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Matsubara J, Chang M, Seilie AM, Murphy SC. Flow Cytometric Sorting of Infected Erythrocytes Demonstrates Reliable Detection of Individual Ring-Stage Plasmodium falciparum Parasites by Plasmodium 18S rRNA Reverse Transcription Polymerase Chain Reaction. Am J Trop Med Hyg 2022; 106:tpmd211226. [PMID: 35405648 PMCID: PMC9209910 DOI: 10.4269/ajtmh.21-1226] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2021] [Accepted: 01/17/2022] [Indexed: 11/12/2022] Open
Abstract
Molecular diagnostic tests for Plasmodium falciparum parasites are increasingly used to enable ultrasensitive detection of infection in clinical trials and field surveillance studies. Ribonucleic acid (RNA)-based assays targeting 18S rRNA are particularly sensitive with limits of detection reported to comprise a single infected red blood cell (RBC) in a relatively large volume of blood. However, the validation testing at such limiting concentrations is hampered by the so-called Poisson distribution of such rare events, which can lead laboratorians to inaccurately set the limit of detection higher (i.e., less sensitive) than the assay can actually detect. Here we set out to formally demonstrate the analytical sensitivity of the Plasmodium 18S rRNA quantitative reverse transcription PCR (qRT-PCR). Fluorescence-activated cell sorting (FACS) was used on synchronous P. falciparum cultures doubly stained for DNA and RNA and was followed by qRT-PCR on the individual sorted cells spiked with negative whole blood. Over 95% of individual single-ring infected RBCs were detected by qRT-PCR. The formally measured median 18S rRNA content per individual ring-stage P. falciparum parasite was 9,550 copies (interquartile range 8,130-12,300). Thus, one can confidently rely on Plasmodium 18S rRNA qRT-PCR to detect one parasite per 50-µL blood sample.
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Affiliation(s)
- Jokichi Matsubara
- Department of Laboratory Medicine and Pathology, University of Washington, Seattle, Washington
- Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, Washington
| | - Ming Chang
- Department of Laboratory Medicine and Pathology, University of Washington, Seattle, Washington
- Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, Washington
| | - Annette M Seilie
- Department of Laboratory Medicine and Pathology, University of Washington, Seattle, Washington
- Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, Washington
| | - Sean C Murphy
- Department of Laboratory Medicine and Pathology, University of Washington, Seattle, Washington
- Center for Emerging and Re-emerging Infectious Diseases, University of Washington, Seattle, Washington
- Department of Microbiology, University of Washington, Seattle, Washington
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Shears MJ, Seilie AM, Kim Lee Sim B, Hoffman SL, Murphy SC. Quantification of Plasmodium knowlesi versus Plasmodium falciparum in the rhesus liver: implications for malaria vaccine studies in rhesus models. Malar J 2020; 19:313. [PMID: 32867784 PMCID: PMC7457220 DOI: 10.1186/s12936-020-03385-4] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2020] [Accepted: 07/13/2020] [Indexed: 02/07/2023] Open
Abstract
Background Rhesus macaques are valuable pre-clinical models for malaria vaccine development. The Plasmodium knowlesi/rhesus and Plasmodium falciparum/rhesus models are two established platforms for malaria vaccine testing, and both have previously been used to assess live-attenuated sporozoite vaccines. However, there is evidence that the susceptibility of the rhesus liver to P. knowlesi versus P. falciparum sporozoites likely differs, potentially complicating comparisons between these two platforms. Methods To quantify the differing susceptibility of rhesus to P. knowlesi and P. falciparum sporozoites, animals were infected by direct venous inoculation of purified, cryopreserved wild-type P. knowlesi sporozoites (PkSPZ) or P. falciparum sporozoites (PfSPZ). The entire liver was collected 5 days post-infection, and parasite burden in each liver lobe was quantified using an ultrasensitive Plasmodium 18S rRNA RT-PCR biomarker assay. The potential of using 18S rRNA copy number in the rhesus liver to directly measure the efficacy of vaccines targeting P. falciparum sporozoites and liver stages was also theoretically evaluated. Results Infection of rhesus with a high dose of PkSPZ led to consistently high burden liver stage infections (range 9.5–10.1 log10 copies 18S rRNA/g of liver), with similar amounts of parasite 18S rRNA detected in every liver lobe. Inoculation of rhesus with high doses of PfSPZ led to more variable, lower liver burdens (range 4.9–6.6 log10 copies 18S rRNA/g of liver in infected lobes), with parasite 18S rRNA below the limit of detection in some liver lobes. The low signal and heterogeneity of liver burden in the PfSPZ-infected animals indicates that even this extremely sensitive molecular assay cannot be used to assess reliably vaccine efficacy in the P. falciparum/rhesus platform. Conclusions Detection of 18S rRNA in the liver following high dose intravenous PfSPZ confirmed that rhesus are modestly susceptible to wild-type P. falciparum sporozoites. However, comparison of 18S rRNA RT-PCR biomarker signal indicates that the P. falciparum liver burden was 3–5 logs lower than in PkSPZ-infected animals. Quantification of this difference in liver stage burden will help guide and interpret data from pre-clinical studies of live-attenuated sporozoite vaccines in rhesus models.
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Affiliation(s)
- Melanie J Shears
- Department of Laboratory Medicine and Pathology, University of Washington, 750 Republican St., F870, Seattle, WA, 98109, USA.,Center for Emerging and Re-emerging Infectious Diseases, University of Washington, 750 Republican St., Seattle, WA, 98109, USA
| | - Annette M Seilie
- Department of Laboratory Medicine and Pathology, University of Washington, 750 Republican St., F870, Seattle, WA, 98109, USA.,Center for Emerging and Re-emerging Infectious Diseases, University of Washington, 750 Republican St., Seattle, WA, 98109, USA
| | - B Kim Lee Sim
- Sanaria, Inc., 9800 Medical Center Drive, Suite A209, Rockville, MD, 20850, USA
| | - Stephen L Hoffman
- Sanaria, Inc., 9800 Medical Center Drive, Suite A209, Rockville, MD, 20850, USA
| | - Sean C Murphy
- Department of Laboratory Medicine and Pathology, University of Washington, 750 Republican St., F870, Seattle, WA, 98109, USA. .,Center for Emerging and Re-emerging Infectious Diseases, University of Washington, 750 Republican St., Seattle, WA, 98109, USA. .,Washington National Primate Research Center, University of Washington, 1959 NE Pacific St., Seattle, WA, 98195, USA.
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