Antibodies that define NANA-independent MN-system antigens

Mouse models of IgG- and IgM-mediated hemolysis

Well-characterized mouse models of allo-immune antibody-mediated hemolysis would provide a valuable approach for gaining greater insight into the pathophysiology of hemolytic transfusion reactions. To this end, mouse red blood cells (mRBCs) from human glycophorin A transgenic (hGPA-Tg) donor mice were transfused into non-Tg recipients that had been passively immunized with IgG or IgM hGPA-specific monoclonal antibodies (mAbs). In this novel murine "blood group system," mRBCs from hGPA-Tg mice are "antigen positive" and mRBCs from non-Tg mice are "antigen negative." Passive immunization of non-Tg mice with the IgG1 10F7 and IgG3 NaM10-2H12 anti-hGPA mAbs each induced rapid clearance of incompatible transfused hGPA-Tg-mRBCs in a dose-response manner. Using various knockout mice as transfusion recipients, both the complement system and activating Fcgamma receptors were found to be important in the clearance of incompatible mRBCs by each of these IgG mAbs. In addition, the IgM E4 anti-hGPA mAb induced complement-dependent intravascular hemolysis of transfused incompatible hGPA-Tg-mRBCs accompanied by gross hemoglobinuria. These initial studies validate the relevance of these new mouse models for addressing important questions in the field of transfusion medicine.

Maternal alloimmunization against the rare platelet-specific antigen HPA-9b (Max a) is an important cause of neonatal alloimmune thrombocytopenia

BACKGROUND

Neonatal alloimmune thrombocytopenia (NATP) is an important cause of morbidity and mortality in the newborn. Optimal management of subsequent pregnancies requires knowledge of the alloantigen that caused maternal immunization, but this is possible only in a minority of cases. This study investigated whether this can be explained in part by maternal immunization against the "rare" alloantigen HPA-9b (Max(a)), implicated previously only in a single NATP case.

STUDY DESIGN AND METHODS

Archived paternal DNA from unresolved cases of NATP and normal individuals was typed for platelet (PLT)-specific antigens with real-time polymerase chain reaction and direct sequencing. PLT-specific alloantibodies were characterized by flow cytometry and solid-phase enzyme-linked immunosorbent assay. Recombinant GPIIb/IIIa was expressed in stably transfected Chinese hamster ovary cells. Clinical information was obtained directly from attending physicians.

RESULTS

Six of 217 fathers were positive for the presence of HPA-9b (Max(a)), an incidence about seven times that in the general population. In each of five cases studied, maternal serum samples reacted with intact paternal PLTs and paternal GPIIb/IIIa. Only one of three serum samples tested recognized recombinant GPIIb/IIIa carrying the HPA-9b (Max(a)) mutation. These seemingly discrepant reactions may reflect different requirements for oligosaccharides linked to residues close to the mutation in GPIIb that determines HPA-9b (Max(a)). NATP in the affected children was severe and was associated with intracranial hemorrhage in three of six infants on whom information was obtained.

CONCLUSIONS

Maternal immunization against HPA-9b (Max(a)) is an important cause of NATP and should be considered in cases of apparent NATP not resolved on the basis of maternal-fetal incompatibility for "common" PLT antigens.

Alteration of amino acid residues at the L-chain N-terminus and in complementarity-determining region 3 increases affinity of a recombinant F(ab) for the human N blood group antigen

BACKGROUND

To examine the fine specificity of glycopeptide-specific antibodies, this study focused on the human MN blood group system. F(ab) phage display methods were previously used to construct an F(ab) family in which the H-chain Fd fragment was held constant whereas the L chains were "shuffled." This yielded two related F(ab), N92 and NNA7, with low and high affinity for N, respectively. Although their L-chain sequences are very similar, sharing 92 percent amino acid identity, there are intriguing differences at the N-terminus and in complementarity-determining region 3 (CDR3) at positions 89, 91, 92, and 96.

STUDY DESIGN AND METHODS

Site-directed mutagenesis, ELISA, and hemagglutination were used to examine the contributions of these variations to antibody affinity.

