Cloning of a Human IgM Autoantibody Bearing a Cross-Reactive Idiotype in a λ. Expression Vector: A New Approach to Studying Autoantibodies

The monoclonal antibody 4B4 is a human IgM,κ which expresses a cross-reactive lupus-associated idiotype and has anti-Sm binding activity. We find that the V_L nucleotide sequence of 4B4, like the 4B4 V_H region, is encoded by unmutated germline genes. The 4B4 V_H and V_L were cloned into the ImmunoZap λ expression vector to produce three recombinant immunoglobulin polypeptides. These recombinant polypeptides expressed, respectively, either the 4B4 V_H or V_L alone or a V_H/V_L heterodimer. ELISA showed that the V_H/V_L heterodimer retained anti-Sm antibody activity. The 4B4 idiotype was found predominantly on the V_H. This report describes: (i) a method for producing recombinant immunoglobulin molecules from an IgM-secreting B cell line and (ii) the ability of recombinant antibody fragments expressed in Escherichia coli to retain the structural and antigenic properties of the native molecule.