Association between human serum-induced crisis forms in cultured Plasmodium falciparum and clinical immunity to malaria in Sudan.

Clinical histories with regard to falciparum malaria were collected from adults living in holo-, hyper-, and hypoendemic areas of Sudan and matched to serum samples which were assayed for antiparasitic activity in cultures of Plasmodium falciparum. The adult population of the endemic areas could be divided into three groups based on oral histories: those who never experience falciparum malaria; those with a childhood history of malaria, who experience only mild occasional malaria as adults; and those who suffer serious recurring malaria symptoms. In vitro parasite inhibition was greatest with sera from individuals with no clinical histories of malaria, and generally, more inhibition was noted in sera from holoendemic versus hyperendemic areas. Serum from hypoendemic urban Khartoum was not inhibitory. There was no relationship between serum indirect fluorescent antibody titers and parasite inhibition, but there was strong association between clinical immunity and intraerythrocytic parasite inhibition resulting in "crisis" forms. Purified immunoglobulin G was not strongly associated with crisis forms, which were consistently associated with fractions of immune serum remaining after immunoglobulin removal. Thus, it appears that clinical immunity to malaria in Sudan is based on nonantibody serum factors, possibly associated with cell-mediated immunity. Human leukocyte alpha-interferon had no inhibitory effects on cultured P. falciparum. Some umbilical cord sera were profoundly inhibitory, producing crisis forms, whereas others were not inhibitory, suggesting that factors that induce crisis forms may play a role in protecting neonates from falciparum malaria.