RESULTS

Studies with the N92-S91G and NNA7-G91S mutants demonstrated that the Gly at position 91 was critically important for ensuring high affinity. Indeed, the affinity of N92-S91G was almost as high as N92TM, in which all four CDR3 residues were changed to match NNA7. N-terminal L-chain differences were surprisingly important in determining affinity. For example, when the N-terminus of N92 was changed to match that of NNA7, affinity increased approximately 30-fold.

CONCLUSION

Specific residues at the L-chain N-terminus and in CDR3 significantly affected F(ab) affinity for N. Future structural studies of these F(ab), alone and complexed with this glycopeptide antigen, will provide further insights into these phenomena.

Adherence of erythrocytes during exflagellation of Plasmodium falciparum microgametes is dependent on erythrocyte surface sialic acid and glycophorins

Malaria male gametocytes within a newly ingested infected blood meal in the mosquito midgut emerge from erythrocytes and extrude approximately eight flagellar microgametes in a process termed exflagellation. In culture, and in blood removed from infected patients, emerging microgametes avidly adhere to neighboring uninfected and infected erythrocytes, as well as to emerged female macrogametes, creating "exflagellation centers". The mechanism of erythrocyte adherence is not known nor has it been determined for what purpose microgametes may bind to erythrocytes. The proposition of a function underlying erythrocyte adherence is supported by the observation of species-specificity in adhesion: microgametes of the human malaria Plasmodium falciparum can bind human erythrocytes but not chicken erythrocytes, whereas avian host Plasmodium gallinaceum microgametes bind chicken but not human erythrocytes. In this study we developed a binding assay in which normal, enzyme-treated, variant or null erythrocytes are identified by a cell surface fluorescent label and assayed for adherence to exflagellating microgametes. Neuraminidase, trypsin or ficin treatment of human erythrocytes eliminated their ability to adhere to Plasmodium falciparum microgametes, suggesting a role of sialic acid and one or more glycophorins in the binding to a putative gamete receptor. Using nulls lacking glycophorin A [En(a-)], glycophorin B (S-s-U-) or a combination of glycophorin A and B (Mk/Mk) we showed that erythrocytes lacking glycophorin B retain the ability to bind but a lack of glycophorin A reduced adherence by exflagellating microgametes. We propose that either the sialic acid moiety of glycophorins, predominantly glycophorin A, or a more complex interaction involving the glycophorin peptide backbone, is the erythrocyte receptor for adhesion to microgametes.

Biochemical characterization of the O-glycans on recombinant glycophorin A expressed in Chinese hamster ovary cells

Alterations in N- and O-linked glycosylation affect cell surface expression and antigenicity of recombinant glycophorin A expressed in transfected Chinese hamster ovary (CHO) cells. To understand these effects further, glycophorin A was purified by immunoaffinity chromatography from transfected wild type and glycosylation deficient CHO cells. The O-glycans were characterized both biochemically, using gel filtration and high performance anion exchange chromatography, and immunologically, using carbohydrate specific monoclonal antibodies to probe Western blots. The O-glycans of human erythrocyte glycophorin A consist mainly of short oligosaccharides with one, two, or three sialic acid residues linked to a common disaccharide core, Gal beta 1-3GalNAc alpha 1-Ser/Thr, with the disialylated structure being the most abundant. With the exception of the trisialylated derivative, the same structures were found on recombinant glycophorin A expressed by wild type CHO cells. However, in contrast to human erythrocyte glycophorin A, the monosialylated oligosaccharide was the most abundant structure on the recombinant protein. Furthermore, recombinant glycophorin A was shown to express a small amount of the Tn antigen (GalNAc alpha 1-Ser/Thr). Recombinant glycophorin A had the same O-glycan composition, whether purified from clones expressing high or moderate levels of the recombinant glycoprotein. This indicates that the level of expression of the transfected glycoprotein did not affect its O-glycan composition. Deletion of the N-linked glycosylation site at Asn26, by introducing the Mi.I mutation (Thr28-->Met) by site-directed mutagenesis, did not markedly affect the O-glycan composition of the resulting recombinant glycoprotein expressed in wild type CHO cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Human erythrocyte glycophorins: protein and gene structure analyses

