Abstract
BackgroundThe circumsporozoite protein (CSP) of Plasmodium is a key surface antigen that induces antibodies and T-cells, conferring immune protection in animal models and humans. However, much of the work on CSP and immunity has been developed based on studies using rodent or non-human primate CSP antigens, which may not be entirely translatable to CSP expressed by human malaria parasites, especially considering the host specificity of the different species.MethodsUsing a genetically engineered strain of Plasmodium berghei that expresses luciferase, GFP and the Plasmodium falciparum orthologue of CSP, the effect of laboratory preparation, mosquito treatment and mouse factors on sporozoite infectivity was assessed using an in vivo bioluminescence assay on mice. This assay was compared with a PCR-based protection assay using an already described monoclonal antibody that can provide sterile protection against sporozoite challenge.ResultsBioluminescence assay demonstrated similar detection levels of the quantity and kinetics of liver-stage infection, compared to PCR-based detection. This assay was used to evaluate treatment of sporozoite and delivery method on mouse infectivity, as well as the effects of age, sex and strain of mice. Finally, this assay was used to test the protective capacity of monoclonal antibody AB317; results strongly recapitulate the findings of previous work on this antibody.ConclusionsThe PbGFP-Luc line and in vivo bioluminescence imaging provide highly sensitive read-outs of liver-stage infection in mice, and this method can be useful to reliably evaluate potency of pre-erythrocytic interventions.
Highlights
The circumsporozoite protein (CSP) of Plasmodium is a key surface antigen that induces antibodies and T-cells, conferring immune protection in animal models and humans
Measuring parasite liver load by RT‐qPCR and bioluminescence One of the most sensitive and quantitative assays to evaluate the development of Plasmodium liver stages is reverse transcriptase quantitative polymerase chain reac‐ tion (RT-qPCR) measuring plasmodial 18S ribosomal RNA in the livers of sporozoite-infected mice [9]. Another assay has been described based on the use of parasite lines expressing luciferase that allows the estimation of parasite burden by measuring parasite specific in vivo bioluminescence in the liver [10]. The sensitivity of both assays was compared in experiments with mice injected intravenously with varying numbers of transgenic P. berghei sporozoites expressing luciferase and the P. falciparum CSP (Fig. 1a)
While the background with bioluminescence is higher, it appears that this measurement in individual mice shows less variability between mice of the same group compared to the RT-qPCR measurements
Summary
The circumsporozoite protein (CSP) of Plasmodium is a key surface antigen that induces antibodies and T-cells, conferring immune protection in animal models and humans. Much of the work on CSP and immunity has been developed based on studies using rodent or non-human primate CSP antigens, which may not be entirely translatable to CSP expressed by human malaria parasites, especially considering the host specificity of the different species. The development of transgenic rodent parasites in which the murine CSP is replaced by the P. falciparum orthologue helps to overcome some of these limitations This strategy has previously been used to evaluate the immunogenicity and efficacy of vaccine constructs against P. falciparum [3, 4], and Plasmodium vivax CSP [5,6,7], but a detailed analysis of the key features of such an assay has not been reported. An important objective is to develop a high throughput assay that may allow the evaluation of multiple immune or pharmacological interventions, permitting a quantitative comparison of reagents that may affect sporozoite infectivity and/or development of the liver stage
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