Comparative analyses of functional antibody-mediated inhibition with anti-circumsporozoite monoclonal antibodies against transgenic Plasmodium berghei
Malaria Journal volume 22, Article number: 335 (2023)
Acquired functional inhibitory antibodies are one of several humoral immune mechanisms used to neutralize foreign pathogens. In vitro bioassays are useful tools for quantifying antibody-mediated inhibition and evaluating anti-parasite immune antibodies. However, a gap remains in understanding of how antibody-mediated inhibition in vitro translates to inhibition in vivo. In this study, two well-characterized transgenic Plasmodium berghei parasite lines, PbmCh-luc and Pb-PfCSP(r), and murine monoclonal antibodies (mAbs) specific to P. berghei and Plasmodium falciparum circumsporozoite protein (CSP), 3D11 and 2A10, respectively, were used to evaluate antibody-mediated inhibition of parasite development in both in vitro and in vivo functional assays.
IC50 values of mAbs were determined using an established inhibition of liver-stage development assay (ILSDA). For the in vivo inhibition assay, mice were passively immunized by transfer of the mAbs and subsequently challenged with 5.0 × 103 sporozoites via tail vein injection. The infection burden in both assays was quantified by luminescence and qRT-PCR of P. berghei 18S rRNA normalized to host GAPDH.
The IC50 values quantified by relative luminescence of mAbs 3D11 and 2A10 were 0.396 µg/ml and 0.093 µg/ml, respectively, against transgenic lines in vitro. Using the highest (> 90%) inhibitory antibody concentrations in a passive transfer, an IC50 of 233.8 µg/ml and 181.5 µg/ml for mAbs 3D11 and 2A10, respectively, was observed in vivo. At 25 µg (250 µg/ml), the 2A10 antibody significantly inhibited liver burden in mice compared to control. Additionally, qRT-PCR of P. berghei 18S rRNA served as a secondary validation of liver burden quantification.
Results from both experimental models, ILSDA and in vivo challenge, demonstrated that increased concentrations of the homologous anti-CSP repeat mAbs increased parasite inhibition. However, differences in antibody IC50 values between parasite lines did not allow a direct correlation between the inhibition of sporozoite invasion in vitro by ILSDA and the inhibition of mouse liver stage burden. Further studies are needed to establish the conditions for confident predictions for the in vitro ILSDA to be a predictor of in vivo outcomes using this model system.
In 2021, malaria infections accounted for an estimated 247 million cases and 619,000 deaths, with the majority of the burden afflicting the African region . This global disease is transmitted to humans by the bite of an infected Anopheles spp. mosquito during a blood meal, releasing sporozoites into the dermis and initiating an infection . These sporozoites then migrate to blood vessels by active gliding motility [3,4,5] to reach the liver sinusoid. The sporozoites traverse through hepatic tissue before invading a hepatocyte to begin asexual schizogony [6, 7]. The rupture of hepatocytes releases merozoites which invade red blood cells initiating the blood-stage infection. Although the blood stage is associated with morbidity and mortality from malaria, the sporozoites and liver-stage (LS) forms of the pre-erythrocytic (PE) stage represent ideal therapeutic targets for disease protection [8, 9]. RTS,S/AS01 took advantage of this stage’s vulnerability, leading to the first malaria vaccine. RTS,S targets Plasmodium falciparum CSP and achieves 70.6% efficacy against severe malaria when given seasonally with chemopreventative drugs [10,11,12].
CSP is a multifunctional parasite protein essential for sporozoite development and hepatic infections [13,14,15]. The N-terminal region of CSP consists of a junctional region and conserved pentamer ‘KLKQP’ termed region 1 . CSP’s C-terminal domain comprises of a known cell-adhesive motif called type I thrombospondin repeat (TSR), termed region II, and a GPI anchor . In between the N- and C-terminal domains is a central immunodominant repeat region comprising of 4 to 8 tandem amino acid repeats [16, 17]. Earlier studies have shown that these repeats are species-specific, and antibodies targeting the repeat region immobilize sporozoites and block hepatocyte invasion [16, 18, 19]. Later studies corroborated these findings with sporozoite challenge studies in mice and humans, correlating this protection with anti-CSP sera [20,21,22,23,24]. These early challenge studies provided a rationale for CSP as a PE stage vaccine candidate .
