Plasmodium falciparum is a blood parasite that lives for the most part inside red cells. It is responsible for the death of 1-2 million people through malaria every year. The mechanisms by which the parasite invades red cells are complex and not completely understood. For many years it has been known that proteins called glycophorins are used by the parasite to gain entry into the red cell. However, the existence of another protein that allows entry independent of glycophorins has been suspected for nearly as long. The identity of the alternative protein has been a difficult mystery to solve.
Researchers have now identified an alternative protein used by P. falciparum to invade red blood cells. The results may aid the development of a future vaccine for malaria. The researchers identified complement receptor 1 as the protein that enables P. falciparum to invade red blood cells. Proteins called glycophorins are used by the parasite to gain entry into the red cell. However, because infection can take place without glycophorins, researchers suspected that another protein is also involved. Complement receptor 1 (CR1), also known to help protect red cells from attack by the immune system, has been suspected of having other roles in the development of malaria complications. The team was able to demonstrate that this protein is important in the invasion of red cells by using several laboratory strains of malaria as well as strains obtained from Kenya.
P. falciparum may use the CR1 protein instead of glycophorins if the parasite encounters a variant that lacks the glycophorin receptor; if the immune system mounts a response against parasite proteins involved in the dominant pathway due to a previous infection; or if the host were treated with a vaccine that blocks the glycophorin pathway. The recognition of the additional role of complement receptor 1 in red cell invasion will allow the definitive identification of malaria proteins that interact with it and that could be used in a future vaccine cocktail to block red cell invasion.
Complement Receptor 1 Is a Sialic Acid-Independent Erythrocyte Receptor of Plasmodium falciparum. 2010 PLoS Pathog 6(6): e1000968. doi:10.1371/journal.ppat.1000968
Plasmodium falciparum is a highly lethal malaria parasite of humans. A major portion of its life cycle is dedicated to invading and multiplying inside erythrocytes. The molecular mechanisms of erythrocyte invasion are incompletely understood. P. falciparum depends heavily on sialic acid present on glycophorins to invade erythrocytes. However, a significant proportion of laboratory and field isolates are also able to invade erythrocytes in a sialic acid-independent manner. The identity of the erythrocyte sialic acid-independent receptor has been a mystery for decades. We report here that the complement receptor 1 (CR1) is a sialic acid-independent receptor for the invasion of erythrocytes by P. falciparum. We show that soluble CR1 (sCR1) as well as polyclonal and monoclonal antibodies against CR1 inhibit sialic acid-independent invasion in a variety of laboratory strains and wild isolates, and that merozoites interact directly with CR1 on the erythrocyte surface and with sCR1- coated microspheres. Also, the invasion of neuraminidase-treated erythrocytes correlates with the level of CR1 expression. Finally, both sialic acid-independent and dependent strains invade CR1 transgenic mouse erythrocytes preferentially over wild-type erythrocytes but invasion by the latter is more sensitive to neuraminidase. These results suggest that both sialic acid-dependent and independent strains interact with CR1 in the normal red cell during the invasion process. However, only sialic acid-independent strains can do so without the presence of glycophorin sialic acid. Our results close a longstanding and important gap in the understanding of the mechanism of erythrocyte invasion by P. falciparum that will eventually make possible the development of an effective blood stage vaccine.