Inhibition of in vitro growth of Plasmodium falciparum by immune serum from monkeys.

Three owl monkeys that had been immunized against the Camp strain of Plasmodium falciparum by infection were treated with chloroquine and rechallenged with parasites. Immune serum caused a dose-dependent, time-dependent inhibition of in vitro parasite growth. Heat-inactivation eliminated nonspecific inhibition by normal monkey serum without diminishing immune inhibition. Purified IgG from immune serum inhibited parasite growth. Serum taken immediately before the second challenge did not inhibit growth in vitro at a 1:10 dilution, although the monkeys successfully resisted the in vivo challenge. However, immune sera from all three monkeys taken two to four weeks after in vivo challenges were inhibitory, but sometimes detection required 20% serum. Growth inhibition in vitro by 10% serum was a poor predictor of in vivo protective immunity. Undiluted blood containing higher antibody levels (which are boosted by challenge), combined with additional immune mechanisms, may explain the protection observed in vivo.