Vaccination of monkeys with recombinant Plasmodium falciparum apical membrane antigen 1 confers protection against blood-stage malaria

Abstract

A major challenge facing malaria vaccine development programs is identifying efficacious combinations of antigens. To date, merozoite surface protein 1 (MSP1) is regarded as the leading asexual vaccine candidate. Apical membrane antigen 1 (AMA1) has been identified as another leading candidate for an asexual malaria vaccine, but without any direct in vivo evidence that a recombinant form of Plasmodium falciparum AMA1 would have efficacy. We evaluated the efficacy of a form of P. falciparum AMA1, produced in Pichia pastoris, by vaccinating Aotus vociferans monkeys and then challenging them with P. falciparum parasites. Significant protection from this otherwise lethal challenge with P. falciparum was observed. Five of six animals had delayed patency; two of these remained subpatent for the course of the infection, and two controlled parasite growth at <0.75% of red blood cells parasitized. The protection induced by AMA1 was superior to that obtained with a form of MSP1 used in the same trial. The protection induced by a combination vaccine of AMA1 and MSP1 was not superior to the protection obtained with AMA1 alone, although the immunity generated appeared to operate against both vaccine components.

FIG. 1.

Daily parasitemias in A. vociferans monkeys after challenge. Monkeys were vaccinated three times with 100 μg of Pvs25H (solid lines) or nothing (dotted lines) (A), 100 μg of AMA1 (B), 100 μg of MSP1₄₂ (C), or 100 μg of AMA1 plus 100 μg of bvMSP1₄₂ (D). Two weeks after the final vaccination, they were challenged with 5 × 10⁴ P. falciparum FVO parasites, freshly passaged from a donor monkey. Thin blood films were taken daily and stained with Giemsa stain. Parasitemia was determined as the percentage of infected RBCs in 50 (for <0.1% parasitemia) or 10 (for >0.1% parasitemia) high-power magnification fields. Monkeys were treated either at day 28, at >5.0% parasitemia, or when their hematocrit (H) fell below 25%. +, monkey T1115 died of heart problems at a parasitemia of 4.6%. Due to the late appearance of parasites, monkeys T1112 and T964 were monitored until days 36 and 33, respectively.

FIG. 2.

Reciprocal antibody titers in vaccinated monkeys. (A) Antibody titers to AMA1 in monkeys receiving AMA1either alone or in combination with MSP1₄₂. (B) Antibody titers to MSP1₄₂ in monkeys receiving MSP1₄₂ either alone or in combination with AMA1. Shown are individual titers (•), geometric mean titers (▾), and standard deviations (error bars). ELISA titers are expressed in arbitrary antibody units, calculated by reference to a standard Aotus serum against the plate antigen. A serum dilution of the reciprocal of the antibody units reported would give an approximate absorbance at 405 nm of 1.0.