α2-Macroglobulin Can Crosslink Multiple Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) Molecules and May Facilitate Adhesion of Parasitized Erythrocytes

Abstract

Rosetting, the adhesion of Plasmodium falciparum-infected erythrocytes to uninfected erythrocytes, involves clonal variants of the parasite protein P. falciparum erythrocyte membrane protein 1 (PfEMP1) and soluble serum factors. While rosetting is a well-known phenotypic marker of parasites associated with severe malaria, the reason for this association remains unclear, as do the molecular details of the interaction between the infected erythrocyte (IE) and the adhering erythrocytes. Here, we identify for the first time a single serum factor, the abundant serum protease inhibitor α2-macroglobulin (α2M), which is both required and sufficient for rosetting mediated by the PfEMP1 protein HB3VAR06 and some other rosette-mediating PfEMP1 proteins. We map the α2M binding site to the C terminal end of HB3VAR06, and demonstrate that α2M can bind at least four HB3VAR06 proteins, plausibly augmenting their combined avidity for host receptors. IgM has previously been identified as a rosette-facilitating soluble factor that acts in a similar way, but it cannot induce rosetting on its own. This is in contrast to α2M and probably due to the more limited cross-linking potential of IgM. Nevertheless, we show that IgM works synergistically with α2M and markedly lowers the concentration of α2M required for rosetting. Finally, HB3VAR06+ IEs share the capacity to bind α2M with subsets of genotypically distinct P. falciparum isolates forming rosettes in vitro and of patient parasite isolates ex vivo. Together, our results are evidence that P. falciparum parasites exploit α2M (and IgM) to expand the repertoire of host receptors available for PfEMP1-mediated IE adhesion, such as the erythrocyte carbohydrate moieties that lead to formation of rosettes. It is likely that this mechanism also affects IE adhesion to receptors on vascular endothelium. The study opens opportunities for broad-ranging immunological interventions targeting the α2M—(and IgM-) binding domains of PfEMP1, which would be independent of the host receptor specificity of clinically important PfEMP1 antigens.

Severe adult malaria is associated with specific PfEMP1 adhesion types and high parasite biomass

“…Thus, it will be important to explore if both rosetting and nonrosetting PfEMP1 variants are contributing to adult SM. Although DC6 binding properties are uncharacterized, recent studies have shown that DBLζ and DBLe mediate binding to IgM and α2-macroglobulin, and it has been hypothesized that binding to these serum factors can cross-link multiple PfEMP1 to increase the binding affinity of N-terminal domains (54,55). Further sequencing of DC6 variants will be necessary to assess whether DC6 and DC8 are part of the same or different proteins in severe isolates, and to determine the binding properties of DBLζ and DBLe Indian domains to serum factors.…”

Plasmodium falciparum var genes expressed in children with severe malaria encode CIDR α1 domains

“…PECAM1 binding (Treutiger et al , ) has been associated with PfEMP1 containing DC5 (Berger et al , ), but var transcripts encoding DC5 PfEMP1 were not found in high abundance in the studied children. Binding to IgM (Semblat et al , , ; Stevenson et al , ) and alpha‐2‐macroglobulin (Stevenson et al , ) has been mapped to C‐terminal PfEMP1 domains, DBLε and/or DBLζ, which exhibited higher transcript proportions in children with uncomplicated malaria (Fig EV1).…”

Severe malaria: what’s new on the pathogenesis front?

“…While the role of IgM binding in rosetting remains unclear, it appears to strengthen the bond of the central IE with the surrounding erythrocytes. Stevenson et al (2015) recently reported that the serum protein α 2 -macroglobulin (α 2 M) is able to induce rosetting in vitro and ex vivo, using several parasite isolates. In contrast to IgM, α 2 M elicits rosetting alone, while the presence of IgM significantly lowers the concentration of α 2 M required.…”