Robust, reproducible, industrialized, standard membrane feeding assay for assessing the transmission blocking activity of vaccines and drugs against Plasmodium falciparum

Sexual erythrocytic and mosquito stage VIMTs are being developed with the expectation that they will play an important role in malaria elimination campaigns, and ultimately in malaria eradication. Such vaccine development has been carried out largely in research laboratories with limited capacity for scaling up the SMFA and for repeated throughput under highly controlled conditions, and often do not or cannot establish and maintain the rigorous quality control measures required of clinical laboratories or manufacturing facilities. By contrast, Sanaria routinely manufactures PfSPZ that meet all regulatory standards [9,17-25]. To do this Sanaria has established robust P. falciparum culture conditions and gametocyte production, coordinated with mosquito rearing and artificial membrane feeding methods that reproducibly yield highly infected A. stephensi mosquitoes. In order to monitor P. falciparum infections in mosquitoes Sanaria also routinely measures oocyst infection intensity and prevalence, and these assessments are performed and analysed by a team of trained staff members. Previous studies have developed medium to high throughput assays for mosquito and sexual stages of malaria that can be assessed in vitro [26] and a version of the SMFA that, after the feeding event, utilizes a P. falciparum strain derived from NF54 that has been genetically modified to express Green Fluorescent Protein-Luciferase for semi-automated downstream processing [27]. While such assays do provide higher throughput alternatives to the downstream (i.e. oocyst detection) elements of the SMFA, they are restricted to the genetically modified strain for assessment of transmission blocking effects. Consequently, the SMFA remains the gold standard for determining the effects of drugs or vaccines on transmission of multiple strains of P. falciparum from human host to mosquito vector. By exploiting its Good Manufacturing Practice (GMP) and Good Laboratory Practice (GLP) capabilities, Sanaria has developed an industrialized SMFA capacity to screen and evaluate SSM-VIMT and TB drug candidates that are in preclinical and clinical development, and can assess hundreds of samples in a relatively short period of time. This paper analyses the data generated from blinded SMFAs on 188 serum samples, performed semi-continuously, to conduct SMFA at an industrial scale with a high degree of reproducibility.

The efficacy of sexual erythrocytic and mosquito stage VIMTs [28], drugs [29], and other targeted interventions (e.g., [30]) are calculated based on percent reductions in infection (oocyst) intensity, infection (oocyst) prevalence or both. However, efficacy estimates from an assay system operated infrequently at widely spaced time intervals are subject to uncertainty from variables that impact each individual SMFA. These variables include oocyst intensity and prevalence in the control group, the number of experiments, and the number of mosquitoes analysed per experiment [31]. Moreover, it is inappropriate to calculate TBA-I from arithmetic means, though this has been used in previous studies [32,33], because oocyst infection intensity follows a negative binomial distribution at low intensities, but is closer to a Poisson distribution at high intensities [16,34,35]. Therefore, oocyst intensity should be presented as geometric mean if the data for any of the samples in the analysis are skewed, and TBA-I should be calculated based on geometric mean intensity of infection. In the present study the geometric mean oocyst intensities were therefore used to determine TBA-I for all samples.

While TBA of a given test sample is expressed typically as the percent reduction in oocyst infection intensity compared to a negative control [7,11,36], investigators have also analysed effects of antibodies or drugs on infection prevalence rates (TBA-P), measuring the proportion of fed mosquitoes exhibiting one or more oocysts [16,26,35] or any detectable sporozoites [37]. To provide a more complete picture of the effects of different TB treatments, both TBA-I and TBA-P oocyst infection data derived from dissections of 14–28 mosquitoes per sample are reported. As has been reported in previous studies [31], TBA based on measurement of TBA-I is greater than that based on measurement of TBA-P: 81.55% TBA-I versus 12.89% TBA-P for anti-Pfs25 antiserum MRA39, and 94.63% TBA-I versus 63.52% TBA-P for anti-Pfs25 antiserum MRA38. Within any given experiment, the consistent proportional reduction after Pfs25 antibody treatment compared to the respective controls indicates that the infectivity of the cultures was correlated for each independent experiment. Since the average number of oocysts in the control was not correlated with the percent reduction (Figure 2), this means that the dilution used (1:54 final dilution) of the control antibody contained sufficient activity to be able to quantify transmission reduction at even high control infection rates. Furthermore, the narrowness of these confidence intervals supports the reproducibility of results for this assay. The relationship between effects on infection intensity and prevalence is an important consideration in evaluating transmission-blocking interventions when the ultimate goal is to reduce prevalence rates to levels that can interrupt transmission in the field, in other words reduce the basic reproductive rate, R₀, to a value less than 1 [38].

