Establishing RTS,S/AS01 as a benchmark for comparison to next-generation malaria vaccines in a mouse model

Abstract

New strategies are needed to reduce the incidence of malaria, and promising approaches include vaccines targeting the circumsporozoite protein (CSP). To improve upon the malaria vaccine, RTS,S/AS01, it is essential to standardize preclinical assays to measure the potency of next-generation vaccines against this benchmark. We focus on RTS,S/AS01-induced antibody responses and functional activity in conjunction with robust statistical analyses. Transgenic Plasmodium berghei sporozoites containing full-length P. falciparum CSP (tgPb-PfCSP) allow two assessments of efficacy: quantitative reduction in liver infection following intravenous challenge, and sterile protection from mosquito bite challenge. Two or three doses of RTS,S/AS01 were given intramuscularly at 3-week intervals, with challenge 2-weeks after the last vaccination. Minimal inter- and intra-assay variability indicates the reproducibility of the methods. Importantly, the range of this model is suitable for screening more potent vaccines. Levels of induced anti-CSP antibody 2A10 equivalency were also associated with activity: 105 μg/mL (95% CI: 68.8, 141) reduced liver infection by 50%, whereas 285 μg/mL (95% CI: 166, 404) is required for 50% sterile protection from mosquito bite challenge. Additionally, the liver burden model was able to differentiate between protected and non-protected human plasma samples from a controlled human malaria infection study, supporting these models’ relevance and predictive capability. Comparison in animal models of CSP-based vaccine candidates to RTS,S/AS01 is now possible under well controlled conditions. Assessment of the quality of induced antibodies, likely a determinant of durability of protection in humans, should be possible using these methods.

How to Accelerate Early Stage of Malaria Vaccine Development by Optimizing Functional Assays

“…In some of the abovementioned CoP studies for RTS,S, when the participants were divided into protective or non-protective groups, there were significant differences in the immunological readouts between the two groups. However, substantial overlap was observed between the two groups, e.g., some people with higher responses could be in the non-protective group, while other people with lower responses could be in the protective group [ 19 , 20 , 21 , 22 , 24 , 27 ]. If we cannot determine the threshold value, then a receiver operating characteristic curve analysis (at each of different levels of immunological measurement, determine the chance to correctly classify “protective” individuals as “protective” or misclassify “protective” individuals as “non-protective”), such as was done in one of aforementioned studies [ 22 ], will be beneficial for future vaccine development.…”

“…This sounds obvious, however, only limited publications are available for assay qualification or validation in the malaria field. Based on the definitions above, there is no publication discussing assay validation, and only a few assays were qualified, such as ELISA for RTS,S [ 41 ], a mouse challenge model using transgenic P. berghei parasites expressing Pf CSP [ 27 , 42 ], GIA [ 43 ], and SMFA [ 44 ], and some additional studies have evaluated a part of assay precision (e.g., [ 45 , 46 , 47 , 48 ]). Thus, currently, it is very challenging to interpret published results by researchers from different laboratories.…”