Chemically humanized murine monoclonal antibody against a cell nuclear antigen: usefulness in autoimmune diagnostics

The cellular nuclear antigen SS-B/La is known to be a major antigenic target to an autoantibody in patients with Sjogren's syndrome and systemic lupus erythematosus. It is useful to detect an anti-SS-B/La antibody from patients' sera in a clinical point of view. We purified SS-B/La from rabbit thymus acetone powder by affinity chromatography with a murine anti-SS-B/La monoclonal antibody (1C3-H7). An enzyme-linked immunosorbent assay method, in which SS-B/La was used to coat a plate, was also successfully established. It is difficult to obtain a large volume of patient's serum with high antibody titer and high specificity as a positive control. We investigated whether or not a positive control from human could be replaced by a murine monoclonal antibody to SS-B/La. The 1C3-H7 was conjugated with a human IgG Fc' fragment using N-gamma-maleimidobutyryloxysuccinimide as a cross-linker. The chemically humanized murine monoclonal antibody (1C3-Fc') was recognized by antiserum specific for human IgG Fc fragment. 1C3-Fc' reacted to SS-B/La but not to other antigens. Furthermore, the titration curve of this conjugate ran parallel with those of patients' sera specific for SS-B/La. It is concluded that a chemically humanized murine monoclonal antibody is useful as a positive control in place of a human patient's serum.