The biochemistry of blood group antigens--some recent advances

Targeted gene deletion demonstrates that the cell adhesion molecule ICAM-4 is critical for erythroblastic island formation

Erythroid progenitors differentiate in erythroblastic islands, bone marrow niches composed of erythroblasts surrounding a central macrophage. Evidence suggests that within islands adhesive interactions regulate erythropoiesis and apoptosis. We are exploring whether erythroid intercellular adhesion molecule 4 (ICAM-4), an immunoglobulin superfamily member, participates in island formation. Earlier, we identified alpha(V) integrins as ICAM-4 counterreceptors. Because macrophages express alpha(V), ICAM-4 potentially mediates island attachments. To test this, we generated ICAM-4 knock-out mice and developed quantitative, live cell techniques for harvesting intact islands and for re-forming islands in vitro. We observed a 47% decrease in islands reconstituted from ICAM-4 null marrow compared to wild-type marrow. We also found a striking decrease in islands formed in vivo in knock-out mice. Further, peptides that block ICAM-4/alpha(V) adhesion produced a 53% to 57% decrease in reconstituted islands, strongly suggesting that ICAM-4 binding to macrophage alpha(V) functions in island integrity. Importantly, we documented that alpha(V) integrin is expressed in macrophages isolated from erythroblastic islands. Collectively, these data provide convincing evidence that ICAM-4 is critical in erythroblastic island formation via ICAM-4/alpha(V) adhesion and also demonstrate that the novel experimental strategies we developed will be valuable in exploring molecular mechanisms of erythroblastic island formation and their functional role in regulating erythropoiesis.

Safety of the blood supply in Latin America

Appropriate selection of donors, use of sensitive screening tests, and the application of a mandatory quality assurance system are essential to maintain the safety of the blood supply. Laws, decrees, norms, and/or regulations covering most of these aspects of blood transfusion exist in 16 of the 17 countries in Latin America that are the subject of this review. In 17 countries, there is an information system that, although still incomplete (there are no official reports on adverse events and incidents), allows us to establish progress made on the status of the blood supply since 1993. Most advances originated in increased screening coverage for infectious diseases and better quality assurance. However, in 2001 to 2002, tainted blood may have caused infections in 12 of the 17 countries; no country reached the number of donors considered adequate, i.e., 5% of the population, to avoid blood shortages, or decreased significantly the number of blood banks, although larger blood banks are more efficient and take advantage of economies of scale. In those years, paid donors still existed in four countries and replacement donors made up >75% of the blood donors in another eight countries. In addition, countries did not report the number of voluntary donors who were repeat donors, i.e., the healthiest category. In spite of progress made, more improvements are needed.

Molecular characterization of cold agglutinins

An unusual bias involving the exclusive usage of the V4-34 gene segment by pathogenic antibodies with I/i antigen specificity has been documented in the literature. In addition, all unmutated and several mutated V4-34 encoded antibodies have been shown to be reactive with the anti-idiotypic monoclonal antibody 9G4. The 9G4 Id, therefore, is a marker for V4-34 gene segment expression. Based on these two correlations, it became vital to localize and characterize the nature of the 9G4 Id and to determine the relationship between the Id and I binding. Mutational analysis indicated that the 9G4 Id is located in framework region 1 (FR1) of V4-34 encoded antibodies. Two distinct sections of FR1, encompassing amino acid residues 6-12 and 23-25, form the 9G4 Id. Mutational analysis demonstrated that both FR1 and CDRH3 were required for I binding. When either one was disrupted, the mutant antibody could not bind I. This indicates that I binds through a framework region, and not exclusively through CDRH3. This renders the I interaction with the V4-34 encoded portion of immunoglobulins unconventional, with characteristics similar to superantigen binding to immunoglobulin through FR. When the FR1 DNA sequence of V4-34 was exchanged for FR1 sequences from other VH families I binding was lost, providing a structural explanation for this restricted VH usage. An understanding of the localization and structure of the 9G4 Id and the requirements of V4-34 encoded antibodies for I binding provide insights into the structure of pathogenic antibodies and their requirements for binding antigen. This information should be useful in analyzing new interactions such as the lytic activity of some V4-34 encoded antibodies for B cells.

