In order to assess the protective effects of anti-Pf155/RESA
antibodies of different specificities in vivo, passive immunizations of Aotus monkeys were performed.
Antibodies reactive with the Pf155/RESA repeat sequences (EENV)2 and EENVEHDA were isolated from the
immunoglobulin G (
IgG) fraction of a pool of plasmas from Liberia by affinity chromatography on synthetic
peptides. The two fractions of
antibodies differed in specificity but displayed similar capacities to inhibit merozoite invasion in Plasmodium falciparum in vitro cultures. Four groups of monkeys (named groups I to IV) were injected with (i) 160 mg of total control
IgG, (ii) 2 mg of
IgG affinity purified on (EENV)2, (iii) 2 mg of
IgG affinity purified on EENVEHDA, and (iv) 160 mg of total immune
IgG, respectively. The monkeys were then challenged with P. falciparum-infected erythrocytes, and the levels of
parasitemia and hematocrits as well as other serological parameters were determined daily. Although all groups developed
parasitemia, groups II and IV tended to show lower mean daily levels. Three monkeys of group II and two monkeys (each) of groups III and IV self cured the
infections, but so did one monkey from the group treated with control
IgG (group I). The serum levels of transfused
antibodies were low at the peak of
parasitemia, suggesting that clearance of parasites was mediated by immune responses mounted by the monkeys. The results indicate that
antibodies to
epitopes formed by repeats of Pf155/RESA may depress P. falciparum
parasitemias and thus that immunogens based on such repeats should be suitable components in a
subunit vaccine against asexual stages of P. falciparum.