Humoral immune responses in volunteers immunized with irradiated Plasmodium falciparum sporozoites.

Volunteers immunized with gamma-irradiated Plasmodium falciparum sporozoites serve as the gold standard for protective immunity against mosquito-borne malaria transmission and provide a relevant model for studying protective immune effector mechanisms. During a 7-12 month period, we immunized four volunteers via the bites of irradiated, infected mosquitoes. Following these exposures to attenuated sporozoites, all four volunteers developed antibodies to sporozoites as measured by an immunofluorescence assay and by an enzyme-linked immunosorbent assay using the circumsporozoite (CS) protein repeat-based molecule R32LR as capture antigen. Three volunteers also developed antibodies against the nonrepeating (flanking) regions of the CS protein; the level of these antibodies paralleled the serum activity to inhibit sporozoite invasion of hepatoma cells in vitro. These three volunteers were protected against malaria transmitted by the bites of five infected mosquitoes. Two of these protected volunteers received additional immunizing doses of irradiated sporozoites and were subsequently protected against challenge with a heterologous P. falciparum clone. No detectable fluctuations were observed in circulating levels of tumor necrosis factor, interferon-gamma, or interleukin-6 during the course of this study. Analysis of the humoral and cellular immune responses of these protected volunteers is expected to yield important clues to additional targets of immunity against the pre-erythrocytic stages of malaria parasites.