Complete sequence of the genes encoding the V_H and V_L regions of low- and high- affinity monoclonal lgM and lgA1 rheumatoid factors produced by CD5⁺ B cells from a rheumatoid arthritis patient

We have characterized the V_H and V_L genes of three low-affinity polyreactive and two hlgh-affinity monoreactive IgM and lgA1 rheumatoid factor (RF) mAb generated using circulating CD5⁺ B cells from a single rheumatoid arthritis patient. We found that four and one RF mAb utilized genes of the V_HIV and V_HIII families, respectively. The V_HIV gene usage by these RF mAb differs from the preferential V_HIII, V_HI, and, to a lesser extent, V_HII gene usage by the IgM with RF activity found In patients with mixed cryoglobulinemia, Waldenstrom's macroglobulinemia, and other monoclonal gammopathies. In addition, in contrast to the preponderant xL chain usage by the RF in these patients, a λL chain was utilized by all RF mAb from our rheumatoid arthritis patient. Two RF mAbs utilized V_λl, two V_λIV, and one V_λlll L chains. The V_H genes of the two low-affinity polyreactive IgM RF mAb were in germllne configuration. When compared with the deduced amino acid sequence of the putatively corresponding genomic segment, the V_H gene of the high-affinity monoreactive IgM RF mAb displayed five amino acid differences, all of which are in the complementarity determining regions (CDR), possibly the result of a process of somatic point mutation and clonal selection driven by Ag. The unavailability of the corresponding genomic V_H segment sequences made it impossible to infer whether the V_H genes utilized by the two lgA1 RF were in a germline or somatically mutated configuration. Sequencing of the genes encoding the H chain CDR3 (D segments) revealed that all three low-affinity polyreactive RF mAb displayed a much longer D segment (36-45 bases) than their high-affinity monoreactive counterparts (15-24 bases), raising the possibility that a long D segment may be one of the factors involved in antibody polyreactivity.