The production of human anti-red blood cell (RBC) Igs in vitro from immunized individuals would greatly facilitate the genetic analysis of the human immune response to RBC antigens and also provide useful serologic reagents. Technical difficulties inherent in human B-cell immortalization have led to the development of molecular approaches that bypass the need for cell transformation. By cloning human Ig gene segments into bacterial expression vectors, libraries are created of filamentous phage particles displaying Fab fragments on their surfaces. Libraries have been screened with purified, soluble antigen and selected clones genetically manipulated in Escherichia coli to produce soluble Fab fragments. Our goal has been to adapt this technique to the study of RBC autoantibodies and alloantibodies that have specificities against unpurifiable membrane-bound antigens. To test the feasibility of this approach, two sets of phage were created, one set expressing a human anti-Rh(D) Ig and the other expressing a human antitetanus toxoid Ig. After verifying the presence of functional phage-displayed Fabs through biochemical, flow cytometric, and electron microscopic analyses, a model library was constructed comprising one anti-Rh(D)- expressing phage per 10(4) antitetanus toxoid-expressing phage. A method was developed for screening the library with intact Rh(D)- positive RBCs. After four rounds of panning, anti-Rh(D) specificity was enriched more than 10,000-fold to a final frequency of approximately 100%. Plasmid DNA derived from anti-Rh(D) phage was used to produce milligram quantities of soluble recombinant anti-Rh(D) Fabs purified by nitrogen cavitation and nickel-chelation affinity chromatography. The authenticity of the Fabs was confirmed by sodium dodecyl sulfate- polyacrylamide gel electrophoresis and immunoblotting, which showed bands with molecular weights of approximately 50 kD and 26 kD under nonreducing and reducing conditions, respectively. Binding of recombinant anti-Rh(D) Fabs to Rh(D)-positive RBCs was demonstrated by flow cytometry and by an agglutination assay. Our results suggest that repertoire cloning by cell-surface enrichment may have broad application to the study of the human immune response to erythroid antigens in addition to membrane-bound antigens present on other hematopoietic cells.

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