An in vitro assay to measure antibody-mediated inhibition of malaria sporozoite infection.

A large research effort is currently underway to find an effective and affordable malaria vaccine. Tools that allow the evaluation of the host immune response are essential to vaccine development. This thesis focuses on an assay to evaluate antibody-mediated inhibition of sporozoite infection: the Inhibition of Sporozoite Invasion assay (ISI). ISI measures antibody capacity to prevent sporozoite infection in an in vitro system that reflects the actual infection context. This assay was first described in 1982 by Hollingdale et al., but the assay has been affected by many limitations, particularly time-consuming and subjective microscopy readouts. The recent development of transgenic parasites detectable by flow cytometry (due to the expression of reporter proteins such as GFP) provides a readout which highly increases the throughput of the technique. The objective of this thesis is to further develop the ISI assay and adapt it to evaluate humoral responses in candidate screening and clinical studies. Chapter 1 describes the process to optimise a protocol for a flow cytometry based ISI assay. In Chapter 2, mouse, macaque, and human serum samples (obtained after vaccination with a viral vector vaccine expressing a P. falciparum antigen) were tested using P. berghei transgenic parasites expressing P. falciparum antigens. In studies with mouse samples, the assay proved to be useful comparing the functional capacity of antibodies generated against different antigens and with different vaccination strategies, such as the adjuvant or vaccination dose. A positive increase of antibody functionality after vaccination was shown for some mouse, macaque, and human volunteer samples, and interesting correlations were found for ISI assay data and other antibody assays and clinical data.