Human RBCs glycophorins are integral membrane proteins rich in sialic acids that carry blood group antigenic determinants and serve as ligands for viruses, bacteria, and parasites. These molecules have long been used as a general model of membrane proteins and as markers to study normal and pathological differentiation of the erythroid tissue. The RBC glycophorins known as GPA, GPB, GPC, GPD, and GPE have recently been fully characterized at both the protein and the DNA levels, and these studies have demonstrated conclusively that these molecules can be subdivided into two groups that are distinguished by distinct properties. The first group includes the major proteins GPA and GPB, which carry the MN and Ss blood group antigens, respectively, and a recently characterized protein, GPE, presumably expressed at a low level on RBCs. All three proteins are structurally homologous and are essentially erythroid specific. The respective genes are also strikingly homologous up to a transition site defined by an Alu repeat sequence located about 1 Kb downstream from the exon encoding the transmembrane regions. Downstream of the transition site, the GPB and GPE sequences are still homologous, but diverge completely from those of GPA. The three glycophorin genes are organized in tandem on chromosome 4q28-q31, and define a small gene cluster that presumably evolved by duplication from a common ancestral gene. Most likely two sequential duplications occurred, the first, about 9 to 35 million years ago, generated a direct precursor of the GPA gene, and the second, about 5 to 21 million years ago, generated the GPB and GPE genes and that involved a gene that acquired its specific 3' end by homologous recombination through Alu repeats. Numerous variants of GPA and GPB usually detected by abnormal expression of the blood group MNSs antigens are known. An increasing number of these variants have been structurally defined by protein and molecular genetic analyses, and have been shown to result from point mutations, gene deletions, hybrid gene fusion products generated by unequal crossing-over (not at Alu repeats), and microconversion events. The second group of RBC membrane glycophorins includes the minor proteins GPC and GPD both of which carry blood group Gerbich antigens. Protein and nucleic acid analysis indicated that GPD is a truncated form of GPC in its N-terminal region, and that both proteins are produced by a unique gene called GE (Gerbich), which is present as a single copy per haploid genome and is located on chromosome 2q14-q21.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemistry and molecular biology of MNSs blood group antigens

This chapter has reviewed the nature of antigens of the MNSs blood group system. The structures of the proteins and the molecular features and organization of glycophorin genes were described, emphasizing their domain arrangement and the extensive sequence homology that indicates that their common and variant alleles belong to a single gene family. Methods currently used to examine these antigens are immunoblotting and DNA typing. The majority of variant genes are hybrids of parent glycophorin genes in a variety of arrangements; they contain no other sequences but those of the parent genes. The structures of the hybrids are summarized in Figure 8. Several hybrids appear to have arisen by unequal homologous recombination but others appear to have occurred through gene conversion. In this system the molecular genetic basis for a single variant phenotype may differ, as documented by gene rearrangements that appear to have occurred, as separate events, at different sites in the same intron; this has resulted in protein structures (hence phenotypes) that are identical. For example, unequal homologous recombination occurring within intron 3 can have given rise to only a limited number of phenotypes, namely alpha M-delta S, alpha N-delta S, alpha M-delta S, alpha N-delta S and delta-alpha. In addition, different sites of an exon may have been involved in gene rearrangements through gene conversion leading to nearly identical protein structures, yet different serological phenotypes. Thus, gene conversion could be more significant for generation of antigenic diversification as the number of possible new alleles is quite large. The participation of the HGpE gene in these rearrangements would make this number even larger. New sites and the expressed pseudoexon have created the epitopes of the variant phenotypes, and sequences specific for several variant antisera have been identified. Thus, the molecular basis for several serological reactions involving this system is now better understood.

