An ideal vaccine should elicit a long lasting immune response against the natural parasite, both at the T- and B-cell level. The immune response should occur in all individuals and be directed against determinants that do not vary in the natural parasite population. A major problem in designing synthetic peptide vaccines is that T cells generally recognize peptide antigens only in association with one or a few of the many variants of major histocompatibility complex (MHC) antigens. During the characterization of epitopes of the malaria parasite Plasmodium falciparum that are recognized by human T cells, we analysed a sequence of the circumsporozoite protein, and found that synthetic peptides corresponding to this sequence are recognized by T cells in association with many different MHC class II molecules, both in mouse and in man. This region of the circumsporozoite protein is invariant in different parasite isolates. Peptides derived from this region should be capable of inducing T-cell responses in individuals of most HLA-DR types, and may represent good candidates for inclusion in an effective anti-malaria peptide vaccine.

SUMMARYA non-polymorphic antigen associated with the rhoptry organelles of Plasmodium falciparum has been purified by immunoaffinity chromatography. The antigen, RAP-1 (rhoptry associated protein-1), which is defined by monoclonal antibodies which inhibit parasite growth in vitro, is a multi-component antigen consisting of four major proteins of 80, 65, 42 and 40 kDa and two minor proteins of 77 and 70 kDa. These proteins were electro-eluted from preparative sodium dodecyl sulphate polyacrylamide gels and protected Saimiri sciureus monkeys from a lethal blood-stage infection of P. falciparum malaria. Sera from the protected animals recognized only proteins of the RAP-1 antigen when used to probe a Western blot of total parasite protein extract, confirming that RAP-1 is responsible for eliciting the protective immune response.

There is currently a need for vaccine development to improve the immunogenicity of protective epitopes, which themselves are often poorly immunogenic. Although the immunogenicity of these epitopes can be enhanced by linking them to highly immunogenic carriers, such carriers derived from current vaccines have not proven to be generally effective. One reason may be related to epitope-specific suppression, in which prior vaccination with a protein can inhibit the antibody response to new epitopes linked to the protein. To circumvent such inhibition, a peptide from tetanus toxoid was identified that, when linked to a B cell epitope and injected into tetanus toxoid-primed recipients, retained sequences for carrier but not suppressor function. The antibody response to the B cell epitope was enhanced. This may be a general method for taking advantage of previous vaccinations in the development of new vaccines.

To circumvent problems associated with polymorphic vaccine candidates for Plasmodium falciparum malaria, we evaluated recombinant proteins representing sequences from relatively highly conserved regions of the precursor to the major merozoite surface proteins, gp190, for their ability to protect Saimiri monkeys against malaria challenge. Recombinant proteins represented amino acid residues 147 to 321 (p190-1) or 147 to 321 and 1060 to 1195 (p190-3), and their efficacy was compared with that of native gpl90 and its processed products. All antigens were derived from P. falciparum Ki, a Thai isolate, while the challenge strain was Palo Alto (from Uganda, Africa), which contains, with the exception of the N-terminal 375 amino acids, which are almost identical to the Kl sequence, essentially the MAD-20 allelic form of gpl90. By 12 days following challenge, each control monkey required drug treatment. Three monkeys injected with p190-3 required therapy, while one cleared the parasites without therapy. Two monkeys injected with p190-1 received therapy on day 14, while the remaining two cleared the parasites without therapy. Of four animals injected with native gpl90, because of health reasons unrelated to malaria, one was not challenged with parasites and one was removed from the study 8 days after challenge when its parasitemia was 1.1% (parasitemias in control animals ranged from 4.3 to 9%); the remaining two cleared the parasites after maximum parasitemias of 0.45 and 0.53%. The highest levels of antiparasite antibody were produced by animals immunized with native gpl90. There was a significant correlation between monkeys which did not require drug treatment and antiparasite antibody. These results may suggest that native gpl90 and/or its processed products can provide excellent protection against heterologous challenge and that antibody is important for protection. The challenge for vaccine development is to identify the protective sequence(s).

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