Erythroid cell adhesion molecules Lutheran and LW in health and disease

The Lutheran and LW glycoproteins are blood group-active proteins found at the surface of human red cells. The Lutheran glycoprotein (Lu gp) is a member of the immunoglobulin superfamily (IgSF) that binds the extracellular matrix protein laminin, in particular, laminin isoforms containing the alpha 5 subunit. The LW glycoprotein (LW gp), also an IgSF member, has substantial sequence homology with the family of intercellular adhesion molecules (ICAMs). LW gp binds the integrin very late antigen-4 (VLA-4, alpha 4 beta 1) and alpha V-containing integrins. Studies on the expression of LW and Lu gps during erythropoiesis utilizing in vitro cultures of haemopoietic progenitor cells have shown that LW gp expression precedes that of Lu gp. These observations have led to the suggestion that LW gp on erythroblasts may interact with VLA-4 on macrophages to stabilize erythroblastic islands in normal bone marrow and that Lu gp may facilitate trafficking of more mature erythroid cells to the sinusoidal endothelium where alpha 5-containing laminins are known to be expressed. Levels of Lu gp and LW gp expression on sickle red cells are greater than on normal red cells and sickle red cells adhere to alpha 5-containing laminins. These data suggest that the Lu and LW molecules may contribute to the vaso-occlusive events associated with episodes of acute pain in sickle cell disease.

Cellular and molecular mechanisms of senescent erythrocyte phagocytosis by macrophages. A review

Human red blood cells (RBCs) have a life-span of 120 days in circulation, after which they are phagocytized by resident macrophages. Extensive studies have been undertaken by many investigators in order to elucidate the cellular and molecular mechanisms of the erythrophagocytosis. The critical questions addressed by physiologists, clinicians and biochemists are: 'which of the many traumatic blemishes that appear on the erythrocyte surface as it winds its way through the circulation is the primary signal for clearance of the effete RBC from the circulation?', or 'What is the critical signal that it, and it alone, will activate the resident macrophage to adhere to and engulf it?'. Numerous, and often conflicting, hypotheses have been proposed. Each investigator focusing on but one of the many modifications that afflict the cell surface of the ageing erythrocyte, viz changes in either or both the carbohydrate or peptidic moieties of glycoproteins; abolishment of the pre-existing asymmetry in the lipid bilayer with the exposure of phosphatidylserine residues; or alterations in spectrin, to mention but a few. Many of these investigators also have invoked an intermediary role for auto-immune antibodies that recognise the change(s) on the erythrocyte surface and thereby serve as opsonins as a prelude to the erythrophagocytosis. The objective of the present review is to evaluate the data in support of the various hypotheses, and to submit some of our own recent observations involving the use of flow cytometric procedures that: i) provide evidence that the cell surface sialic acid serves as a determinant of the life-span; ii) characterise the senescent erythrocyte population that is specifically captured and phagocytized by macrophages (utilising the rapid and sensitive procedure we developed for quantification of in vitro erythrophagocytosis); and finally iii) provide evidence for the existence of an alternative pathway that is independent of immunoglobulins.

Blood group antigens defined by the amino acid sequences of red cell surface proteins

The antigens of 18 blood group systems are expressed on proteins that are intrinsic to the red cell. The proteins which carry the antigens of these systems have been identified and primary sequence information is available for all but two (SC, DO). Several different functional groups are evident. Antigens of the DI, CO, RH, XK and JK systems are located on proteins which have the structure of membrane transport proteins. The FY antigens mark a cytokine receptor. The IN, LW, XG antigens are associated with molecules which have adhesion functions and the LU glycoprotein also has a structure which suggests a role in adhesion. YT and KEL antigens are located on cell surface enzymes and the CR and KN antigen on molecules involved in complement regulation. Finally, the MN and GE antigens are located on sialic acid-rich glycoproteins (glycophorins A, B and C/D respectively), a group of molecules which do not, as yet, have a clearly defined function. The molecular basis of antigens in several blood group systems have been defined and shown to depend upon the amino acid sequence.