In vitro, liver-stage functional assays are essential for the preclinical evaluation of PE-stage vaccine candidates against malaria parasites [26,27,28,29,30,31,32]. These assays initially characterize antibodies that can interrupt critical sporozoite invasive phenotypes, such as gliding, cell traversal, and liver-stage development [32,33,34,35,36,37,38]. Similarly, rodent models have been used for decades to evaluate the safety, immunogenicity, and efficacy of vaccine candidates and elucidate parasite biology [39,40,41,42,43,44]. These models are advantageous when assessing correlates of humoral protection against the PE stage in vivo . As antibody-mediated inhibition in vitro reflects the potential for inhibition in vivo ; it is important to understand the predictive power of in vitro bioassays to in vivo outcomes.
Here, the correlation of antibody-mediated inhibition between two well-characterized anti-CSP mAbs against the central repeat regions, 3D11 (Plasmodium berghei) and 2A10 (P. falciparum), was analysed in vitro and in vivo, using transgenic P. berghei sporozoites [47,48,49,50]. These antibodies were selected based on data from previous studies which demonstrated the functional inhibition of these mAbs against critical PE stage phenotypes [32, 51]. Functional inhibition was characterized by performing sporozoite live gliding motility and inhibition of liver-stage development assays (ILSDA) followed by a homologous challenge in mice. This study provides valuable insight into the use of in vitro bioassays to predict anti-CSP antibody function in vivo. However, a direct correlation between the in vitro assays and challenge model could not be established suggesting the need for further studies to precisely identify in vitro assay factors that are reliable predictors of in vivo outcomes.
4 to 6 weeks female BALB/c mice (Envigo) were maintained under pathogen-free conditions following the guidelines set by the Institutional Animal Care and Use Committee (IACUC) protocol IS000010943R approved by the University of South Florida Ethics Committee.
Transgenic Plasmodium berghei parasite lines
Two P. berghei transgenic parasite lines were used. The first, a PbANKA-PfCSP(r) PbCSP termed Pb-PfCSP(r) (2257cl2), expressing PfCSP (PF3D7_0304600) in place of endogenous PbCSP (PBANKA_0403200) and a GFP-luciferase fusion reporter (GFP::LucPbeef1α) introduced at a neutral 230p locus. A second transgenic line PbANKA-mCherryhsp70 + Luceef1α termed PbmCh-luc (1868cl1), that express mCherry and luciferase reporters under hsp70 and eef1α promoters, respectively at neutral 230p locus. Transgenic lines were obtained from the repository www.pberghei.eu labelled RMgm-4110 (Pb-PfCSP(r)) and RMgm-1320 (PbmCh-luc). Transgenic line PbmCh-luc was chosen as a pseudo-wildtype (‘WT’) instead of P. berghei-ANKA to facilitate in vitro and in vivo quantification. Both lines were previously described [52,53,54].
6 to 8 weeks female BALB/c mice (Envigo) were injected intraperitonially with 100 to 150 µl of cryopreserved parasitized blood of either Pb-PfCSP(r) or PbmCh-luc. At 0.3% to 0.5% gametocytaemia, anaesthetized mice were fed to 3- to-5-day-old female Anopheles stephensi mosquitoes. The blood-fed mosquitoes were maintained at 21 °C in 70–80% relative humidity. The infectivity status of the blood-fed mosquitoes was assessed on days 10–12 for oocyst formation in the midgut and on days 18–21 for sporozoites in the salivary glands. The mosquitoes were anaesthetized by cold treatment and surface sterilized before salivary gland dissection, as previously described . Sporozoites were prepared aseptically and quantified for experiments.
Passive immunizations and challenge
Groups of mice (n = 4) were administered intraperitoneally (IP) with 25, 12.5, or 2.5 µg of 3D11 and 2A10 in 100 µL of PBS 30 min before challenge. These concentrations were equivalent to 250, 125, and 25 µg/ml concentrations in vitro. Control mice received PBS. Following mAb injection, mice were challenged with 100 µl of 5.0 × 103 sporozoites resuspended in RPMI (Corning) via tail IV injections. Bioluminescence was measured at 44 h post infection (hpi) imaged using in vivo imaging system (IVIS) (PerkinElmer) as described below. Liver samples were collected for RNA isolation and snap-frozen in liquid nitrogen.
