Both VH and VL chains of polyreactive IgM antibody are required for polyreactivity: expression of Fab in Escherichia coli.

Monoclonal polyreactive antibodies can bind to many structurally dissimilar self and non-self antigens. Neither the precise antigen-binding site on the polyreactive antibody molecule nor the molecular basis of polyreactivity has been elucidated. The present study was initiated to see whether antibody genes encoding the Fab fragment of a human monoclonal polyreactive IgM antibody (MoAb 67) could be efficiently expressed in Escherichia coli, and whether the bacterially expressed Fab fragments possessed biological activity. cDNA encoding the variable domains of the heavy and light chains of MoAb 67 were cloned, amplified by polymerase chain reaction (PCR) and expressed in E. coli. Neither the recombinant heavy nor light chain showed antigen-binding activity. In contrast, the recombinant Fab 67 fragment showed the same antigen-binding reactivity profile as the native IgM antibody. It is concluded that the antigen-binding activity of polyreactive antibodies resides in the Fab fragment, and that both the heavy and light chains are required for activity.