The dataset generated in this study is unique in terms of scale and consistency of method, and species and strains of parasite and vector. This consistency is reflected in the typically high reproducibility between results of duplicate feeding events (Figure 3) regardless of control infectivity and TBA-I or TBA-P. Duplicate rather than the typical triplicate assessments of a given sample were performed to allow higher throughput of the SMFA and was based on the high confidence in, and consistency of, the SMFA at Sanaria. Figure 3 now provides a powerful decision tool for future SMFAs at Sanaria. The occasional large difference between duplicate assays can be seen in the figure and will allow Sanaria to perform additional feeds on test samples where the difference between results is considered to be unacceptably high.

To conduct routine SMFAs requires the ability to continuously generate adequate numbers of highly infected mosquitoes. Sanaria has established a facility with fully trained staff executing procedures that allow continuous production of A. stephensi mosquitoes sustaining high P. falciparum oocyst infection prevalence rates and intensities. A. stephensi strain SDA500 mosquitoes and P. falciparum strain NF54 gametocytes are produced continuously and used to generate infected mosquitoes according to standard operating procedures described in a Biologics Master Files submitted to the US FDA. In 74 independently performed SMFAs the geometric mean oocyst infection intensity was 46.1 oocysts/mosquito and mean infection prevalence of 94.2% for negative controls (Table 1). Consistent infections are achieved through optimized and rigorously controlled mosquito colony maintenance procedures and gametocyte culture protocols that include quality control monitoring of stage V gametocytaemia, microgametocyte:macrogametocyte ratio and exflagellation activity.

Variable TBA outcomes can be problematic in SMFA. To address this, it is necessary to analyse a sufficient number of mosquitoes per treatment within a single SMFA to derive reproducible measurements of TBA. Examination of positive controls from 74 independent SMFAs using two different anti-Pfs25 antisera [39] demonstrated that feeding 30–35 mosquitoes and analysing a target 25 (actual range 14–28) of these mosquitoes for oocyst infection levels was sufficient to reliably measure TBA outcomes (Table 3, Figure 2A, B). Further, it is clear that even sample sizes below 20 mosquitoes can be sufficient to analyse the results under the highly controlled SMFA conditions described here. Using both prevalence and intensity in a fitted model provides the optimum use of all the available data rather than restricting analysis to a single parameter. It demonstrates the high consistency of the Sanaria’s SMFA and resulting mosquito infections, and enables pooling of subsets of data to be interrogated. Regardless of sample size, all data fitted to the robust intensity-prevalence relationship generated in Figure 4F. These findings are in contrast with those based on simulation models [33] and the Plasmodium berghei rodent parasite model [16], which suggested that using fewer than 50 mosquitoes per feed provided unrealistic TBA values. The reasons for this discrepancy are unclear, but likely reflect the greater reproducibility of SMFAs performed semi-continuously under good manufacturing (GxP)-like conditions with tightly controlled parameters.

A further important consideration to ensure that meaningful TBA assessments can be obtained from SMFA is to establish positive control antibody titres appropriate for the levels of oocyst intensity achieved in negative control assays. This was addressed in the present study by plotting TBA in positive controls against mean infection (oocyst) intensity and mean infection (oocyst) prevalence in the negative controls. At the antibody dilution used (1:54), the percent reduction in oocyst numbers (infection intensity) was similar for low and high numbers of oocysts in controls (Figure 2A, B). However, particularly for antibody, MRA38, there was an indication that as the oocyst prevalence increased in the controls, the percent reduction in oocyst numbers in mosquitoes fed the MRA38 antibody decreased (Figure 2D).

The P. falciparum SMFA is both time consuming and labour intensive. It requires the coordinated availability of Anopheles mosquitoes and infective gametocyte stage parasites. It is an expensive assay to perform, and for preclinical research and development is best carried out in a facility in which highly infected mosquitoes can be safely produced, maintained and manipulated. Nevertheless, it represents the best current assay for measuring the TB potential of candidate antibodies, drugs or other targeted interventions. The current study represents, as far as can be ascertained, the first report of an industrial scale application of the SMFA to a large number of TB experimental samples conducted semi-continuously and highly reproducibly over an extended time period.