Visualization and quantification of luciferase activity in mice challenged with transgenic P. berghei
Liver-stage luciferase activity was visualized using an intensified-charge-coupled device (I-CCD) video camera of the IVIS. Mice were anaesthetized using isoflurane anesthesia system (XGI-8, Xenogen). Following this, mice were injected intraperitoneally (IP) with d-luciferin (100 mg/kg of body weight) (Xenolight, Perkin Elmer,) in PBS. Measurements were performed within 5 min of injection. Bioluminescence imaging was acquired on auto-exposure. Imaging data were analysed using the living image 3.0 (Xenogen) software. Quantitative luminescence reads with equal region of interest (ROI) across all mice abdomens were measured. Total flux (photons/second [p/s]) values were calculated from the ROI and used to determine relative infectivity.
Hepatoma cells (HC-04) were maintained in Minimum Essential Media (Gibco) and Ham’s F12 nutrient mix (Gibco), supplemented with 30 mM HEPES (Gibco), 10% heat-inactivated fetal bovine serum (Sigma-Aldrich), 2 mM l-Glutamine (Gibco) and 40 µg/ml Gentamicin (Sigma-Aldrich) at 37 °C (5% CO2). At ~ 90% confluency, cells were split 1:6 into a pre-coated collagen flask (Corning™) every 3–4 days. The cell line was authenticated via American Type Culture Collection Human STR Profiling Service (ATCC) and tested negative for mycoplasma contamination (Fisher Scientific).
Live gliding motility assay
The live gliding motility assay was performed as previously described with minor changes . Briefly, 384-well glass-bottom plates (Greiner Bio-One) were placed at 37 °C in a CO2 incubator. The mAbs were diluted (500, 5 and 0.5 μg/ml) in 6% BSA in RPMI-1640 medium (Corning) in 80 μl and incubated at 37 °C in a CO2 incubator. After salivary gland dissection and sporozoite preparation, 80 μl of sporozoites (150 sporozoites/µl) was added to each mAb dilution and control and 40 μl was dispensed to each well in triplicates. Plates were centrifuged at 200×g for 3 min and imaged using Cell Insight CX7 high-content imaging system (Thermo Fisher Scientific) in brightfield. Wells were imaged at 20X at the rate of one frame/second for 60 s. Acquired images were exported for analysis and sporozoites were manually quantified as gliding and non-gliding. Gliding was defined as attached movement, and percent gliding was calculated by dividing total gliding sporozoites by total sporozoites.
Inhibition of liver-stage development assay (ILSDA)
ILSDA was carried out as previously described with minor changes . HC-04 cells were seeded at a density of 8000 cells per well in a 384 well plate (Greiner Bio-One), 24 h before infection with sporozoites. After the aseptic preparation of sporozoites, 75 sporozoites/µl were incubated at room temperature with mAbs at a final concentration of 250, 125, 25, 2.5, 0.25, and 0.025 μg/ml for 20 min. 2A10 and 3D11 mAbs were an isotype control for PbmCh-luc and Pb-PfCSP, respectively. No antibody-treated sporozoites served as controls and were used for data normalization. Next, 3.0 × 103 sporozoites from each treatment were added to wells with HC-04 cells in triplicates. The plate was centrifuged at 200×g for 5 min, and sporozoites were allowed to invade HC-04 cells for 1 h at 37 °C in CO2 incubator. After one hour, the plates were washed with media to remove uninvaded sporozoites and a subsequent media change at 24 hpi. At 48 hpi, infected HC-04 wells were incubated with 1 × Xenolight (PerkinElmer) for 10 min at 37 °C. The plates were imaged on the CLARIOstarPlus (BMG LABTECH) immediately after Xenolight incubation for relative luminescence units (RLU) output.
