Structural basis of malaria RIFIN binding by LILRB1-containing antibodies.

Some Plasmodium falciparum repetitive interspersed families of polypeptides (RIFINs)-variant surface antigens that are expressed on infected erythrocytes1-bind to the inhibitory receptor LAIR1, and insertion of DNA that encodes LAIR1 into immunoglobulin genes generates RIFIN-specific antibodies2,3. Here we address the general relevance of this finding by searching for antibodies that incorporate LILRB1, another inhibitory receptor that binds to ß2 microglobulin and RIFINs through their apical domains4,5. By screening plasma from a cohort of donors from Mali, we identified individuals with LILRB1-containing antibodies. B cell clones isolated from three donors showed large DNA insertions in the switch region that encodes non-apical LILRB1 extracellular domain 3 and 4 (D3D4) or D3 alone in the variable-constant (VH-CH1) elbow. Through mass spectrometry and binding assays, we identified a large set of RIFINs that bind to LILRB1 D3. Crystal and cryo-electron microscopy structures of a RIFIN in complex with either LILRB1 D3D4 or a D3D4-containing antibody Fab revealed a mode of RIFIN-LILRB1 D3 interaction that is similar to that of RIFIN-LAIR1. The Fab showed an unconventional triangular architecture with the inserted LILRB1 domains opening up the VH-CH1 elbow without affecting VH-VL or CH1-CL pairing. Collectively, these findings show that RIFINs bind to LILRB1 through D3 and illustrate, with a naturally selected example, the general principle of creating novel antibodies by inserting receptor domains into the VH-CH1 elbow.

Structure of Plasmodium falciparum Rh5-CyRPA-Ripr invasion complex.

Plasmodium falciparum causes the severe form of malaria that has high levels of mortality in humans. Blood-stage merozoites of P. falciparum invade erythrocytes, and this requires interactions between multiple ligands from the parasite and receptors in hosts. These interactions include the binding of the Rh5-CyRPA-Ripr complex with the erythrocyte receptor basigin1,2, which is an essential step for entry into human erythrocytes. Here we show that the Rh5-CyRPA-Ripr complex binds the erythrocyte cell line JK-1 significantly better than does Rh5 alone, and that this binding occurs through the insertion of Rh5 and Ripr into host membranes as a complex with high molecular weight. We report a cryo-electron microscopy structure of the Rh5-CyRPA-Ripr complex at subnanometre resolution, which reveals the organization of this essential invasion complex and the mode of interactions between members of the complex, and shows that CyRPA is a critical mediator of complex assembly. Our structure identifies blades 4-6 of the ß-propeller of CyRPA as contact sites for Rh5 and Ripr. The limited contacts between Rh5-CyRPA and CyRPA-Ripr are consistent with the dissociation of Rh5 and Ripr from CyRPA for membrane insertion. A comparision of the crystal structure of Rh5-basigin with the cryo-electron microscopy structure of Rh5-CyRPA-Ripr suggests that Rh5 and Ripr are positioned parallel to the erythrocyte membrane before membrane insertion. This provides information on the function of this complex, and thereby provides insights into invasion by P. falciparum.

Giardia lamblia: absence of cyst antigens and reduced secretory vesicle formation and bile salt uptake in an encystation-deficient subline.

Encystation of Giardia lamblia entails the appearance of a number of new antigens, as well as formation of a novel class of large encystation-specific secretory vesicles (ESV) that transport stage-specific proteins to the nascent cyst wall. The monoclonal antibody GCSA-1, which was raised against purified cyst walls, recognizes protein species of approximately 26-46 kDa that are regulated by exposure to bile (plus lactic acid) and alkaline pH, the factors that induce encystation. The GCSA-1 epitope is maximally expressed after approximately 14 hr of encystation and localizes to the interior, but not the membrane of the ESV as shown by frozen section immunoelectron microscopy. To further understand the process of encystation, we compared two sublines of strain WB that differ in their ability to encyst in vitro. Water-resistant cysts were not detected in subline A6 under conditions in which subline C6 formed approximately 2 x 10(5) cysts/ml. Moreover, subline A6 did not form ESV efficiently or detectably express antigens recognized by mAb GCSA-1 or by polyclonal anti-cyst sera. Finally, uptake of the bile salt taurocholate by A6 was reduced 4- to 20-fold, compared with that of C6, although transport by both strains was sodium-dependent and regulated by bile salt starvation. The decrease in bile salt uptake by A6 may be related to its defect in encystation.