Title: The host-parasite interface
Description: Invasion of the malaria parasite, Plasmodium falciparum, into human erythrocytes is a complex, incompletely understood process. The merozoite ligand-erythrocyte receptor engagement used by a parasite defines an invasion pathway, which is characterized by its sensitivity to various enzymes. The highly polymorphic nature of parasite ligands and host receptors allows for myriad complementary associations. Here we explore the factors that affect ligand-receptor engagement and consequent invasion pathway utilization. Invasion pathway utilization is somewhat plastic, for phenotypic switching allows normally sialic acid-dependent parasites to invade via sialic acid-independent means. PfRh2b is a sialic acid-independent invasion ligand that shares ∼7.5 kb of sequence with PfRh2a. Disruption of these paralogs reveals that the most robust phenotypic switch occurs during concurrent PfRh2a and PfRh2b expression, implying a coordinated action between the proteins and divulging a previously unknown role for PfRh2a in invasion. Of the paralogs, only PfRh2b affects chymotrypsin- and trypsin-mediated invasion. As these proteins share identical amino termini, the effects of PfRh2b on these invasion phenotypes must map to its divergent carboxy-terminal sequence. Genetic modification of this region yields a dominant negative phenotype.

Since stable PfRh2b knockouts have been engineered in multiple parasite lines, this result was unexpected. The contributions of PfRh2a and PfRh2b to phenotypic switching are physiologically relevant, for sialic acid levels decline during in vivo erythrocyte aging, which is accelerated in the presence of P. falciparum. Erythrocyte invasion without regard for sialic acid content increases cell availability, and decreased erythrocyte selectivity is correlated with increased disease severity. This versatility masks an underlying preference for younger cells, for we and others have observed decreased parasitemia in increasingly older cells. Given the association between ligand reliance and enzyme-sensitivity status, we investigated whether invasion pathway utilization influences erythrocyte age preferences. Our data show that decreased invasion efficiencies in older erythrocytes occur irrespectively of invasion pathway utilization, highlighting a host-specific influence on erythrocyte invasion that extends to field isolates. Discovery of genotypes that precipitate less efficient phenotypic switching, the lethality of perturbations to PfRh2b, and decreased invasion efficiencies in aged cells betray vulnerabilities inherent to the parasitization of human erythrocytes by P. falciparum merozoites.