RNA isolation, cDNA synthesis, and qRT-PCR
RNA was isolated from Pb-PfCSP and PbmCh-luc infected liver samples for qRT-PCR. Infected liver samples were homogenized in TRIzol (Fisher Scientific) on a TissueLyser II (Qiagen). RNA was coprecipitated with glycogen (Sigma-Aldrich) at − 20 °C overnight and treated with DNase (Fisher Scientific). cDNA was synthesized using SuperScript III Platinum two-step quantitative reverse transcription-PCR (qRT-PCR) kit (Invitrogen), according to manufacturer’s instructions. The LS burden was quantified by absolute copy number of P. berghei 18S rRNA (PBANKA_0521221) normalized to murine GAPDH (glyceraldehyde-3-phosphate dehydrogenase) (NM_001289726.2) copy number as previously described .
Sporozoite surface protein essential for liver stage development (SPELD, PBANKA_0910900) and liver-specific protein 2 (LISP2, (PBANKA_1003000) served as early and late LS markers, respectively (Additional file 1: Figure S1) . Primer sequences for all genes are listed in Additional file 1: Table S1 . PCR-amplified target cDNA inserted into pJET1.2 plasmid (Thermo Fisher Scientific) and standards were prepared at tenfold dilutions from 108 through 101. cDNA samples were quantified by Luna® Universal qPCR Master Mix (New England BioLabs) along with the standards and copy numbers were used to determine the gene expression.
Significance between means was determined by a two-way ANOVA followed by a Tukey’s multiple comparisons post hoc test with relevant comparisons for sporozoite gliding assay. Where indicated, a Kruskal–Wallis test with Dunn’s multiple comparisons and a two-tailed Mann–Whitney test were used for analysing non-parametric data. IC50 curves were calculated using nonlinear regression dose–response modelling with concentrations converted to log10 in ILSDA. Correlation coefficients were determined using a two-tailed Spearman’s correlation. All graphs presented are displayed as means with standard error of the mean (SEM.), and data were analysed using GraphPad Prism version 9 (GraphPad Software, San Diego, California, USA). % Inhibition was calculated by the equation below:
where the antibody-mediated inhibition being measured is represented as ‘x’.
Monoclonal antibodies against CSP block the live gliding of transgenic sporozoites
To determine the range of mAb concentrations that achieved 100% to no inhibition in vitro, functional inhibition against transgenic P. berghei sporozoites in a live gliding assay was evaluated first (Fig. 1). The antibodies 3D11 (anti-PbCSP) and 2D10 (anti-PfCSP) inhibited sporozoite gliding in a dose-dependent manner and were not cross-reactive as PbmCh-luc exposed to 2A10 and Pb-PfCSP(r) exposed to 3D11 exhibited gliding motility similar to control (Fig. 1a and b). At a concentration of 2.5 µg/ml, mAbs 3D11 and 2A10 showed 69% and 59% inhibition to PbmCh-luc and Pb-PfCSP(r) sporozoites respectively. Complete inhibition of PbmCh-luc and Pb-PfCSP(r) live sporozoite gliding was observed at a concentration of 250 µg/ml for mAbs 3D11 and 2A10, respectively (Fig. 1a and b).
Determination of the half-maximal inhibitory concentration (IC 50 ) of anti-CSP mAbs by ILSDA
Then, an ILSDA was performed to determine the IC50 of anti-CSP mAbs and assess the inhibition of LS forms. Luminescence of all concentrations tested and qRT-PCR for the highest inhibitory concentrations quantified parasite burden (Fig. 2a–e). Through relative luminescence, the IC50 values were determined to be 0.396 µg/ml and 0.093 µg/ml for 3D11 and 2A10, respectively (Fig. 2c and f). Therefore, 2A10 appeared to have approximately fourfold higher inhibition against Pb-PfCSP(r) than 3D11 against PbmCh-luc (Fig. 2c and f). High levels of inhibition (> 93%) by 3D11 at 250, 125, and 25 µg/ml were observed in PbmCh-luc LS forms. At comparable concentrations of 2A10, near complete inhibition (> 92%) was observed of Pb-PfCSP(r) LS forms (Fig. 2).
Decreased transgenic parasite liver burden in mice that received anti-CSP mAbs
To assess the functional inhibition in vivo, mice that were given the mAbs at the highest inhibitory concentrations (250, 125, and 25 µg/ml) received sporozoite challenge infections. Upon challenge with transgenic P. berghei, IVIS imaging was done at 44 h as shown (Fig. 3a and d). Total flux (p/s) and qRT-PCR quantified liver burden (Fig. 3b and e) while control mice were used to normalize data and calculate percent inhibition (Fig. 3c and f). 2A10 significantly inhibited liver-stage development at 25 µg compared to 2.5 µg (p = 0.0148) in Pb-PfCSP(r) infected mice (Fig. 3f). However, no significant reduction in liver burden was observed in mice that received 3D11 (Fig. 3c).
