Critical evaluation of different methods for measuring the functional activity of antibodies against malaria blood stage antigens

Antibodies are thought to be the primary immune effectors in the defense against erythrocytic stage Plasmodium falciparum. Thus, malaria vaccines directed to blood stages of infection are evaluated based on their ability to induce antibodies with anti-parasite activity. Such antibodies may have different effector functions (e.g., inhibition of invasion or inhibition of parasite growth/development) depending on the target antigen. We evaluated four methods with regards to their ability to differentiate between invasion and/or growth inhibitory activities of antibodies specific for two distinct blood stage antigens: AMA1. and MSP1(42). We conclude that antibodies induced by these vaccine candidates have different modes of action that vary not only by the antigen, but also by the strain of parasite being tested. Analysis based on parasitemia and viability was essential for defining the full range of anti-parasite activities in immune sera.