The human immune response to Plasmodium falciparum includes both antibodies that inhibit merozoite surface protein 1 secondary processing and blocking antibodies

Abstract

Malaria merozoite surface protein 1 (MSP1) is cleaved in an essential step during erythrocyte invasion. The responses of children to natural malaria infection included antibodies that inhibit this cleavage and others that block the binding of these inhibitory antibodies. There was no correlation between the titer of the antibody to the 19-kDa fragment of MSP1 and its inhibitory activity. These findings have implications for the design of MSP1-based vaccines.

FIG. 1.

Prevalence of malaria parasitemia among 343 children, 10 days to 15 years old, during the dry season (January to March 1999) and among 365 children with the same age distribution at the end of the rainy season (October to November 1999), at Idere and Igbo-Ora, rural towns in southwestern Nigeria. The prevalence was calculated as the percentage of malarial parasite-positive individuals. The actual number of parasitemic cases is shown at the top of each bar. Open bars, dry season; filled bars, wet season.

FIG. 2.

Some plasma samples contain MSP1 secondary-processing-inhibitory antibodies. Fifty samples (dry season set) chosen at random were analyzed for their ability to inhibit the processing of MSP1₄₂ in P. falciparum FCB-1 merozoite preparations. The samples comprised merozoites resuspended in (i) reaction buffer alone (positive control) (no Ab), (ii) reaction buffer followed by immediate solubilization in detergent (zero hour), (iii) reaction buffer after pretreatment with a 10% (vol/vol) plasma sample as indicated (remaining samples shown, individually labeled), (iv) reaction buffer after pretreatment with MAb 12.8 (data not shown), and (v) reaction buffer containing 1 mM phenylmethylsulfonyl fluoride (negative control, data not shown). After 1 h, the samples were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and blotted onto nitrocellulose, and MSP1₄₂ and MSP1₃₃ were identified and quantified. The results (vertical bars, percent MSP1 processing) are shown for 20 plasma samples. The remainder of the 50 samples analyzed showed either no effect or enhanced MSP1 processing. The MSP1₁₉-specific antibody titers of the individual samples are also shown as log reciprocal antibody titers (⧫).

FIG. 3.

Human plasma samples compete with processing-inhibitory MAbs for binding to MSP1₁₉. A competitive ELISA was used to determine whether or not antibodies developed during natural infection with P. falciparum could competitively block the binding of MAbs 12.8 and 12.10. The results presented are for the same plasma samples as those described in the legend to Fig. 2 and arranged in the same order: (i) labeled antibody bound in the absence of a competitor (no Ab), (ii) labeled antibody bound in the presence of a competitor MAb (12.8), and (iii) labeled antibody bound in the presence of human plasma samples as indicated (remaining samples shown, individually labeled). The binding of MAb 12.8 to antigen is expressed as a percentage of the amount bound in the absence of a competitor; similar results were obtained for biotinylated MAb 12.10 (data not shown).