Mode of Action of Invasion-Inhibitory Antibodies Directed against Apical Membrane Antigen 1 of Plasmodium falciparum

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Abstract
Antibodies against apical membrane antigen 1 (AMA-1) of Plasmodium falciparum inhibit merozoite invasion into erythrocytes. Invasion-inhibitory polyclonal AMA-1 antibodies inhibit secondary proteolytic processing and surface redistribution of AMA-1 on merozoites. We present evidence supporting inhibition of processing and redistribution as probable causes of inhibition of invasion by polyclonal antibodies. Polyclonal anti-AMA-1 was much more inhibitory than monoclonal antibody (MAb) 4G2dc1 in an invasion assay. Although both polyclonal and monoclonal immunoglobulin G (IgG) inhibited secondary processing of the 66-kDa form of AMA-1, only polyclonal IgG caused its anomalous processing, inhibited its redistribution, and cross-linked soluble forms of AMA-1 on merozoites. Moreover, Fab fragments of polyclonal IgG that fail to cross-link did not show the enhancement of inhibitory effect over intact IgG, as observed in the case of Fab fragments of MAb 4G2dc1. We propose that although blocking of biologically important sites is a common direct mode of action of anti-AMA-1 antibodies, blocking of AMA-1 secondary processing and redistribution are additional indirect inhibitory mechanisms by which polyclonal IgG inhibits invasion. We also report a processing inhibition assay that uses a C-terminal AMA-1-specific MAb, 28G2dc1, to detect merozoite-bound remnants of processing (∼20 kDa from normal processing to 48 and 44 kDa and ∼10 kDa from anomalous processing to a 52-kDa soluble form of AMA-1). The ratio of intensity of 10-kDa bands to the sum of 10- and 20-kDa bands was positively correlated with inhibition of invasion by polyclonal antibodies. This assay may serve as an important immunochemical correlate for inhibition of invasion.

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