Sensitive real-time PCR detection of Plasmodium falciparum parasites in whole blood by erythrocyte membrane protein 1 gene amplification

Abstract Background Malaria remains a global public health problem responsible for 445,000 deaths in 2016. While microscopy remains the mainstay of malaria diagnosis, highly sensitive molecular methods for detection of low-grade sub-microscopic infections are needed for surveillance studies and identifying asymptomatic reservoirs of malaria transmission. Methods The Plasmodium falciparum genome sequence was analysed to identify high copy number genes that improve P. falciparum parasite detection in blood by RT-PCR. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1)-specific primers were evaluated for P. falciparum detection in hospital-based microscopically positive dried blood spots and field-acquired whole blood from asymptomatic individuals from Ghana. Results PfEMP1 outperformed the Pf18S sequence for amplification-based P. falciparum detection. PfEMP1 primers exhibited sevenfold higher sensitivity compared to Pf18S primers for parasite genomic DNA. Probit analysis established a 95% detection threshold of 9.3 parasites/mL for PfEMP1 compared to 98.2 parasites/mL for Pf18S primers. The PfEMP1 primers also demonstrated superior clinical sensitivity, identifying 100% (20/20) of dried blood spot samples and 70% (69/98) of asymptomatic individuals as positive versus 55% (11/20) and 54% (53/98), respectively, for Pf18S amplification. Conclusions These results establish PfEMP1 as a novel amplification target for highly sensitive detection of both acute infections from filter paper samples and submicroscopic asymptomatic low-grade infections

A no film slot blot for the detection of developing P. falciparum oocysts in mosquitoes.

Non-microscopy-based assays for sensitive and rapid detection of Plasmodium infection in mosquitoes are needed to allow rapid and high throughput measurement of transmission intensity and malaria control program effectiveness. Here, we report on a modified enhanced chemiluminescence-based slot blot assay for detection of Plasmodium falciparum (Pf) circumsporozite protein (PfCSP) expressed on parasite oocysts developing inside the mosquito midgut. This modified assay has several novel features that include eliminating the need for exposure to autoradiography (AR) film, as well as utilizing a novel high affinity anti-CSP antibody, and optimizing assay procedures resulting in significant reduction in the time required to perform the assay. The chemiluminescent signal for the detection of PfCSP in mosquito samples was captured digitally utilizing the C-Digit blot scanner that, allowed the detection of 0.01 pg of recombinant P. falciparum CSP and as few as 0.02 P. falciparum oocysts in a little over two hours. The earlier ECL-SB detected rCSP and oocysts and took approximately 5 h to perform. Whole mosquito lysates from both high and low prevalence-infected mosquito populations were prepared and evaluated for PfCSP detection on the ECL-SB by both AR film and digital data capture and analysis. There was a 100% agreement between the AR film and the C-Digit scanner methods for PfCSP detection in randomly sampled mosquitoes. This novel "No Film" Slot Blot assay obviates the need for AR film exposure and development and significantly reduces the assay time enabling widespread use in field settings

A slot blot immunoassay for quantitative detection of Plasmodium falciparum circumsporozoite protein in mosquito midgut oocyst.

There is still a need for sensitive and reproducible immunoassays for quantitative detection of malarial antigens in preclinical and clinical phases of vaccine development and in epidemiology and surveillance studies, particularly in the vector host. Here we report the results of sensitivity and reproducibility studies for a research-grade, quantitative enhanced chemiluminescent-based slot blot assay (ECL-SB) for detection of both recombinant Plasmodium falciparum circumsporozoite protein (rPfCSP) and native PfCSP from Oocysts (Pf Oocyst) developing in the midguts of Anopheles stephensi mosquitoes. The ECL-SB detects as little as 1.25 pg of rPfCSP (linear range of quantitation 2.5-20 pg; R2 = 0.9505). We also find the earliest detectable expression of native PfCSP in Pf Oocyst by ECL-SB occurs on day 7 post feeding with infected blood meal. The ECL-SB was able to detect approximately as few as 0.5 day 8 Pf Oocysts (linear quantitation range 1-4, R2 = 0.9795) and determined that one Pf Oocyst expressed approximately 2.0 pg (0.5-3 pg) of native PfCSP, suggesting a similar range of detection for recombinant and native forms of Pf CSP. The ECL-SB is highly reproducible; the Coefficient of Variation (CV) for inter-assay variability for rPf CSP and native PfCSP were 1.74% and 1.32%, respectively. The CVs for intra-assay variability performed on three days for rPf CSP were 2.41%, 0.82% and 2% and for native Pf CSP 1.52%, 0.57%, and 1.86%, respectively. In addition, the ECL-SB was comparable to microscopy in determining the P. falciparum prevalence in mosquito populations that distinctly contained either high and low midgut Pf Oocyst burden. In whole mosquito samples, estimations of positivity for P. falciparum in the high and low burden groups were 83.3% and 23.3% by ECL-SB and 85.7% and 27.6% by microscopy. Based on its performance characteristics, ECL-SB could be valuable in vaccine development and to measure the parasite prevalence in mosquitoes and transmission-blocking interventions in endemic areas

blocking

Comparison of different blocking times. Serially diluted oocyst lysates were analyzed by slot blot utilizing either 1 hour or 0.5 hour incubation with blocking buffer. All other conditions were held constant. IODs were obtained from exposed film.<br

oocyst comp2.tif

Comparison of mAb 2A10 and C3103 for the detection of PfOocyst. Stock oocyst preparations from pooled mosquito lysates were serially diluted and processed by film blotting. Integrated optical densities were obtained from scanned film.<br

Labeling of primary antibody enhances detection of <i>Pf</i>Oocysts in the slot blot assay.

<p>We evaluated whether conjugation of primary mAb 2A10 with alkaline phosphatase (1°-AP) enhanced sensitivity for the detection of <i>Pf</i>oocyst prepared from mosquito lysates when compared to the use of AP-conjugated secondary antibody (2°-AP). Detection limits were compared for each protocol by fitting the band intensities of serially diluted oocysts to a Michaelis-Menten regression curve and establishing a cutoff intensity threshold of mean + 2 SD from unfed mosquito specimens run on the same blot. Labeled primary antibody displayed overall higher band intensities across the range of oocyst dilutions examined and achieved lower limits of detection than the typical sandwich antibody format (0.009 oocyst versus 0.02 oocyst, respectively). The removal of an additional antibody incubation step also contributed to an overall shorter assay time in the newly developed slot blot protocol.</p

Sensitivity comparison between 2A10 and C3103 mAbs.

<p>Twofold serial dilutions of r<i>Pf</i>CSP protein were transferred to a nitrocellulose membrane using the slot blot apparatus and incubated with AP-conjugated 2A10 or C3103 mAb. Chemiluminescent signal was captured using autoradiography film and band intensities were quantitated using ImageJ software. While both antibodies adhere very well to the Michaelis-Menten curve fit, the C3103 antibody developed in our laboratory exhibited approximately 2-fold enhanced detection of CSP antigen.</p