BOTH SM AND DNA ARE SELECTING ANTIGENS IN THE ANTI-SM B-CELL RESPONSEIN AUTOIMMUNE MRL LPR MICE/

More than half of the anti-Sm hybridomas isolated from MRL/Mp-lpr/lpr (MRL/lpr) mice produce Abs that also bind ssDNA, and half of these bin d dsDNA. Intraclonal comparisons indicate that DNA is a selecting Ag f or at least some dual-binding clones. To determine whether Sm itself i s a selecting Ag for anti-Sm, we have identified the somatic mutations within the expressed V-H and V kappa genes of eight anti-Sm hybridoma s, six of which do not bind DNA. We find that these V genes have betwe en 0 and 12 somatic mutations each, and that four hybridomas possess a higher number of heavy or light chain CDR replacement (R) mutations t han expected by chance, suggesting that these anti-Sm-producing B cell s have undergone Ag selection. To demonstrate directly the effect of s omatic mutation on Sm binding, we have engineered the unmutated counte rpart of Ab 2-12, an Sm-specific hybridoma Ab with a nonrandom distrib ution of V kappa CDR R mutations, and compared its ability to bind Sm and ssDNA with that of the originally isolated 2-12 Ab. We find that t he unmutated Ab has a much lower avidity for Sm than the mutant, but, unlike the mutant, it binds ssDNA. We conclude that Sm can drive clona l expansion in the anti-Sm response, and that Sm-only binding B cells can arise from Sm/DNA dual-binding B cell clonal precursors. These dat a also suggest that dual binding is not necessary to sustain clonal ex pansion. Thus, this response is unique in that it can be driven by eit her of two Ags.