Identification by immunoblotting of the erythrocyte membrane sialoglycoproteins that carry the Vw and Mur antigens

Immunoblotting of the separated membrane components of MiI erythrocytes with anti-Vw identified a band of Mr 40,000 with a mobility close to that of beta-sialoglycoprotein corresponding to the abnormal alpha-sialoglycoprotein present in MiI cells. A comparison of results obtained when MiIII erythrocytes were immunoblotted with anti-Mur, anti-s and the monoclonal antibody R1.3, indicated that the Mur antigen is located on the abnormal delta-sialoglycoprotein of Mr 36,000 in MiIII erythrocytes. Prior treatment of MiI erythrocytes with neuraminidase resulted in an increase in the intensity of staining of the anti-Vw reactive component. This was consistent with the enhanced reactions observed in haemagglutination tests with neuraminidase-treated erythrocytes. The mobility of the Vw component was reduced when erythrocytes were pre-treated with low concentrations of neuraminidase but increased when higher concentrations of neuraminidase were used.

Isolation and characterization of BanLec-I, a mannoside-binding lectin from Musa paradisiac (banana)

A lectin (BanLec-I) from banana (Musa paradisiac) with a binding specificity for oligomannosidic glycans of size classes higher than (Man)6GlcNAc was isolated and purified by affinity chromatography on a Sephadex G-75 column. It did not agglutinate untreated human or sheep erythrocytes, but it did agglutinate rabbit erythrocytes. BanLec-I stimulated T-cell proliferation. On size-exclusion chromatography, BanLec-I has a molecular mass of approx. 27 kDa, and on SDS/PAGE the molecular mass is approx. 13 kDa. The isoelectric point is 7.2-7.5. BanLec-I was found to be very effective as a probe in detecting glycoproteins, e.g. on nitrocellulose blots.

Human-type blood group activities on chimpanzee erythrocytes with special reference to M and N

Human-type blood group activities on the red blood cells (RBCs) of three chimpanzees were individually examined with commercial mouse monoclonal antibodies (anti-A, -B, -H, -M, -N, -Lea, and -Leb) as well as lectins (UEA-I and VGA) and conventional polyclonal antisera for the systems ABO, MN, Lewis, Rh-Hr, P, Kell, Kidd, Duffy, and Lutheran. For further analysis of the MN antigens, treatment of the RBCs with sialidase, trypsin, and chymotrypsin were employed. The activities recognized among the three chimpanzees were A, H, M, N, Leb, c, S, k, and Jka. The RBCs of the three individuals possessed the A antigen which showed the same serologic activity as the human A1. Those chimpanzee RBCs showed higher H-activity than the human A1 RBCs. The Lewis b activity was revealed by the absorption-elution method. The RBCs of the three individuals showed a reactivity to the polyclonal anti-M reagents, which was affected by both the sialidase and trypsin treatment. The RBCs of two individuals were agglutinated with the monoclonal anti-N. The receptor was sensitive to sialidase and chymotrypsin. The RBCs of the three individuals, however, did not react with the monoclonal anti-M or with one of the polyclonal anti-N. These results indicate structural differences in the glycophorins and MN antigens between the human and chimpanzee.

Evidence for the occurrence of human erythrocyte membrane sialoglycoproteins in human kidney endothelial cells

Several mouse monoclonal antibodies recognising different epitopes on human erythrocyte sialoglycoproteins alpha and beta (syn: glycophorins A and C, respectively) react with human renal endothelium. Those monoclonal anti-blood group M and anti-blood group N antibodies which recognised the M/N epitopes on sialidase-treated human erythrocytes also reacted with human renal endothelium whilst those which recognise the M/N epitopes on native, but not on sialidase-treated human erythrocytes were not reactive with human renal endothelium. These results provide evidence for the occurrence of alpha and beta molecules in renal endothelium and suggest that alpha in renal endothelium is incompletely sialylated.

Haemoglobin M-Hyde Park associated with polyagglutinable red blood cells in a South African family

Twelve of 35 members tested in a large ethnically-mixed South African family were found to have both haemoglobin M type Hyde Park and persistent polyagglutinable red blood cells. The characteristics of the polyagglutination have not been recorded previously. The cells of affected family members were not agglutinated by Arachis hypogea, Dolichos biflorus or Salvia sclarea, but were agglutinated weakly by Salvia horminum and BSII (GSII) and reacted strongly with Glycine soja and Sophora japonica lectins. BSI (GSI) lectin agglutinated the group A but not the group O cells. The N and MN cells were agglutinated more strongly than normal by Vicia graminea, other anti-N lectins and human anti-N but the M and MN cells reacted as expected with human anti-M. The name 'Hyde Park' is provisionally suggested for this type of polyagglutination, although it appears unlikely that the evidently complete association between the polyagglutination and the variant haemoglobin is the result of a single genetic mutation. More likely, the connection has a post-genetic origin, perhaps showing that bonds, possibly affected adversely by precocious senescence, normally occur between the haemoglobin and alpha-sialoglycoprotein molecules in red blood cells.

