Molecular mapping of the 8.12 sle-associated idiotype specificity at the single amino acid level

The 8.12 idiotype is expressed in elevated titer in the serum of patients with systemic lupus erythematosus and is a marker for a subpopulation of anti-DNA antibodies that possess a VλII encoded light chain. This study utilized a eukaryotic expression system to identify the structural basis for expression of this idiotype. Reversion of the 8.12+ DSC light chain to the hs1v215.23/DPL11 germline gene reveals that the 8.12 idiotype is encoded in the germline. The 8.12+ DSC and the 8.12− AS17 light chains, both belonging to the VλII family, were subjected to site directed mutagenesis, to localize amino acids important for expression of the 8.12 idiotype. Point mutations were performed in CDR1, CDR2, FR3 and CDR3, in positions where the 8.12+ DSC differs from the 8.12− AS17. Amino acids in CDR1 and the CDR2 proximal region of FR3, but not the J proximal region of CDR3, play a crucial role in 8.12 reactivity. The 3-D structure of Mcg, a human IgG1, with which DSC shares a sequence homology of 92.3% has been examined to visualize the effect of each of the mutations and to identify the surface on DSC that comprises the idiotype.