Isolation of an IgG anti-B from a human Fab-phage display library

BACKGROUND: ABO incompatibility is a common cause for mild hemolysis in the newborn, ranging from 1 in 30 to 1 in 150 births. Fortunately, hemolysis r equiring transfusion is rare and restricted to blood group O mothers, becau se blood group A and B individuals make poor IgG anti-B and anti-A response s. No human IgG ABO antibody sequences have been reported, in part because of the difficulty in obtaining human IgG hybridomas. Phage-display technolo gy may be able to circumvent these difficulties, but its application to car bohydrate antigens is poorly studied. STUDY DESIGN AND METHODS: A human IgG1 phage-display Fab library was constr ucted from splenocytes derived from a nonhyperimmunized blood group O perso n, and panned against group B RBCs. RESULTS: After five rounds of panning, essentially all phage bound to group B RBCs. Nucleotide sequence analysis of a single monoclonal IgG1 lambda ph age, FB5.7, revealed a highly mutated VH4 family heavy chain, and a nearly germline VL7 family lambda light chain. The Fab agglutinated group B, but n ot group A, random-donor RBCs. However, group B ELISA reactivity could be i nhibited by soluble B-trisaccharide, soluble A-trisaccharide, galactose, an d N-acetyl galactosamine. Similarly, galactose and N-acetyl galactosamine w ere able to inhibit group B RBC agglutination. CONCLUSION: FB5.7 is the first human IgG ABO MoAb described. Alhough it beh aves serologically like a group B-specific antibody, it demonstrates intera ction with both the A and B epitopes. Phage-display technology can be used to better define the relationship between antibody genotype and phenotype i n anti-carbohydrate responses in nonhyperimmunized hosts, and thus to impro ve our understanding of the composition of the antibody repertoire.