Possible role of the carbohydrate residues on the structure of the N-terminus of glycophorin AM

Natural-abundance 13C nuclear magnetic resonance (13C-n.m.r.) was used to study the effect of monoglycosylation on the structure and dynamics of a pentapeptide related to the N-terminus of glycophorin AM. The results of this study indicate that a single point of glycosylation, on the pentapeptide, can significantly affect its structure. Moreover, glycosylation of this pentapeptide also affects its dynamic motion in solution. This study further defines the role that the carbohydrate residue plays in determining the structure about the N-terminus of glycophorin AM.

Two blood group M epitopes disclosed by monoclonal antibodies

Two cloned mouse hybridomas, designated G8 and E3, produced anti-M of immunoglobulin classes IgG2b and IgG1, respectively. No discrepancies were observed in testing over 5,000 normal donor blood samples with appropriately diluted G8 and E3 culture supernatant fluids in parallel with rabbit anti-M and anti-N typing reagents. The specificity and titer of antibodies produced by G8 and E3 were minimally affected by changes in temperature (37 degrees C, 22 degrees C, 4 degrees C). G8 and E3 showed reduced activity with type MM red cells that had been treated with either neuraminidase or papain, but differences were observed in the susceptibility of the respective epitopes to treatment with neuraminidase. Furthermore, G8 and E3 exhibited different specificities when used to test the red cells of nonhuman primates and erythrocytes of the rare MgMg human blood type. These results indicate the existence of at least two M antigen epitopes.

Individuals lacking the Gerbich blood-group antigen have alterations in the human erythrocyte membrane sialoglycoproteins beta and gamma

Membranes from erythrocytes with a new Gerbich (Ge)-negative phenotype (Leach phenotype), as well as those from two other Ge-negative phenotypes, were examined. Whereas cells of the Leach phenotype apparently lack three minor sialoglycoproteins (beta, beta 1 and gamma), the membranes of Ge- Yus- and Ge- Yus+ erythrocytes apparently lack beta- and gamma-sialoglycoproteins but contain additional diffusely migrating components of apparent Mr 30 500-34 500 and 32 500-36 500 respectively. Immunoprecipitation experiments showed that the abnormal components of both Ge- Yus- and Ge- Yus+ erythrocytes reacted with two monoclonal antibodies, BRIC 4 and BRIC 10. These antibodies have been shown to react with sialoglycoproteins beta and beta 1 in normal erythrocytes. Cytoskeletal preparations from Ge- Yus- and Ge- Yus+ erythrocyte membranes contained the abnormal components. In contrast with cells of the Leach phenotype, which are elliptocytic, Ge- Yus- and Ge- Yus+ were of normal shape, despite their apparent lack of beta- and gamma-sialoglycoproteins. It seems likely that the abnormal components in these cells contribute to their normal shape. Ovalocytic erythrocytes were shown to incorporate more radioactivity in the sialoglycoprotein-beta 1 region than normal erythrocytes after labelling by the periodate/NaB3H4 technique. It is suggested that abnormal components in Ge- Yus- and Ge- Yus+ erythrocytes result from chromosomal misalignment with unequal crossing-over at meiosis between the genes giving rise to beta-, beta 1- and gamma-sialoglycoproteins.

