Sporozoite motility as a quantitative readout for anti-CSP antibody inhibition

Abstract

Antibodies can prevent malaria by neutralizing the infectious Plasmodium falciparum sporozoites (SPZ) before they establish an infection in the liver. Circumsporozoite protein (CSP), the most abundant surface protein of SPZ is the leading candidate for passive (and subunit) immunization approaches against malaria. Comprehensive assessment of the parasite-inhibitory capacity of anti-CSP monoclonal antibodies (mAbs) is an important step in advancing CSP-based immunization strategies. In this study, we employed a quantitative imaging-based motility assay to quantify the effect of anti-CSP mAbs on SPZ motility, both in vitro and in human skin.Our assay provided a quantitative measure of mAb parasite-inhibitory capacity through measurement of the half-maximal motility inhibitory concentration (IC50M) value for anti-CSP mAbs (IC50M 2A10: 24 nM, IC50M 3SP2: 71 nM). We found a sevenfold discrepancy between the IC50M and the binding saturation concentration measured by ELISA, possibly related to the observed shedding of CSP-mAb complexes during SPZ movement. In a subset of SPZ (5%), in vitro motility was unaffected by the presence of 2A10 while 3SP2 was able to completely block movement. In our ex vivo skin explant model, SPZ proved less susceptible to anti-CSP mAbs compared to SPZ in an in vitro environment. By quantitatively assessing motility, we created a valuable tool that can be used for comprehensive assessment of anti-CSP mAb potency. Insight that will help deepen our understanding of anti-CSP mAb potency and guide selection of the most promising anti-CSP mAbs for downstream clinical development.

Molecular determinants of cross-reactivity and potency by VH3-33 antibodies against the Plasmodium falciparum circumsporozoite protein

“…Anti-PfCSP antibodies have been shown to neutralize sporozoites through various mechanisms along the parasite’s journey through the skin, blood, and liver. 18 , 20 , 49 , 50 , 51 , 52 , 53 Nonetheless, as antibody mode of action was not examined here, the impact of cross-reactivity and binding conformation on different mechanisms remains to be explored; likely through the use of sporozoite imaging and additional in vivo challenge models that more accurately reflect different aspects of sporozoite infection (e.g., inhibition of liver stage development assays using Pf sporozoites). 18 , 20 , 53 , 54 Similarly, it remains unclear whether an antibody’s recognition mode influences its persistence in sera.…”

“… 18 , 20 , 49 , 50 , 51 , 52 , 53 Nonetheless, as antibody mode of action was not examined here, the impact of cross-reactivity and binding conformation on different mechanisms remains to be explored; likely through the use of sporozoite imaging and additional in vivo challenge models that more accurately reflect different aspects of sporozoite infection (e.g., inhibition of liver stage development assays using Pf sporozoites). 18 , 20 , 53 , 54 Similarly, it remains unclear whether an antibody’s recognition mode influences its persistence in sera. Indeed, we noted that the titers of two passively transferred MGG4-like mAbs, Ky15.1 and Ky311, were markedly reduced in animal sera prior to in vivo PbPfCSP challenge.…”