Abstract
BackgroundIndividuals living in endemic areas gradually acquire natural immunity to clinical malaria, largely dependent on antibodies against parasite antigens. There are many studies indicating that the variant antigen PfEMP1 at the surface of the parasitized red blood cell (pRBC) is one of the major targets of the immune response. It is believed that antibodies against PfEMP1 confer protection by blocking sequestration (rosetting and cytoadherence), inducing antibody-dependent cellular-inhibitory effect and opsonizing pRBCs for phagocytosis.MethodsA recombinant NTS-DBL1α domain from a rosette-mediating PfEMP1 was expressed in Escherichia coli. The resulting protein was purified and used for immunization to generate polyclonal (goat) and monoclonal (mouse) antibodies. The antibodies’ ability to opsonize and induce phagocytosis in vitro was tested and contrasted with the presence of opsonizing antibodies naturally acquired during Plasmodium falciparum infection.ResultsAll antibodies recognized the recombinant antigen and the surface of live pRBCs, however, their capacity to opsonize the pRBCs for phagocytosis varied. The monoclonal antibodies isotyped as IgG2b did not induce phagocytosis, while those isotyped as IgG2a were in general very effective, inducing phagocytosis with similar levels as those naturally acquired during P. falciparum infection. These monoclonal antibodies displayed different patterns, some of them showing a concentration-dependent activity while others showed a prozone-like effect. The goat polyclonal antibodies were not able to induce phagocytosis.ConclusionImmunization with an NTS-DBL1-α domain of PfEMP1 generates antibodies that not only have a biological role in rosette disruption but also effectively induce opsonization for phagocytosis of pRBCs with similar activity to naturally acquired antibodies from immune individuals living in a malaria endemic area. Some of the antibodies with high opsonizing activity were not able to disrupt rosettes, indicating that epitopes of the NTS-DBL1-α other than those involved in rosetting are exposed on the pRBC surface and are able to induce functional antibodies. The ability to induce phagocytosis largely depended on the antibody isotype and on the ability to recognize the surface of the pRBC regardless of the rosette-disrupting capacity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-016-1459-3) contains supplementary material, which is available to authorized users.
Highlights
Individuals living in endemic areas gradually acquire natural immunity to clinical malaria, largely dependent on antibodies against parasite antigens
PfEMP1 is used by the parasite to evade clearance from the human host through two main mechanisms: (1) evasion of the host immune response raised against one PfEMP1 variant, switching to new variants through differential expression of approximately 60 distinct members per genome of the var family [3]; and, (2) evasion of the spleen clearance through sequestration of the parasitized red blood cell (pRBC) in the host’s microvasculature [7, 8], which is mediated by the interaction between PfEMP1 and receptors located on the endothelial cell surface or receptors on the uninfected red blood cells (RBCs) surface
To investigate whether antibodies elicited upon animal immunization with the recombinant N-terminal segment (NTS)-DBL1αdomain and antibodies present in adult immune sera from a malariaendemic area were able to opsonize and induce pRBC phagocytosis in vitro, fluorescently labelled FCR3S1.2 pRBCs were pre-incubated with the antibodies and mixed with THP-1 cells
Summary
Individuals living in endemic areas gradually acquire natural immunity to clinical malaria, largely dependent on antibodies against parasite antigens. PfEMP1 is used by the parasite to evade clearance from the human host through two main mechanisms: (1) evasion of the host immune response raised against one PfEMP1 variant, switching to new variants through differential expression of approximately 60 distinct members per genome of the var family [3]; and, (2) evasion of the spleen clearance through sequestration of the pRBCs in the host’s microvasculature [7, 8], which is mediated by the interaction between PfEMP1 and receptors located on the endothelial cell surface (cytoadhesion) or receptors on the uninfected RBC surface (rosetting). Among the different DBL domains, the DBL1α is the most conserved [9, 10] and has been identified as a ligand both for rosetting and cytoadhesion [11,12,13]
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