Mechanisms of band 3 oxidation and clustering in the phagocytosis of Plasmodium falciparum-infected erythrocytes

Abstract

Erythrocytes (RBCs) opsonized by IgG and complement are prevalently recognized and phagocytosed by complement receptor CR1. This mechanism, effective in senescent and damaged RBCs seems to be operative in ring-parasitized RBCs, since infection by Plasmodium falciparum induces stage-dependent binding of auto-antibodies and activated C3 to the RBC membrane. Later, parasite forms are also recognized by non-opsonic receptors, such as scavenger receptor CD36. Malaria parasites induce the oxidative formation of hemichromes which are the trigger for the auto-antigen development. Band 3 protein is the most plausible candidate of the RBC auto-antigen, induced by hemichromes. Auto-antigens isolated from trophozoites were found only in a high-molecular-weight protein aggregates not present in the normal RBC. The immunocomplex was purified by protein-A affinity chromatography, purified proteins digested by trypsin and analyzed by MALDI-TOF. Peptide mapping showed that the main antigen consisted of band 3 protein aggregates that also contained hemichromes, IgGs, complement factor 3 (C3), and traces of spectrin and glycophorin but no parasite proteins. Two cysteines located in the band 3 cytoplasmic domain were found to be particularly reactive to oxidants and mediated band 3 covalent dimerization via disulfide bonds. Thus, parasites promote oxidative alterations in the membrane of the host which lead to exposure of antigenic sites recognized by anti-band 3 auto-antibodies. Formation of band 3 clusters appears to be mediated by cytoplasmic binding of hemichromes and also by direct band 3 oxidation, whereby clustered, oxidized and antigenic band 3 was underglycosylated.