Deadly Malaria Parasites Hijack CD55 to Invade Human Erythrocytes

Malaria remains a major global health threat and is particularly devastating in Africa, where it mainly kills children younger than five years who have not yet developed partial immunity to the disease. Plasmodium falciparum is the most lethal of the human malaria parasites and is able to rapidly develop resistance to antimalaria drugs, including the current artemisinin combination therapy. This therefore fuels the ongoing, urgent need to develop novel treatments, especially since there is currently no vaccine available. The red blood cell (RBC) stage of the parasite lifecycle is responsible for the pathogenesis of malaria, but the complex process of invasion and the concomitant interactions between host erythrocyte receptors and parasite ligands are not fully understood. Another layer of complexity is added in the case of P. falciparum since it possesses several ligands that can target different receptors on the RBC surface, which facilitates entry into genetically variable human hosts and has contributed to the success of this deadly pathogen.

Mature erythrocytes are terminally differentiated cells with a limited proteome and no nucleus; therefore, genetic manipulation of the genome to identify factors influencing host susceptibility to infection is not possible. To overcome this challenge, Dr. Elizabeth Egan and colleagues from Harvard University exploited two recent technological advances of RNA interference and ex vivo production of RBCs to design a forward hematopoietic stem cell–based genetic screen. Since all the known host receptors for parasite invasion are erythrocyte membrane proteins encoding blood group antigens, the researchers selectively targeted this group of genes. They transduced hematopoietic progenitor cells with a pooled lentivirus short hairpin RNA (shRNA) library to knock down selected genes. These genetically manipulated cells were induced to proliferate and differentiate, and at the orthochromatic erythroblast stage, when they were mature enough to sustain parasite development, they were infected with P. falciparum parasites expressing green fluorescent protein. Infected RBCs were isolated, and the relative amounts of each shRNA in the population were quantified by deep sequencing and compared with the profile of control knockdown cells not exposed to parasites.

Several hits were identified, including genes encoding proteins that are known to be involved in the invasion process, such as basigin and complement receptor 1 (CR-1); but the top-ranked gene was CD55 or decay-accelerating factor (DAF). CD55 is a 70kDa glycoprotein, which is attached to the erythrocyte membrane by a glycosylphosphatidylinositol (GPI) anchor and carries the Cromer blood group antigens. It regulates the complement system and prevents complement-mediated destruction of erythrocytes. It is interesting to note that CD55 on epithelial cells serves as a receptor for some viral and bacterial pathogens.

To verify that malaria parasites utilize CD55 to invade RBCs, the authors used shRNA to knock down CD55 and demonstrated reduced invasion. Further evidence was provided by infecting CD55 null cells from a Japanese patient with laboratory and clinical strains of P. falciparum indicating that CD55 is essential for the successful entry and development of parasites. The invasion process is complex and may be divided into several steps, including recognition of the host cell, reorientation, irreversible binding, and tight junction formation just prior to entry. Experiments with cytochalasin suggested that CD55 plays a role in the later stages of committed attachment, but the details still need to be determined.