Quantitative Plasmodium sporozoite neutralization assay (TSNA).

The circumsporozoite (CS) protein is the major surface protein of Plasmodium sporozoites. Antibodies to the immunodominant repeat domain of CS immobilize sporozoites and prevent infection of hepatocytes. Plasmodium falciparum vaccines containing CS repeats are undergoing human trials in endemic areas, and proof of efficacy has been obtained. The correlates of protection are under investigation. Levels of anti-repeat antibodies in the serum of the human volunteers have been measured mostly by enzyme-linked immunosorbent assay (ELISA) and IFA. Assays that measure the effect of the serum antibodies on parasite infectivity (serum neutralization assays SNAs) are not usually performed because they require a susceptible host and P. falciparum sporozoites are highly infectious only to humans. To overcome this limitation, we developed a new assay named transgenic sporozoite neutralization assay (TSNA) that uses as neutralization target, a transgenic rodent malaria parasite Plasmodium berghei that bears the P. falciparum CS repeats [CS(Pf)]. Following incubation with human serum, CS(Pf) infectivity of HepG2 cells is evaluated by real-time PCR. We have compared ELISA titers and TSNAs in a limited number of sera from humans immunized with (T1B)4 MAP, a peptide vaccine containing P. falciparum CS repeats. A comparison between the two assays did not reach significance (p=0.175) when analyzed by non-parametric Spearman correlation method. Ongoing human trials of CS-based vaccines should provide an opportunity to determine whether TSNAs will provide better correlates of protective immunity than ELISA assays.