes for HLA class I antigens that are found in most individuals from Caucasian and African populations. Most of these epitopes are in conserved regions of P falciparum. CTL peptide epitopes were found in a further two antigens, thrombospondin-related anonymous protein and sporozoite threonine and asparagine rich protein, indicating that a subunit vaccine designed to induce a protective CTL response may need to include parts of several parasite antigens. However, CTL levels in both children with malaria and in semi-immune adults from an endemic area were low suggesting that boosting these low levels by immunisation might provide substantial or even complete protection against infection and disease.

Conservation of the Plasmodium falciparum sporozoite surface protein gene, STARP, in field isolates and distinct species of Plasmodium

The extent of structural conservation of the Plasmodium falciparum sporozoite surface protein gene, STARP, recently characterized in the T9/96 clone, has been analyzed using the polymerase chain reaction. Results from Ivory Coast and Thai clones, field isolates originating from Brazil and Kenya and laboratory-maintained strains strongly suggest that this gene has a highly conserved structure throughout this species. This structure includes a complex repetitive central domain consisting of a mosaic region followed by tandem 45-amino acid-encoding (Rp45) and 10-amino acid-encoding (Rp10) repeat regions. Limited size variation in this domain appeared to result from highly localized duplication events in the Rp45 and Rp10 regions. No size variation was observed in the 5' and 3' coding non-repetitive regions, but minor size polymorphism was found in the single intron at the 5' end of the gene. No evidence was found of distinct families of polymorphic types, as has been observed with the blood-stage MSA-1, MSA-2 and S-antigens. The sequence of the STARP homologue in the phylogenetically close chimpanzee parasite, Plasmodium reichenowi, has also been elucidated and reveals high sequence conservation, although interesting differences were detected in the composition of the Rp10 region, known in P. falciparum to contain B- and T-cell epitopes. Finally, DNA hybridization reveals the presence in rodent malaria species of sequences containing homology to the STARP non-repetitive (though not the repetitive) regions, which would suggest that a similar, conserved gene may exist in these species.