SM AND DNA-BINDING BY DUAL REACTIVE B-CELLS REQUIRES DISTINCT V-H, V-KAPPA, AND V-H CDR3 STRUCTURES

We have previously demonstrated an overlap of the anti-Sm and anti-DNA responses in MRL/Mp-lpr/lpr mice. The Ab produced by many anti-Sm hyb ridomas bind DNA and are encoded by Ig V genes used by anti-DNA hybrid omas. In addition, some anti-Sm Ab that bind DNA have acquired mutatio ns that improve DNA binding, indicating that DNA is a selecting Ag in the anti-Sm response. To gain insight into the basis for the dual bind ing ability of these Ab, we coexpressed the H chain from the anti-Sm h ybridoma 2-12 with nine different L. chains. Hybridoma 2-12 binds Sm b ut not DNA, yet expresses the same J558 V-H gene as three anti-Sm hybr idomas that bind ssDNA and at least one anti-DNA hybridoma that does n ot bind Sm. We found that most of the transfectoma Ab bind Sm, but the ir avidities vary over more than 3 orders of magnitude. Five of the ni ne transfectoma Ab bind ssDNA, and none bind dsDNA. In general, the ab ility to bind each Ag follows the binding ability of the hybridoma fro m which the L chain is derived. H Chain swapping experiments indicate that the H chain, V-H CDR3 in particular, contributes to the binding o f both Sm and DNA. We conclude that Sm and DNA select for distinct fea tures of V-H, V-K, and V-H CDR3, suggesting selection by both Ag in th e anti-Sm response.