Molecular characterization of a human anti-Rh(D) antibody with a DH segment encoded by a germ-line sequence

Section 5: Structural/genetic analysis of mAbs to blood group antigens. Coordinator's report

The heavy and light chain immunoglobulin variable region nucleotide sequences for 219 mAbs to human red blood cells were collected from workshop participants, published reports, and Genbank. Information regarding antigen specificity, species of origin, method of cloning, and other relevant serological properties was correlated with the sequence data. Immunoglobulin sequences were analyzed to determine the heavy- and light-chain immunoglobulin genes used and the overall extent of somatic mutation from germline configuration. Approximately 50% of the sequences encoded antibodies with Rh(D) specificity with the remaining sequences encoding mAbs to other Rh-related antigens, antigens of the ABO, MNS, and Kell blood group systems, and several others. Surprisingly, no sequence data were available for mAbs with specificity for a number of common Rh antigens, common Kell antigens, or antigens of the Lewis, Kidd, or Duffy blood group systems. The majority of mAbs were of human origin but included a significant number of macaque mAbs, murine mAbs, and a small number of synthetically-designed recombinant antibodies. Both cellular (EBV-transformation, cell fusion) and molecular (phage display) approaches were used for antibody cloning. Analysis of certain groups of sequences demonstrated patterns of immunoglobulin gene restriction, repertoire shift, and somatic mutation. Analysis of other mAbs demonstrated the value of antibody sequence data for the design and production of novel reagents useful in blood group serology.

Erythropoiesis: Splenic Immunoglobulin Variable Region Genes Encoding Red Blood Cell Binding Fab Fragments in Autoimmune Hemolytic Anemia

An absolute requirement for the V(H(4-34) ) immunoglobulin (Ig) variable (V) region heavy chain (V(H) ) gene has been demonstrated in pathogenic cold agglutinin autoantibodies. Investigation of IgG binding anti-Rhesus (Rh) alloantibodies provides further evidence of V gene restriction in red blood cell (RBC) binding antibodies and demonstrates that the V(H(4-34) ) gene used to form cold agglutinins may also encode RBC antibodies of varied specificities. We reasoned that a similar V gene restriction may be evident in the gene segments encoding IgG anti-RBC autoantibodies mediating autoimmune hemolytic anemia (AIHA). To further examine this question IgG Fab fragment phage display libraries were constructed from the spleen of a patient with AIHA. The index autoantibody appeared to have incomplete anti-C specificity and bound all panel RBCs except Rh null. The Fab fragment phage display libraries were therefore panned twice on CDE/CDe RBCs and binding phage were eluted. Binding of the phage displayed Fab fragments to RBCs was confirmed by immunoflourescence and flow cytometry. Specificity was confirmed by the absence of binding to Rh null cells, murine RBCs and to human peripheral blood lymphocytes. Molecular analysis of Ig V genes encoding the pan RBC binding Fab fragments revealed a relative V(H) gene restriction and evidence of somatic mutation. The V(H(3) ) family member V(H(26) ) was prominent in RBC binding Fabs. The V(H(3) ) family member hV3005 and the V(H(4) ) family DP-65 gene segments also encoded RBC binding Fabs. The J(H(4) ) gene segment was present in all binding clones. Varied kappa and lambda light chain (V(L) ) genes were identified by sequencing and no single light chain was prevalent. Three of the ten V(L) and two of the three V(H) identified by sequencing appeared to derive from germline genes previously noted to have RBC binding specificity. We conclude that splenic Ig V genes can encode pan RBC binding antibodies with specificities similar to autoantibodies found in AIHA and that V(H) gene segment utilization by these antibodies is derived from a limited pool of somatically mutated V(H) gene segments.

V(D)J germline gene repertoire analysis of monoclonal D antibodies and the implications for D epitope specificity

BACKGROUND

The D antigen is a highly immunogenic human RBC antigen. Alloimmunization against the D antigen produces high-affinity antibodies that cause hemolytic transfusion reactions and HDN.

STUDY DESIGN AND METHODS

Cloning and subsequent sequence analysis of 11 new samples of monoclonal anti-D was performed in an attempt to identify V(D)J germline gene usage. Sequences were compared and analyzed with 37 previously published samples of anti-D for identification of V(H) and V(L) pairings, canonical structures, and conformation of restricted germline gene usage.

RESULTS

The V(H) and V(L) pairings used by the new D MoAbs resulted in seven canonical combinations, three of which had not been described previously. Preferential usage of gene segments from the VH3 and VH4 families and of D3, D6, JH6, and DPK9 germline gene segments was also determined. Three samples of anti-D from different donors were found to use similar V(H) and V(kappa) germline genes, despite the fact that two of the antibodies recognized epD6/7 and the third recognized epD1. From the cumulative analysis of the anti-D IgG, 24 V(H) and V(L) gene pairings were identified, resulting in only 10 canonical structures.