Correlation of functional inhibition of anti-CSP mAbs between in vitro and in vivo assays
Lastly, since a dose-dependent antibody reduction of both parasite lines were observed at equivalent concentrations in vivo, the relationship between functional inhibition in vitro to in vivo was explored further (Fig. 4). Additionally, as relative luminescence possessed a good correlation with qRT-PCR of infected mice liver samples (Additional file 1: Figure S2), the focus was narrowed to comparing inhibition by bioluminescence across both assays. Although the highest concentrations (250, 125, and 25 µg/ml) tested in vitro achieved 92–100% inhibition of liver-stage invasion, in vivo the efficacy of these concentrations ranged widely from ~ 0–60% for 3D11 and ~ 30–68% for 2A10. Hence, no significant correlation was observed between the two assays at overlapping concentrations determined by luminescence (Fig. 4a and b).
Functional assays replicate critical invasive sporozoite phenotypes in vitro [32, 34,35,36,37,38, 46, 55, 60]. These bio-assays are paramount in initially quantifying functional antibody-mediated inhibition. Additionally, rodent malaria challenge models are used to characterize humoral immune responses [39, 61]. Thus, it is necessary to elucidate how antibody-mediated inhibition in vitro translates into inhibition in vivo. To accomplish this, the study utilized two well-characterized transgenic lines (PbmCh-luc  and Pb-PfCSP(r) [52, 62]) and inhibitory anti-CSP monoclonal antibodies. The inhibition potential and concentration of mAbs 2A10 and 3D11 were validated by comparative analysis in a live sporozoite gliding assay (Fig. 1). Monoclonal antibodies were not cross-reactive as the B-cell epitopes of 3D11 and 2A10 are the immunodominant central repeat region of P. berghei and P. falciparum CSP, respectively [48, 49, 63, 64]. Hence, 2A10 mAbs bind strongly,KD of 2.7 ± 2.1 nM , to ‘NANP’ repeats in P. falciparum CSP while 3D11 mAbs have a high affinity,KD of 159 ± 47 nM , for repeats ‘PAPP’, ‘NAND’, and ‘NPND’ in P. berghei CSP [63, 64].
Next, IC50 concentrations were determined against in vitro liver-stage development using HC-04 cells. The observed disparity between IC50 values of 2A10 and 3D11, despite antibodies exhibiting comparable inhibition at the highest concentrations tested, may be related to differences in epitope binding affinity and avidity which was not explored in this study (Fig. 2). The IC50 value for 2A10 against transgenic Pb-PfCSP(r) (Fig. 2) was similar to previously published data for 2A10 against P. falciparum LS forms in primary human hepatocytes . However, inhibition varied marginally in Rodríguez-Galán et al., using 2A10 mAbs and Pb-PfCSP(r), with an observed IC50 of 0.4 µg/ml . This discrepancy was only observed for 2A10. The functional inhibition for 3D11 was shown to be effective at 1 µg/ml against both PfCSP@PbUIS4  and PbmCh-luc (Fig. 2). Similar inhibitory concentrations for 3D11 were reported using a P. berghei sporozoite cell traversal assay . Therefore, these results corroborate previously published studies where both 2A10 and 3D11 had similar functional inhibition in vitro.
Following determination of the IC50 in vitro, an in vivo challenge was conducted. The mouse study was restricted to the highest inhibitory concentrations (> 90%) as the lower concentrations assessed in vitro had less than 80% inhibitory. Previous studies required a higher dose of 2A10 to reduce liver burden compared to the in vivo data from this study [45, 63, 67]. Transgenic P. berghei liver burden was reduced by 75% to 82% in C57BL/6 mice that received 300 µg of 2A10 . However, in humanized FRG HuHep mice, 87% to 99% inhibition was achieved after a passive transfer of 500 µg of 2A10 mAbs and challenged with 2 × 106 P. falciparum sporozoites . In a mosquito-bite challenge, 51% inhibition of liver-stage burden was observed after 24 h of mice being passively given 150 µg of 2A10 . Despite a similar reduction in liver burden, reduced sporozoite speed, and blood vessel invasion in mice passively immunized with 150 µg and 25 µg of 3D11 , a significant reduction in liver burden was not observed for 3D11 at the concentrations tested.
