Correlation of antibody multireactivity with variable region primary structure among murine anti‐erythrocyte autoantibodies

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Abstract
A high proportion of the antibodies in the preimmune repertoire bind to several unrelated antigens and are considered to be multireactive. This property is reportedly associated with the antibodies produced by CD5+ B lymphocytes. Because many antibodies specific for bromelain‐treated mouse red blood cells (BrMRBC) derive from CD5+ B cells, we tested monoclonal antibodies of this specificity for multireactivity. Two variable region combinations, VH11/Vϰ9 and VH12/Vϰ4, account for >80% of this repertoire, but none of these antibodies exhibited a multireactive phenotype. In contrast, three anti‐BrMRBC binding antibodies belonging to the J558 family (BrM1, BrM8, and CH12) showed varying degrees of multireactivity, and bound both highly negatively and positively charged antigens. The amino acid sequences of the VH regions of these antibodies are highly homologous (greater than 85 % identical) and they possess large VH‐D‐J junctions with extensive N‐region insertions. The ϰ chains of two of these antibodies utilize an identical Vϰ gene segment, while the third uses a very different Vϰ with only 50 % homology. The entire H chain V regions of these antibodies are unusually basic, with isoelectric points of 9.5–10, a feature which might be important in promoting interactions with acidic epitopes. The multi‐reactive antibodies also contain regions with a high concentration of hydroxylside chain amino acids, especially in their VH‐D‐J junctions. This region also contains acidic amino acid residues, which may be important in binding of positively charged epitopes. We propose that an open, accessible binding site and a charge polarity may be features which facilitate the binding of charged epitopes, providing a structural basis for multireactivity of at least some antibodies.

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