Differential effects of human serum and cells on the growth of Plasmodium falciparum adapted to serum-free in vitro culture conditions

A single Plasmodium falciparum isolate was adapted for growth in serum-free culture medium. The parasitemia increased from 0.5% to 20% on day 7 after thawing. The asexual forms of the parasites appeared morphologically normal and pigment formation was comparable with that seen under standard conditions with serum present. Parasites were coincubated in 96-well plates with serum, peripheral blood mononuclear cells (PBMC), and PBMC in the presence of autologous serum from healthy non-immune individuals (n = 12), healthy semi-immune individuals (n = 12), and malaria patients (n = 7). Growth was monitored for six days. The concentration of interleukin-6 and interferon-gamma (IFN-gamma) in supernatants from the continuous cultures were measured by a bioassay and an enzyme-amplified sensitivity immunoassay. The results of this study showed that parasites cultured in serum-free medium in the presence of PBMC develop more rapidly, particularly with cells from malaria patients, compared with parasites cultured alone. The growth of parasites was different if 10% autologous serum was added to the culture. Parasite growth with sera from acutely infected individuals was similar with that with sera from aparasitemic, non-immune individuals, and both supported significantly higher parasite growth over the six-day culture period compared with sera from the uninfected semi-immune individuals. Production of IFN-gamma by cells from nonimmune individuals and malaria patients was higher when cultures did not contain autologous serum. Nonimmune donor cells produced high amounts of IFN-gamma, but cells from the semi-immune donors produced little of this cytokine. There was no marked inhibition of parasite growth with any combination of serum and cells over six days of culture. A difference between the groups was observed after two days of culture, when growth with cells and serum from the uninfected, semi-immune group was significantly lower than that from the nonimmune group, but this was not subsequently sustained. The results of the study show that continuous cultivation of P. falciparum in serum-free medium provides a novel in vitro model to study mechanisms of the interplay between components of the human immune system and the malarial parasite, in which any possible influence of human serum is removed.