CONCLUSIONS

Despite the potential for diversity, only a minority of V(H) and V(L) germline genes are used by anti-D. Consequently, V(H) and V(L) pairings and the resulting canonical structures are similarly restricted.

Genetic and Immunological Properties of Phage-Displayed Human Anti-Rh(D) Antibodies: Implications for Rh(D) Epitope Topology

Understanding anti-Rh(D) antibodies on a molecular level would facilitate the genetic analysis of the human immune response to Rh(D), lead to the design of therapeutically useful reagents that modulate antibody binding, and provide relevant information regarding the structural organization of Rh(D) epitopes. Previously, we described a Fab/phage display-based method for producing a large array of anti-Rh(D) antibodies from the peripheral blood lymphocytes of a single alloimmunized donor. In the current study, we present a detailed analysis of 83 randomly selected clones. Sequence analysis showed the presence of 28 unique γ1 heavy chain and 41 unique light chain gene segments. These paired to produce 53 unique Fabs that had specificity for at least half of the major Rh(D) epitopes. Surprisingly, despite this diversity, only 4 closely related heavy chain germline genes were used (VH3-30, VH3-30.3, VH3-33, and VH3-21). Similarly, nearly all Vκ light chains (15/18) were derived from one germline gene (DPK9). λ light chains showed a more diverse VL gene usage, but all (23/23) used the identical Jλ2 gene. Several Fabs that differed in epitope specificity used identical heavy chains but different light chains. In particular, 2 such clones differed by only 3 residues, which resulted in a change from epD2 to epD3 specificity. These results suggest a model in which footprints of anti-Rh(D) antibodies are essentially identical to one another, and Rh(D) epitopes, as classically defined by panels of Rh(D) variant cells, are not discrete entities. Furthermore, these data imply that the epitope specificity of an anti-Rh(D) antibody can change during the course of somatic mutation. From a clinical perspective, this process, which we term epitope migration, has significance for the design of agents that modulate antibody production and for the creation of mimetics that block antibody binding in the settings of transfusion reactions and hemolytic disease of the newborn.

Restricted Use of Cationic Germline VH Gene Segments in Human Rh(D) Red Cell Antibodies

The human red cell Rh(D) antigen elicits the production of high-affinity IgG antibodies, which can prevent blood transfusion and cause hemolytic disease of the newborn. It has been known for 20 years that Rh(D) antibodies are among the most positively charged human serum IgGs. Analysis by IEF of 9 human anti-Rh(D) monoclonal antibodies showed that their isoelectric points (pI) (8.3 to 8.6) were also significantly higher than the average pI of serum IgGs (7.0 to 8.5). Sequencing of the anti-Rh(D) H and L chains cDNAs showed a preferential use of VH1 , VH3, JH6, and Vκ1 gene segments. The high pIs in IEF were correlated with a higher number of cationic amino acid residues in the H chain V regions without clustering in the complementary determining region. Computer analysis indicated that the germline VH used in anti-Rh(D) was selected among the most cationic segments available in the human VH repertoire or expressed in normal B cells. These results indicate that the selection of cationic VH segments may be an important early step in the formation of clinically relevant anti-Rh(D) and other red cell antibodies, possibly to facilitate epitope binding in the negatively charged red cell membrane environment.

Three new cross-reacting idiotopes as markers for the products of two distinct human VH3 genes expressed in the early repertoire

This article describes characterization of three new cross-reacting idiotopes, as recognized by mouse MoAbs, on human antibodies utilizing VH3 genes that are expressed in the early repertoire. Two of the mouse MoAbs (3H7 and 3H1) were raised against a human MoAb utilizing the DP47 (VH26) VH3 gene, whilst the third (7B4) was raised against a DP46 (GLSJ2) gene product. Evidence for the anti-idiotypic specificity of the mouse MoAbs was provided by their reactivity with the immunizing IgM, but not with Fc mu, and by their specific inhibition of the binding between each immunizing antibody and its antigen. The three anti-idiotypic MoAbs were shown to be VH-specific reagents by the independence of their reactivity upon the L-chain type, or the antigenic specificity of the human MoAbs tested. Specificity of each mouse MoAb for VH3 gene-products was demonstrated by its sole cross-reactivity with VH3 proteins. Each anti-Id had a different reactivity pattern with a panel of MoAbs utilizing different VH3 genes. By relating the VH sequences of the tested VH3 proteins to their germline counterparts, 3H7 and 3H1 appeared to be specific for DP47-encoded proteins, although 3H1 had weak cross-reactivities with a few other VH3 gene-products. 7B4 appeared to be specific for antibodies utilizing DP46-related genes. Both 3H7 and 3H1 were also completely different to B6 and D12, two previously described MoAbs that also recognize VH3 proteins. Although 7B4 was similar to B6 and D12 in its binding to DP46-related gene products, B6 and D12 additionally recognized non DP46-related proteins and were thus different to 7B4.