Mapping a common interaction site used by Plasmodium falciparum Duffy binding-like domains to bind diverse host receptors.

The Duffy binding-like (DBL) domain is a key adhesive module in Plasmodium falciparum, present in both erythrocyte invasion ligands (EBLs) and the large and diverse P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of cytoadherence receptors. DBL domains bind a variety of different host receptors, including intercellular adhesion molecule 1 (ICAM-1), a receptor interaction that may have a role in infected erythrocyte binding to cerebral blood vessels and cerebral malaria. In this study, we expressed the nearly full complement of DBLbeta-C2 domains from the IT4/25/5 (IT4) parasite isolate and showed that ICAM-1-binding domains (DBLbeta-C2(ICAM-1)) were confined to group B and group C PfEMP1 proteins and were not present in group A, suggesting that ICAM-1 selection pressure differs between PfEMP1 groups. To further dissect the molecular determinants of binding, we modelled a DBLbeta-C2(ICAM-1) domain on a solved DBL structure and created alanine substitution mutants in two DBLbeta-C2(ICAM-1) domains. This analysis indicates that the DBLbeta-C2::ICAM-1 interaction maps to the equivalent glycan binding region of EBLs, and suggests a general model for how DBL domains evolve under dual selection for host receptor binding and immune evasion.

Global genetic diversity and evolution of var genes associated with placental and severe childhood malaria.

In Plasmodium falciparum, var genes encode adhesive proteins that are transported to the surface of infected erythrocytes and act as major virulence determinants for infected erythrocyte binding and immune evasion. Var genes are highly diverse and can be classified into five major groups (UpsA, B, C, D, and E). Previous serological studies have suggested that the UpsA var group may contain common antigenic types that have important roles in severe childhood malaria. Here, our analysis found that UpsA vars are highly diverse between 22 world-wide parasite isolates, although they could be grouped into two broad clusters that may be separately recombining. By comparison, orthologs of the UpsA-linked Type 3 var and UpsE-linked var2csa were detected in nearly all parasite isolates, and a var2csa ortholog was also present in a chimpanzee malaria P. reichenowi that diverged from P. falciparum approximately 5-7 million years ago. Although the specific function of Type 3 var genes is unknown, var2csa is a leading candidate for a pregnancy associated malaria vaccine. Compared to typical var genes, var2csa is unusually conserved but still had only 54-94% amino acid identity in extracellular binding regions. However, var2csa alleles have extensive gene mosaicism within polymorphic blocks that are shared between world-wide parasite isolates and recognizable in P. rechenowi suggesting a high rate of self-self recombination and an ancient and globally-related pool of var2csa polymorphism. These studies aid our understanding of the evolutionary mechanisms that shape var diversity and will be important to the development of vaccines against pregnancy associated malaria and severe malaria.