Human alloantibodies detecting a red cell antigen apparently identical to MER2

The functional importance of blood group-active molecules in human red blood cells

Antigens of 23 of the 30 human blood group systems are defined by the amino acid sequence of red cell membrane proteins. The antigens of DI, RH, RHAG, MNS, GE and CO systems are carried on blood group-active proteins (Band 3, D and CE polypeptides, RhAG, Glycophorins A and B, Glycophorins C and D and Aquaporin 1, respectively) which are expressed at high levels (>200,000 copies/red cell). These major proteins contribute to essential red cell functions either directly as membrane transporters and by providing linkage to the underlying red cell skeleton or by facilitating the membrane assembly of the protein complexes involved in these processes. The proteins expressing antigens of the remaining 17 blood group systems are much less abundant (<20,000 copies/red cell) and their functional importance for the circulating red cell is largely unknown. Human gene knock-outs (null phenotypes) have been described for many of these minor blood group-active proteins, but only absence of Kx glycoprotein has been clearly linked with pathology directly related to the function of circulating red cells. Recent evidence suggesting the normal quality control system for glycoprotein synthesis is altered during the latter stages of red cell production raises the possibility that many of these low abundance blood group-active proteins are vestigial. In sickle cell disease and polycythaemia vera, elevated Lutheran glycoprotein expression may contribute to pathology. Dyserythropoiesis with reduced antigen expression can result from mutations in the erythroid transcription factors GATA-1 and EKLF.

Two MER2-negative individuals with the same novel CD151 mutation and evidence for clinical significance of anti-MER2

BACKGROUND

MER2 (RAPH1), the only antigen of the RAPH blood group system, is located on the tetraspanin CD151. Only four examples of alloanti-MER2 are known. We report here two new examples of alloanti-MER2, in women of Pakistani and Turkish origin, one of whom showed signs of a hemolytic transfusion reaction (HTR) after transfusion of 3 units of red cells (RBCs).

STUDY DESIGN AND METHODS

Standard serologic methods were used. A monocyte monolayer assay (MMA) was used to assess the potential clinical significance of one of the antibodies. All exons and flanking intronic sequences of CD151 were amplified and sequenced. A homology model for CD151 second extracellular loop (EC2) was constructed based on the crystal structure of CD81.

RESULTS

RBCs of both patients did not react with alloanti-MER2, and neither of their antibodies reacted with MER2-negative RBCs. The MMA results suggested that the antibody that appeared to have caused an HTR had the potential to be clinically significant. Both patients were homozygous for a 511C>T mutation in CD151 encoding an Arg171Cys change. This change did not result in any significant structural rearrangement in the protein model.

CONCLUSIONS

Two MER2-negative patients with anti-MER2 are homozygous for the same novel mutation encoding an amino acid substitution in the EC2 of CD151. One of the antibodies may have been responsible for an HTR, and crossmatch-compatible RBCs should be recommended for transfusion to patients with anti-MER2.

Red blood cell blood group antigens: structure and function

Red blood cell (RBC) blood group antigens are polymorphic, inherited, carbohydrate or protein structures located on the extracellular surface of the RBC membrane. They contribute to the architecture of the RBC membrane, and their individual function(s) are being slowly revealed. The biological qualities assigned to these RBC membrane structures are based on observed physiological alteration in RBCs that lack the component, by documenting similarities in its protein sequence (predicted from the nucleotide sequence of the gene) to proteins of known function and by extrapolation to identified functional homologues in other cells. The varied roles of RBC antigens include membrane structural integrity, the transport of molecules through the membrane, as receptors for extracellular ligands, adhesion molecules, enzymes, complement components and regulators, and in glycocalyx formation.

Blood groups and their function

The function(s) assigned to red blood cell membrane components is based on an observed effect in the red cells that lack the component, comparison of the protein sequence (predicted from the nucleotide sequence of the gene) to proteins of known function, and extrapolation of function of the component in other cells. The functions are varied and include membrane structure, transport, receptor, adhesion, enzyme activity, complement components, complement regulation and glycocalyx formation. Several components have more than one function.

Lutheran antigens, CD44-related antigens, and Lutheran regulatory genes

The Lutheran (Lu) blood group antigens are a family of human erythrocyte antigens which reside on two closely-related erythrocyte integral membrane proteins. Sixteen Lutheran or so-called para-Lutheran antigens have thus far been described, and human antisera to many of them have been shown to immunoblot two proteins, of 78 and 85 kDa. Lu cDNA encodes an integral membrane protein of 597 amino acids that is a member of the Ig superfamily. Lu proteins comprise five Ig superfamily domains, along with a single transmembrane domain and a cytoplasmic domain of about 60 amino acids. The two proteins seen in biochemical studies of red cell membranes appear to be derived from 2 mRNA species that differ only in their 3' ends, suggesting that they arise from alternate splicing of a single preRNA. Three genetic backgrounds for the Lu(a-b-) [Lu null] phenotype have been described. A recessive Lu null phenotype is rarely observed as a result of homozygosity for two amorphic LU alleles. However, the most common Lu(a-b-) phenotype appears to be caused by an independently segregating, dominant gene, designated In (Lu), which inhibits expression of all Lutheran antigens to nearly undetectable levels. This gene also affects the expression of other cell surface proteins and blood group antigens that are genetically unlinked to the Lutheran locus, including CD44 and MER2. CD44, a member of the cartilage link family of proteins, bears the In and AnWj blood group antigens. A widely distributed protein CD44 is expressed at normal levels on all tissues except erythrocytes in the presence of the In (Lu) gene. A second Lutheran regulatory gene, XS2, is responsible for the third Lu(a-b-) phenotype, which exhibits an X-linked inheritance pattern. The XS2 gene down-regulates but does not abolish expression of LU genes and does not affect expression of CD44.