MONOCLONAL IGM RHEUMATOID-FACTOR SECRETED BY CDS-NEGATIVE B-CELLS DURING MIXED CRYOGLOBULINEMIA - EVIDENCE FOR SOMATIC MUTATIONS AND INTRACLONAL DIVERSITY OF THE EXPRESSED V-H REGION GENE

Mixed cryoglobulinemia is usually considered to be a nonmalignant huma n B cell proliferation that produces a monoclonal IgM rheumatoid facto r (RF). Important immunologic similarities and differences were descri bed between the monoclonal B cells during mixed cryoglobulinemia and d uring malignant chronic lymphocytic leukemia (CLL): high frequency of the same V-H and V kappa gene usage by both types of monoclonal B cell s producing IgM with RF activity, apparent intraclonal homogeneity, bu t different expression of the pan T cell CD5 Ag. The description of an unusual CD5-negative B cell CLL case secreting a mutated IgM RF led t he authors to suggest that the usage of non-mutated germline Ig genes is a property of cells derived from the CD5 lineage or stage of differ entiation, rather than an intrinsic property of CLL or of IgM RF-produ cing cells in general. Because mixed cryoglobulinemia cells are usuall y CD5-negative, it was of interest to test for the existence of mutati ons in the V-H and V kappa regions, as well as for the intraclonal hom ogeneity of the expressed Ig genes. In this study, we used the PCR tec hnique to analyze the monoclonal rheumatoid factor (mRF) V genes from a patient with mixed cryoglobulinemia. We show that the CD5-negative m onoclonal B cells express a slightly mutated V kappa 3 gene, but a mor e mutated V(H)1 gene whose genomic counterpart was shown to be the 51p 1 germline gene. The sequence analysis of several independent clones s hows some degree of intraclonal diversity, suggesting the existence of a clonal filiation. These results are discussed in terms of the origi n of the monoclonal B cell during mixed cryoglobulinemia and CLL.