In the current study, the difference between 3D11 inhibition of PbmCh-luc in vitro and in vivo was lower than between 2A10 inhibition of Pb-PfCSP(r) in vitro and in vivo (Fig. 4). These findings suggest the potential for alternative antibody-mediated mechanisms of inhibition in vivo versus in vitro. Previously, in a cell traversal assay, a group of antibodies including 2A10 demonstrated differential inhibition in vivo that was not observed in vitro . High affinity and avidity of mAbs determined in vitro were better predictors of protective efficacy in vivo [46, 69]. This study provides valuable insight into the predictably of in vitro bioassays to in vivo model outcomes; however, some limitations include the fact that antibody concentration after IP injections was not explored thereby limiting potential correlation to the dosage of mAbs administered. Secondly, the sample size was not enough to make definitive conclusions and lastly, mice were challenged intravenously, restricting inhibitory mAb activity to a short timeframe of sporozoite inoculum in blood. Route of sporozoite administration plays an essential role as antibodies effectively inhibit sporozoites at the site of injection [61, 68, 70, 71]. Future studies will explore alternative routes of infection including mosquito bites, intradermal or subcutaneous injections, replicating other humoral mechanisms in vitro with a larger cohort of mice, and comparing inhibition in vitro across primary cells, immortal cell lines and other rodent models [72, 73]. This comprehensive approach would give a better understanding of factors influencing the translatability of antibody-mediated inhibition in vitro to in vivo outcomes.
This study evaluated antibody-mediated inhibition of transgenic P. berghei LS forms in in vitro and in vivo assays. While 2A10 mAbs had higher inhibitory activity in vitro and in vivo than 3D11 mAbs, the discrepancy in inhibition between both assays was more pronounced for 2A10 than 3D11. Despite some similarities between antibody inhibition titration curves, no significant correlation was established in parasite burden between these assays by luminescence. While not optimal for predicting in vivo outcomes, in vitro assays remain a useful preliminary screening tool as high throughput cost-effective laboratory assays for vaccine discovery.
Availability of data and materials
The data analysed from this study are available from the corresponding author upon reasonable request. All relevant data are present in the manuscript.
National Institute of Allergy and Infectious Disease
National Institutes of Health
Bovine serum albumin
Inhibition of liver-stage development assay
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We thank the animal facility at the University of South Florida for maintaining the animals. We would also like to thank Alison Roth, Ph.D., for her continued assistance with the liver-stage assays and Justin Gibbons, Ph.D., for consultation on data analysis. We would also like to thank Chris Janse, Ph.D. for providing both transgenic lines PbmCh-luc (1868cl1) and Pb-PfCSP(r) (2257cl2). The following reagents were obtained from BEI resources, NIAID, NIH: Hybridoma 2A10 producing mouse monoclonal anti-P. falciparum Circumsporozoite (MRA-183) subclass IgG2a, contributed by Elizabeth Nardin. Hybridoma 3D11 producing mouse monoclonal anti-P. berghei CSP (MRA-100) subclass IgG1 contributed by Victor Nussenzweig. HC-04, hepatocyte (human) (MRA-975) contributed by Jetsumon Sattabongkot Prachumsri .
This study was supported by the National Institutes of Health (UO1: 5U01AI155361-02 to J.H.A.) and does not represent the official views of the National Institutes of Health.
The authors declare no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Gene expression levels of early and late liver-stage P. berghei biomarkers. Figure S2. Correlation between relative luminescence and P. berghei 18S rRNA expression in infected mice. Table S1. Primers used in qRT-PCR for LS quantification and maturity biomarkers.
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Nicholas, J., Kolli, S.K., Subramani, P.A. et al. Comparative analyses of functional antibody-mediated inhibition with anti-circumsporozoite monoclonal antibodies against transgenic Plasmodium berghei. Malar J 22, 335 (2023). https://doi.org/10.1186/s12936-023-04765-2