Evasion of Classical Complement Pathway Activation on Plasmodium falciparum-Infected Erythrocytes Opsonized by PfEMP1-Specific IgG.

Mads Delbo Larsen,Rafael Bayarri-Olmos,Sisse Bolm Ditlev,Maria Del Pilar Quintana,Peter Garred,Michael Fokuo Ofori,Lars Hviid

doi:10.3389/fimmu.2018.03088

Abstract

Members of the PfEMP1 protein family are expressed on the surface of P. falciparum-infected erythrocytes (IEs), where they contribute to the pathogenesis of malaria and are important targets of acquired immunity. Although the PfEMP1-specific antibody response is dominated by the opsonizing and complement-fixing subclasses IgG1 and IgG3, activation of the classical complement pathway by antibody-opsonized IEs does not appear to be a major immune effector mechanism. To study the molecular background for this, we used ELISA and flow cytometry to assess activation of the classical component pathway by recombinant and native PfEMP1 antigen opsonized by polyclonal and monoclonal PfEMP1-specific human IgG. Polyclonal IgG specific for VAR2CSA-type PfEMP1 purified from a pool of human immune plasma efficiently activated the classical complement pathway when bound to recombinant PfEMP1 in ELISA. In contrast, no activation of complement could be detected by flow cytometry when the same IgG preparation was used to opsonize IEs expressing the corresponding native PfEMP1 antigen. After engineering of a VAR2CSA-specific monoclonal antibody to facilitate its on-target hexamerization, complement activation was detectable in an ELISA optimized for uniform orientation of the immobilized antigen. In contrast, the antibody remained unable to activate complement when bound to native VAR2CSA on IEs. Our data suggest that the display of PfEMP1 proteins on IEs is optimized to prevent activation of the classical complement pathway, and thus represents a hitherto unappreciated parasite strategy to evade acquired immunity to malaria.

Highlights

Malaria remains a major health problem with an estimated 219 million cases and 435,000 deaths in 2017 alone [1]
Binding of C1q (Figure 1A), deposition of C4 (Figure 1B) and C3 (Figure 1C), and formation of terminal complement complex (TCC) (Figure 1D) were detected in recombinant fulllength ectodomain of the VAR2CSA-type PfEMP1 IT4VAR04 (FV2)-ELISA after incubation with human IgG purified from a plasma pool with known high FV2-reactivity, followed by non-immune human serum (NHS) as a source of complement components
These findings suggest that the distribution of PfEMP1 on infected erythrocyte (IE) inhibits classical complement activation, possibly due to the clustered, knob-restricted distribution of the antigen

Summary

Introduction

Malaria remains a major health problem with an estimated 219 million cases and 435,000 deaths in 2017 alone [1]. The particular virulence of P. falciparum is related to the unique ability of this parasite to express members of a family of clonally variant surface antigens called P. falciparum erythrocyte membrane protein 1 (PfEMP1) on the surface of the infected erythrocytes (IEs) [3]. This enables sequestration of IEs in the microvasculature, mediated by interaction of PfEMP1 with vascular host receptors such as CD36, endothelial protein C receptor (EPCR), and oncofetal chondroitin sulfate [4,5,6,7]. We show that polyclonal, and even monoclonal, IgG can activate the classical complement pathway when bound to surface-bound recombinant PfEMP1, such activation does not occur when the IgG is bound to native PfEMP1 expressed on the IE surface

Methods

Results

Conclusion