Background: The most severe form of malaria is caused by the intraerythrocytic protozoan parasite Plasmodium falciparum. The parasite exports multiple proteins to the erythrocyte membrane, including members of the protein family P. falciparum erythrocyte membrane protein 1 (PfEMP-1). Expression of PfEMP-1 on the infected erythrocyte (IE) surface is restricted to electron dense protrusions called knobs. The significance of this is unknown, but it is suspected to be related to the adhesive function of PfEMP-1, which is important for the pathogenesis of the disease.
PfEMP-1 is a primary target of the protective antibodies that are gradually acquired in response to P. falciparum-IEs. Although this response is dominated by IgG1 and IgG3, complement-mediated attack following activation of the classical pathway does not appear to be a major effector mechanism. We hypothesized that this is related to the knob-restricted expression of PfEMP-1 on IEs, which may interfere with binding and activation of C1.
Materials and methods: We used recombinant PfEMP1 proteins, PfEMP1-specific human monoclonal IgG and human immune sera, P. falciparum-IEs, phagocytic cell lines, and specific complement reagents to study the above hypothesis by ELISA and flow cytometry based methods.
Results and conclusions: PfEMP1-specific monoclonal IgG and immune sera can activate complement, when tested by ELISA and using deposition of several complement components as readout. In contrast, deposition of complement was not detectable by flow cytometry when using IEs expressing the same PfEMP1 proteins. Results of further experiments assessing phagocytosis of antibody-opsonized IEs in the presence and absence of complement, and the impact of Fc modifications of PfEMP1-specific IgG to enhance hexamer formation will be presented. Our preliminary conclusion is that the knob-restricted expression of PfEMP1 on the IE surface may serve as a hitherto unappreciated immune evasion mechanism employed by P. falciparum parasites.