Colocalization of Rh polypeptides and the aminophospholipid transporter in dilauroylphosphatidylcholine-induced erythrocyte vesicles

Analysis and clinical relevance of microparticles from red blood cells

PURPOSE OF REVIEW

The mechanisms involved in the formation of red blood cell (RBC) microparticles in vivo as well as during erythrocyte storage are reviewed, and the potential role of microparticles in transfusion medicine is described.

RECENT FINDINGS

Microparticles release is an integral part of the erythrocyte ageing process, preventing early removal of RBCs. Proteomics analyses have outlined the key role of band 3-ankyrin anchoring complex and the occurrence of selective RBC membrane remodelling mechanisms in microparticles formation. The presence of several RBC antigens, expressed on microparticles, has been demonstrated. The potential deleterious effects of RBC microparticles in transfused recipients, including hypercoagulability, microcirculation impairment and immunosuppression, are discussed.

SUMMARY

Formation and role of RBC microparticles are far from being completely understood. Combining various approaches to elucidate these mechanisms could improve blood product quality and transfusion safety. Implementation of RBC microparticles as biomarkers in the laboratory routine needs to overcome technical barriers involved in their analysis.

Epitope mapping of four monoclonal antibodies specific for the human RhD antigen

RhD is a highly immunogenic erythrocyte membrane protein, implicated in hemolytic disease of the newborn and other hemolytic disorders. Anti-RhD antibodies are used in the treatment of such disease states. Six mutant forms of recombinant RhD were stably expressed in K562 cells, and these cells were used to investigate epitope specificities of four anti-RhD monoclonal antibodies (mAbs). Amino acid substitutions were made in the exofacial loops of RhD to the corresponding residues found in the related RhCE polypeptide; M169L/M170R and I172F in the third loop, F223V and E233Q in the fourth loop, and D350H and G353W/A354N in the sixth loop. Each mAb was found to have a unique fine specificity and recognized multiple distant sites within RhD. The mAbs also differed in how they recognized individual amino acids in the exofacial loops of RhD.

RH blood group system and molecular basis of Rh-deficiency

Rhesus (Rh) antigens are defined by a complex association of membrane polypeptides that are missing or severely deficient from the red cells of rare Rhnull individuals who suffer a clinical syndrome of varying severity characterized by abnormalities of the red cell shape, cation transport and membrane phospholipid organization. The Rhnull phenotype is an inherited condition that may arise from homozygosity either for a 'suppressor' gene unrelated to the RH locus ('regulator type') or for a silent allele at the RH locus itself ('amorph type'). A current model suggests that the proteins of the Rh complex (Rh, RhAG, CD47, LW, GPB) are assembled by non-covalent bonds and that it is not assembled or transported to the cell surface when one subunit is missing. Rh and RhAG proteins belong to the same protein family and are quantitatively the major components that form the core of the complex, which is firmly linked to the membrane skeleton. Molecular analysis of Rhnull individuals has revealed that abnormalities occur only at the RHAG and RH loci, without alteration of the genes encoding the accessory chains. Mutations of the RHAG gene, but not of RH, occur in all Rhnull individuals of the regulator type (including Rhmod) investigated so far (13 cases), strongly suggesting that RHAG mutants act as 'suppressors' and not as transcriptional regulators of the RH genes and that variable expression of the RHAG alleles may account for the Rhmod phenotypes (exhibiting weak expression of Rh antigens). Conversely, mutations of the RHCE gene, but not of RHAG, occur in two unrelated Rhnull individuals of the amorph type, supporting the view that RH mutants result from a 'silent' allele at the RH locus. These findings strongly support the Rh complex model since when either the Rh or RhAG protein is missing, the assembly and/or transport of the Rh complex is defective. Transcriptional as well as post-transcriptional mechanisms may account for the molecular abnormalities, but experimental evidence based on expression models is required to test these hypotheses, in the hope that they may help to clarify the biological role of the Rh proteins in the red cell membrane.

Phosphatidylserine exposure and aminophospholipid translocase activity in Rh-deficient erythrocytes

Endogenous phosphatidylserine (PS) exposure and lipid transport activity have been investigated for seven unrelated cases of Rhnull erythrocytes. Endogenous PS exposure was measured by prothrombinase activity. Out of six cases studied, two Rhnull samples exhibited abnormal aminophospholipid exposure, as suggested by the measurement of a lower Km of factor Xa for prothrombin. Aminophospholipid translocase activity was measured through the transbilayer redistribution of spin-labelled analogues of phospholipids. Provided that incubation conditions allow the maintainance of intracellular ATP level, no difference was observed between Rhnull and control erythrocytes, clearly indicating that the aminophospholipid translocase and Rh polypeptides are different molecular species.