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J. Exp. Med.,
Volume 186, Number 10, November 17, 1997 1689-1699
By

From the * Division of Parasitology, National Institute for Medical Research, London NW7 1AA,
United Kingdom; and Merozoite surface protein-1 (MSP-1) of the human malaria parasite Plasmodium falciparum undergoes at least two endoproteolytic cleavage events during merozoite maturation and release,
and erythrocyte invasion. We have previously demonstrated that mAbs which inhibit erythrocyte invasion and are specific for epitopes within a membrane-proximal, COOH-terminal domain of MSP-1 (MSP-119) prevent the critical secondary processing step which occurs on the
surface of the extracellular merozoite at around the time of erythrocyte invasion. Certain other
anti-MSP-119 mAbs, which themselves inhibit neither erythrocyte invasion nor MSP-1 secondary processing, block the processing-inhibitory activity of the first group of antibodies and
are termed blocking antibodies. We have now directly quantitated antibody-mediated inhibition of MSP-1 secondary processing and invasion, and the effects on this of blocking antibodies. We show that blocking antibodies function by competing with the binding of processing-inhibitory antibodies to their epitopes on the merozoite. Polyclonal rabbit antibodies specific
for certain MSP-1 sequences outside of MSP-119 also act as blocking antibodies. Most significantly, affinity-purified, naturally acquired human antibodies specific for epitopes within the
NH2-terminal 83-kD domain of MSP-1 very effectively block the processing-inhibitory activity of the anti-MSP-119 mAb 12.8. The presence of these blocking antibodies also completely
abrogates the inhibitory effect of mAb 12.8 on erythrocyte invasion by the parasite in vitro.
Blocking antibodies therefore (a) are part of the human response to malarial infection; (b) can
be induced by MSP-1 structures unrelated to the MSP-119 target of processing-inhibitory antibodies; and (c) have the potential to abolish protection mediated by anti-MSP-119 antibodies.
Our results suggest that an effective MSP-119-based falciparum malaria vaccine should aim to
induce an antibody response that prevents MSP-1 processing on the merozoite surface.
Institute of Cell, Animal and Population Biology, University of Edinburgh,
Ashworth Laboratories, Edinburgh EH9 3JT, United Kingdom
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