Structural analysis of the Ss sialoglycoprotein specific for Henshaw blood group from human erythrocyte membranes

The N-terminal structures of the MN and Ss erythrocyte membrane sialoglycoproteins (glycophorins A, B) from two Henshaw (He) blood-group heterozygotes were determined by manual sequencing of tryptic glycopeptides and various secondary fragments. No structural alteration of the MN glycoprotein could be detected. The He-specific portion of the Ss glycoprotein was found to exhibit the N-terminal sequence Trp-Ser+-Thr+-Ser+-Gly-(+ = glycosylation). Thus it differs at three positions from its normal counterpart which possesses 'N' activity and exhibits the N-terminal structure Leu-Ser+-Thr+-Thr+-Glu-. Analysis of the Ss glycoprotein from 15 He-negative erythrocyte samples did not reveal any of the three He-specific structural alterations. The presence of a glycine residue at the fifth position of the blood-group-M-active MN glycoprotein as well as in the He-specific Ss glycoprotein provides an explanation for the occurrence of antisera (anti-Me) reacting with the M and He antigens.

An interesting monoclonal anti-N produced following immunization with human group O, NN erythrocytes

Spleen cells from Balb/c mice given multiple injections of intact human erythrocytes (group O, NN) were fused with NS1 myeloma cells. Culture fluids from the resulting hybrid cells were screened for agglutinating antibody against a panel of erythrocytes. One cell line, 2/23, secreted an IgM antibody which reacted more strongly with NN than with MM cells. Neuraminidase or papain treatment of erythrocytes abolished agglutination whereas trypsin treatment did not. Reactions with U- erythrocytes of different MN phenotypes confirmed the anti-N specificity of monoclonal antibody 2/23. This is the first report of monoclonal anti-N stimulated by the immunization of mice with intact erythrocytes.

Further examples of human anti-Me found in sera of Israeli donors

A number of Israeli donors had anti-M in their sera, a proportion of which cross-reacted with N He(+) red cells. Anti-Me was detected and while the reactivity with M and He determinants could be separated by using trypsin-treated red cells, the cross-reactivity for M and He determinants was complete in absorption experiments. One serum had anti-M separable from anti-Me and another apparent anti-M was absorbed by trypsin-treated N He(+) red cells.

Monoclonal antibodies specific for the M- and N-forms of human glycophorin A

Four mouse monoclonal antibodies directed against the red cell membrane protein glycophorin A have been isolated and characterized. They are produced by hybridomas derived from SP2/0 myeloma cells and spleen cells from Biozzi mice immunized with a mixture of human erythrocytes from homozygous blood group M and N individuals. These antibodies recognize and bind to purified glycophorin A and to glycophorin on the red cell surface. All are of the IgGl, kappa light chain subclass and bind to determinants presented on the 39 amino acid, trypsin-sensitive, N-terminal peptide of glycophorin A. Three display differential specificities for the two allelic forms of glycophorin A; two are exquisitely specific for the M-form and one preferentially binds the N-form. Treatment of red cells with neuraminidase, which removes N-acetylneuraminic acid from glycophorin A, abolishes the binding of these three antibodies. The binding of the N-specific antibody is also sensitive to modification of the amino-terminal residue of the antigen. The fourth antibody binds equally well to both the M- and N-forms as well as to neuraminidase-treated red cells; thus it recognizes a public, N-acetylneuraminic acid independent glycophorin A determinant.

Possible role of the carbohydrate residues in the display of the MN blood group determinants by glycophorin A

Heterozygous glycophorin AM,N and homozygous glycophorin AM were reductively methylated with 13C-enriched formaldehyde in the presence of cyanoborohydride. Total reductive methylation modified the five lysine residues, and the N-terminal amino acid residues (serine and leucine) of glycophorins AM and AN, respectively. The 13C resonances of the incorporated labels were monitored as a function of the degree of glycosylation of the glycoprotein. While minimal, if any, structural changes were observed near the N-terminal amino acid upon removal of alpha-D-N-acetylneuraminic acid residues, gross structural changes were observed when most of the oligosaccharide chains were removed. We also found that progressive methylation of the lysine residues of glycophorin AM may influence either the chemical shift of one of the nonequivalent methyl groups of the N alpha, N-[13C]dimethyl serine residue, or one of the two states of glycophorin AM.

Mouse monoclonal anti-N. I. Production and serological characterization

Two examples of mouse monoclonal anti-N are described. The antibodies were derived from mice immunized with sialoglycoprotein extracts of group O MM ss erythrocyte membranes and the probable stimulus for immunization was glycophorin B associated N-antigen. Both antibodies reacted as direct agglutinins but appeared to recognize different epitopes with one having a greater dependence on sialic acid. The antibodies could prove to be valuable alternatives to those reagents used currently for N-blood grouping.

