The Cross-Reactive Idiotopes Recognized by the Monoclonal Antibodies 9G4 and LCI are Located in Framework Region 1 of Two Non-Overlapping Subsets of Human VH4 Family Encoded Antibodies

The monoclonal anti-idiotopic antibodies LCI and 9G4 bind two non-overlapping sets of V_H4 encoded antibodies. 9G4 exclusively binds V_H4–21 encoded antibodies, while LCI binds antibodies derived from V_H4 family gene segments V71-2, V71-4, V_H4-18, V_H72-I and V2-1. The V_H4–21 gene segment is utilized by most cold agglutinin (CA) antibodies with I/i specificity, while antibodies encoded by other V_H4 gene segments are associated not with CA disease, but primarily with rheumatoid-factor (RF) activity.
We previously determined that the idiotope to which 9G4 binds in V_H4-21-derived antibodies is located in framework region l (FR1). In the present study, by using mutational analysis involving individual framework- and complementarity-determining region exchanges between V_H4-2I- and V71-2-encoded antibodies, we have found that the idiotope to which LCI binds in V71-2-derived antibodies also maps to FR1. The LC1 idiotope is heavy (H)-chain associated, but requires pairing with a light (L) chain for LCI binding. Recombinant antibodies composed of a variety of kappa (k) and lambda L chains paired with either a V71-2 or V_H4–21 chain were produced in the baculovirus expression system. LCI bound all of the k-containing antibodies but did not bind the V71-2-encoded H chain alone nor to the two A-containing antibodies. This experiment demonstrates that not all light chains exert equivalent influence on the conformation of the H-chain idiotope. These results indicate that the FR1 of V_H4-encoded antibodies is immunogenic and suggest a physiological role of FR l during an immune response.