Idiotypic Determinants Used in the Analysis of Antibody Diversification and as Regulatory Targets" zy K. BEYREUTHER, J. BOVENS, M. BRUGGEMANN, R. DILDROP, G. KELSOE, U. KRAWINKEL, C. MULLER, S. NISHIKAWA, A. RADBRUCH, M. RETH, M. SIEKEVITZ, T. TAKEMORI, H. TESCH, G. WILDNER, S. ZAISS, AND K. RAJEWSKYb zyx Institute for Genetics University zyxw of Cologne Cologne, Federal Republic of Germany INTRODUCTION zyxw In 1973, Imanishi and Makela found that the primary immune response of C57BL/6 mice against the hapten 4-hydroxy-3-nitrophenylacetyl (NP) is dominated by antibodies that bind the cross-reacting hapten 5-iodo-NP (NIP) with higher affinity than the immunizing hapten (heteroclicity).' The highly restricted nature of the response was also later demonstrated in isoelectric focusing analysis and was shown to be controlled by the IgH Iocus.*~~ The vast majority of primary anti-NP antibodies of IgHbmice carry zyxwvu X chains: in contrast to the 95% K-bearing antibodies in normal mouse immunoglobulin, and express a recurrent idiotype (called NPb) that is controlled by both the Ighband the zyxwvu IgX locus.s-' The monoclonal antibody technique permitted us to analyze the NPb idiotype at the level of individual antibody molecules. The idiotype turned out to be expressed by a large family of closely related, but distinct, antibody species, the NPb antibody family.8*9 The variable (V) regions of three monoclonal antibodies of our collection were sequenced by A. Bothwell in D. Baltimore's group at the DNA level and the corresponding V genes in the germ line were identified (references 10 and 11, and A. Bothwell, M. Paskind, and D. Baltimore, personal communication). Two of the antibodies, isolated from the primary response and designated BI-8 (IgM, XI) and BI-48 (IgG,, XI), carried a V, sequence identical to that of a germ line VH gene, V186-2. The third antibody, S43 (IgG2,, XI) had been isolated from a hyperimmune response. Its VHregion was also clearly derived from V186-2, but it carried ten point mutations resulting in seven amino acid substitutions. All three antibodies had drastically different D segments, expressed either JH 1 or JH2, and carried A1 chains. The V, region of antibody BI-8 was germ line encoded, whereas that of antibody S43 carried two amino acid substitutions.'0s'' We have recently learned from Dr. H. Sakano and Dr. K. Karjalainen (personal communication) that two further anti-NP antibodies of C57BL/6 origin also carry a V186-2 encoded V, region. Their D segments are 'This work was supported by the Deutsche Forschungsgemeinschaft through SFB 74, the 'Correspondence to: K. Rajewsky, Institut fur Genetik. Universitat zu KBln, Weyertal 121, W' senschaftsministerium des Landes Nordrhein-Westfalen, and the Fazit-Foundation. D-5000 Koln 41, F.R.G. 121