may increase 10- or 20-fold during the next cycle of asexual development, and parasitemias of about 1 percent may be associated with severe disease and some risk of death, despite appropriate treatment. Vaccines that control infection only after higher levels of parasitemia are reached would not be recognized in such a trial, even though such vaccines might be beneficial in partially immune individuals who were tolerant to modest levels of parasitemia. Candidate vaccine antigens may therefore need to be tested twice before efficacy is known: once in nonimmune volunteers and a second time in individuals living in an endemic area. This adds a further layer of logistical complexity to the vaccine evaluation process.
Antigen Complexity Malaria infection stimulates immune responses to a vast array of asexual blood-stage antigens. It is difficult to predict a priori whether a particular antigen will stimulate a protective immune response. Many of these antigens show considerable sequence variability, and some appear to undergo antigenic variation. In addition, many are too large to be expressed efficiently in recombinant vector systems, and because of their size they are difficult to analyze with use of synthetic peptides. A detailed look at one blood-stage antigen will illustrate many of the problems that are faced.
MSA-1 is a glycoprotein (Holder, 1988) anchored by myristic acid to the surface of the schizont (Haldar et al., 1985). When the schizont ruptures, the antigen is processed into a number of discrete products residing on the surface of the merozoite, most of which disappear at the time the merozoite invades red blood cells. Only the carboxy-terminus of MSA-1 carries over into the ring-form trophozoite stage of the parasite (see Figure 9-1). MSA-1 from different P. falciparum isolates varies in size from 180 to 220 kilodaltons, and homologues are present in all Plasmodia species studied. The protein is encoded by a single exon on parasite chromosome 9 (Holder et al., 1985; Mackay et al., 1985; Tanabe et al., 1987; Peterson et al., 1988a).
Monoclonal antibody-binding studies have demonstrated that MSA-1 from different strains contains both conserved and variable regions (McBride et al., 1985). Gene-sequencing studies have confirmed this, and results from several groups have allowed the gene coding region to be subdivided into conserved, semivariable, and variable blocks (Tanabe et al., 1987; Peterson et al., 1988a,b). The variable blocks are generally one of two types (alleles), raising the possibility that a limited number of antigens could be used to protect against all variants of MSA-1. In addition to the two alleles, there are isolate-specific mutations that might allow the parasite to evade allele-specific immunity.