Analysis of patterns of growth inhibition of P. falciparum in synchronised cultures induced by serum from children and adults from Madang, Papua New Guinea.

In this study we have modified a micro Plasmodium falciparum in vitro growth inhibition assay to allow dissection of growth inhibition induced by test sera into two categories: inhibition of intracellular schizont growth and inhibition of uptake of merozoites into a second cohort of erythrocytes. This was achieved using morphology-controlled synchronised cultures with incubation times restricted to cover either ring to schizont development or schizont to ring development. Sera tested were obtained from a large prospective study of healthy residents of Madang, Papua New Guinea, who were carefully documented with respect to presence of malarial parasites in the blood, spleen size, age, sex and history of fever. In the ring to schizont assay sera from all the children tested inhibited parasitic growth by at least 20%, compared to only 10 of 39 adults tested (P less than 0.0005). In the schizont to ring assay 20 of 39 adult sera tested inhibited the uptake of 14C-isoleucine into P. falciparum protein compared to 3 of the 15 children's sera tested (P less than 0.01). These results are interpreted as reflecting a switch from non-specific mechanisms of resistance with increasing age.