Structural basis of a conserved idiotope expressed by an autoreactive human B cell lymphoma. Evidence that a VH CDR3 mutation alters idiotypy and specificity

Our laboratory has previously investigated the relationship of autoimmune disease and B cell neoplasia in a patient with a diffuse, well differentiated splenic B cell lymphoma and associated autoimmune hemolysis due to an anti-Pr2 antibody. EBV-immortalized B cell clones, established from this lymphoma, were shown to secrete the same pathologic anti-Pr2 antibody. The antiidiotypic mAb, RI.1, defined a private Id (IdRI.1) of the anti-Pr2 antibody that was related to the Ag-binding site and was expressed by both the lymphoma and derived cell lines. This unique Id was expressed by the majority of splenic tumor B cells and also was conserved over a period of 4 yr. In this report, the structural basis of IdRI.1 expression was investigated by analysis of Id- variants isolated by flow microfluorimetry using RI.1. Six Id- cell lines that secrete IgM kappa but lack Pr2 specificity were generated from an Id+ cell line, LS2. These lines were shown to be related to LS2 and the lymphoma by karyotype and by restriction fragment analysis of Ig gene rearrangements. Shared and unshared nucleotide substitutions in the VH and VL regions of the six independent clones were used to construct a genealogic tree relating the Id- clonal members to a common Id+ precursor. The tree illustrates that the base changes occurred sequentially, suggesting that they were introduced by somatic point mutation. Only one VH CDR3 bp difference from the LS2 nucleic acid sequence is common to all Id- sequences, resulting in an amino acid substitution of cysteine 108 to tyrosine. Taken together, these findings suggest that both the expression of IdRI.1 and Ag binding are affected by a single mutation localized to the D region of the anti-Pr2 antibody.