Mg and Mc: mutations within the amino-terminal region of glycophorin A

M and N are the two common ("normal") alleles at the MN locus of the MNSs blood group system. The antigens M and N that they determine are located within the amino-terminal region of glycophorin A. In the serologically active and glycosylated (*) fragment of glycophorin AN the sequence is Leu-Ser*-Thr*-Thr*-Glu-, and in that of glycophorin AM it is Ser-Ser*-Thr*-Thr*-Gly-. Mg and Mc are very rare ("variant") alleles of M and N; as to the corresponding antigens, Mg is serologically quite distinct from M and N, while Mc is a compound of both. Erythrocytes of genotypes MgN, MgM, MgMg, and McM, which were the object of the present study, contain normal amounts of glycophorin A in their membrane. In glycophorin AMg the amino-terminal sequence is related to that of glycophorin AN by substitution of asparagine for threonine in position 4, and it is nonglycosylated: Leu-Ser-Thr-Asn-Glu-. The corresponding structure of glycophorin AMc is Ser-Ser*-Thr*-Thr*-Glu-; it is thus closely related to that of glycophorin AN and AM, by substitution of the amino acids in positions 1 or 5, respectively. All of these substitutions can be explained by single base changes. The distinctions in chemical structure not only confirm the location of M and N in this region of glycophorin A, because they are the only differences observed, but also indicate, because they are correlated with the distinctions in antigenic specificity, that M and N are structural genes coding for amino acid sequences. The finding that Mc contains structural features of both M and N suggests that these two forms of glycophorin A have evolved from a common ancestral gene by single base substitutions at sites in the genome coding for amino acids in positions 1 and 5 of the sequence. Carbohydrate structures, however, are also necessary for full expression of antigens M and N. Glycosylation during biosynthesis of residues within the polypeptide appears to depend on a particular protein structure.

A human antiserum reacting with modified blood group M determinants

In the serum of a healthy blood donor, Mar, antibodies were identified that agglutinate blood group M erythrocytes only after their preincubation in glucose-containing solution. This agglutination was inhibited by blood group M glycoprotein preincubated in glucose solution but not by untreated M and N glycoproteins or N glycoprotein pretreated with glucose. A similar "activating' effect on M erythrocytes or M glycoprotein was achieved when glucose was replaced by mannose or N-acetylglucosamine, whereas several other replaced by mannose or N-acetylglucosamine, whereas several other sugars were ineffective. The "activating' effect of glucose on M antigen was time-, temperature-, pH-, and concentration-dependent. The binding of glucose to M glycoprotein was demonstrated under the preincubation conditions used in the above mentioned agglutinations. The results obtained suggest that the antibodies of serum, Mar, react with blood group M determinants modified by formation of a glycosylamine linkage between amino groups of the glycoprotein and glucose or other hexoses with the same configuration at asymmetric carbons 3, 4 and 5 as glucose.

The role of lectins in blood group serology

Many lectins display blood group activity, and extracts from Dolichos biflorus (anti-A1), Ulex europaeus (anti-H), and Vicia graminea (anti-N) seeds provide an alternative to human sera as a source of blood-typing reagents. However, the major application of lectins in blood group serology undoubtedly lies in the recognition and elucidation of red celll polyagglutination. In this respect, lectins from Arachis hypogaea (anti-T/Tk), Salvia sclarea (anti-Tn). Salvia horminum (anti-Tn + Cad). Dolichos biflorus (anti-Tn/Cad) Glycine max, and the N-acetyl-D-glucosminyl-binding lectin, BS II (anti-Tk) from Bandeiraea simplicifolia seeds, provide an invaluable source of reagents for use in investigative immunohemotology. Because of their specific carbohydrate-binding properties, lectins have also been used as probes in studies on the topography of the red cell surface. This latter appliction has provided much information on the structure of the MN, T, and Tn red cell surface receptors and has aided in defining the red cell membrane abnormalities associated with certain uncommon phenotypes within the MN blood group system.