V_H4.21-Encoded Natural Autoantibodies with Anti-i Specificity Mirror those Associated with Cold Hemagglutinin Disease

The Ig V_H regions of virtually all human pathogenic cold agglutinin (CA) anti-i/I autoantibodies are encoded by a single Ig V_H gene segment, V_H4.21, in conjunction with a highly variable CDR3 structure. The ant,-I specificity is often associated with VκIII-encoded L chains, whereas anti-i autoantibodies appear to use a broader array of κ and λ V_L gene segments. B cells expressing V_H4.21 are abundantly present in adult lymphoid tissues from healthy individuals but their relationship with B cells secreting pathogenic CA is unknown. Herein we have analyzed the distribution of V_H4.21-expressing B cells in fetal, neonatal, and adult B cell populations using the monoclonal anti-V_H4.21 antibody 9G4. In addition, we have analyzed the anti-i and anti-I binding capacity and V regions of 19 V_H4.21-encoded mAb secreted by cord and adult blood-derived cell lines from healthy individuals. The results show that V_H4.21 expressing B cells are overrepresented in all repertoires studied and are evenly distributed over cord blood CD5⁺ and CD5^- B cell populations. V_H4.21-encoded H chains strongly predispose for ant.-i binding capacity, regardless of the V_L regions or H chain CDR3 structure. The avidity of some of these antibodies was similar to those of pathogenic CA. In addition, we found evidence for monospecific anti-I binding in antibodies encoded by other members of the V_H4 gene family. We conclude that V_H4.21-encoded antibodies with anti-i specialty from the normal B cell repertoire mirror their pathogenic counterparts and that naturally occurring ant.-I antibodies may be encoded by a more diverse array of V_H4 genes.